=19. Constituents to be Determined.=—The most important point in
the analysis of mineral phosphates is to determine their content of
phosphoric acid. Of equal scientific interest, however, and often of
great commercial importance is the determination of the percentage of
other acids and bases present. The analyst is often called on, in the
examination of these bodies, to make known the content of water both
free and combined, of organic and volatile matter, of carbon dioxid,
sulfur, chlorin, fluorin, silica, iron, alumina, calcium, manganese,
magnesia, and the alkalies. The estimation of some of these bodies
presents problems of considerable difficulty, and it would be vain to
suppose that the best possible methods are now known. Especially is
this the case with the processes which relate to the estimation of the
fluorin, silica, iron, alumina, and lime. The phosphoric acid, however,
which is the chief constituent from a commercial point of view, it
is believed, can now be determined with a high degree of precision.
Often the estimation of some of the less important constituents is of
great interest in determining the origin of the deposits, especially
in the case of fluorin. While the merchant is content with knowing the
percentage of phosphoric acid and the manufacturer asks in addition
only some knowledge of the quantity of iron, alumina, and lime the
analyst in most cases is only content with a complete knowledge of the
constitution of the sample at his disposal.
=20. Direct Estimation of the Phosphoric Acid.=—It often happens, in
the case of a mineral phosphate, that the only determination desired
is of the phosphoric acid. In this instance the analyst may proceed as
follows: If the qualitative test shows the usual amount of phosphoric
acid, two grams of the sample passed through a sieve, with a millimeter
mesh, are placed in a beaker and thoroughly moistened with water. The
addition of water is to secure an even action of the hydrochloric acid
on the carbonates present. The beaker is covered with a watch-glass
and a little hydrochloric acid is added from time to time until all
effervescence has ceased. There are then added about thirty cubic
centimeters of aqua regia and the mixture raised to the boiling-point
on a sand-bath or over a lamp. The heating is continued until chlorin
is no longer given off and solution is complete. The volume of the
solution is then made up to 200 cubic centimeters without filtering,
filtered, and an aliquot part of the filtrate, usually fifty cubic
centimeters, representing half a gram of the original sample, taken
for the determination of the phosphoric acid according to the method
of the Official Agricultural Chemists. The small quantity of insoluble
material does not introduce any appreciable error into the process when
the volume is made up to 200 or 250 cubic centimeters.
=21. Method of the Official Agricultural Chemists for Total Phosphoric
Acid.=—To the hot solution, for every decigram of phosphorus pentoxid
which may be present, add fifty cubic centimeters of the molybdic
solution. Digest at 65° for an hour, filter, and wash with water or
ammonium nitrate solution[11]. Test the filtrate by renewed digestion
with additional molybdate reagent. Dissolve the precipitate on the
filter with ammonia in hot water and wash into a beaker, making the
volume of filtrate and washings not more than 100 cubic centimeters.
Nearly neutralize with hydrochloric acid, cool, and add magnesia
mixture from a burette at the rate of about one drop a second, stirring
vigorously, meanwhile. The quantity of magnesia mixture to be added is
not prescribed in the official method but it should always be in excess
of the amount necessary for complete precipitation. For each decigram
of phosphorus pentoxid, from eight to ten cubic centimeters should
be used. Fifteen minutes after the last of the magnesia mixture has
been stirred in, thirty cubic centimeters of ammonia of 0.95 specific
gravity are added and the beaker set aside for two hours or longer. The
ammonium magnesium phosphate is separated by filtration, dried, ignited
gently at first, and finally over a blast-lamp and weighed as magnesium
pyrophosphate. The factors for calculating the phosphorus pentoxid and
tricalcium phosphate from the weight of pyrophosphate are given below
on the two bases; _viz._, hydrogen equals 1, and oxygen equals 16.
H = 1.
Mg₂P₂O₇ × 0.63976 = P₂O₅
Mg₂P₂O₇ × 1.3964 = Ca₃(PO₄)₂
P₂O₅ × 2.1827 = Ca₃(PO₄)₂
O = 16.
Mg₂P₂O₇ × 0.63792 = P₂O₅
Mg₂P₂O₇ × 1.3926 = Ca₃(PO₄)₂
P₂O₅ × 2.1831 = Ca₃(PO₄)₂
=22. Preparation of Solutions.=—_Molybdic Solution._—Dissolve 100 grams
of molybdic acid in 400 grams or 417 cubic centimeters of ammonia, of
0.96 specific gravity, and pour the solution thus obtained into 1,500
grams or 1,250 cubic centimeters of nitric acid, of 1.20 specific
gravity. Keep the mixture in a warm place for several days, or until
a portion heated to 40° deposits no yellow precipitate of ammonium
phosphomolybdate. Decant the solution from any sediment and preserve in
glass-stoppered vessels.
_Magnesia Mixture._—Dissolve twenty-two grams of recently ignited
calcined magnesia in dilute hydrochloric acid, avoiding an excess of
the latter. Add a little calcined magnesia in excess, and boil a few
minutes to precipitate iron, alumina, and phosphoric acid; filter, add
280 grams of ammonium chlorid, 700 cubic centimeters of ammonia of
specific gravity 0.96, and water enough to make a volume of two liters.
Instead of the solution of twenty-two grams of calcined magnesia, 110
grams of crystallized magnesium chlorid may be used.
_Dilute Ammonia for Washing._—One volume of ammonia, of 0.96 specific
gravity, mixed with three volumes of water, or usually one volume of
concentrated ammonia with six volumes of water.
=23. Use of Tartaric Acid in Phosphoric Acid Estimation.=—In the
presence of iron the molybdate mixture is likely to carry down some
ferric oxid with the yellow precipitate. To prevent this, and also
hinder the separation of molybdic acid in the solution on long
standing, tartaric acid has been recommended.
Jüptner has found that the presence of tartaric acid does not interfere
with the separation of the yellow precipitate, as some authorities
assert.[12] Even 100 grams of the acid in one liter of molybdate
solution produce no disturbing effect. Molybdate solution treated
with tartaric acid did not show any separation of molybdic acid
when kept for a year at room temperatures. The presence of tartaric
acid, therefore, is highly recommended by him to prevent the danger
of obtaining both ferric oxid and molybdic acid with the yellow
precipitate.
=24. Water and Organic Matters.=—The sample, according to the practice
of Chatard, should be ground fine enough to leave no residue on an
eighty mesh sieve, and should be thoroughly mixed by passing it three
times through a forty mesh sieve[13].
Two grams are weighed into a tared platinum crucible. This, with
its lid, is placed in an air-bath at 105°, and heated for at least
three hours. The lid is then put on, and the crucible is placed in
a desiccator and weighed as soon as cold. The loss in weight is the
moisture.
Wyatt recommends that two grams of the fine material be heated in
ground watch-glasses, the edges of which are separated so as to allow
the escape of the moisture.[14] The heating is continued for three
hours at 110°, the watch-glasses then closed and held by the clip,
cooled in a desiccator, and weighed. This method is excellent for very
hygroscopic bodies, but where quick-acting balances are used, scarcely
necessary for a powdered mineral.
The residue from the moisture determination is gradually heated to
full redness over a bunsen, and then ignited over the blast-lamp.
This operation is repeated after weighing until a constant weight is
obtained. The loss (after deducting the percentage of carbon dioxid as
found in another portion) may be taken as water and organic matter.
This method is sufficient for all practical purposes; but when minerals
containing fluorin are strongly ignited, a part of the fluorin is
expelled; hence, if more accurate determinations are required, the loss
of fluorin must be taken into account. In this laboratory it has been
proved that a pure calcium fluorid undergoes progressive decomposition
at a bright red heat with formation of lime.
Wyatt directs that the combined water and organic matters be determined
in the residue from the moisture estimation as follows: The residue is
brushed into a weighed platinum crucible, which is heated over a small
bunsen for ten minutes and then brought to full heat of a blast-lamp
for five minutes. After cooling, the total loss is determined by
weighing. After deducting the carbon dioxid determined in a separate
portion, the residual loss is regarded as due to combined moisture and
organic matter.
=25. Carbon Dioxid.=—Many forms of compact apparatus have been devised
for this estimation, but none of them is satisfactory if accurate
results are desired.[15] Not to mention other objections, many
phosphates must be heated nearly to the boiling-point with dilute acid
to effect complete decomposition of the carbonates. The distillation
method described by Gooch[16] is excellent, and when once the apparatus
is set up, its work will be found to be rapid and satisfactory.
Wyatt regards the estimation of carbon dioxid as one of the most
important for factory use. The carbonates present in a sample indicate
the loss of an equivalent amount of acid in the process of conversion
into superphosphate.[17]
The apparatus employed for estimating carbon dioxid may be any one of
those in ordinary use for this purpose. The principle of the process
depends on the liberation of the gas with a mineral acid, its proper
desiccation, and subsequent absorption by a caustic alkali, best in
solution.
The apparatus of Knorr, described in volume first, page 338, may be
conveniently used. The weight of the sample to be used should be
regulated by the content of carbonate. When this is very high, from
one to two grams will be found sufficient; when low, a larger quantity
must be used. Hydrochloric is preferred as the solvent acid. Those
forms of apparatus which are weighed as a whole and the carbon dioxid
determined by reweighing after its expulsion, are not as reliable as
the absorption apparatus mentioned.
=26. Soluble and Insoluble Matter.=—Five grams of the fine phosphate
are put into a beaker, twenty-five cubic centimeters of nitric acid,
(specific gravity 1.20) and 12.5 cubic centimeters of hydrochloric
acid (specific gravity 1.12) are added. The beaker, covered with a
watch-glass, is placed upon the water-bath for thirty minutes[18]. The
contents of the beaker are well stirred from time to time, and at the
end of the period the beaker is removed from the bath, filled with
cold water, well stirred, and allowed to settle. The solution is next
filtered into a half liter flask, and the residue is thoroughly washed
with cold water, partially dried, and then ignited, (finishing with the
blast-lamp) and brought to constant weight. The figures thus obtained
will, however, be incorrect, because the fluorin liberated during
the solution of the phosphates dissolves a portion of the silica.
Hence, the results are too low. Nevertheless, as the same action would
occur in the manufacture of a superphosphate from the material, the
determination may be considered, as a fair approximation to commercial
practice. The ignited residue must be tested for phosphorus pentoxid.
=27. Preparation of the Solution.=—The flask containing the filtrate
is filled to the mark with cold water, and the solution is thoroughly
mixed by twice pouring into a dry beaker and returning it to the flask.
Cold water is used for washing the residue, since if hot water be used,
the sesquichlorids are apt to become basic and insoluble, and hence to
remain in the residue and on the filter paper. Besides, as the flask
is to be filled to the mark, the contents must be cold before any
volumetric measurements can be made.
=28. Silica and Insoluble Bodies.=—Wyatt describes the following method
for determining the total insoluble or siliceous matters in a mineral
phosphate[19]. Five grams of the fine sample are placed in a porcelain
dish with about thirty cubic centimeters of aqua regia. The dish is
covered with a funnel, placed on a sand-bath and, after solution is
complete, evaporated to dryness with care to prevent sputtering. When
dry the residue is moistened with hydrochloric acid and again dried,
rubbing meanwhile to a fine powder. The heat of the bath is then
increased to 125° and maintained at this temperature for about ten
minutes. When cool, the residue is treated with fifty cubic centimeters
of hydrochloric acid for fifteen minutes. The acid is then diluted and
filtered on a gooch, which is washed with hot water until the filtrate
amounts to a quarter of a liter. The residue in the crucible is dried,
ignited, and weighed. This method, unless the solution be subsequently
boiled with nitric acid, may not retain all the phosphoric acid in the
ortho form.
It is difficult to estimate the total silica by the ordinary methods
of mineral analysis. This is due to the fact that in an acid solution
of a substance containing silicates and fluorids the whole of the
silica or the fluorin, as the case may be may escape as silicofluorid
on evaporation. Again, it is not easy to decompose calcium phosphate
by fusing with sodium carbonate. If an attempt be made to do this,
however, the process should be conducted as follows: A portion of the
sample is ground to an impalpable powder in an agate mortar. From one
to two grams of the substance are mixed with five times its weight
of sodium carbonate and fused with the precautions given in standard
works on quantitative analysis. The fused mass is digested in water,
boiled, and filtered, and the residue washed first with boiling water
and afterwards with ammonium carbonate. The filtrate contains all the
fluorin as sodium fluorid and, in addition to this, sodium carbonate,
silicate, and aluminate. Mix the filtrate with ammonium carbonate and
heat for some time, replacing the ammonium carbonate which evaporates.
Separate by filtration the silicic acid hydrate and aluminum hydroxid
which are formed and wash them with ammonium carbonate. To separate
the last portions of silica from the filtrate, add a solution of zinc
oxid in ammonia. Evaporate until no more ammonia escapes and separate,
by filtration, the zinc silicate and oxid. Determine the silica in
this precipitate by dissolving in nitric acid, evaporating to dryness,
taking up with nitric acid and separating the undissolved silica by
filtration. In the alkaline filtrate the fluorin may be estimated by
the usual method as calcium salt.
=29. Estimation of Lime.=—One hundred cubic centimeters of the solution
(containing one gram of the original substance) are evaporated in a
beaker to about fifty cubic centimeters; ten cubic centimeters of
dilute sulfuric acid (one to five) are added; and the evaporation is
continued on the water-bath until a considerable crop of crystals of
gypsum has formed[20]. The solution is then allowed to cool, when it
generally becomes pasty, owing to the separation of additional gypsum.
When it is cold, 150 cubic centimeters of ninety-five per cent alcohol
are slowly added, with continual stirring, and the whole is allowed
to stand for three hours, being stirred from time to time. After
three hours, it is filtered, with the aid of a filter-pump, into a
distillation flask, and the beautifully crystalline precipitate, which
does not adhere to the beaker, is washed with ninety-five per cent
alcohol. The filter, with the precipitate, is gently removed from the
funnel and inverted into a platinum crucible, so that, by squeezing the
point of the filter, the precipitate is made to fall into the crucible,
and the paper can be pressed down smoothly upon it. On gentle heating
of the crucible, the remaining alcohol burns off, and when the paper
has been completely destroyed, the heat is raised to the full power
of a bunsen for about five minutes. After cooling in a desiccator the
crucible containing the calcium sulfate, is weighed. The filtration may
also be accomplished on asbestos felt.
=30. The Ammonium Oxalate Method.=—This method has been extensively
used in this country in commercial work, and is best carried out as
described by Wyatt.[21] The total filtrates from the iron and alumina
precipitates, secured as described in paragraph =33=, are well mixed
and concentrated to a volume of about 100 cubic centimeters. There
are added about twenty cubic centimeters of a saturated solution of
ammonium oxalate, and after stirring, the mixture is allowed to cool
and remain at rest for six hours. The supernatant liquid is poured
through a filter, the residue washed three times by decantation with
hot water and brought upon the filter. The beaker and precipitate are
washed at least three times. The precipitate is dried and ignited at
low redness for ten minutes. The temperature is then raised by a blast
and the ignition continued for five minutes longer, or until the lime
is obtained as oxid. The precipitate is likely to contain magnesia. The
magnesia is estimated in the filtrates from the lime determination by
first mixing them and concentrating to 100 cubic centimeters, which,
after cooling, are made strongly alkaline with ammonia. After allowing
to stand for twelve hours the ammonium magnesium phosphate is collected
and reduced to magnesium pyrophosphate by the usual processes. If one
gram of the original material has been used the pyrophosphate obtained,
multiplied by 0.36, will give the weight of magnesia contained therein.
=31. Lime Method of Immendorff.=—The tedious processes required
to determine the lime in the presence of iron, alumina, and large
quantities of phosphoric acid are well known to analysts. Immendorff
has published a method, accompanied by the necessary experimental
data, based on the comparative insolubility of calcium oxalate in very
dilute solution of hydrochloric acid. He has shown in the data given
that the lime is all precipitated in the conditions named and that the
precipitate, when properly prepared, is not contaminated with weighable
amounts of the other substances found in the original solution[22].
The ease with which oxalic acid can be determined volumetrically
with potassium permanganate solution aids greatly in the time-saving
advantages of the process.
In a hydrochloric acid solution of a mineral phosphate an aliquot
part of the filtrate representing about 250 milligrams of calcium
oxid, usually about twenty-five cubic centimeters, should be taken
for the analysis. Ammonia is added in slight excess and then the acid
reaction restored with hydrochloric until shown plainly by litmus. The
solution is then heated and the lime thrown down by adding a solution
of ammonium oxalate in excess. In order to secure a greater dilution
of the hydrochloric acid after the precipitation has been made, water
should be added until the volume is half a liter. Before filtering, the
whole should be cooled to room temperature. The precipitate should be
washed first with cold and afterwards with warm water. The well-washed
precipitate is dissolved in hot dilute sulfuric acid and the solution,
while hot, titrated with a standard solution of potassium permanganate
set by a solution of ammonio-ferrous sulfate.
If one cubic centimeter of the permanganate represent 0.005 gram of
iron it will correspond almost exactly to 0.0035 gram of calcium oxid.
_Example._—Sample of rather poor mineral phosphate, five
grams in half a liter. Strength of potassium permanganate, one
cubic centimeter equivalent to 0.00697 gram of iron and to
0.003484 gram of calcium oxid.
Twenty-five cubic centimeters of the solution, representing one
quarter of a gram, in which the lime was precipitated as above
described, required 9.6 cubic centimeters of the potassium
permanganate to saturate the oxalic acid. Then
9.6 × 0.003484 = 0.0334464 gram,
or 13.38 per cent of calcium oxid. The method is also applicable
to basic slags.
=32. Estimation of Iron and Alumina in Mineral Phosphates.=—When
mineral phosphates are to be used for the manufacture of
superphosphates by treatment with sulfuric acid their content of
iron and alumina becomes a matter of importance. By reason of the
poor drying qualities of the sulfates of these bases their presence
in any considerable excess of a few per cent becomes exceedingly
objectionable. The accurate estimation of these ingredients is not
only then a matter of scientific interest but one of great commercial
significance to the manufacturer.
The conventional methods so long in use depending on the precipitation
of the iron and alumina as phosphates in the presence of acetic acid
have been proved to be somewhat unreliable. Not only does the acetic
acid fail to prevent the precipitation of some of the lime, but it
also dissolves more or less of the iron and aluminum phosphates. The
solution of the precipitate and its reprecipitation by the addition of
ammonia, may free the second precipitate from lime, but it increases
the error due to the solubility of the aluminum salt. The methods
recently introduced for the estimation of iron and alumina in presence
of excess of lime and phosphoric acid are not entirely satisfactory,
but are the best which can now be offered.
=33. The Acetate Method.=—The principle of this process is based on the
fact that in a solution containing iron, alumina, lime, and phosphoric
acid the iron and aluminum phosphates can be thrown down in a slightly
acid solution by ammonium acetate while the calcium phosphate remains
in solution. The acidity in the older methods is due to acetic and can
be secured by making the solution slightly alkaline with ammonia and
adding acetic to slight acidity. One of the best methods of conducting
the operation is that of C. Glaser[23]. Glaser’s modification of the
older processes is based on the assumption that at 70° the aluminum
phosphate is quantitatively precipitated by ammonium acetate in a
dilute hydrochloric acid solution and that the mixed precipitates of
iron and aluminum phosphates obtained at this temperature are free of
lime. The operation is conducted in the following manner:
The hydrochloric acid solution of the phosphate must contain no free
chlorin and is treated with a few drops of a methyl orange solution.
Ammonia is added until nearly neutral, but the acid reaction is
retained as shown by the indicator. A few cubic centimeters of ammonium
acetate are added, which produce a yellow coloration of the liquid and
also a complete precipitation of the iron and aluminum phosphates when
warmed to 70°. At this temperature the precipitation of any calcium
phosphate is avoided. A small quantity of the lime may be carried down
mechanically and therefore the precipitate should be dissolved in
hydrochloric acid and the precipitation again made as above after the
addition of some sodium phosphate. If the original solution contain
any free chlorin, as may be the case when aqua regia is employed as
solvent, before beginning the separation, ammonia should be added in
slight excess and the acidity restored by hydrochloric after adding
the indicator. In washing the precipitates, water of not over 70°
must be used. As has been shown by Hess in the work cited in the
next paragraph, the statement of C. Glaser to the effect that the
precipitates obtained as above are free of lime has not been proved to
be strictly correct. The process, however, is a distinct improvement
over the older methods and forms the basis of the amended process given
below, which appears to be sufficiently accurate to entitle the acetate
method to favorable consideration.
=34. Method of Hess.=—Hess has lately made a thorough investigation of
the standard methods of determining the iron and aluminum oxids in the
presence of phosphoric acid and has shown that the assumption that the
composition of the precipitate is represented by the formula Al₂(PO₄)₂
+ Fe₂(PO₄)₂ is erroneous[24].
In the washing of the precipitated iron and aluminum phosphates there
is a progressive decomposition of the compound with the production
of a basic salt. The composition of the precipitate at the end is
dependent chiefly upon the way in which the washing takes place. It
is quite difficult to always secure a washing in exactly the same way
and the final composition of the precipitate varies with almost every
determination. It is not, therefore, an accurate proceeding to take
half the weight of the precipitate as phosphoric acid or as iron oxid
and alumina. In every case it is necessary to dissolve the precipitate
and determine the phosphoric acid in the regular way. Hess proposes the
following method for carrying out the acetate process of separation:
The mineral phosphate should be dissolved in hydrochloric acid and
the solution made up to such a volume as shall contain in each fifty
cubic centimeters, one gram of the original substance. This quantity
of the solution is diluted with two or three times its volume of
water to which a drop of methyl orange solution (1-100) is added, and
ammonia added with constant stirring until the solution is just colored
and still reacts slightly acid. Without taking any account of the
precipitate which is produced by this approximate neutralization of
the solution, there are added fifty cubic centimeters of acid ammonium
acetate which in one liter contains 250 grams of commercial ammonium
acetate. The acidity of the solution is due to an excess of acetic in
the commercial salt. The temperature is then carried to 70° and the
precipitate produced immediately separated by filtration, washed four
times with water below 70°, and again dissolved in dilute hydrochloric
acid. The dissolved precipitate is treated with ten cubic centimeters
of a ten per cent ammonium phosphate solution and again almost
neutralized as described above, twenty-five cubic centimeters of the
ammonium acetate solution added and warmed to 70°.
The precipitate obtained is once more dissolved and precipitated as
above described, and is then collected upon a filter, washed, ignited,
and weighed. The residue after ignition is dissolved in the crucible by
heating with a little concentrated hydrochloric acid, and washed into a
beaker. Any silicic acid present is separated by filtration, ignited,
and weighed, and subtracted from the total weight of the precipitate.
To the filtrate is added ammonia to diminish the acidity, but not
sufficient to produce a precipitate and the clear solution is treated
with thirty cubic centimeters of the ordinary ammoniacal citrate
solution and fifteen cubic centimeters of magnesium mixture, and the
precipitation of the ammonium magnesium phosphate hastened by stirring
with a glass rod.
It is advisable to always make the filtrate from the third
precipitation slightly ammoniacal and to boil it for a long time.
If the operation have been carried on correctly, there occurs only
a slight precipitate of Ca₃P₂O₈ amounting only to a few milligrams.
In some cases it may be necessary to dissolve the precipitate and
reprecipitate the iron and aluminum phosphates a fourth time.
The whole time required for the triple precipitation, according to
Hess, if all the operations be properly conducted, is from three to
four hours. It is therefore possible by this variation of the acetate
method to secure a determination of the iron and alumina as phosphates
in the same time which is occupied by the Glaser-Jones method when the
separation of lime is taken into account.
If the solution of the mineral phosphate employed contain any notable
quantity of organic material, it must be destroyed by boiling with
bromin or some other oxidation agent, before the precipitation by the
acetate method is commenced.
The presence of silicic acid need not be taken into special
consideration since this can be detected and determined in the
phosphate precipitates after they have been ignited and weighed. While
the determinations of the phosphoric acid in Hess’ method were made
by precipitation in the presence of citrate, he found that they agree
perfectly with the previous precipitations with molybdic solution.
=35. Method of Glaser.=—The principle on which this method rests
depends on the preliminary removal of the lime by conversion into
calcium sulfate and its precipitation in the presence of strong
alcohol.[25] It is conducted as follows:
Five grams of the phosphate are dissolved in a mixture of twenty-five
cubic centimeters of nitric acid of 1.2 specific gravity and about 12.5
cubic centimeters of hydrochloric acid of 1.12 specific gravity, and
made up to a volume of half a liter, and filtered. One hundred cubic
centimeters of the filtrate, equivalent to one gram of the substance,
are placed in a quarter liter flask and twenty-five cubic centimeters
of sulfuric acid of 1.84 specific gravity added. The flask is allowed
to stand for about five minutes and meanwhile shaken a few times. About
100 cubic centimeters of alcohol of ninety-five per cent are then
added and the flask filled with alcohol to the mark and well shaken.
A certain degree of concentration takes place and this is compensated
for by lifting the stopper and filling again with alcohol to the mark
and shaking a second time. After allowing to stand for half an hour
the contents of the flask are filtered, 100 cubic centimeters of the
filtrate being equal to four-tenths gram of the substance. This volume,
filtered, is evaporated in a platinum dish until the alcohol is driven
off. The alcohol-free residue is heated to boiling in a beaker with
about fifty cubic centimeters of water. Ammonia is added to alkaline
reaction, but in order to avoid strong effervescence it is not added
during the boiling. The excess of ammonia is evaporated, the flask
allowed to cool, the contents filtered, precipitate and filter washed
with warm water, ignited, and the phosphates of iron and alumina
weighed. Half of the weight of the precipitate represents the weight of
Fe₂O₃ + Al₂O₃. The estimation, as before indicated, should be carried
on without delay, the whole time required not exceeding from one and a
half to two hours.
=36. Jones’ Variation.=—The method of Glaser described above, as
practiced by the German chemists, has been found by Jones to be
inaccurate on account of the alcohol not being added in sufficient
quantity in the precipitation of calcium sulfate and for the additional
reason that the amount of sulfuric acid added is more than is actually
necessary[26]. Jones modifies the method as follows: Ten grams of the
material are dissolved in nitro-hydrochloric acid and the solution made
up to 500 cubic centimeters and filtered. Fifty cubic centimeters of
this solution, representing one gram, are evaporated to twenty-five
cubic centimeters and, while still hot, ten cubic centimeters of dilute
sulfuric acid (one to five) added. The mixture is then well stirred and
cooled. One hundred and fifty cubic centimeters of ninety-five per cent
alcohol are next added and after stirring, the solution is allowed to
stand three hours. The calcium sulfate is collected on a filter, washed
with alcohol, and the filtrate and washings collected in an erlenmeyer.
The washing is completed when the last ten drops, after dilution with
an equal volume of water, are not colored with a drop of methyl orange.
The moist calcium sulfate is transferred to a platinum crucible, the
filter placed on it, the alcohol burned off, the filter incinerated,
and the calcium sulfate ignited and weighed. The contents of the flask
are heated to expel the alcohol, the residue washed into a beaker, made
slightly alkaline with ammonia, and again heated till all the ammonia
is driven off. This treatment is necessary to prevent the precipitate
from being contaminated with magnesia. The precipitate is collected
on a filter, washed four times with hot water, or water containing
ammonium nitrate, dried, ignited, and weighed. One-half of the weight
of the precipitate represents the weight of the ferric and aluminic
oxids.
=37. Estimation of Iron and Alumina in Phosphates by Crispo’s
Method.=—The phosphate of ferric iron is subject to a slight
decomposition in presence of both hot and cold water with a tendency
to the production of basic compounds. It is soluble to a slight extent
in hot and cold acetic acid, almost insoluble in ammonium acetate, and
quite insoluble in ammonium chlorid and nitrate. Aluminum phosphate
is likewise soluble, to a slight degree, in acetic acid and ammonium
acetate, and insoluble in ammonium chlorid and nitrate. The method of
Crispo for the separation of iron and alumina in phosphates is based
on the above properties.[27] Five grams of the mineral phosphate are
dissolved in fifty cubic centimeters of aqua regia, composed of forty
cubic centimeters of hydrochloric acid of 1.10, and ten of nitric acid
of 1.20 specific gravity, and this solution is diluted to half a liter.
To fifty cubic centimeters of the filtered solution are added two of
ammonia (0.96) and fifty of a half saturated solution of ammonium
chlorid, and the whole boiled. The liquid should remain clear, but if
it become cloudy add a little dilute nitric acid, drop by drop, until
the turbidity is removed, and then ten cubic centimeters of a saturated
solution of ammonium acetate, and boil for three minutes, cool, and
filter. The precipitate is washed twice with a ten per cent solution of
ammonium chlorid and redissolved with two cubic centimeters of nitric
acid, and the filter washed with hot water. The phosphoric acid is
separated by forty cubic centimeters of molybdate solution, and the
precipitate washed three or four times with a one per cent nitric acid
solution.
To the filtrate are added fifty cubic centimeters of a one-half
saturated ammonium chlorid solution, ammonia is added in slight excess
to produce precipitation and the mixture boiled for a few minutes.
After filtering, the precipitate is washed with hot water three or
four times, dissolved in two cubic centimeters of nitric acid, and the
filter washed with hot water. Again, fifty cubic centimeters of half
saturated ammonium chlorid are added and the precipitate thrown down
once more by ammonia in slight excess. The precipitate is washed with
hot water and finally ignited and weighed as iron and aluminum oxids.
According to Crispo, the original Glaser method, with its various
modifications, is not to be considered reliable, and the choice lies
between the molybdic method as usually practiced, and his own for
the accurate estimation of iron and alumina. Manganese disturbs the
accuracy of the results unless the directions given are carefully
followed. Manganese phosphate is soluble at all temperatures below
fifty. If then the mixture of the phosphates be allowed to cool before
filtering, the iron and aluminum salts are not contaminated with
manganese. This method of Crispo is somewhat tedious, but it is claimed
that these variations of the molybdic method render it exact in respect
of the determination of iron and alumina.
=38. Method Employed in Geological Survey.=—Chatard gives the
following directions for conducting the Glaser-Jones process[28]: The
distillation flask containing the alcoholic filtrate is connected with
its condenser and heated on a water-bath until no more alcohol comes
over. This distillate, if mixed with a little sodium carbonate and
redistilled over quicklime, can be used over and over again, so that
the expense for alcohol is really very slight, while in the use of the
Glaser method, with its large amount of sulfuric acid, all the alcohol
is lost.
When the distillation is ended the residue in the flask is washed into
a platinum dish and evaporated to a small bulk on the water-bath. The
dark brown color produced is due to the presence of organic matter and
this must be destroyed, as it prevents the complete precipitation of
the phosphate in the subsequent operation.
The organic matter is best destroyed by removing the dish from the
bath, adding a small quantity of pure sodium nitrate, and heating very
carefully over the naked flame, keeping the dish well covered with
a watch-glass to avoid spattering. The mass fuses to a colorless,
viscous liquid, becoming glassy when cooled and is readily soluble in
a hot very dilute solution of nitric acid. The solution transferred
to a beaker is made distinctly alkaline with ammonia and carefully
neutralized with acetic acid, diluted with hot water, boiled, and
the precipitate allowed to settle, after which it is separated by
filtration.
After the precipitate has been completely transferred to the filter,
the washing is completed with a dilute solution of ammonium nitrate.
The precipitate is dried, ignited, cooled, and weighed.
The determinations should be made in pairs, one portion being used
for the estimation of the phosphoric acid by fusing with a little
sodium carbonate, and the other, after fusion with sodium carbonate,
is dissolved with sulfuric acid and the iron reduced and titrated with
potassium permanganate solution. The filtrate from the iron and alumina
determination is evaporated to a small bulk, made strongly ammoniacal
and allowed to stand for some time when the magnesia present separates
as ammonium magnesium phosphate which is determined in the usual way.
If, during the evaporation of the filtrate, any flocculent matter
separate, it should be removed by filtration and examined before
precipitating the magnesia.
=39. Variation of Marioni and Fasselli.=—Glaser’s method has been shown
to be subject to errors by Marioni and Fasselli[29] in the following
respects:
1. The precipitation of a small quantity of calcium phosphate with the
ferric and aluminum phosphates.
2. The possible precipitation of basic phosphates if all the iron and
alumina are not combined with phosphoric acid in the mineral.
3. The partial solubility of ferric and aluminum phosphates in dilute
acetic acid.
4. The decomposition of ferric orthophosphate into soluble acid
phosphate and insoluble basic salt by boiling.
To avoid these errors the following procedure is proposed: From one to
five grams of the phosphate are boiled in a flask for ten minutes with
fifteen cubic centimeters of strong hydrochloric acid, and afterwards
diluted with a double volume of water. A few crystals of potassium
chlorate are added, and several drops of nitric acid, and the liquid
boiled to expel chlorin. It is then filtered and washed until the
volume of the filtered liquid amounts to 150 cubic centimeters. After
cooling, a half gram of ammonium phosphate in solution is added, and
two cubic centimeters of glacial acetic acid, followed by dilute
ammonia, drop by drop, until a slight precipitate persists on stirring.
Again the same quantity of acetic acid is added as above, well shaken,
and left for two hours. The precipitate is collected on a filter and
washed with a one per cent ammonium phosphate solution. The precipitate
is dissolved by a minimum quantity of hydrochloric acid and the
solution collected in the same vessel in which the precipitation took
place. A second precipitation is conducted just as described above.
The precipitate is washed as above described and ignited at a dull red
heat. Half the weight obtained represents the ferric oxid and alumina.
=40. Method of Ogilvie.=—For the separation of alumina from phosphoric
acid Ogilvie recommends that the filtrate from the phosphomolybdate
precipitate be neutralized with ammonia, the precipitate thus formed
redissolved in nitric acid, again precipitated with ammonia, filtered,
ignited, and weighed as aluminum oxid.[30] If iron be present it will,
of course, appear in the product. For use in the examination of mineral
phosphates the method can not have a wide application without amendment.
=41. Method of Krug and McElroy.=—Krug and McElroy show that when
sufficient alcohol is added to precipitate all of the calcium sulfate
in the Glaser method, it will also cause a precipitation of a
considerable quantity of iron, by means of which the calcium sulfate
will be colored.[31] The presence of potassium and ammonium salts also
affects very notably the precipitation of calcium. The method employed
by them, in order to avoid these sources of error, is as follows:
One hundred cubic centimeters, equivalent to one gram of the substance,
in a nitric acid solution, are placed in a half liter flask and a
solution of ammonium molybdate added until all the phosphoric acid
has been precipitated. The addition of ammonium nitrate will hasten
the separation of the ammonium phosphomolybdate. The liquid should be
allowed to stand for twelve hours. The flask is then filled to the
mark, the contents well shaken, filtered through a dry filter, and
duplicate samples of 200 cubic centimeters each of the filtrate taken
for analysis.
A small quantity of ammonium nitrate is dissolved in the liquid, and
ammonia cautiously added, keeping the solution as cool as possible. The
iron and alumina are precipitated as hydroxids. The mixed hydroxids are
collected on a filter, washed with water, the filtrate and washings
being collected in a beaker. The precipitate should be dissolved with a
small quantity of a solution of ammonium nitrate and nitric acid, again
precipitated with ammonia, filtered, washed, ignited, and weighed. This
treatment is for the purpose of excluding all possibility of error from
the presence of molybdic anhydrid. After weighing, the mixed oxids
should be fused with sodium bisulfate, the magma dissolved in water,
and the iron determined volumetrically with potassium permanganate
after reduction to the ferrous state.
McElroy has further shown by experiments in this laboratory that even
the molybdate method of separating the iron and alumina from phosphoric
acid with the improvements as first suggested by Krug and himself, may
not always give reliable results.[32] In a solution containing ferrous
iron equivalent to 56.4 milligrams of ferric oxid, were placed enough
of a solution of sodium phosphate to correspond to 100 milligrams of
phosphorus pentoxid. The precipitate was dissolved by adding nitric
acid, oxidized with bromin water, and the phosphoric acid thrown out
with ammonium molybdate. The precipitate was washed with weak nitric
acid and the combined filtrate and washings neutralized with ammonia.
The resultant precipitate was dissolved in a solution of ammonium
nitrate and nitric acid, filtered, and again precipitated with ammonia.
In two instances the quantities of material recovered after ignition
were 56.9 and 57.3 milligrams, respectively, instead of the theoretical
amount, _viz._, 56.4 milligrams.
When the work was repeated after the addition of 400 milligrams of
calcium oxid the weight of the precipitate recovered was 62.3 and
63.1 milligrams in duplicate determinations. Similar determinations
were made with a known weight, _viz._, 35.6 milligrams of alumina.
The treatment of the mixture was precisely as indicated above for
iron. The quantity of alumina finally obtained was 28.9 and 29.3
milligrams, respectively, in duplicate determinations. When the lime
was added, however, the weights of alumina, recovered, fell to 19.8 and
20.6 milligrams, respectively. These results show that the molybdate
method for the separation of iron and alumina in the presence of a
large excess of lime and phosphoric acid is subject to widely varying
results, but that the error due to the excess of iron in the weighed
product is partly corrected by the one due to deficiency of alumina.
=42. Method of Wyatt.=—A method largely used in this country, both in
private laboratories and by fertilizer factories, for determining iron
and alumina, is described by Wyatt[33]. It is claimed for this method
that, while it may not be strictly accurate, yet it is rapid and easy,
and in the hands of trained analysts yields concordant results. Fifty
cubic centimeters of the first solution of the sample in aqua regia, or
an amount thereof equivalent to one gram of the phosphate, in a beaker,
are rendered alkaline by ammonia. The resulting precipitate is first
redissolved by hydrochloric acid, and a slight alkalinity is again
produced with ammonia. Fifty cubic centimeters of strong acetic acid
are next added, the mixture stirred and placed in a cool place and left
until cold. The precipitate is then separated by filtration and washed
twice with boiling water. The vessel holding the filtrate is replaced
by the beaker in which the precipitation was made. The precipitate
is dissolved in a little fifty per cent hot hydrochloric acid and
the filter washed with hot water. After rendering slightly alkaline,
as in the first instance, the treatment with acetic acid is repeated
as described. The precipitate is washed this time, twice with cold
water containing a little acetic acid and three times with hot water.
The precipitate is dried, ignited, and weighed as iron and aluminum
phosphate. Half of this weight may be taken to represent the quantity
of iron and aluminum oxids.
To separate the iron and alumina the precipitate just described
is dissolved in hot hydrochloric acid, filtered into a 100 cubic
centimeter flask, and made up to the mark by hot wash-water.
The phosphoric acid is determined in one-half of the filtrate and in
the remaining half the iron is reduced with zinc and determined with
potassium permanganate in the usual way. The phosphoric acid and iron
having been thus determined the alumina is estimated by difference. The
chief objection to this process is in the excessive quantity of acetic
acid used and the danger of solution of the precipitated phosphates
caused thereby.
=43. Estimation of the Lime and Magnesia.=—The filtrate and washings
from the first precipitation, (paragraph =41=,) of iron and alumina
in the method of Krug and McElroy, above described, are collected and
sufficient ammonium oxalate is added to precipitate the calcium. The
precipitated calcium is very fine and should be collected on a gooch,
under pressure. The filtrate and washings from the calcium precipitate
are again collected, and a solution of sodium phosphate added to
precipitate the magnesia. The solution must be kept cool and slightly
alkaline with ammonia during the above operations in order to prevent
the separation of molybdic anhydrid.
=44. Estimation of Sulfuric Acid.=—As a rule, sulfates are not abundant
in mineral phosphates. In case the samples are pyritiferous, however,
considerable quantities of sulfuric acid may be found after treatment
with aqua regia.
The acid is precipitated with barium chlorid, in the usual way, in an
aliquot portion of the filtrate first obtained. The precipitate of
barium sulfate is washed with hot water until clean, dried, ignited,
and weighed. If the portion of the filtrate taken represent half a gram
of the original material then the weight of barium sulfate obtained
multiplied by 0.6858 will give the quantity of sulfur trioxid in one
gram.
=45. Estimation of Fluorin by the Method of Berzelius as Modified by
Chatard.=—The method of estimating fluorin as proposed by Berzelius,
has been found quite satisfactory in the laboratory of the Geological
Survey, with the modifications given below.[34]
Two grams of the phosphate are intimately mixed in a large platinum
crucible with three grams of precipitated silica and twelve grams of
pure sodium carbonate, and the mixture is gradually brought to clear
fusion over the blast-lamp. When the fusion is complete the melt is
spread over the walls of the crucible, which is then rapidly cooled
(preferably by a blast of air). If this have been properly done, the
mass separates easily from the crucible, and the subsequent leaching is
hastened. The mass, detached from the crucible, is put into a platinum
dish into which whatever remains adhering to the crucible, or its lid,
is also washed with hot water. A reasonable amount of hot water is now
put into the dish, which is covered and digested on the water-bath
until the mass is thoroughly disintegrated. To hasten this, the
supernatant liquid may, after awhile, be poured off, the residue being
washed into a small porcelain mortar, ground up, returned to the dish
and boiled with fresh water until no hard grains are left. The total
liquid is then filtered, and the residue is washed with hot water.
The filtrate (which should amount to about half a liter) is nearly
neutralized with nitric acid (methyl orange being used as indicator),
some pure sodium bicarbonate is at once added, and the solution (in
a platinum dish, if one large enough is at disposal, otherwise in a
beaker) is placed on the water-bath, when it speedily becomes turbid
through separation of silica. As soon as the solution is warm it is
removed from the bath, stirred, allowed to stand for two or three
hours, and then filtered by means of the filter-pump and washed with
cold water.
The filtrate is concentrated to about a quarter of a liter and nearly
neutralized, as before, some sodium carbonate is added, and the
phosphoric acid is precipitated with silver nitrate in excess. The
precipitate is separated by filtration and washed with hot water, and
the excess of silver in the filtrate is removed with sodium chlorid.
The filtrate from the silver chlorid (after addition of some sodium
bicarbonate) is evaporated to its crystallizing point, then cooled and
diluted with cold water; still more sodium bicarbonate is added, and
the whole is allowed to stand, when additional silica will separate,
and this is to be removed by filtration.
This final solution is nearly neutralized, as before; a little sodium
carbonate solution is added; it is heated to boiling and an excess
of solution of calcium chlorid is added. The precipitate of calcium
fluorid and carbonate must be boiled for a few minutes, when it can
be easily filtered and washed with hot water. The precipitate is then
washed from the filter into a small platinum dish and evaporated to
dryness, while the filter, after being partially dried and used to wipe
off any particles of the precipitate adhering to the dish in which it
was formed, is burned, and the ash is added to the main precipitate.
This, when dry, is ignited, and allowed to cool; dilute acetic acid is
added in excess, and the whole is evaporated to dryness, being kept
on the water-bath until all odor of acetic acid has disappeared. The
residue is then treated with hot water, digested, filtered on a small
filter, washed with hot water, partially dried, put into a crucible,
carefully ignited, and weighed as calcium fluorid. The calcium fluorid
is then dissolved in sulfuric acid by gentle heating and agitation,
evaporated to dryness on a radiator, ignited at full red heat, and
weighed as calcium sulfate. From this weight the equivalent weight of
calcium fluorid should be calculated, and this should be very close
to that actually found as above, but should never exceed it. The
difference, which is generally about a milligram (sometimes more), is
due to silica precipitated with the fluorid. The percentage of fluorin
is, therefore, always calculated from the weight of the sulfate, and
not from that of the fluorid obtained.
The main improvements in this method are the use of sodium bicarbonate
to separate the silica, and the keeping of the earlier solutions as
dilute as possible, which can not be done, if ammonium carbonate be
used for the separation of the silica. These changes make the fluorin
estimation, although still tedious, far more rapid than before, and the
results are very satisfactory.
=46. Modification of Wyatt.=—By reason of the tediousness of the method
of Chatard given above, Wyatt has sought to shorten the process by the
following modification:[35]
Five grams of the finely ground phosphate are fused in a platinum dish
with fifteen grams of the mixed carbonates of sodium and potassium and
three grams of fine sand. After fusing very thoroughly with a strong
heat for a quarter of an hour, the dish is removed from the fire and
cooled. Its contents, dissolved in hot water, are then put into a half
liter flask, and a considerable excess of ammonium carbonate is added
to the liquid. All the soluble silica falls out of solution, and the
flask, after cooling, is made up to the mark with distilled water, well
shaken, and then set aside for twenty-four hours to settle. At the
end of this time 200 cubic centimeters are carefully decanted through
a filter; the filter is well washed, and the filtrate, after being
nearly neutralized with hydrochloric acid, is treated with an excess of
calcium chlorid solution.
The precipitate, consisting of phosphate, fluorid, and some calcium
carbonate, is allowed to settle, and is then carefully washed with
boiling water, first by decantation several times, and finally on
the filter. After being properly dried in the gas-oven, calcined,
and cooled, the residue is treated with acetic acid, placed upon the
water-bath, and evaporated to complete dryness.
The calcium acetate is now well washed out by several treatments
with boiling water, and the residue is brought upon a filter, dried,
calcined, and weighed. The weight represents the calcium phosphate
and fluorid contained in two grams of the original sample; and if
the calcium phosphate in the residue be determined, according to the
usual methods, the difference will be calcium fluorid and may be thus
estimated.
_Example._—Assuming the calcined residue of calcium phosphate and
fluorid in two grains of the original sample to have amounted to one
and six-tenths gram and the calcium phosphate in this quantity to have
been determined as 1.540 gram, the calcium fluorid is thus proved to
be 0.060 gram, and, therefore, 2: 0.60::100: x = 3 per cent calcium
fluorid which, multiplied by 0.4897, gives 1.46 per cent of fluorin.
The above method, while shorter, is not to be preferred to the Chatard
process when great accuracy is desired. All the soluble silica may
not fall out of the solution as Wyatt says. Finally the fluorin
is calculated from small differences in the weight of very heavy
precipitates and all the error of the process may be found affecting
the numbers for fluorin. For commercial purposes, however, the method
is to be recommended for its comparative brevity.