The methods of bringing the potash into solution and of precipitating
the sulfuric acid are the same as for the Halle process described above.
Add then twenty cubic centimeters of the platinum solution and
evaporate the mixture nearly to dryness. Add a sufficient quantity of
eighty per cent alcohol and stir for some time. Allow to stand and then
filter through a gooch dried at 120°. Finally wash with eighty per cent
alcohol, dry at 120°, and weigh.
B. _Method for Potash Superphosphate and other mixed Fertilizers._—The
reagents necessary are the same as under A, and, in addition, a
saturated solution of barium hydrate and a solution of ammonium
carbonate mixed with ammonia.
Boil twenty grams of the substance with water for half an hour, cool,
make up to half a liter and filter. Boil fifty cubic centimeters of
the filtrate, and add barium chlorid till no more precipitate forms.
Mix with baryta water to strong alkaline reaction, cool, make up to
100 cubic centimeters and filter. Raise fifty cubic centimeters of the
filtrate to the boiling temperature and add ammonium carbonate solution
till no more precipitate forms: Cool, make up to 100 cubic centimeters
and filter. Transfer fifty cubic centimeters of the filtrate to a
platinum dish, evaporate and heat the residue, avoiding too high a
temperature, till the ammonia salts are expelled. Dissolve the residue
in water, filter, and treat the filtrate as described under A.
=257. Swedish Methods.=—The Swedish chemists determine the potash
in mineral salts by the platinum chlorid process, but with certain
variations from the processes already given. The manipulation is
conducted as follows:[206]
Weigh one gram of the sample to be examined and pour about 300 cubic
centimeters of hot water over it in a beaker and filter after complete
solution; add one cubic centimeter of hydrochloric acid, heat nearly
to boiling, add dilute barium chlorid solution from a pipette or
burette in a very fine stream stirring, slowly and carefully, till
all sulfuric acid is completely precipitated, and only a trace of the
precipitant is in excess. If the precipitation be conducted in the
way given the barium sulfate will come down in crystalline condition,
and settle rapidly within a few minutes, and almost immediately after
the precipitation is finished may be filtered clear. The filtrate and
washings from the barium sulfate are brought into a liter flask; fill
this to the mark, take out fifty cubic centimeters with a pipette,
evaporate the greater portion on a water-bath in a porcelain dish,
transfer the residue by means of ammonia-free water to a beaker of
fifty cubic centimeters capacity, add ten cubic centimeters of platinic
chlorid solution, stir well with a glass rod, evaporate on a water-bath
to a sirupy condition, allow to cool, and if the residue be too dry,
add a few drops of water to allow the sodium platinochlorid to take
up crystal water with certainty, stir well, add alcohol after a few
minutes, mix carefully, leave the mixture standing for a while in the
beaker covered with a watch glass, stirring occasionally; finally
decant the solution, which must be of a dark yellow color, through a
very small filter, wash the precipitate in the beaker repeatedly with
small quantities of alcohol and decant; then transfer the precipitate
to the filter, wash with alcohol, dry the filter and the precipitate
at a gentle heat till all alcohol has evaporated, carefully transfer
the contents of the filter to a watch glass placed on white glazed
paper; dissolve the potassium platinochlorid still remaining on the
filter in small quantities of boiling water, evaporate the filtrate
on a water-bath in an accurately weighed platinum dish to dryness
and transfer the same to the main portion of the chlorid from the
watch glass. In order to obtain the salt free of the corresponding
combinations of sodium, barium, calcium, and magnesium, which salts,
although soluble in alcohol, may make the salt impure, before weighing,
treat the precipitate twice with small quantities of cold water which
will dissolve these impurities; evaporate the solution after addition
of one cubic centimeter of platinic chlorid nearly to dryness on a
water-bath, treat the residue in the same way as given before, add the
small quantity of potassium platinochlorid which is hereby obtained
together with the main portion to the platinum dish, dry at 130°, and
weigh. Only after having been treated in this way may the precipitated
potassium platinochlorid be considered absolutely pure. The Stassfurt
salts contain magnesia, often in large quantities and as a consequence
the potassium platinochlorid precipitated directly is likely to be
contaminated therewith.
=258. Methods for the Analysis of Carnallit, Kainit, Sylvinit, and
Kieserit.=—The chemists of the German Potash Syndicate use the
following methods in the analysis of the raw products mentioned
above.[207]
(1) _Preparation of the Sample._—It is advisable to take from a large
well mixed mass at least half a kilogram for the analytical sample and
this should be ground to a fine powder in a mill or mortar.
(2) _Estimation of the Potash by the Precipitation Method._—In a half
liter flask are placed 35.70 grams of kainit or sylvinit, or 30.56
grams of carnallit or bergkieserit, which are boiled with 350 cubic
centimeters of water after the addition of ten cubic centimeters of
hydrochloric acid. After cooling the flask is filled to the mark with
water, well shaken, and its contents filtered. Fifty cubic centimeters
of the filtrate are treated in a 200 cubic centimeter flask with a
solution of barium chlorid, the flask filled to the mark, well shaken,
and its contents filtered. Twenty cubic centimeters of the filtrate,
corresponding to 0.3570 or 0.3056 gram of the substance, are treated
with five cubic centimeters of platinic chlorid solution and the
potassium estimated according to the usual methods.
(3) _Estimation of Potash (K₂O) in Raw Potash Salts._—(_a_) For the
determination of potash alone in carnallit, kainit, and sylvinit
one hundred grams of the well-mixed sample are put into a graduated
flask holding one liter and dissolved by boiling with half a liter
of water, acidulated with ten cubic centimeters of hydrochloric. The
purpose of adding hydrochloric acid is to bring any polyhalit that
might be present in the salts into solution and which it is difficult
to dissolve in pure water. After dissolving and cooling the flask is
filled up to the mark. The solution, after mixing, is filtered through
a dry filter and 100 cubic centimeters of the filtrate, corresponding
to ten grams substance, are put into a half liter flask by means of
a pipette. After the addition of 200-300 cubic centimeters of water
the solution is heated to boiling and the sulfuric acid accurately
precipitated with normal barium chlorid solution, containing 104
grams of the dry salt in one liter. The volume of the precipitate
is calculated from the amount of barium solution used and from the
specific gravity of the barium sulfate. After cooling, the flask is
filled up with water as far above the mark as equals the volume of the
calculated barium precipitate, and, after thorough mixing, the solution
is filtered again through a dry filter. Fifty cubic centimeters of this
filtrate, corresponding to one gram substance, are evaporated upon the
water-bath with a sufficient amount of platinic chlorid. The residue of
potassium platinochlorid is washed with ninety per cent alcohol, dried
at 120°, and weighed.
(_b_) If it be desired to determine separately the quantity of
potash present in the form of sulfate and in the form of chlorid,
as for example in kainit and in sulfate of potash, or if it is to
be determined whether potassium sulfate is in combination with a
proportionate amount of magnesium chlorid, as in kainit, or in
combination with magnesium sulfate alone, as in schönit, it then
becomes necessary to determine besides potash the percentages of
chlorin, sulfuric acid, lime, magnesia, the total alkalies, water,
and the residue insoluble in water. For this purpose 100 grams of the
sample are dissolved, the solution is filtered, the filter washed,
and the filtrate made up to one liter; a part of the liquid is taken
for the determination of sulfuric acid; by precipitating with barium
chlorid, and another part for the determination of lime and magnesia.
For the determination of the alkali chlorids, 100 cubic centimeters
of the solution, corresponding to ten grams substance, are acidulated
with hydrochloric, and, after heating to boiling, the sulfuric acid
is completely precipitated with barium chlorid, with the precaution
of using not more of the barium solution than is necessary for the
complete precipitation. Fifty cubic centimeters of the filtered
solution, corresponding to one gram substance, are evaporated to
dryness in order to drive off the hydrochloric acid. Magnesium chlorid
is decomposed by igniting with oxalic acid or with mercuric oxid. After
ignition, the residue is moistened with a little ammonium carbonate for
the purpose of converting the calcium oxid that may have been formed
into calcium carbonate. The alkali chlorids, which are entirely free
of lime and magnesia, are weighed, and potassium chlorid is determined
by means of platinic chlorid. The amount of sodium chlorid is obtained
by deducting potassium chlorid from the mixed chlorids. For the water
determination five grams of the sample are ignited and the loss of
weight is determined. The ignited mass is dissolved in water, and for
the purpose of determining the quantity of magnesium chlorid that may
have been decomposed by the ignition the percentage of chlorin is
determined by titration. The difference in the contents of chlorin
before and after ignition is subtracted from the loss in weight, after
allowance has been made for the absorption of oxygen and for the loss
of hydrogen. The rest is water. The results obtained are calculated in
the following manner: From the total amount of the sulfuric acid found,
that portion is deducted which is combined with calcium as calcium
sulfate; the rest of the sulfuric acid is divided into two equal parts
for the purpose of calculating the contents of potassium sulfate and
magnesium sulfate, according to the molecular proportion in which these
salts are present in kainit and in schönit. If there be an excess of
potash left uncombined with sulfuric acid, then it is in the form of
potassium chlorid; likewise the amount of magnesia, uncombined with
sulfuric acid, is to be reckoned as magnesium chlorid. The result of
this calculation will tell how much potash is in the form of kainit
(K₂SO₄, MgSO₄, MgCl₂ with 6H₂O) and how much of it is in the form of
schönit (K₂SO₄, MgSO₄, with 6H₂O) and how much in the form of potassium
chlorid. The sodium is reckoned as sodium chlorid.
(_c_) In calculating the contents of potash, of potassium chlorid, and
of potassium sulfate from the weighed potassium platinochlorid, the
factors 0.1928, 0.3056, and 0.3566 are used, assuming that the atomic
weight of platinum is 197.18.
(_d_) The two methods which have been described under _a_ and _b_, and
which are in common use in the Stassfurt potash industry, _i. e._, the
so-called precipitation method, and the oxalic acid method, give almost
identical results. The first method, however, deserves preference on
account of greater simplicity in cases where potash alone is to be
determined. Finkner’s method likewise gives results which agree well
with the results obtained by the customary methods. It consists in
evaporating the salt solution with a sufficient quantity of platinic
chlorid without previously removing the sulfuric acid, reducing the
potassium platinochlorid, and weighing the metallic platinum.
The following are the results of comparative analyses:
1. After the precipitation method 22.02 per cent KCl
2. After the oxalic acid method 22.03 per cent KCl
3. After Finkner’s method 22.01 per cent KCl
In another sample of carnallit the following results were obtained:
1. After the precipitation method 17.88 per cent KCl
2. After the oxalic acid method 17.88 per cent KCl
In a third sample of carnallit the content of potassium chlorid was as
follows:
1. After the precipitation method 18.44 per cent
2. After the oxalic acid method 18.38 per cent
The Anhalt chemists object to precipitating the sulfuric acid and
alkaline earths with barium oxid and ammonium carbonate, and afterwards
the potash with platinic chlorid. The results obtained with this method
are, according to them, very inaccurate, and always too low. This is
explained by the fact that it is impossible to precipitate sulfuric
acid without at the same time precipitating some of the potash, unless
it be in an acid solution.
A separation of the alkaline earths, if potash alone is to be
determined, is superfluous, for the reason that calcium and magnesium
platinochlorid are soluble in ninety per cent alcohol, even with more
facility than sodium platinochlorid.
=259. Methods for Concentrated Potash Salts.=—In the preceding
paragraphs have been given the methods used by the Stassfurt syndicate
for the estimation of potash in the raw salts as they come from the
mines. Following are the methods used by the same syndicate for the
concentrated approximately pure compounds and the other salts which
accompany them.
_Potassium Chlorid._—The following process is used for the estimation
of potassium and other constituents of the high grade chlorids of
commerce. In a half liter flask are placed 7.6405 grams of the finely
powdered sample, which is dissolved and made up to the mark. With
salts which contain more than half a per cent of sulfuric acid the
preliminary conversion of the sulfates into the corresponding chlorin
compounds, by precipitation with barium chlorid solution, is necessary.
Twenty cubic centimeters of the above solution, corresponding to
0.3056 gram of the salt, are placed in a flat porcelain dish having
a diameter of about ten centimeters and, after the addition of five
cubic centimeters of the platinic chlorid solution, evaporated on
the water-bath with constant stirring until, after cooling, the
sirupy liquid passes quickly into a fine crystalline condition. The
residue is rubbed into a fine powder with a glass rod, mixed with
twenty cubic centimeters of ninety-six per cent alcohol and dried at
120° to a constant weight. It is weighed while warm and brought on
a moistened filter with alcohol, care being taken that the liquid
does not touch the edge of the filter. The filtration can be carried
on under a moderate pressure. The complete washing of the potassium
platinochlorid can be easily accomplished upon the filter. The filter
and the precipitate, after as much of the alcohol wash has been removed
as is possible, are dried at 120° to constant weight and weighed while
still warm. One milligram of the potassium platinochlorid thus obtained
corresponds to a tenth per cent of potassium chlorid.
_Estimation of Sodium Chlorid._—For the estimation of the sodium
chlorid which may be present in the potassium chlorid twelve and a half
grams of the latter salt are dissolved in a quarter liter flask with
twenty-five cubic centimeters of boiling water after the addition of a
little potassium carbonate for the purpose of converting the magnesium
and calcium compounds into carbonates. After filtration 100 cubic
centimeters corresponding to five grams of the salt are evaporated to
dryness in a porcelain or platinum dish after the addition of a few
drops of concentrated hydrochloric acid in order to convert any calcium
carbonate which may be present into chlorid. The residue is gently
ignited and weighed. In this mixture of potassium and sodium chlorids
the potassium chlorid may be estimated in the usual way and the sodium
chlorid determined by difference or the respective proportions of
the two bases may be calculated after the determination of the total
chlorin by precipitation with a standard solution of silver nitrate.
_Estimation of Magnesium Chlorid._—In order to estimate the amount of
magnesium chlorid in high grade muriate of potash, twenty-five grams
of the latter salt are dissolved in a half liter flask and treated
with ten cubic centimeters of a normal solution of potash lye. The
flask is then filled to the mark with water, thoroughly shaken and its
contents filtered. Fifty cubic centimeters of the filtrate are then
titrated with one-tenth normal sulfuric acid. The calcium compounds
which remain in solution do not influence the result. The quantity
of magnesium chlorid originally present corresponds to the number of
cubic centimeters of the normal potash lye which has disappeared in
the operation. The reaction which takes place is represented by the
following equation:
MgCl₂ + 2KOH = MgO₂H₂ + 2KCl.
_Potassium Sulfate._—The quantity of potassium sulfate contained in the
high grade sulfates of commerce is determined in the following manner:
In a half liter flask are placed 8.9235 grams of the finely ground
sample which is dissolved in about 350 cubic centimeters of boiling
water after the addition of twenty cubic centimeters of hydrochloric
acid. The sulfuric acid is thrown out by the addition, drop by drop, of
a barium chlorid solution, the contents of the flask being kept boiling
meanwhile and thoroughly stirred. From time to time the addition
of the barium chlorid is stopped and the upper part of the liquid
allowed to become clear by the subsidence of the barium sulfate. It is
then noticed whether or not an additional drop of the barium chlorid
solution produces a turbidity. Any excess of barium chlorid is removed
by the careful addition of sulfuric acid. After the precipitation is
complete and the contents of the flask are cooled, it is filled up to
the mark with water and its contents filtered. Twenty cubic centimeters
of the filtrate, corresponding to 0.357 gram of the original salt are
precipitated by platinic chlorid in the usual manner and the resulting
potassium platinochlorid collected and weighed. One milligram of the
potassium platinochlorid thus obtained corresponds to one-tenth per
cent of potassium sulfate in the original salt. To the percentage of
potassium sulfate thus found three-tenths per cent are to be added for
a correction when high grade potassium sulfate is taken. If the sample
be a high grade sulfate of potassium and magnesium no correction should
be applied.
_Estimation of Potassium Chlorid and Potassium Sulfate in Calcined
Manurial Salts._—In these salts 15.281 grams for potassium chlorid
or 17.847 grams for potassium sulfate are dissolved in a half liter
flask after the addition of ten cubic centimeters of hydrochloric
acid. The flask is filled to the mark and its contents filtered and
250 cubic centimeters placed in a half liter flask and treated with
barium chlorid solution as indicated above. The rest of the operation
is exactly as has been described. In each case one milligram of the
potassium platinochlorid corresponds to one-tenth per cent of the
desired salt.
_Estimation of Magnesium Sulfate in Kieserit._—Ten grams of the finely
powdered kieserit are boiled for one hour in a half liter flask
two-thirds full of water. After cooling, from fifty to sixty cubic
centimeters of double normal potash lye and twenty cubic centimeters of
a ten per cent neutral potassium oxalate solution are added, the flask
filled to the mark, and after being well shaken and standing for a
quarter of an hour, filtered. The reaction is represented by the formula
MgSO₄ + 2KOH = MgO₂H₂ + K₂SO₄.
Fifty cubic centimeters of the filtrate are then titrated with
one-tenth normal sulfuric acid. To the percentage of magnesium sulfate
found by this process two-tenths per cent are to be added as a
correction.
_Barium Chlorid Solution._—Dissolve 122 grams of crystallized barium
chlorid in water in a liter flask. Add fifty cubic centimeters of
hydrochloric acid and water to the mark and shake well.
=260. The Barium Oxalate Method.=—The principle of this process, worked
out by Schweitzer and Lungwitz[208] is based on the fact that in an
ammoniacal solution, by means of barium oxalate, all the alkaline
earths can be precipitated as oxalates, and sulfuric acid in similar
circumstances can be thrown down as a barium salt and the iron and
alumina as hydroxids. The reagents used to secure this precipitation
are ammonia and barium oxalate.
For the determination of potash in a superphosphate the analytical
process is conducted as follows: Ten grams of the superphosphate are
mixed with half a liter of water and fifteen grams of barium oxalate
dissolved in hydrochloric acid.
The mixture is boiled for twenty minutes and treated with some hydrogen
peroxid to oxidize any ferrous iron that may be present. Afterwards
the solution is made alkaline with ammonia. After cooling, it is made
up to a given volume (half a liter) and filtered. An aliquot part of
the filtrate is evaporated to dryness, ignited, extracted with hot
water and, after the addition of a few drops of hydrochloric acid, the
potassium is precipitated with platinic chlorid, and collected and
weighed in the usual manner: Or the ignited residue may be dissolved
directly in dilute hydrochloric acid and the rest of the process
carried out as indicated.
In kainit the process is conducted as follows: Ten grams of the
powdered sample are treated with a hydrochloric acid solution of the
barium oxalate containing ten grams of the salt. The rest of the
operation is conducted as described above. In the use of this method
it is important that always enough of the barium oxalate solution be
employed to fully saturate all the sulfuric acid which may be present.
=261. Method of DeRoode for Kainit.=—All the potash contained
in kainit, according to de Roode, passes readily into aqueous
solution.[209] On evaporating this aqueous solution to a pasty
condition with enough platinic chlorid to unite with all the halogens
present all the other bodies can be washed out of the potassium
platinochlorid by ammonium chlorid solution and the pure platinum salt
thus obtained, which is washed and dried in the usual way. De Roode
therefore asserts that it is quite useless to previously precipitate
the solution of kainit with barium chlorid, ammonium oxalate, or
carbonate. Before the addition of alcohol to the residue obtained by
evaporation with platinic chlorid the sodium sulfate present renders
the platinum salt sticky and difficult to wash, but the disturbing
sodium compound can be readily removed by washing with ammonium chlorid
solution.
The method of direct treatment has the advantage of avoiding the
occlusion of potash in other precipitates and the danger of loss on
ignition. The method as used by de Roode gives results about one-tenth
per cent higher than are obtained by the official processes.
=262. The Calcium Chlorid Method.=—Huston has proposed the addition of
calcium chlorid to the solution of a fertilizer in the determination
of potash, in order to furnish sufficient calcium to form tricalcium
phosphate with all the phosphoric acid present, and thereby permit
of the use of platinum dishes in the lindo-gladding method.[210] In
testing this process de Roode found that when sufficient calcium
chlorid was added to combine with all the phosphoric acid present and
then ammonia added in excess and a portion of the solution filtered,
no test for phosphoric acid could be obtained; but, that if in
addition to the calcium chlorid and ammonia, some ammonium oxalate or
carbonate was added, a filtered portion of the solution gave a test
for phosphoric acid.[211] This is accounted for by the fact that the
calcium phosphate, which is precipitated by the ammonia, is changed by
the ammonium oxalate or carbonate into calcium oxalate or carbonate
and ammonium phosphate, so that the very object for which the calcium
chlorid was added is defeated by the addition of the ammonium oxalate
or carbonate. In order to make the use of calcium chlorid effective
it is necessary to filter the liquid from the precipitate formed by
the calcium chlorid and ammonia and then add the ammonium oxalate or
carbonate to the filtrate. This necessitates two separate filtrations
and makes the proposed method of Huston as long as the old process.
=263. Rapid Control Method for Potash Salts.=—For rapid control work
where great accuracy is not required Albert recommends that the finely
ground substance be placed in a liter flask and about 400 cubic
centimeters of water added and three cubic centimeters of hydrochloric
acid.[212] After boiling, barium chlorid is added drop by drop as long
as a precipitate is produced. After cooling, the flask is filled to
the mark and shaken and its contents filtered through a dry filter. An
aliquot portion of the filtrate is evaporated with platinum chlorid
solution in a smooth porcelain dish almost to dryness and the mass
treated with alcohol, filtered through a weighed filter, and well
washed with alcohol. The filter is then dried in an air-bath to a
constant weight. For the different kinds of potash materials on the
market the following proportions are recommended:
_Kainit or Carnallit._—Twenty grams in one liter: Fifty cubic
centimeters of the filtrate are evaporated with forty of platinic
chlorid solution. The weight of potassium platinochlorid obtained ×
19.3 gives the per cent of K₂O.
_Sulfate of Potash._—Fifteen grams in one liter: Twenty cubic
centimeters of the solution are evaporated with fifteen of platinic
chlorid. The weight of potassium platinochlorid obtained × 64.33 gives
the per cent of K₂O.
_Potassium Chlorid._—Ten grams in one liter: Twenty-five cubic
centimeters are evaporated with fifteen of platinic chlorid solution.
The weight of the precipitate obtained × 77.2 gives the per cent of K₂O.
=264. Weighing the Precipitate as Metallic Platinum.=—Hilgard calls
attention to the difficulty of weighing the double chlorid of platinum
and potash as such, although he acknowledges that in the gooch this
weighing can be made with great accuracy.[213] He prefers to estimate
the platinum in the metallic state and uses for this purpose a platinum
crucible the inside of which, half way up from the bottom, is coated
with a layer of platinum sponge, which is conveniently prepared by
the decomposition of a few decigrams of the platinum double salt by
inclining the crucible and rotating it during the progress of the
reduction, using about a quarter of an hour in all. The platinum sponge
produced in this way greatly favors the decomposition of the double
salt for analytical purposes. The decomposition of the salt takes place
quickly and quietly and at conveniently low temperatures.
When the decomposition is ended the crucible is strongly heated so as
to hold the platinum sponge, which is produced, together sufficiently
to prevent its being removed in the subsequent washing of the crucible
by decantation. By the ignition at a high temperature necessary to
secure this, the greater part of the calcium chlorid is volatilized.
After cooling, a few drops of concentrated hydrochloric acid are placed
in the crucible and if the slightest yellow color be shown the acid is
evaporated and the ignition repeated, with the addition of a little
oxalic acid. In most cases the slight yellow color produced comes
from a trace of iron and will therefore appear again after the second
ignition. The crucible is subsequently washed by repeated decantations,
finally with boiling water, and after drying is ignited and weighed.
The advantage of this process is that without further trouble the
reduced metal is completely freed of any salts of the alkaline earths,
etc., which have been carried down with it and also from any of the
uncombined sodium chlorid which may not have been washed out by the
alcohol. In fact, the results obtained in this way are nearly always
lower than those obtained through the direct weighing of the double
salt, and the wash water which is first poured off contains, as a rule,
traces of the alkaline earths and almost without exception some sodium
chlorid. Correction for the filter ash is unnecessary because the ash
is completely dissolved by the treatment received. The platinum sponge
which is collected in the crucible in this way is removed in case it
does not adhere to the sides and the crucible is then ready for the
next operation.
=265. Sources of Error in the Platinum Method.=—In the comparative work
done in the determination of potash by the members of the Association
of Official Agricultural Chemists there has been noted, from year to
year, marked differences in the data obtained by different analysts.
Such differences often are due to personal errors, or a failure to
accurately follow the directions for manipulation. Sometimes, however,
they are due to sources of error in the processes employed. In the
platinum method these sources of error have been long known to exist.
Chief among these is the remarkable facility with which potash becomes
incorporated with the precipitates of other bodies. The character
and magnitude of some of these errors have lately been studied by
Robinson.[214]
Many precipitates occlude potash and hold it so firmly that it cannot
be washed out with hot water although the potash compounds present
in the precipitate are perfectly soluble. It appears to be a kind of
molecular adhesion. Barium sulfate has this property of attaching
potash molecules in a high degree, and ferric and aluminic compounds
only to a slightly less extent. To reduce the losses, consequent on the
conditions just mentioned, to a minimum, the sulfuric acid and earthy
bases should be very slowly precipitated, with violent agitation, at a
boiling temperature.
Another source of loss in the platinum method arises from the
use of a solution of ammonium chlorid for washing the potassium
platinochlorid precipitate. There is danger here, not only of the
solution of the impurities present in the precipitate, but also
of a double decomposition by means of which some ammonium may be
substituted for the potassium in the washed product. In the official
method, moreover, there is danger of securing a final precipitate
which may contain traces of calcium and magnesium sulfates when these
bodies are abundantly present in the sample taken for analysis. The
careful analyst must guard against these sources of error, but it is
probably true that he will never secure a practically chemically pure
precipitate of potassium platinochlorid when working on the mixed
fertilizers found in commerce.
=266. Effect of Concentration on the Accuracy of Potash
Analysis.=—Winton has also studied the sources of error in the
determination of potash as platinochlorid, especially with reference
to the effect of the concentration of the solution at the time of
precipitation.[215]
He finds that the method of precipitating in concentrated solutions and
drying the potassium platinochlorid at 130°, depends for its accuracy
upon the mutual compensation of three errors; _viz._, (1) to the
solubility of the potassium salt in eighty per cent alcohol, (2) to the
presence of water in the crystals which is not driven off at 130°, and
(3) the use of a factor based on the wrong atomic weight of platinum.
He finds, further, that the error due to the presence of water occluded
in the crystals can be reduced to a minimum, and the process of drying
greatly simplified, by adding the solution of platinum chlorid to the
potash solution in a dilute condition, not exceeding one per cent
in strength. The potassium platinochlorid thus produced can be very
effectively dried at 100°. The error due to the solubility of the
salt in eighty per cent alcohol can also be greatly reduced by using
ninety-five per cent alcohol. The error due to the wrong factor, based
on the old atomic weight of platinum, _viz._, 0.3056, can be corrected
by using the factor based on the recently determined atomic weight of
platinum, _viz._, 195, which is 0.30688.
=267. Differences in Crystalline Form.=—Winton has also observed a
distinct difference in the crystals of potassium platinochlorid when
obtained from concentrated and dilute solutions.[216] When platinic
chlorid is added to a concentrated solution of potassium chlorid, a
large part of the salt which is formed is precipitated in a pulverulent
state, the remainder being deposited on evaporation. After treating
with alcohol, filtering, and drying, the double salt is found in the
state of a fine powder which, when examined under the microscope, is
found to consist largely of radiating crystals. The characteristic form
is one having six arms formed by the intersection, at right angles, of
three bars. Numerous globular cavities in the crystals are observed in
which mother liquid is enclosed. For this reason the salt is not easily
dried at 100°, but when so dried loses additional moisture at 130°,
and still more at 160°. The total additional loss, after drying at
100°, from this cause may amount to as much as six-tenths per cent of
potassium chlorid.
When, however, the solution of the potassium salt is so dilute that
no precipitate at all is formed on the addition of platinic chlorid,
the double salt is all deposited, as well as formed slowly, during
the evaporation and occurs exclusively as octahedra. These octahedra
are comparatively free of cavities, and give up practically all their
moisture when dried at 100°. A method of procedure therefore for
potash determination, based on the above principle of the addition of
the reagent to dilute solutions, and drying the double salt produced
upon evaporation, after washing with ninety-five per cent alcohol at
100°, and using the factor 0.30688 for potassium chlorid and 0.1939
for potassium oxid, gives good results and is regarded as better than
any of the methods which prescribe the addition of platinic chlorid to
highly concentrated potash solutions.
=268. Factors for Potash Estimation.=—The factor now in use by the
official chemists to convert potassium platinochlorid into potash (K₂O)
is 0.19308, and for potassium chlorid 0.3056.
Wolfbauer gives the differences which may arise by computing the potash
from its platino-double chlorid by the different values assigned to the
atomic weight of platinum.[217]
The common factor used to obtain potassium chlorid from potassium
platinochlorid is based on the atomic weight 197.18 and is derived from
the formula:
2(39.13 + 35.46) 149.18
------------------------------ = ------ = 0.30557.
2 × 39.13 + 197.18 + 6 × 35.46 = 488.20
The variations arising from taking other assigned values for the atomic
weight of platinum are shown in the following table:
Factor for potassium Relation to factor
chlorid from 0.30557
in per cent
Atomic Determined
weight of or Potassium Potassium
Platinum. calculated platino- Platinum. platino- Platinum.
by chlorid. chlorid.
197.18 Berzelius 0.30557 0.75658 100.00 100.00
197.88 Andrews 0.30517 0.75390 99.86 99.65
195.06 Haberstadt 0.30690 0.76468 100.44 101.07
Seubert }
194.87 and Clark } 0.30700 0.76555 100.47 101.20
The factor 0.3056 is regarded as the best for the computation from
potassium platinochlorid and 0.7566 from platinum. It is also suggested
that it is better to make the computation from the reduced platinum
than from the double salt.
=269. Recovery of the Platinum Waste and Preparation of the Platinic
Chlorid Solution.=—(1) _By Reduction in Alkaline Alcohol._—All
filtrates containing platinic chlorid, all precipitates of potassium
platinochlorid and all residues of metallic platinum should be
carefully preserved and the platinum recovered therefrom by the
following process: The platinum residues are placed in a large
porcelain dish. Since these residues contain a large amount of alcohol
they should be diluted with about one-third their volume of water, and
when boiling treated with some sodium carbonate. The solid potassium
platinochlorids should not be added until the liquid is boiling, and
then only little by little. The heating on the water-bath is continued
until the liquid floating over the platinum sponge is quite clear and
only slightly yellow. The liquid is then poured off and the reduced
platinum purified by boiling with hydrochloric acid and water. It is
then dried and ignited to destroy any organic matter which may be
present. It is advisable to boil the finely divided platinum once with
strong nitric acid, and after this is poured off the solution of the
platinum is effected in a large porcelain dish over a water-bath by
adding about four times its weight of hydrochloric acid, warming, and
adding nitric acid, little by little. After the platinum is in solution
the evaporation is continued until a drop of the liquid, removed by a
glass rod, quickly solidifies. The crystalline mass which is formed
on cooling is taken up with water and filtered, and then a sufficient
amount of water added so that each ten cubic centimeters will contain
one gram of platinum. The specific gravity of this solution is 1.18 at
ordinary temperatures. Special care must be taken that the solution
contains neither platinous chlorid nor nitrogen compounds. If the first
named compound be present it should be converted into platinic chlorid
by treatment with fuming hydrochloric acid and a little nitric acid.
The last mentioned compound may be removed by evaporating successively
with hydrochloric acid and water. If the platinic chlorid be made
from waste platinum, the danger of contamination with iridium must be
considered. In such a case the platinum should be separated as ammonium
platinochlorid, which can afterwards be reduced as above indicated.
A convenient test of the purity of platinic chlorid solution is
accomplished by the precipitation of a known weight of chemically pure
potassium salt.
(2) _By Reduction in Nascent Hydrogen._—The platinum residues,
filtrates containing platinum, etc., are collected in a large flask and
evaporated in a large dish on a water-bath, and reduced by means of
zinc and hydrochloric acid to metallic platinum, the mass being warmed
until all the zinc has been dissolved. The supernatant liquid standing
over the spongy platinum is decanted and the spongy mass boiled twice
with distilled water. The spongy platinum is then brought on a filter
and washed till the filtrate shows no acid reaction. The filter and
platinum sponge are next incinerated in a platinum dish and the residue
weighed. The weighed mass of pure platinum is dissolved in hydrochloric
acid, with the addition of as little nitric acid as possible, and,
after cooling, filtered. The filtrate is afterwards evaporated in a
porcelain dish on a water-bath to a sirupy consistence, taken up with
water and filtered. To this filtrate enough water is now added to make
the solution correspond to one gram of metallic platinum in ten cubic
centimeters.