=249. Classification of Methods.=—To detect the presence of potash in
a mixture the aid of the spectroscope may be invoked. In the scale of
the spectrum divided into 170 parts, on which the sodium line falls at
50, potassium gives three faint rather broad bands, two red, falling
at 17 and 27, and one plum-colored band, near the extreme right of
the spectrum, at 153. Potassium, however, does not give brilliant and
well-marked spectral bands, such as are afforded by its associates
rubidium, caesium, sodium, and lithium. A convenient qualitative test
which, for practical purposes will be quite sufficient, may be secured
by dipping a platinum loop into a strong acid solution of the supposed
potash compound, and viewing through a piece of cobalt glass, the
coloration produced thereby when held in the flame of a bunsen. The
red-purple tint thereby produced should be compared with that coming
from a pure potash salt similarly treated. If a fertilizer sample
give no indication of potash when treated as above it may be safely
concluded that it does not contain any weighable quantity of potash.
For the estimation of the percentage of potash present in a given
sample it may be safely assumed that all of value in agriculture will
be given up to an aqueous or slightly acid solution if organic matter
have been destroyed as indicated in a previous paragraph. In the case
of minerals insoluble in a dilute acid the potash may be determined by
some one of the processes given in the first volume.[200] The potash
having been obtained in an aqueous or slightly acid (hydrochloric)
solution, it may be determined either by precipitation as potassium
platinochlorid or as potassium perchlorate. The former method is the
one which has been almost exclusively used by analysis in the past, but
the latter one is coming into prominence and by reason of the greater
economy attending its practice and the excellent results obtained by
some analysts, demands a generous consideration.
=250. The Platinic Chlorid Method.=—The principle of this method
rests on the great insolubility of the potassium platinochlorid
in strong alcohol and the easy solubility of some of its commonly
attending salts; _viz._, sodium, etc., in the same reagent. Before the
precipitation of the potash it is necessary to remove the bases of the
earths, sulfates, etc. Barium chlorid and hydroxid, ammonium oxalate or
carbonate, sulfuric acid, etc., are used in conjunction or successively
to effect these purposes in the manner hereinafter described. The
filtrate and washings containing the potash are evaporated to dryness
and gently ignited to expel excess of ammonium salts and in the residue
taken up with water and acidulated with hydrochloric acid, the potash
is precipitated with platinic chlorid solution. The best methods of
executing the analysis follow.
=251. The Official Agricultural Method.=—This method is based on the
processes at first proposed by Lindo[201] and Gladding,[202] and
is given below as adapted to mixed fertilizers and mineral potash
salts.[203]
(1) _In Superphosphates._—Boil ten grams with 300 cubic centimeters of
water thirty minutes. To the hot solution add ammonia in slight excess,
and then a sufficient quantity of ammonium oxalate to precipitate all
the lime present; cool and make up to half a liter, mix thoroughly,
and filter through a dry filter; evaporate fifty cubic centimeters,
corresponding to one gram, nearly to dryness, add one cubic centimeter
of dilute sulfuric acid (1 to 1), evaporate to dryness and ignite to
whiteness. As all the potash is in form of sulfate, no loss need be
apprehended by volatilization of potash, and a full red heat must
be maintained until the residue is perfectly white. This residue
is dissolved in hot water, plus a few drops of hydrochloric acid,
and a slight excess of platinum solution is added. This solution is
then evaporated to a thick paste in a small dish, and eighty per
cent alcohol added. In evaporating, special precaution should be
taken to prevent absorption of ammonia. The precipitate is washed
thoroughly with alcohol by decantation and on the filter, as usual.
The washing should be continued even after the filtrate is colorless.
Ten cubic centimeters of the ammonium chlorid solution, prepared as
hereinafter directed, are run through the filter, or the washing may
be performed in the dish. The ten cubic centimeters will contain the
bulk of the impurities, and are thrown away. Fresh portions of ten
cubic centimeters of the ammonium chlorid are run through the filter
several times (5 or 6). The filter is then washed thoroughly with pure
alcohol, dried, and weighed as usual. Care should be taken that the
precipitate is perfectly soluble in water. The platinum solution used
contains one gram of metallic platinum in every ten cubic centimeters.
To prepare the washing solution of ammonium chlorid, place in a bottle
500 cubic centimeters of water and 100 grams of ammonium chlorid and
shake till dissolved. Now pulverize five or ten grains of potassium
platinochlorid, put in the bottle and shake at intervals for six or
eight hours; let settle over night, then filter off the liquid into
a second bottle. The first bottle is then ready for preparation of a
fresh supply when needed.
(2) _Potassium Chlorids._—In the analysis of these salts an aliquot
portion of the solution, containing a half gram, is evaporated with
forty cubic centimeters of the platinum solution and a few drops of
hydrochloric acid, and washed as before.
(3) _Potassium Sulfate, Kainit, Etc._—In the analysis of kainit,
dissolve ten grams of the pulverized salt in 300 cubic centimeters of
boiling water, add ammonia to slight excess, then a sufficient quantity
of ammonium oxalate to throw down all lime present; cool and make up
to half a liter, mix thoroughly, and filter on a dry filter; from
twenty-five cubic centimeters, corresponding to a half gram, proceed
to remove the ammonia, as in the analysis of superphosphates; dissolve
the residue in hot water, plus a few drops of hydrochloric acid, and
add fifteen cubic centimeters of platinum solution. In the analysis
of high-grade sulfate and of double-manure salt (potassium sulfate,
magnesium sulfate, containing about twenty-seven per cent of potassium
oxid), make up the solution as above, but omit the precipitation,
evaporation, etc.; to an aliquot part equal to a half gram add fifteen
cubic centimeters of platinum solution. In all cases special care
must be taken in the washing with alcohol to remove all the double
platinum sodium chlorid which may be present. The washing should be
continued some time after the filtrate is colorless. Twenty-five cubic
centimeters of the ammonium chlorid solution are employed instead of
ten cubic centimeters, and the twenty-five cubic centimeters poured
through at least six times to remove all sulfates and chlorids. Wash
finally with alcohol; dry and weigh as usual.
=252. Alternate Method for Potash.=—Boil ten grams of the prepared
sample for thirty minutes with 300 cubic centimeters of water, and,
after cooling and without filtering, make up to one liter and filter
through a dry filter. If the sample have ten per cent of potassium
oxid, use fifty cubic centimeters of the filtrate; if less than ten per
cent of potassium oxid (ordinary potash fertilizers), use 100 cubic
centimeters of the filtrate. In each case make the volume up to 150
cubic centimeters, heat to 100°, and add, drop by drop with constant
stirring, a slight excess of barium chlorid, and, without filtering,
in the same manner add barium hydrate in slight excess. Filter while
hot and wash until the precipitate is free of chlorids. Add to the
filtrate one cubic centimeter of strong ammonium hydrate, and then a
saturated solution of ammonium carbonate, until the excess of barium is
precipitated. Heat and add, in fine powder, a half gram of pure oxalic
acid or 0.75 gram of ammonium oxalate. Filter, wash free of chlorids,
evaporate the filtrate to dryness in a platinum dish, and ignite
carefully over the free flame, below red heat, until all volatile
matter is driven off.
The residue is digested with hot water, filtered through a small
filter, and washed with successive small portions of water until the
filtrate amounts to thirty cubic centimeters or more. To this filtrate,
add two drops of hydrochloric acid, in a porcelain dish, and from five
to ten cubic centimeters of a solution of ten grams of platinic chlorid
in 100 cubic centimeters of water. The mixture is evaporated on a
water-bath to a thick sirup, as above, treated with alcohol of eighty
per cent strength, washed by decantation, collected in a gooch or other
form of filter, washed with strong alcohol, afterwards with five cubic
centimeters of ether, dried for thirty minutes at 100°, and weighed.
It is desirable, if there be an appearance of foreign matter in the
double salt, that it should be washed, according to the previous
method, with ten cubic centimeters of the half-concentrated solution of
ammonium chlorid, which has been saturated by shaking with potassium
platinochlorid.
=253. Method of Solution for Organic Compounds.=—In case the potash is
contained in organic compounds, like tobacco stems, cottonseed hulls,
etc., weigh ten grams, saturate with strong sulfuric acid, and ignite
in a muffle to destroy organic matter. Add a little strong hydrochloric
acid to moisten the mass and warm slightly so as to loosen it in the
dish. Proceed then as in the lindo-gladding or alternate method.
=254. Factors.=—The use of the factors 0.3056 for converting potassium
platinochlorid to potassium chlorid and 0.19308 for converting it to
potassium oxid is advised. The latter number is almost identical with
that used by the Halle and Stassfurt chemists _viz._, 0.1927 and 0.1928
respectively.
=255. Methods Used at the Halle Station.=—(1) _In Kainits and other
Mineral Salts of Potash._[204]—Five grams of the prepared sample are
boiled for half an hour in a half liter flask with from twenty to
thirty cubic centimeters of concentrated hydrochloric acid and 100
cubic centimeters of water, and afterwards as much water added as is
necessary to fill the flask about three quarters full, and the sulfuric
acid is then precipitated with barium chlorid. To avoid an excess of
barium chlorid the solution used is of known strength and is added
first in such quantity as would precipitate the sulfuric acid from
a kainit of low sulfuric acid content. The mixture is then boiled,
allowed to settle and tried with a dropping tube containing barium
chlorid. If a further precipitate be given a few drops more of barium
chlorid solution are added, again boiled and allowed to settle. This
is continued until barium chlorid gives no precipitation. After the
barium chlorid gives no more precipitate a drop of dilute sulfuric acid
is added to test for excess of barium. The operation is continued with
the sulfuric acid until it no longer gives a precipitate of barium
sulfate. By the alternate use of the barium chlorid and sulfuric acid
the exact neutral point can soon be secured. When this point is reached
the liquid is allowed to cool, the flask is filled to the mark, its
contents filtered, and of the filtrate fifty cubic centimeters, equal
to half a gram of the substance, taken for further estimation.
This quantity is evaporated on a water-bath to a sirupy consistence
in a porcelain dish with ten cubic centimeters of platinic chlorid.
The platinic chlorid solution should contain one gram of platinum
in each ten cubic centimeters. The residue is treated with eighty
per cent alcohol and, with stirring, allowed to stand for an hour.
The precipitate is then collected on a gooch, either of platinum or
porcelain, washed about eight times with eighty per cent alcohol
and the potassium platinochlorid dried for two hours at 100°. After
weighing the precipitate is dissolved in hot water and the residue
washed under pressure, first with hot water and then with alcohol.
The crucible with the asbestos felt is dried at 100° and weighed. Any
impurities which the double salt may have carried down with it are
left on the filter and the weight of the original precipitate can thus
be corrected. The weight of potassium platinochlorid is multiplied by
0.1927 and the product corresponds to the weight of K₂O in the sample
taken.
(2) _Estimation of Potash in Guanos and Other Fertilizers containing
Organic Substances._—Ten grams of the substances are carefully
incinerated at a low temperature in a platinum dish. After ignition
the contents of the dish are placed in a half liter flask and boiled
for an hour with hydrochloric acid and a few drops of nitric acid. The
sulfuric acid can then be precipitated directly with barium chlorid, or
better, allow the flask to cool, fill to the mark, filter and treat an
aliquot part of the filtrate with barium chlorid as described above.
The filtrate from the separated sulfate of barium is neutralized with
ammonia and all the bases, with the exception of magnesia and the
alkalies, precipitated with ammonium carbonate; boil, fill to the
mark and filter. Of this filtrate evaporate from 100 to 200 cubic
centimeters in a platinum dish. After evaporation the ammonium salts
are driven off by careful ignition, the residue taken up with hot water
and filtered through as small a filter as possible into a porcelain
dish; the magnesia remaining in the precipitate. The filtrate is
acidified with a few drops of hydrochloric acid, ten cubic centimeters
of platinic chlorid added and the further determination conducted as
with kainit.
=256. Dutch Method.=—The process used at the Royal Agricultural Station
of Holland is almost identical with that employed at Halle.[205]
A. _Method for Stassfurt and other Potash Salts._—The necessary
reagents are: