Dissolve the substance in which the phosphoric acid is to be estimated in concentrated nitric acid, add, at least, eight times as much tinfoil as there is phosphoric acid present, and warm the mixture for five or six hours, until the precipitate has completely subsided, leaving the supernatant fluid quite clear. Wash by decantation combined with filtration, rinse the washed precipitate into a platinum dish, and digest with a small quantity of highly concentrated solution of potassa. The product of this operation is a mixture of metastannate and phosphate of potassa, which, upon addition of hot water, dissolves to a clear fluid, and even very readily if there has not been too much hydrate of potassa used. Dissolve, in the same way, the trifling particles of the precipitate which may still adhere to the filter, add this solution to that of the precipitate, transfer the whole fluid to a weighed flask of 1000 grammes capacity, and dilute with water to make the fluid up to about 900 grammes; saturate now with sulphuretted hydrogen, adding also some pentasulphide of ammonium, and then acetic acid, until the bisulphide of tin is precipitated, and the fluid slightly acid. Replace the flask now on the scale, add water, until the contents weigh 1000 grammes (or some other round number), shake, allow it to stand from 12 to 16 hours, filter the supernatant clear fluid into a porcelain dish, and weigh the flask again, which now contains the rest of the fluid, together with the sulphide of tin. The weight found gives, by simple subtraction, also the quantity of the filtrate in which the phosphoric acid is to be estimated. The proportion which this bears to the total quantity of the fluid, viz., 1000 grammes, minus the weight of the sulphide of tin (which may be calculated with sufficient accuracy from the amount of tin originally used, or may be estimated in the direct way), is easily found.

Wash the filter used in transferring the clear fluid from the flask to the porcelain dish, and add the rinsings to the filtrate in the latter; evaporate the fluid until a small quantity only is left, and then estimate the phosphoric acid in this according to the directions of b, a. The way here recommended of effecting the separation of the phosphoric acid fluid from the sulphide of tin must be resorted to of necessity, since in the filtration and washing of the sulphide, no matter whether with pure water or water containing sulphuretted hydrogen, a small quantity of tin would inevitably be dissolved. Results accurate.

c. Determination as Phosphate of Sesquioxide of Uranium.

Leconte, A. Arendt, and W. Knop's method,* very suitable in presence of alkalies and alkaline earths, but not in presence of any notable amount of alumina; in presence of sesquioxide of iron, the method can be applied only with certain modifications (see § 135, h, y). Dissolve the phosphate under examination in acetic acid. If you have a nitric or hydrochloric acid solution, remove the greater portion of the free acid by evaporation, add ammonia until red litmus paper dipt into it turns very distinctly blue, and then redissolve the precipitate formed in acetic acid;

ciple free from defects, yet presents certain practical difficulties, owing principally to the large proportion of tin foil required for the process (eight times the quantity of the phosphoric acid), which makes the presence of even slight impurities in the tin a source of considerable error. These remarks of Reissig completely coincide with the results of my own experiments.

Leconte was the first to recommend the method of precipitating phosphoric acid from acetic acid solution by means of a salt of uranium ("Jahresb." von Liebig und Kopp, 1853, 642); A. Arendt and W. Knop have subsequently subjected it to a careful and searching examination ("Chem. Centralbl.,” 1856, 769, 803, and 1857, 177).

if mineral acids were present, add also some acetate of ammonia. Mix the fluid now with solution of acetate of sesquioxide of uranium, and heat the mixture to boiling, which will cause the phosphoric acid to separate, in form of yellow phosphate of sesquioxide of uranium and

ammonia.

Wash the precipitate by decantation combined with filtration, taking care to boil the mixture after every fresh addition of water; the operation may be materially facilitated, by adding, immediately after precipitation, as soon as the liquid has cooled a little, 2 or 3 drops of chloroform, and giving the mixture a vigorous shake, or boiling it once or twice. Dry the precipitate, and ignite as directed § 53. It is advisable to moisten the ignited mass with nitric acid, and then ignite once more. For the properties of the precipitate and residue, see $93, e. Should it be necessary to dissolve the ignited residue again, for the purpose of reprecipitating it, this can be done only after fusing it previously with a large excess of carbonate of soda and potassa, and thereby converting the pyrophosphoric into tribasic phosphoric acid. Results accurate; compare Experiment No. 84.

d. Determination as Basic Phosphate of Sesquioxide of Iron.

a. Proceed exactly as in the determination of arsenic acid as arsenate of sesquioxide of iron, by Kobell's modification of Berthier's method (§ 127, 3, b). The results are accurate.

B. Mix the acid fluid containing the phosphoric acid with an excess of solution of sesquichloride of iron of known strength, add, if necessary, sufficient alkali to neutralize the greater portion of the free acid, mix with acetate of soda in excess, and boil. If the quantity of solution of - sesquichloride of iron added was sufficient, the precipitate must be brownish-red. This precipitate consists of basic phosphate and basic acetate of sesquioxide of iron, and contains the whole of the phosphoric acid and of the sesquioxide of iron. Filter off boiling, wash with boiling water mixed with some acetate of ammonia, dry carefully, and ignite in a platinum crucible with access of air (§ 53). Moisten the residue left upon ignition with strong nitric acid, evaporate this at a gentle heat, and ignite again. Should this operation have increased the weight, which, however, is not usually the case, it must be repeated, until the weight remains constant. Deduct from the weight of the residue that of the sesquioxide of iron contained in the solution added; the difference is the phosphoric acid.

This modification of Schulze's method was first recommended by A. Müller ("Journ. f. prakt. Chem.," 47, 341); it has been adopted also by Way and Ogston, in their analyses of ashes ("Journal of the Royal Agricultural Society," VIII. Part I.). By the use of a solution of sesquichloride of iron of known strength, the estimation of the sesquioxide of iron in the residue (which would have to be effected in the manner described § 113, 2, a), is dispensed with.

7. J. Weeren's method, suitable for the estimation of the phosphoric acid in phosphates of the alkalies and alkaline earths ("Journ. f. prakt Chem.," 67, 8). Mix the nitric acid solution of the phosphate under examination, which must contain no other strong acid, with a solution of nitrate of sesquioxide of iron of known strength, in sufficient proportion to ensure the formation of a basic salt; evaporate the mixture to dryness, heat the residue to 320° F., until no more nitric acid fumes escape, treat with hot water until all nitrates of the alkalies and alkaline

earths are removed,* collect the ochreous precipitate on a filter, dry, ignite (§53), weigh, and deduct from the weight the quantity of sesquioxide of iron added.

e. Determination as Basic Phosphate of Magnesia (3 Mg O, PO). Fr. Schulze's method, suitable more particularly to effect the separation of phosphoric acid from the alkalies ("Journ. f. prakt. Chem.," 63, 440).

Mix the solution of the alkaline phosphate, which contains chloride of ammonium, with a weighed excess of pure magnesia, evaporate to dryness, ignite the residue until the chloride of ammonium is expelled, and separate the magnesia, which is still present in form of chloride of magnesium, by means of oxide of mercury (§ 104, 3, b). Treat the ignited residue with water, filter the solution of the chlorides of the alkali metals, wash the precipitate, dry, ignite, and weigh. The excess of weight over that of the magnesia used shows the quantity of the phosphoric acid present in the analysed phosphates. Results satisfactory. f. Determination as Phosphate of Silver, see § 135, a.

g. Determination by Volumetric Analysis.

Although it would be most desirable to have a good volumetrical method for determining phosphoric acid, considering how very often the analyst has to determine this acid, yet, up to the present time, this object remains still unattained. Certain methods have, indeed, been proposed, but they lead only to an approximative estimation of the acid, and are accordingly used only in the analysis of urine, for want of more definite and accurate processes. They are based upon the insolubility of phosphate of sesquioxide of iron (Fe, O,, PO,) in acetic acid, or also in a hydrochloric acid solution mixed with an excess of acetate of soda. Liebig ("Annal. d. Chem. u. Pharm.," 78, 150) recommends to acidify the phosphoric acid fluid with acetic acid, and to add acetate of soda, then the least excess of a solution of known strength of sesquichloride of iron, free from acid, or iron alum. The point when the iron solution has been added just in sufficient excess, is ascertained by pressing, some time after the last addition, a glass rod moistened with the fluid against a double filter paper, laid on another filter paper, moistened with solution of ferrocyanide of potassium and resting on a porcelain plate. If the iron solution has been added in excess, a blue color will make its appearance in a few seconds. The great difficulty in this process, which otherwise has the advantage of promptness in its favor, is to hit this point with a proper degree of exactness. The most effective way is to take, to about 0-2 grm. of phosphoric acid, 10 cubic centimetres of concentrated acetic acid and a sufficient quantity of acetate of soda.-Räwsky (“Journ. f. prakt. Chem.," 41, 365) recommends the following method: add to the acid solution (which, with the exception of sesquioxide of iron, must contain no bases forming with phosphoric acid compounds insoluble in acetic acid), ammonia, until the free acid is nearly neutralized, then acetate of sesquioxide of iron† in the least possible excess. The phosphate of sesquioxide of iron is deposited in the form of a faintly yellowish white precipitate. Filter, and wash carefully with cold water-an operation which in the case of larger quantities of substance takes much time;

* In presence of magnesia, warming with a solution of nitrate of ammonia is advisable. A solution of iron alum (1:10), mixed with an equal quantity of solution of acetate of soda (1:10), and to which it is as well to add some free acetic acid, answers the same purpose.

dissolve the precipitate in hydrochloric acid, and determine the iron in the solution in the manner directed § 113, 2, a. Räwsky proceeds upon the assumption that the precipitate has the composition PO,, Fe, O,, and calculates for every 700 of iron 900 (rigorously 887-5, the equivalent of phosphorus being 387.5) of phosphoric acid. This assumption is not correct, however, as the proportion of iron in the precipitate varies, being the higher, the greater the excess of solution of iron used.

For a more detailed account of these methods, I refer to Neubauer and Vogel's "Anleitung zur Analyse des Harns," second edition, page 109; and Dunklenberg's paper on the subject, in "Annal. d. Chem. u. Pharm.," 93, 88.

II. SEPARATION OF PHOSPHORIC ACID FROM THE BASES.

$ 135.

a. From the Fixed Alkalies (see also b, e, l, m).

a. The method I., d, in one of its modifications, is resorted to, or method I., e. The alkalies are found in the filtrate as nitrates or chlorides.

B. The method I., b, a, is applied, and the separation of the magnesia from the alkalies in the filtrate is effected in the manner described § 153. 7. Salts composed after the formula 3 M O, PO, are dissolved in water and the solution is precipitated with neutral solution of silver; the yellow precipitate formed 3 Ag O, PO,, is washed, dried, and ignited in the manner described § 53.

Phosphates composed after the formula 2 M O, HO, PO,, are ignited, the residue dissolved in water, and precipitated with neutral solution of silver. The fluid is filtered off from the precipitate, which, in this case, consists of pyrophosphate of silver 2 AgO, PO,, and the latter washed, dried, and ignited (§ 53). For the properties of the precipitated phosphates of silver, see § 93, 4.

The bases in the filtrates are determined after the removal of the excess of silver (see § 162).

The results are accurate; this method is particularly convenient, on account of the facility with which the alkalies may be estimated in the filtrate.

b. From the whole of the Alkalies see also (a, e, l, m).

a. The separation of the phosphoric acid is effected by the method described § 134, d, a, and the baryta and the alkalies in the filtrate are separated as directed § 153.

B. The aqueous solution is mixed with acetate of lead in slight excess, and the precipitate formed allowed to subside; the fluid is then filtered off, and the alkalies in the filtrate are separated from the excess of the salt of lead used in the process, as directed in § 162. The quantity of the phosphoric acid originally present in the analysed compound may be calculated from the loss; but it may also be determined in the direct way, by treating the washed precipitate of phosphate of lead according to § 135, c. c. From Baryta, Strontia, Lime, and Oxide of Lead.

The compound under examination is dissolved in hydrochloric or nitric acid, and the solution precipitated with sulphuric acid in slight In the separation of phosphoric acid from strontia, lime, and oxide of lead, alcohol is added with the sulphuric acid. The phosphoric

excess.

acid in the filtrate is determined according to § 134, b, a (in the case of strontia, lime, and oxide of lead, after previous removal of the alcohol by evaporation). The determination of the phosphoric acid is effected with the most accurate results by saturating the fluid with carbonate of soda, evaporating to dryness, and fusing the residue with carbonate of soda and potassa. The fused mass is then dissolved in water, and the further process conducted as in § 134, b, a.

d. From Magnesia (see also e, i, l, and m).

The phosphoric acid is separated as in § 134, d, a; and the magnesia and baryta in the filtrate are separated in the manner described § 154.

e. From the whole of the Alkaline Earths (comp. also § 135, i, l, and m). a. Separate phosphoric acid as phosphate of sesquioxide of uranium, as directed in § 134, c, and the sesquioxide of uranium from the alkaline earths, in the filtrate, according to the directions given in the supplement to §§ 160 and 161. This is an excellent method.

B. The phosphoric acid is separated by one of the methods given in §134 d, and y. If the method d, B is applied, the alkaline earths remain in solution as chlorides with the alkaline acetate and alkaline chloride; if d, y, they are obtained in solution as nitrates.

are satisfactory.

The results

7. Dissolve in the least possible amount of nitric acid, add acetate of lead in slight excess, let the precipitate formed subside, and filter; wash the precipitate, which consists of phosphate and basic nitrate of lead, dry, iguite (§ 53), and weigh. The residue is phosphate of lead + oxide of lead, or in other terms, phosphoric acid +x oxide of lead. Put the crucible, with its contents, into a beaker, pour moderately dilute nitric acid over it, and warm, until solution is effected; then decant the fluid into another glass, wash, add the rinsings to the solution, and determine the oxide of lead in it as sulphate (§ 116, 2). Calculate from this the oxide of lead, and deduct the result from the weight of the first residue the difference gives the quantity of the phosphoric acid. In the fluid filtered off from the first precipitate, the bases are separated from the excess of the salt of lead used, in the manner described (§ 162). This method also gives satisfactory results.

f. From Alumina (see also § 135, i, and m.)

If

a. (Otto and Fresenius, applicable also in presence of sesquioxide of iron.) Dissolve in hydrochloric or nitric acid, dilute a little, add a tolerable quantity of tartaric acid, and then ammonia in excess. you have added sufficient tartaric acid, the fluid must now appear clear. Add, in slight excess, a clear solution of sulphate of magnesia mixed with chloride of ammonium and ammonia, and let the mixture stand at rest for 12 hours; then filter, and wash the precipitate with dilute solution of ammonia; to free it completely from alumina, redissolve it in hydrochloric acid, add a little tartaric acid, and reprecipitate with ammonia. Treat the precipitate now as directed in § 134, b, a. To obtain the alumina contained in the filtrate, add some nitrate of potassa and a sufficient quantity of carbonate of soda to effect the decomposition of the chloride of ammonium,* evaporate to dryness, and ignite the residue in a platinum vessel. Dissolve in nitric or

The ignition of alumina in presence of chloride of ammonium would entail loss by the formation and escape of chloride of aluminium (H. Rose).