(which is only rarely the case)-and the silicic acid is separated, in the usual way, by evaporation, the whole of the fluorine may volatilize.

a. Berzelius's method.

Fuse the elutriated substance with 4 parts of carbonate of soda, 197 for some time, at a strong red heat; digest the mass in water, boil, filter, and wash, first with boiling water, then with solution of carbonate of ammonia. The filtrate contains all the fluorine as fluoride of sodium, and, besides this, carbonate of soda, silicate of soda, and aluminate of soda. Mix the filtrate with carbonate of ammonia and heat the mixture, replacing the carbonate of ammonia which evaporates. Filter off the precipitate of hydrate of silicic acid and hydrate of alumina, and wash with carbonate of ammonia. Heat the filtrate until the carbonate of ammonia is completely expelled, and determine the fluorine as directed § 138. To separate the silicic acid, decompose the two precipitates with hydrochloric acid as directed § 140, II., a.*

B. Wöhler's method (suitable only for the analysis of substances 198 which contain a large proportion of fluorine and are readily decomposed by sulphuric acid).

Reduce the compound under examination to the very finest powder, introduce this into a small flask, pour pure sulphuric acid over it, close the flask quickly with a perforated cork into which a small tube with chloride of calcium is fitted, weigh the apparatus with the greatest despatch, and then apply heat until the evolution of fumes of fluoride of silicon (Si Fl) ceases; remove the last remaining traces of the gas from the flask, by an exhausting syringe, let the apparatus cool, and then weigh. The loss indicates the weight of the fluoride of silicon expelled in the process. Deduce from this the quantity both of the fluorine and of the silicon, calculate the latter as silicic acid, and add the quantity found to the weight of the silicic acid in the residue.

6. FLUORIDES, SILICATES, AND PHOSPHATES, IN PRESENCE OF

EACH OTHER.

Native compounds of fluorides, silicates, and phosphates are not 199 uncommon. They are decomposed as in 197. Complete decomposition of the phosphates is not always effected in this process, as phosphate of lime, for instance, is only partially decomposed by fusion with carbonate of soda. The solution remaining after the separation and removal of the silicic acid and the volatilization of the carbonate of ammonia, contains-in presence of phosphatesbesides fluoride of sodium and carbonate of soda, also phosphate of soda.

Neutralize the fluid nearly with hydrochloric acid, precipitate with chloride of calcium, filter, dry, and ignite the precipitate, which consists of fluoride of calcium, phosphate of lime, and carbonate of lime; treat the residue with acetic acid in excess, and evaporate on the water-bath to dryness and complete expulsion of

* The whole of the silicic acid may be removed from the filtrate by treating with carbonate of ammonia: addition of carbonate of zinc and ammonia, as recommended by Berzelius, and afterwards by Regnault, appears therefore superfluous (H. Rose).

the acetic acid; treat the acetate of lime, into which the carbonate has been converted by the last operation, with water, weigh the residue, which consists of phosphate of lime and fluoride of calcium, and effect its ulterior decomposition as directed in 192. In the original residue of the first operation and in the precipitate thrown down by carbonate of ammonia, determine the silicic acid, the rest of the phosphoric acid, and the bases.

7. SILICIC ACID FROM ALL OTHER ACIDS.

a. In Compounds which are decomposed by Hydrochloric Acid. Decompose the substance by digestion with hydrochloric acid or 200 nitric acid, evaporate on the water-bath* to dryness (§ 140, II., a.), and treat the residue, according to circumstances, with water, hydrochloric acid, or nitric acid; filter the fluid from the residuary silicic acid, and determine the other acids in the filtrate. In presence of boracic acid, or fluorine, this method is inapplicable, and the process described in b (201) is employed instead. If carbonates are present, the carbonic acid is determined in a separate portion of the substance.

b. In Compounds which are not decomposed by Hydrochloric
Acid.

Decompose the substance by ignition with carbonate of soda and 201 potassa (§ 140, II., b, a.), and either treat the residue at once cautiously with dilute hydrochloric or nitric acid, and the solution thus obtained as in a (200); or treat the residue with water, precipitate the silicic acid from the solution by heating with bicarbonate of ammonia, filter, add the precipitate to the undissolved residue, and determine the silicic acid, in the united mass, by treating with hydrochloric acid, and proceeding as directed § 140, II., a. Determine the other acids in the filtrate. Which of these two methods may be preferable in particular cases, depends upon the nature of the bases, and upon the relative proportion which the silicic acid bears to the latter. In presence of boracic acid and fluorine, the latter method alone is applicable.

8. CARBONIC ACID FROM ALL OTHER ACIDS.

When carbonates are heated with stronger acids, the carbonic 202 acid is expelled; the presence of carbonates, therefore, does not interfere with the quantitative estimation of most other acids. And as, on the other hand, the carbonic acid is determined by the loss of weight or by combination of the expelled acid, the presence of salts of non-volatile acids does not interfere with the determination of the carbonic acid. Accordingly, compounds containing carbonates, sulphates, phosphates, &c., are analysed in two separate portions, the carbonic acid being determined in one, the other acids in the other sample. In presence of fluorides, one of the weak non-volatile acids, such as tartaric acid or citric acid, must be employed to expel the carbonic acid; since, were sulphuric acid or hydrochloric acid used for the purpose, part of the liberated hydrofluoric acid would escape with the carbonic acid. The process described in § 139, II., e, ß, may be employed, without modification, in presence of fluorides. If, as will occasionally happen in

II.

A higher temperature would not answer.

сс

an analysis, a mixed precipitate of fluoride of calcium and carbonate of lime is thrown down from a solution, the two salts may be separated by evaporating the mixed precipitate with acetic acid to dryness, and treating the residue with water: the acetate of lime formed from the carbonate is dissolved, the fluoride of calcium is left undissolved.

I. SEPARATION OF THE ACIDS OF THE SECOND GROUP FROM THOSE OF THE FIRST.

§ 167.

a. Separation of all the Acids of the Second Group from those of the First.

Mix the dilute solution of the compound under examination with 203 nitric acid, add solution of nitrate of silver in excess, and filter the fluid from the insoluble chloride, bromide, iodide, &c., of silver. The filtrate contains the whole of the acids of the first group, the silver salts of these acids being soluble in water or in nitric acid. Carbonic acid must, under all circumstances, be determined in a separate portion. If this is done as directed § 139, II., e, in presence of chlorides, hydrochloric acid will escape with the carbonic acid. In accurate analyses this is prevented by adding a solution of sulphate of silver in slight excess, or, according to Vohl, by adding some finely levigated oxide of mercury; which addition will also prevent the escape of sulphuretted hydrogen in presence of sulphides; the latter object may be attained also by adding some neutral chromate of potassa.

b. Separation of some of the Acids of the Second Group from Acids of the First Group.

As it is often inconvenient for the ulterior separation of the acids 204 of the second group to have them all in the form of insoluble silver compounds, the analysis is sometimes effected by separating first the acid of the first group, then that of the second. If the quantity of disposable substance is large enough, the most convenient way generally is to determine the several acids, e.g., sulphuric acid, phosphoric acid, chlorine, sulphuretted hydrogen, &c., in separate portions.

Of the infinite number of combinations that may present themselves we will here consider only the most important.

1. SULPHURIC ACID may be separated from chlorine, bromine, 205 iodine, and cyanogen, by precipitation with a salt of baryta. If the acids of the second group are to be determined in the same portion, nitrate of baryta or acetate of baryta is used instead of chloride of barium. In presence of sulphuretted hydrogen, sulphuric acid cannot be determined in this way, as part of the solution of the sulphuretted hydrogen would be converted into sulphuric acid by the oxygen of the air. The error thus introduced into the process may be very considerable (Fresenius, "Journ. f. prakt. Chem.,"

70, 9). The sulphuretted hydrogen must, therefore, first be removed by addition of chloride of copper, and the sulphuric acid determined in the filtrate; or, the sulphuretted hydrogen must be completely oxidized and converted into sulphuric acid by chlorine, and a corresponding deduction afterwards made in calculating the quantity of the sulphuric acid.

2. PHOSPHORIC ACID may be precipitated by means of nitrate of 206 magnesia and ammonia, after addition of nitrate of ammonia ; oxalic acid by nitrate of lime; chlorine, bromine, iodine, &c., are determined in the filtrate.

3. CHLORINE IN SILICATES.

a. If the silicates dissolve in dilute nitric acid, precipitate the 207 solution with nitrate of silver, without applying heat, remove the excess of silver from the filtrate by dilute hydrochloric acid, still without applying heat, and then separate the silicic acid in the usual way.

b. If the silicate becomes gelatinous upon its decomposition with nitric acid, dilute, allow to deposit, filter, wash the separated silicic acid, and treat the filtrate as in a.

c. If nitric acid fails to decompose the silicates, mix the substance with carbonate of soda and potassa, and moisten the mass with water; dry in the crucible, fuse, boil with water, remove the silicic acid which may have dissolved, by means of carbonate of ammonia (185), and then precipitate, after addition of nitric acid, with nitrate of silver (H. Rose).

4. CHLORIDES IN PRESENCE OF FLUORIDES.

If the substance is soluble in water, the separation may be 208 effected as directed in 203; but it is more convenient to precipitate the fluorine with nitrate of lime, and the chlorine in the filtrate with solution of nitrate of silver. Insoluble compounds are fused with carbonate of soda and silicic acid (see 185).

5. CHLORINE IN PRESENCE OF FLUORINE, IN SILICATES. Proceed as directed 197. Saturate the alkaline filtrate nearly 209 with nitric acid, precipitate with nitrate of lime, separate the fluoride of calcium and the carbonate of lime as directed in 202, and precipitate the chlorine in the filtrate by solution of nitrate of silver.

6. SULPHIDES IN SILICATES.

If the substance is decomposed by acids, reduce it to the very 210 finest powder, and treat with fuming nitric acid (§ 148, II., 2, a). When the sulphur is completely oxidized, dilute, filter off the silicic acid, add carbonate of ammonia to the filtrate, to remove the portion of silicic acid which may possibly have dissolved, filter again, and determine in the filtrate the sulphuric acid formed. If the substance is not decomposed by acids, fuse with 4 parts of carbonate of soda and 1 part of nitrate of potassa, boil the fused mass with water, filter, remove the dissolved silicic acid from the filtrate by carbonate of ammonia (185), filter again, and determine in the filtrate the sulphuric acid produced from the sulphur.

Supplement.

ANALYSIS OF COMPOUNDS CONTAINING SULPHIDES OF THE ALKALI METALS, CARBONATES, SULPHATES, AND HYPOSULPHITES.

§ 168.

The following method was first employed by G. Werther ("Journ. 211 f. prakt. Chem.," 55, 22), in the examination of gunpowder residues.

Put the substance into a flask, pour over it water, in which a sufficient quantity of carbonate of oxide of cadmium* is suspended; insert the cork, and shake the vessel frequently. The sulphide of the alkali metal decomposes completely with the carbonate of cadmium. Filter the yellowish precipitate off, and treat with dilute acetic acid (not with hydrochloric acid); the carbonate of cadmium dissolves, the sulphide of cadmium is left undissolved. Oxidize the latter with chlorate of potassa and nitric acid (§ 148, II., 2, a, ß), and precipitate with chloride of barium the sulphuric acid formed from the sulphide.

2

=

Heat the fluid filtered from the yellow precipitate, and mix with solution of neutral nitrate of silver. The precipitate thrown down by that reagent consists of carbonate of silver and sulphide of silver (K O, S,O, + Ag O, NO, KO SO, + Ag S+ NO,). Remove the former salt by means of ammonia, and precipitate from the ammoniacal solution the silver-after acidifying with nitric acid-by means of chloride of sodium. Each equivalent of chloride of silver so obtained corresponds to an equivalent of carbonate.† Dissolve the sulphide of silver in dilute boiling nitric acid, determine the silver in the solution as chloride of silver, and calculate from the result the quantity of the hyposulphite; 1 equivalent of Ag Cl corresponds to 2 equivalents of sulphur in hyposulphurous acid, and accordingly to 1 equivalent of hyposulphite (K 0, S ̧0).

From the fluid filtered from the sulphide and carbonate of silver, remove first the excess of silver by means of hydrochloric acid, and then precipitate the sulphuric acid by a salt of baryta. From the weight of the sulphuric acid found you have, of course, to deduct an amount corresponding to the quantity of that acid resulting from the decomposition of the hyposulphurous acid, and accordingly for 1 equivalent of chloride of silver formed from the sulphide, 0.28 eq. of sulphuric acid. The difference gives the amount of sulphuric acid originally present in the analysed compound. By way of control, you may determine, in the fluid filtered off from the sulphate of baryta, the alkali as sulphate as directed in § 97 or $98.

II. SEPARATION OF THE ACIDS OF THE SECOND GROUP FROM

EACH OTHER.

§ 169.

1. CHLORINE FROM BROMINE.

All the methods of direct analysis hitherto proposed to effect the 212 *To obtain the carbonate of oxide of cadmium free from alkali, carbonate of ammonia must be used as precipitant.

A quantity equivalent to the sulphide found has to be deducted from this (K S+Cd 0, CO,=CdS + KO, CO,),