off from the precipitate, and the latter is washed first with cold water, and finally with a warm solution of ammonia; this operation, depriving the precipitate in a very great measure of its acid, causes it to acquire a deeper tint. The washed precipitate is dried upon the filter in the funnel, the filter and its contents are introduced into a platinum crucible, and the latter is heated, at first covered, subsequently uncovered, with a stream of air directed upon its contents, until the filter is completely incinerated, and the precipitate converted into red peroxide of iron. The object of washing the precipitate with ammonia is to remove part of the succinic acid, since, were this neglected, and the precipitate simply washed with water, part of the peroxide of iron might readily be reduced upon the ignition of the succinate. Properties of the precipitates, vide § 55. This method yields very accurate results.

d. By direct ignition.

The compound to be examined is exposed, in a covered crucible, to a gentle heat at first, which is gradually increased to the highest degree of intensity until the weight of the remaining peroxide ceases to vary, even to the slightest extent.

FIFTH GROUP.

OXIDE OF SILVER-OXIDE OF LEAD-PROTOXIDE OF MERCURY

PEROXIDE OF MERCURY-OXIDE OF COPPER
MUTH-OXIDE OF CADMIUM.

OXIDE OF BIS

Metallic silver and those of the silver compounds which are insoluble in water, are best dissolved in nitric acid (of course excepting those which are insoluble in this acid); dilute nitric

acid suffices generally; sulphuret of silver, however, requires a more concentrated acid. It is best to make the solution in a flask. Chloride, bromide, and iodide of silver are insoluble in water and in nitric acid; but it is not necessary to dissolve these salts in order to analyse them (vide infra). The silver in the chloride, bromide, and iodide, may, however, be readily reduced by means of zinc or iron, and water acidulated with hydrochloric acid or sulphuric acid. It is most advisable, for this purpose, to introduce these salts into a porcelain crucible; to pour some water over the fused mass, to throw a piece of clean zinc or iron upon it, and to add some hydrochloric or sulphuric acid. The spongy silver obtained by these means is to be carefully washed, first with dilute sulphuric acid, and subsequently with water; it is then finally to be dissolved in nitric acid.

b. Determination.

Silver may be weighed as chloride of silver, sulphuret of silver, cyanide of silver, or metallic silver.

1. CHLORIDE OF SILVER. 2. SULPHURET OF SILVER.

3. CYANIDE OF SILVER.

All compounds of silver without exception.

4. METALLIC SILVER.

Oxide of silver and several of its compounds with readily volatile acids; salts of silver with organic acids;

and, finally, chloride of silver.

The method 4 is the most convenient, and is preferred to the other methods in all cases, where its application is admissible. The method 1 is that most generally applied; 2 and 3 serve mostly to separate oxide of silver from other bases.

1. Determination of silver as chloride of silver.

a. In the humid way.

Two methods may be pursued to determine silver as chloride in the humid way.

The principle of the one of these methods is to weigh the

precipitated chloride of silver; the principle of the other is to infer the proportion of silver present in a solution, by determining the quantity of common salt (in solution) which is required to precipitate the silver as chloride. The application of the latter method, however, is more exclusively confined to the workshops of the mint, and is but rarely had recourse to in the laboratory of the chemist; I deem it unnecessary, therefore, to describe it here more in detail.

With regard to the first method, the precipitated chloride of silver may be separated from the supernatant fluid either by decantation or by filtration; the former is generally preferred when we have a copious amount of precipitate before us, the latter answers better for small quantities.

a. Determination by decantation.

The moderately diluted solution of the compound under examination is introduced into a tall flask, with long neck and narrow mouth, and some nitric acid added to it; this mixture is heated to about 140°, and hydrochloric acid is added to it, as long as a precipitate is formed. The mouth of the flask is then closed with a perfectly smooth cork, (or, better still, with a well-ground glass stopper,) and the flask is forcibly shaken until the precipitated chloride of silver has subsided, united into coherent lumps, and the supernatant fluid has become clear. The minute particles of chloride of silver, which may still adhere to the sides of the neck of the flask, are then rinsed down by means of a syringe bottle, and the stopper is loosely replaced. The flask is then placed on a moderately warm spot in the sand-bath until the precipitate has completely subsided, and the supernatant fluid has become perfectly clear and transparent, which generally requires several hours. The clear fluid is then slowly and cautiously decanted into a beaker-glass, so as to retain every particle of the chloride in the flask, whence it is to be carefully transferred into a porcelain crucible, with steep and smooth sides; the last particles of chloride of silver which remain adhering to the bottom and sides of the flask are rinsed into the neck of the latter with a little

water, (inverting the flask, and closing its mouth with the finger,) and thence likewise transferred into the crucible.

When the chloride of silver has completely subsided in the crucible, (which is greatly accelerated by exposing the latter to the heat of a water-bath,) the clear supernatant fluid is carefully decanted (with the aid of a glass rod) into the same beaker-glass which already contains the liquid of the first decantation. The chloride of silver in the crucible is moistened with a few drops of nitric acid, and subsequently drenched with hot distilled water; the chloride is again allowed to subside, the clear supernatant fluid again decanted, and the same operation repeated until the decanted fluid ceases to exhibit the slightest turbidity upon the addition of a few drops of solution of nitrate of silver. The last portions of supernatant fluid are then carefully and cautiously decanted or removed by means of a small syphon; the chloride is thoroughly dried in a water-bath, and subsequently heated to incipient fusion, over the flame of a spirit-lamp, taking care to apply a very gentle heat at first; as soon as the chloride begins to fuse round the border, the crucible is removed from the flame, allowed to cool, and finally weighed.

To remove the fused mass from the crucible, completely and without injury to the latter, a fragment of iron or zinc is placed upon the chloride, and highly dilute hydrochloric or sulphuric acid poured over it. The crucible is finally washed, dried, and weighed. Should the liquids successively decanted from the chloride of silver not be perfectly clear and transparent, they are to be kept standing in a warm place until the last particles of chloride have completely subsided, (which frequently requires several hours ;) the clear supernatant fluid is then decanted, and the subsided chloride added to the bulk of the precipitate in the crucible.

B. Determination by filtration.

The chloride of silver is precipitated and allowed to subside as at a.; the supernatant fluid is then passed through a small filter to which the precipitate likewise is subsequently transferred, with

the aid of a little hot water acidulated with nitric acid; the precipitate, when collected on the filter, is washed, first with water impregnated with nitric acid, and finally with pure water, thoroughly dried, ignited, (§ 31,) and weighed. For the properties of the precipitate, vide § 56.

Both methods, if properly performed, yield very accurate results, although a trifling loss of substance is incurred in B., owing to part of the minute portion of chloride of silver which, notwithstanding the most careful scraping, will invariably adhere to the filter, being reduced upon the ignition of the ashes of the latter; this may be remedied, however, by adding the filter-ashes to the chloride in the crucible, pouring a little dilute nitric acid over them, heating for some time, adding a few drops of hydrochloric acid, evaporating, drying, and igniting as directed sub. a. b. In the dry way.

This method serves more exclusively for the analysis of iodide and bromide of silver, although it is adapted likewise for the quantitative estimation of other salts of silver.

The process is conducted in the apparatus illustrated by Plate XXXII.