of manganese, chlorine is evolved, recognisable by its peculiar irritant smell:—

H2SO4 + MnO2 + 2HCl

=

MnSO4 + 2H2O + Cl2.

It bleaches litmus, and produces a purpling of starch paper moistened with iodide of potassium solution.

Bromides evolve a mixture of hydrobromic acid with bromine vapour, the latter recognisable by its brown colour, irritant smell, and power of bleaching litmus.

Iodides yield iodine vapour, recognisable by its violet colour, and by its rendering starch paper purple.

Fluorides give off pungent fumes of hydrofluoric acid, which redden litmus and etch glass (vide par. 106).

Acetates give off acetic acid vapours, which redden litmus. On the addition of a little alcohol the original sour smell of the acid is changed into the fruity smell of acetic ether.

d. Borates do not react visibly with sulphuric acid, but on adding alcohol, and then setting fire to the mixture in a capsule, the flame presents a marked green colour.

Oxalates and cyanides are decomposed by heated sulphuric acid with liberation of carbonous oxide gas, accompanied in the case of the former salts with carbonic anhydride; but as the evolution of gas is liable to be overlooked, and the gas itself does not present any striking property, the oxalates and cyanides are here included among the salts with which sulphuric acid produces no obvious effect.

Phosphates, arsenates, sulphates, silicates, and oxides do not react visibly with sulphuric acid; except that with some peroxides there is evolution of oxygen, transformable into that of chlorine on the addition of chloride of sodium or hydrochloric acid.

(49.) The various liquid tests for the acids should be applied by preference to an aqueous solution of the original substance. But if insoluble in water, and consequently neither a nitrate nor a chloride, it may be dissolved in dilute nitric or hydrochloric acid, and the tests applied to the solution so formed.

The presence of certain basic metals interferes occasionally with the described reactions of several of the acids. Thus solution of hydrochloric acid or any chloride, when tested with nitrate of silver, gives a white precipitate of chloride of silver, said to be readily soluble in ammonia. But on adding nitrate of silver to solution of mercuric chloride, and treating the resultant white precipitate with ammonia, there is no obvious solution produced, because, although the precipitated chloride of silver does actually and completely dissolve, a white insoluble mercurammonium compound is simultaneously thrown down from the mutual reaction of the ammonia and mercuric salt.

Bearing in mind, however, the interference likely to be produced by the presence of the particular metal previously detected, the tests for sulphuric, hydrochloric, and nitric acids can for the most part be satisfactorily applied to any of their salts.

But in testing for phosphoric, oxalic, and tartaric acids more particularly, and for other acids when any difficulty occurs, it is important that the base of the salt should be one of the alkali metals, potassium, or sodium, or ammonium.

The bases of the first group may be got rid of by saturating the solution with sulphuretted hydrogen gas, filtering, evaporating the filtrate until it ceases to smell, and neutralising it with carbonate of sodium or potassium.

The bases of the second group, together with barium, strontium, calcium, and magnesium, may be removed by adding to the acidulous, or occasionally to the aqueous, solution of the substance an excess of carbonate of sodium or potassium, boiling for some time, and filtering. The filtrate can afterwards be neutralised with nitric or hydrochloric acid, which may be conveniently added drop by drop from a pipette.

(50.) The following abridged table shows the action of some general reagents upon the dissolved salts of the principal acids. By its aid and that of the preceding table, the student will rarely have much difficulty in quickly discovering the acid constituent of his substance, although, indeed, the course of examination is not so systematic as that for the bases. A more complete table

for the detection of the acids would include the reactions of a few organic salts of comparatively rare occurrence, such as the formates, succinates, citrates, meconates, gallates, tannates, and ferridcyanides; of a few mineral salts of similarly rare occurrence, namely, the iodates, seleniates, and silico-fluorides; of a few mineral salts rarely met with in the soluble form, namely, silicates and fluorides; and of the previously detected arsenates. It would also mention the several precipitates given by nitrate of silver and nitrate of barium respectively, which disappear on acidification, and have, as a rule, but little practical interest.

TABLE V.-COURSE FOR DETECTION OF THE ACIDS.

(51.) a. Nitrate of silver causes precipitates in the solutions of very many classes of salts, the majority of silver salts being more or less insoluble in water. The reactions consist in an exchange of the silver of the nitrate of silver for the metal or quasi-metal of the dissolved salt under examination, as illustrated by the following examples :—

All the precipitates produced by nitrate of silver disappear upon the addition of nitric acid, or do not form in presence of free nitric acid, except the chloride, cyanide, bromide, iodide, and sulphide.

With chlorides the silver precipitate is white, becoming slate-coloured on exposure to light, and soluble in ammonia before discolouration. Upon heating the original substance with peroxide of manganese and sulphuric acid, chlorine gas is evolved.

With simple* cyanides the silver precipitate is white, soluble in ammonia, and in boiling concentrated nitric acid: a portion of the precipitate washed by decantation may be treated with yellow sulphide of ammonium, whereby sulphide of silver and sulphocyanate of animonium are produced, which last strikes a deep red colour on the addition of a ferric salt. An excess of sulphide of ammonium must be avoided, or afterwards got rid of by evaporating to dryness.

With bromides the silver precipitate is white, and with difficulty soluble in ammonia. Bromine is liberated from the original salt, when treated with sulphuric acid, and from the solution of the salt when treated with a few drops of nitro-hydrochloric acid or chlorine-water, whereby the liquid assumes a red-brown colour rendered more evident upon the addition of a little ether or chloroform, which dissolves out the bromine to form a deep brown

stratum.

With iodides the silver precipitate has a pale yellow colour, and is insoluble in, but turned white by, ammonia. Iodine is liberated from the salt when treated with sulphuric acid, and from the solution of the salt when treated with a few drops of yellow nitric or nitro-hydrochloric acid or chlorine-water, its presence being manifested by the purple colour it produces on starched paper, or with dilute starch paste added to the liquid, or by the pink or crimson colour it imparts to chloroform.

* Ferrocyanide and sulphocyanate of silver occur as white precipitates, and the ferridcyanide as a brown precipitate, all unaffected by nitric acid, but soluble ferrocyanides, ferridcyanides, and sulphocyanates are recognised immediately by their behaviour with iron solutions.

H

With sulphides the silver precipitate is black, and insoluble in ammonia, but soluble in boiling nitric acid. Nitro-prusside of sodium added to an alkaline sulphide produces a deep purple colouration.

Of the silver precipitates which disappear on acidification with nitric acid, the hydrate is brown, the chromate dark red, the arsenate brick-red, the phosphate bright yellow, though sometimes white, the carbonate pale yellow, and the rest white. The oxalate is insoluble in acetic acid; the acetate is thrown down from concentrated solutions only; the tartrates, formates, and sulphites are reduced to the metallic state on boiling; while the borates, benzoates, and citrates do not exhibit any characteristic property.

(52.) B. Nitrate or chloride of barium precipitates the solutions of many classes of salts, most barium salts being insoluble or sparingly soluble in water. The reaction consists in an exchange of the barium of the nitrate or chloride of barium for the metal or quasi-metal of the salt under examination, thus:

=

2NaNO3 + BaSO4

= KCI + HCI + BaSO4

Ba(NO3)2 + Na2SO4 Bách, + KHSO With sulphates the barium precipitate is white, and, if in any quantity, opaque. It does not disappear upon the addition of nitric or hydrochloric acid, but is nevertheless slightly soluble in concentrated nitric acid. The seleniate and silicofluoride of barium also occur as white precipitates unaffected by acidification, (vide par. 95). Of the barium precipitates which dissolve in nitric or hydrochloric acid, the chromate is yellow and the remainder are white. The carbonate and sulphite dissolve with effervescence. The arsenate, borate, and tartrate do not form in the presence of ammoniacal solutions, and, when once thrown down, disappear more or less readily on the addition of chloride of ammonium. The oxalate and phosphate exhibit no characteristic properties.

(53) y. With a few exceptions, chloride of calcium causes precipitates with the several classes of salts which are precipitated by chloride or nitrate of barium; but while sulphate of barium