EXAMINATION FOR BASIC GROUPS.

(29.) The bases are divided into three principal groups, as

The first object of the student must be to ascertain the group to which the base of the salt under his examination belongs.

The members of the first group are precipitated from their acid solutions by sulphydric acid. The members of the second group are not precipitated from their acid solutions by sulphydric acid, but are precipitated from their neutral solutions by sulphide of ammonium. The members of the third group are precipitated neither by sulphydric acid nor by sulphide of ammonium.

Having ascertained to which particular group the base of his salt belongs, he will proceed according to the directions of the table pertaining to that group, in order to identify the individual member thereof with which he is dealing. Should, therefore, sulphydric acid produce a precipitate, in an acidified solution, the student will proceed according to Table I. Should it produce no obvious precipitate, he will render the solution nearly neutral by ammonia, and then add sulphide of ammonium. Should this reagent produce a precipitate he will proceed according to Table II. Should no precipitate be produced by either of the above reagents, he will proceed according to Table III.

§ III.-EXAMINATION FOR BASES OF GROUP I.

(30.) To recognise the presence of some member of this group by means of sulphuretted hydrogen or sulphydric acid, the solution to be tested should be moderately acid. In solutions which are too acid, sulphuretted hydrogen may not give any precipitate, despite the presence of a member of the group; and in solutions which are neutral or alkaline, it may give a precipitate even in the absence of every member of the group; inasmuch as sulphuretted hydrogen precipitates some members of the second group from their neutral or alkaline solutions.

AQUEOUS SOLUTIONS. These must consequently be acidulated before being treated with sulphuretted hydrogen. A few drops of either nitric or hydrochloric acid will answer the purpose, but the use of the latter acid is generally preferable. The addition of hydrochloric acid, however, sometimes produces a permanent white precipitate, in which case the presence of silver, or lead, or mercury is indicated. Solutions of silver invariably yield a precipitate with hydrochloric acid, solutions of lead and mercury only under certain conditions. But in the event of hydrochloric acid producing a precipitate, it will suffice for the student to distinguish between the above three metals, without following out the directions of the general table for the group (page 68.) The SILVER precipitate is soluble in excess of ammonia.

The MERCURY precipitate is turned black by excess of ammonia. The LEAD precipitate is unaffected by ammonia, but is soluble in boiling water; and, on cooling, is deposited therefrom in crystalline needles.

These three precipitates are blackened by sulphuretted hydrogen, and are not produced by nitric acid, properties distinguishing them from all other precipitates which hydrochloric acid occasionally produces.

The acidification of a solution of tartar-emetic, with either hydrochloric or nitric acid, is attended with the production of a white turbidity, which, however, disappears on gently warming

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the liquid with a little more acid. Moreover, the acidification, by either hydrochloric or nitric acid, of various alkaline solutions, not unfrequently gives rise to whitish precipitates, which sometimes disappear in an excess of acid, and, at other times, remain. Among those which are permanent, the principal are sulphur, from the decomposition of an alkaline persulphide, &c.; silica, from the decomposition of an alkaline silicate; and boric acid from the decomposition of an alkaline borate; but this last precipitate is readily soluble in boiling water.

SOLUTIONS IN ACID. When the solution of the original substance has been made in an acid, it is important to get rid of any great excess of acid; or, at any rate, to reduce its activity.

a. By mere dilution with water. It is generally advisable to dilute somewhat considerably solutions which have been made by means of an acid.

B. By evaporation. The solution may be evaporated down to a small bulk, and then be diluted with water. This process is especially necessary when the solution has been made with nitro-muriatic acid.

When a large

7. By neutralisation with ammonia. quantity of acid has been employed to effect the solution of a substance, it is occasionally useful to neutralise some of the excess of acid with ammonia.

The acid solution of the substance, whether or not evaporated down, or partly neutralised, should, after dilution with water, be perfectly bright. If not bright, it must be rendered so by filtration.

The addition of water to an acid solution sometimes produces an obvious white precipitate, in which case the dilution should be very slight or be dispensed with altogether. The formation of a white precipitate on the addition of water indicates the presence of ANTIMONY Or BISMUTH. The precipitate produced in solutions of the former metal is dissolved by tartaric acid and turned of an orange colour by sulphuretted hydrogen; while that produced in solutions of the latter metal is not dissolved by tartaric acid, and is turned black by sulphuretted hydrogen. Water does not

invariably cause a precipitate in solutions containing antimony or bismuth, but in the event of a precipitate being produced, it will suffice to distinguish between the above two metals without proceeding according to the general table for the group (page 68).

The acidified solution of the substance in water, or the diluted solution of the substance in acid, is to be treated with sulphuretted hydrogen. It may be sufficient to add sulphuretted hydrogen water to the solution, but it is always preferable to use a current of the gas itself. The production of a coloured precipitate is indicative of the presence of some member of the first group; in which case, the gas should be passed into the liquid until it smells permanently even after agitation. A little water should next be added, and the whole well shaken or stirred, to promote the subsequent subsidence of the precipitate, which, on setting the tube aside for a few minutes, will soon collect at the bottom. The supernatant liquid may then be poured off, and the precipitate treated according to the directions of Table I. ß.

Sulphuretted hydrogen, when added to certain solutions, not containing any member of the first group, sometimes produces a more or less considerable yellowish-white turbidity, due to a liberation of finely divided sulphur, effected by some per-oxidated or per-chlorinetted compound, thus:

In yellow solutions, this white turbidity often appears decidedly yellow, from the colour of the liquid through which it is seen. When, simultaneously with the liberation of sulphur, a brownishyellow solution becomes paler or colourless, the presence of a per-salt of iron may be generally inferred; but when it becomes of a marked green colour the presence of chromic acid is indicated.

TABLE I.

(31.) Examination of a solution containing some one member of the first group; namely, TIN, ARSENIC, ANTIMONY, BISMUTH, SILVER, MERCURY, LEAD, COPPER, or CADMIUM; all of which metals are precipitated from their acid solutions by Sulphuretted hydrogen gas (a), or its solution in water.

B. Having treated the not too acid solution of the substance with excess of sulphuretted hydrogen, and poured off or filtered off the supernatant liquid, warm the precipitate with some solution of Sulphide of Ammonium.