tion of a precipitate with sulphuretted hydrogen, before the reaction with sulphide of ammonium can be depended on as a general test for members of the second group.

The solution to be tested with sulphide of ammonium should be nearly neutral, but it may be slightly alkaline, or slightly acid without disadvantage. It must not, however, be decidedly acid; for if so, there may not only be no precipitate produced when some member of the group is present, but, owing to a customary impurity in the reagent, there may even be a precipitate produced when every member of the group is absent. This last occurrence is consequent upon a deposition of sulphur from the mutual decomposition of the acid solution and the reagent, quite irrespective of the presence of any metal. Pure colourless sulphide or sulphydrate of ammonium, indeed, is not precipitated by mere acid solutions; but the yellow persulphide of ammonium, into which it becomes gradually converted, is decomposed by all acid liquids with precipitation of sulphur, thus:

2HCl + (NH4)2S2

=

2NH4Cl + H2S + S.

The aqueous solution of a salt may be examined for members of the second group, by adding sulphide of ammonium at once; but a solution of the salt in acid must be rendered neutral, or nearly neutral, with ammonia before applying the test. The addition of even an excess of ammonia to the acid liquid is no disadvantage. It will sometimes, indeed, produce a precipitate, but the formation of a precipitate by ammonia, equally with the formation of a precipitate by sulphide of ammonium, indicates the presence of a member of the second group; though the non-production of a precipitate by ammonia does not prove the absence of all members of the group. The precipitate produced by ammonia generally differs in its character, and frequently in its appearance, from that produced by sulphide of ammonium, but the formation of a precipitate by ammonia will not interfere with the action of the more characteristic reagent for the group.

The acidulous solution of the substance which has been tested with sulphuretted hydrogen, but which has not yielded any precipitate therewith, may be examined for members of the second group by treatment with ammonia. In this case, one portion of the ammonia neutralises the excess of acid, while another portion combines with the sulphuretted hydrogen to form sulphide or sulphydrate of ammonium, which serves to precipitate any member of the group, thus:

There are certain salts of barium, strontium, calcium, and magnesium which do not dissolve in water, but which are readily soluble in dilute mineral acids-nitric or hydrochloric, for instance without, at the same time, undergoing any obvious decomposition. Hence, when such an acid solution is neutralised by ammonia, or by sulphide of ammonium, the salts are reprecipitated in their original condition; so that, although the alkaline earths strictly belong to the third group, they are occasionally precipitated along with the proper members of the second. These salts are principally the fluoride of calcium-the oxalates of calcium, strontium, and barium—and the phosphates of magnesium, calcium, strontium, and barium. In Table II. they are referred to under the general term of earthy salts; and the mode of distinguishing them from one another is described in par. 59.

TABLE II.

(36.) Examination of a solution containing some one member of the second group of bases; namely, NICKEL, COBALT, MANGANESE, IRON, CHROMIUM, ALUMINUM, or ZINC; all of which bodies are precipitated by Sulphide of Ammonium (a), from their neutral, or nearly neutral, solutions.*

B. Add gradually a considerable excess of aqueous Potash to a portion of the original solution. In any case a precipitate will be formed, which may either remain or be redissolved.

To work successfully, the student must supplement the brief directions of the different tables, and more especially of this table, by the explanations and limitations of the succeeding letterpress.

(37) a. Sulphide of ammonium reacts with salts pertaining to this group, to form precipitates of various characters and aspects. The precipitate is white in solutions of zinc, aluminum, and the earthy salts; black in solutions of iron, nickel, and cobalt; greenish in solutions of chrome; and buff-coloured in those of manganese. The student must not, however, attach too much importance to the colour of a precipitate, as it is a quality very liable to be interfered with by accidental circumstances. For instance, the presence of a trace of iron, occurring as an impurity, may effect a great alteration in the characteristic appearance of precipitates due to chrome, manganese, zinc, aluminum, or earthy salts, respectively, by imparting to them a black, grey, or greenish colour. From its transparency, the precipitate produced in aluminous solutions is very liable to be overlooked.

The salts of nickel, cobalt, manganese, iron and zinc are precipitated by sulphide of ammonium, in the form of sulphides, according to the general equation,

From their neutral solutions these metals are precipitated very imperfectly by sulphuretted hydrogen, in consequence of the formation during the reaction of hydrochloric or some other acid, in which the respective sulphides are soluble, thus:

But the sulphides of nickel, cobalt, and zinc may be precipitated completely from solutions which are acid only with acetic acid, and in which, owing to the addition of an alkaline acetate, no stronger acid than the acetic can be set free during the reaction; though even then sulphide of iron can be but partially precipitated, and sulphide of manganese not at all.

Although the sulphides of nickel and cobalt are not precipitated in the presence of hydrochloric acid, yet, when once produced, they can only be dissolved in the acid with considerable difficulty; but they are readily soluble in nitric acid. The sulphides of

zinc, iron, and manganese, however, are easily dissolved by cold hydrochloric acid, and that of manganese by acetic acid.

The sulphides of aluminum and chromium cannot be produced in the moist way. Hence the salts of these metals are precipitated by sulphide of ammonium in the form, not of sulphides, but of hydrated sesquioxides, with liberation of sulphuretted hydrogen, thus:

=

2AlCl3 + 3(NH4)2S + 6H2O 6NH4Cl + 3H2S + Al2O3.3H2O.

The earthy salts are precipitated as such by a mere neutralisation of the acid in which they were dissolved, thus (vide page 75):

Ca3(PO4)2·4HCl + 2(NH4)2S

=

Ca3(PO4)2 + 4NH4Cl + 2H2S.

(38.) 6. From protosalts of nickel, cobalt, manganese, iron, and zinc, potash throws down the respective prothydrates, precisely as it does the prothydrates of the metals of the first group:

MCI, + 2KHO = 2KCI + ΜΟ.Η.Ο.

From the tri- or sesquisalts of iron, chromium, and aluminum, it precipitates the respective sesquihydrates, thus:—

2MC13 + 6KHO = 6KCI + M2O3.3H2O.

The earthy salts are precipitated as such by the neutralisation of the acid in which they were dissolved.

The appearance of the manganese precipitate is very characteristic. From being quite white it becomes rapidly brown by an absorption of atmospheric oxygen. The precipitate given by potash with perfectly pure protosalts of iron is greenish-white, but the precipitate ordinarily obtained has a dark olive-green colour, becoming ochrey-red by exposure to air: with mixed proto- and sesqui-salts, a black precipitate, and with pure sesquisalts a red-brown precipitate is produced. The hydrates of chromium, aluminum, and zinc are completely soluble in excess