2. Metals that are precipitated from their acid solutions, by 'sulphuretted hydrogen, in the form of sulphides that are insoluble in ammo nium sulphide, (NH4)IS. (Mercury, lead, bis muth, cadmium, copper and silver).
3. Metals that are precipitated from their acid solutions, by sulphuretted hydrogen, in i the form of sulphides that are soluble in am monium sulphide. (Arsenic, antimony and tin).
4. Metals whose sulphides are insoluble in water, and which are precipitated from neu tral solutions, by ammonium sulphide, either as sulphides or as hydrates. (Iron, manganese, co balt, nickel and zinc are precipitated as sul phides; aluminum and chromium as hydrates).
5. Metals which, upon addition of ammo nium carbonate, are thrown down in the form of carbonates that are insoluble in ammonium chloride, (Barium, strontium and calcium).
6. Metals which, upon addition of ammo nium carbonate, are thrown down in the form of carbonates that are soluble in ammonium chloride. (This group contains the single metal magnesium).
7. Metals which remain in solution when treated by any or all of the foregoing reagents, and all of whose important compounds are sol uble. (Potassium, sodium, lithium and monium).
The separation of the groups that are de fined above may be effected as follows: Hydro chloric acid is added to the solution under examination, drop by drop, until no further .precipitation takes place. The white precipi tate which may occur at this stage consists of the chlorides of the metals of Group 1, and is to be preserved for further examination. The solution is filtered, and a drop or two of hydro chloric acid is added to the clear filtrate, to make sure that this reagent is incapable of inducing further precipitation. Sulphuretted hydrogen gas is then passed through the filtrate until the liquid smells strongly of the gas. If a precipitate is formed, it will consist of the sulphides of the metals of the second and third groups. Silver, however, will not (in general) be present, because it is ordinarily completely precipitated in the first group, and has there fore been already removed by filtration. Lead may not be completely precipitated in the first group, and hence its sulphide may occur in the present precipitate. The mercury in the present precipitate represents mercuric mercury in the original solution The mixed sulphides of Groups 2 and 3 are removed filtration and are washed until the wash water is no longer acid.
They are then boiled with ammonium sulphide. This reagent leaves the sulphides of Group 2 undissolved, but dissolves those of Group 3. Filtration then leaves the sulphides of Group 2 on the filter paper; and when the filtrate is acidified by the addition of hydrochloric acid, the sulphides of Group 3 are thrown down again.
The clear filtrate from which Groups 2 and 3 were removed by sulphuretted hydrogen gas is next made slightly alkaline by the addition of ammonia and heated almost to boiling. Ammo nium sulphide is then added, and the whole is kept warm for some time. The sulphides (or hydrates) of Group 4 are thus precipitated, and may be isolated by filtration. The filtrate from this operation is next boiled to expel all the sulphuretted hydrogen and ammonia and ammonium chloride are added. The solution is then heated to the boiling point and treated with a solution of ammonium carbonate (NFL).00a. This causes the precipitation of the carbonates of Group 5, which are removed by filtration. The filtrate is boiled, and then treated with a solution of sodium phosphate, Isla:HBO., to which one-sixth of its volume of ammonia has been added. Magnesium, the metal of the sixth group, is thrown down (if present) in the form of a white crystalline pre cipitate having the formula Mg,(NH.),P,Os. This is again removed by filtration, and the filtrate will contain, in solution, the metals of Group 7.
The several groups being thus separated, it remains to examine each group by itself, to see how its components may be isolated or other wise recognized.
Group 1. The chlorides of this group, as ob tained in pursuing the general scheme outlined above, may be separated very readily. Thus if the precipitate of mixed chlorides be treated with boiling water, the chlorides of silver and mer cury will he unaffected; hut the chloride of lead will dissolve, and may therefore be isolated by filtration. The mixed chlorides of silver and mercury may be separated by treatment with hot ammonia. This reagent dissolves silver chloride, which is again precipitated in the form of chloride upon neutralizing its solution with nitric acid. The hot ammonia does not dissolve mercurous chloride but transforms it into a black substance that contains mercury, chlorine, nitrogen and hydrogen, and remains behind upon the filter papee.