« PreviousContinue »
into their solutions : the metals zinc and iron are especially active in this respect, they simply act by assuming the place of the precipitated metal, thus—CuCl+Fe=FeCi+Cu.
The salts of the metals contained in this subdivision are sometimes called “salts of the heavy metals.”
The first six metals of this subdivision are of frequent occurrence; and of the remainder, tin, antimony, platinum, and gold are the most important. The most abundant are copper, silver, lead, mercury, and tin; but others, such as cadmium, bismuth, and antimony, as well as some of quite the rarer ones, such as tungsten, molybdenum, and vanadium, exercise so very important an influence upon the commoner metals when alloyed with them, as to demand such attention from the student as may enable him to recognize these rarer bodies by their reactions : it is equally, if not more important, that he should be able accurately to recognize the presence of platinum and gold, on account of the great value set upon those metals.
It has just been mentioned that by the action of an alkaline sulphide, such as sulphide of ammonium (in which agent some of the sulphides of the present group are insoluble, while the remainder are soluble), we can separate this extensive subdivision into two sections; we shall adopt this method of arrangement, and here give a list of the members of each section. Another reaction is advantageously made use of for the still further subdivision of the former of these sections, namely, the formation of the insoluble argentic, mercurous, and plumbic chlorides by the addition of hydrochloric acid; the other chlorides of the group being soluble, are thus separated from these three bodies, and are subsequently precipitated in the form of sulphides.
SECTION I.--SALTS OF CADMIUM, COPPER, SILVER, MERCURY,
LEAD, BISMUTH, AND PALLADIUM. Salts of metals which form sulphides insoluble in sulphide of ammonium (NH), S).
SECTION II.-SALTS OF TIN, ANTIMONY, ARSENIC, PLATINUM,
RHODIUM, RUTHENIUM, IRIDIUM, OSMIUM, GOLD, TUNGSTEN, MOLYBDENUM, AND VANADIUM.
Salts of metals which form sulphides soluble in sulphide of ammonium ([NH,], 8).
The tests employed for the recognition of the members of this group are the following :-hydrochloric acid or a soluble chloride, iodide of potassium, cyanide of potassium, chromate of potassium, the hydrates of potassium and ammonium, sulphydrate and sulphide of ammonium, the carbonates of potassium and ammonium, oxalic acid and oxalate of potassium, sulphuric acid or a soluble sulphate, ferrocyanide and ferricyanide of potassium, phosphate of sodium, and hydrosulphuric acid.
The group test is hydrosulphuric acid (HS) in an acid solution.
SECTION I.–Salts of metals which form sulphides insoluble in
Sulphide of ammonium ([NH,], S). SALTS OF CADMIUM, COPPER, SILVER, MERCURY, LEAD,
BISMUTH, AND PALLADIUM.
SALTS OF CADMIUM. Solution for the reactions :-chloride of cadmium (CaCl) in water.
Cadmium is a metal which presents so many features analogous to those of zinc, the last member of the preceding subdivision, that its most fitting place is at the beginning of the present group.
Cadmium, like many of the succeeding metals, appears to form an oxide containing half as much oxygen as the common oxide (Cd,O), and the formula of which consequently is Cd, 0; the former oxide (Cd, O) only is the representative or startingpoint of the large number of salts which this metal yields.
Salts of cadmium, when heated before the blowpipe on platinum foil and in the oxidizing flame, undergo no change excepting the separation of the red-brown oxide, which remains in
fusible; but if heated on a reducing surface such as charcoal affords, and in the inner flame of the blowpipe, these compounds are reduced to the metallic state, while the metal volatilizes, becoming oxidized as it passes through the flame into the air, and then condenses on a distant part of the charcoal as a red-brown incrustation of oxide (Cd, O). It imparts no colour to the blowpipe flame; when heated, however, with borax on a platinum wire and in the oxidizing flame, a transparent bead is produced, which, if saturated with the cadmium salt, becomes milk-white on cooling. In the reducing flame the metal is reduced, and if sufficiently heated, volatilized; before this is effected, however, the wire will probably be spoilt from the cadmium having formed a fusible alloy with the platinum. It yields no characteristic reaction with nitrate of cobalt. The salts of cadmium are white, unless the acid-radical introduces colour.
The chief insoluble salts by means of which cadmium is recognized are the hydrate, the sulphide, the carbonate, the oxalate, the ferrocyanide, the ferricyanide, and the phosphate.
A bar or plate of metallic iron introduced into the solution of a cadmium salt, does not precipitate the metal ; but a bar of zinc removes the cadmium from its solution, the zinc combining with the acid-radical.
THE CHLORIDE is soluble. THE IODIDE is readily soluble.
THE CYANIDE, according to some chemists, is soluble; according to others, cyanide of potassium produces a precipitate with sulphate of cadmium (Cd, SO2), soluble in excess of the precipitant and in warm ammonia-water, but insoluble in solutions of other ammonium salts.
The Hydrate is produced by the action of hydrate of potassium, and is partially precipitated also by hydrate of ammonium : it is a white precipitate.
Its formula is CDHO.
other ammonium salts, and readily in acids, suffering decomposition.
The Sulphide is produced by the action of hydrosulphuric acid or the sulphides of potassium and ammonium on solutions of cadmium salts; it is precipitated even from solutions acidified by dilute mineral acids, and of course also separates from neutral or alkaline solutions. It is a brilliant rich yellow precipitate.
Its formula is Cd, S.
It is very slightly soluble in hydrate of ammonium, but dissolves easily in concentrated hydrochloric or nitric acids, and even in dilute hydrochloric at the boiling temperature, although precipitable from its acid solution by hydrosulphuric acid gas.
THE SULPHATE is soluble.
THE CARBONATE is produced by the action of the neutral or acid carbonates of potassium or ammonium; it is a white precipitate.
Its formula, dried at 100° C., is Cd, CO.
It is insoluble in water, and in excess of its precipitants, but readily soluble in solutions of ammonium salts, and with decomposition, in acids.
THE OXALATE is produced by the action of oxalic acid or alkaline oxalates on solutions of cadmium salts: it is a white crystalline powder.
Its formula, dried at 100° C., is Cd,C,0+2aq.
It dissolves in the hydrate and in most other ammonium salts; it is nearly insoluble in water, and in solution of oxalic acid.
THE FERROCYANIDE is produced by the action of ferrocyanide of potassium : it is a yellowish-white precipitate.
Its formula is Cd, Cfy.
It is soluble in hydrate of ammonium, but does not completely dissolve in other ammonium salts.
THE FERRICYANIDE is produced by the action of ferricyanide of potassium : it is a pale yellow precipitate.
Its formula is Cd, Cfdy.
It dissolves readily in hydrate of ammonium and in most other ammonium salts.
THE PHOSPHATE is produced by the action of phosphate of sodium: it is a white precipitate.
Its formula is probably Cd, PO..
The other reagents of the present and three preceding subdivisions produce no characteristic reactions in solutions of cadmium salts.
The chief methods employed for the detection of cadmium are these,—the formation of the red-brown incrustation on charcoal, and the precipitation of the hydrate, readily soluble in ammonia, and of the brilliant yellow sulphide, insoluble in alkaline sulphides.
SALTS OF COPPER, The metal copper presents some analogy with cadmium, differing, however, from it, among other characters, in forming two distinct and well-defined series of salts, the cuprous and cupric salts, of which the oxides Cu, 0 and Cu,0, and the chlorides Cu, Cl and CuCl may be considered as representatives.
Salts of copper when heated are converted into the oxide (Cu, O), and before the blowpipe on charcoal may be reduced to the metallic state with the flame alone, although the reduction of the oxide may be much more readily effected if it be heated with carbonate of sodium. In this and in all cases in which it is sought to reduce a metal in order to ascertain whether the operation has been successful or otherwise, it is better to remove the mass from the charcoal, to powder it finely in a mortar, to stir up the powdered mass with water, and to allow the mixture to rest in order that the metallic particles may settle. Lastly, the liquid is to be poured off while it still holds the impurities present in suspension, and the agitation with fresh quantities of water, and decantation, to be repeated until the metallic particles are at length obtained perfectly pure and free from the charcoal, &c. which at first obscured their presence and rendered it doubtful. By this process, also, the malleability or brittleness of the metal under examination can be ascertained; for if, under the pestle, the globule of metal flattens out into a lamina, it may