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Iron (Fe). The purest iron met with in commerce is in the form of pianoforte wire: it should be preserved in a closely-stoppered bottle, together with some caustic lime to prevent oxidation.
Zinc (Zn). One chief impurity of this metal is lead, which, however, does not ordinarily interfere with its applications. The most detrimental impurity is arsenic, and the zinc employed in analysis should always be free from this substance : some specimens of commercial zinc are quite free from it. It should be either granulated or cut into small strips.
Copper (Cu). The most convenient form in which to employ this metal is that of foil or turnings.
SALTS OF POTASSIUM.
Iodide of potassium (KI). This salt is found of sufficient purity in commerce for ordinary operations: its chief contamination is carbonate of potassium (K, CO2), which may be separated by digesting the crude salt in hot strong alcohol, filtering, and crystallizing out the iodide. To make a solution for the purposes of testing, 1 part of salt may be dissolved in 10 parts of water.
Nitrate of potassium KNO3). Commercial nitre is sufficiently pure for ordinary use : its great impurities are chloride and sulphate of potassium, from which it may be freed by repeated crystallizations.
Chromate of potassium (KCrO2). The salt met with in commerce is pure enough for the purposes to which it is applied. As the bichromate of potassium is a salt of more common occurrence, it may be used for the preparation of the neutral chromate by adding to every 100 parts of bichromate dissolved in water, 47 parts of dry carbonate of potassium (K, CO). The salt may then be crystallized. For use, 1 part of the crystals should be dissolved in 10 of water.
Cyanide of potassium (K[CN] or KCy). The cyanide prepared by throwing a mixture of 8 parts of dried ferrocyanide of potassium with 3 of dried carbonate of potassium into a crucible heated to low redness, is sufficiently pure for ordinary analytical purposes. When so heated it decomposes thus,
2(K, FeCyz)+K, C0,=5KCy+KCyO+2Fe+CO. When its contents are in a state of tranquil fusion the crucible is removed from the fire and allowed to stand, so that the particles of iron may subside: the fused mass is then poured out upon an iron plate. It consists of cyanide and cyanate of potassium, and should be kept in a well-stoppered
bottle. The cyanide may be obtained pure by digesting the mass in hot alcohol, in which the cyanate is nearly insoluble.
Sulphocyanide of potassium (K(CNS) or K (CyS] or KCsy). This salt is prepared by fusing 46 parts of ferrocyanide of potassium, 17 of carbonate of potassium, and 32 of sulphur in a covered iron crucible or pan. The fused mass, when cold, is boiled with alcohol; the sulphocyanide crystallizes out on cooling. 1 part should be dissolved in 10 of water.
Silicate of potassium (KSIO,?). A solution of silicate of potassium made by fusing 1 part of pure quartzsand (Si, 0%) with 4 parts of carbonate of potassium, and dissolving the resulting mass in boiling water, is occasionally employed in analysis. The fused mass should be first pounded and washed with cold water.
Acetate of potassium (K[C, H, 0,] or KĀ). The commercial salt is sufficiently pure for all ordinary analytical pur.' poses. 1 part should be dissolved in 4 of water.
Hydrate of potassium (KHO). The salt met with in commerce contains many impurities, of which the principal are the chloride, sulphate, and silicate of potassium; these do not generally interfere with its use. But it also contains very frequently carbonate of potassium, alumina (Al, 0), and oxide of lead; the presence of these substances is often very troublesome in analytical processes. The last impurity is derived from the flint-glass bottles in which the solution of hydrate of potassium is often kept: German glass bottles should be used instead. To obtain the pure hydrate, it is only necessary to dissolve the commercial salt in alcohol, in which its impurities are insoluble, and then by evaporating the clear part of the solution in a silver dish, the hydrate is obtained perfectly pure, if care has been taken to preserve it from the carbonic acid of the air. The following is a method of preparing a solution of this substance:-10 parts of carbonate of potassium are dissolved in 100 parts of water and the solution heated to boiling in a silver or bright iron vessel provided with a lid: 8 parts of good freshly burnt lime (Ca, O) are slaked in another covered vessel, and the hydrate formed added by degrees to the boiling solution of carbonate of potassium, the mixture being constantly stirred. The change may be represented thus :
K, CO +2(CaHO)=Ca, CO, +2KHO. The mixture is boiled for a few minutes, the lid of the vessel remaining on; the liquid is then allowed to rest until all the carbonate of calcium formed has settled, when the clear solution of potassa or hydrate of potassium is poured off into a well-stoppered bottle.
Sulphydrate of potassium (KHS). This salt is prepared by passing sulphuretted hydrogen (H, S) into a solution of hydrate of potassium until the liquid has a strong odour of the gas.
Sulphate of potassium (K, SO2). The commercial salt is sufficiently pure for analytical purposes ; it is
sometimes, however, expedient to recrystallize it. For a solution for testing, 1 part is to be dissolved in about 12 of water.
Acid metantimoniate of potassium (K, H, Sb, 0,). To prepare this salt, the neutral antimoniate is at first procured by throwing a mixture of 1 part of antimony (Sb) with 4 parts of nitre (KNO,) into a red-hot crucible; when the mass is cold, the excess of nitre is removed from it by digesting the fused mass in tepid water; the insoluble residue still left is then boiled with water for some time, and is thereby dissolved : its solution is then evaporated to the consistence of a syrup, and solid hydrate of potassium added to convert the antimoniate into metantimoniate: the liquid is further evaporated until, upon a drop being taken out upon a glass rod, it readily crystallizes ; it is then allowed to cool, when a crystalline mass is obtained containing the acid and neutral antimoniates of potassium. The crystals are then dried on filter-paper and preserved dry in a stoppered bottle for use. When required for testing, 1 part is dissolved in about 20 of water at a gentle heat, the solution cooled and filtered. During this process of solution the neutral salt is converted by the action of water into the acid metantimoniate, hydrate of potassium being produced at the same time. Ferrocyanide of potassium (K,[FeC, N3] or K, [FeCy3] or
K, Cfy+1}aq)*. This salt is met with in great purity in commerce: its solution becomes somewhat alkaline on keeping. 1 part of salt should be dissolved in 12 parts of water.
Carbonates of potassium and of sodium (K, CO, +Na, CO2). The separate salts are mixed in equivalent proportions. The mixture fuses at a lower temperature than either of its constituents.
Ferricyanide of potassium (K[Fe, C.N.] or K3 [Fe, Cyc] or K, Cfdy).
The commercial salt is of sufficient purity. This substance is prepared by passing chlorine gas into a solution of 1 part of ferrocyanide of potassium (K, Cfy) in 9 parts of water until a drop of the liquid no longer produces a blue precipitate or colour in a solution of perchloride of iron (Fe, Cly). It should be crystallized several times: the crystals are of a fine deep red colour: 1 part of them should be dissolved in 10 of water for use as a test.
SALTS OF SODIUM.
Acetate of sodium (Na[C,H,O,) or NaĀ). This salt is met with in commerce of sufficient purity for ordinary analytical operations. Being much cheaper than acetate of potassium, it is generally employed in its stead, although it cannot be so advantageously used in experiments where free oxalic acid is present, since the oxalate of sodium which is then formed is comparatively insoluble. 1 part is to be dissolved in 4 of water.
* The symbol aq=H,O, and is generally employed to denote water of crystallization.
Hydrate of sodium (NaHO). . This salt is frequently substituted for hydrate of potassium, on account of its greater cheapness. It is subject to the same impurities as the hydrate of potassium, and may be freed from them in a similar manner.
Sulphydrate of sodium (NaHS). This salt is also used instead of the sulphydrate of potassium, and may be prepared in a similar manner.
Sulphite of sodium (Na, SO, +10 aq). This salt may be prepared by passing sulphurous acid gas (S02) (produced by boiling copper turnings in a flask with concentrated sulphuric acid, and passing the gas through water in a wash-bottle) into an aqueous solution of carbonate of sodium, until carbonic acid gas is no longer evolved. The solution should be evaporated with as little exposure to the air as possible, on account of the great tendency of the sulphite to pass into sulphate, and finally allowed to crystallize. The solution for testing should contain about 1 part of sulphite in 5 parts of water.
Carbonate of sodium (Na, CO2). The commercial salt usually contains an admixture of sulphate and chloride of sodium, from which it is difficult to purify it. An easy method of preparing pure carbonate is by precipitating a comparatively pure specimen of a sodium salt with oxalic acid or a soluble oxalate. The oxalate of sodium is nearly insoluble, and by washing the precipitate obtained as before mentioned with water, only a small quantity of oxalate is lost, whilst the impurities are washed away. By ignition of this oxalate the pure carbonate is obtained, thus
Na, C,0.=Na, CO, +CO. Biborate of sodium or borax (Na, Bo, 04+10 aq). The commercial salt may be employed for ordinary analytical operations, It can be purified by crystallization.
Phosphate of sodium (Na, HPO4+12 aq). For ordinary purposes the commercial phosphate may be employed; it contains sulphate of sodium, from which it may be purified by crystallization. For use as a test 1 part is to be dissolved in 10 parts of water. Phosphate of sodium, ammonium and hydrogen, or microcosmic salt or
phosphorus salt (Na NH, H PO, +4aq). This salt is prepared by boiling a solution of 6 parts of phosphate of sodium (the reagent just described) in 2 parts of water, and adding 1 part of powdered chloride of ammonium (NH, CI). Chloride of sodium separates and is removed by filtration, while the filtrate on concentration yields crystals of microcosmic salt.
SALTS OF AMMONIUM.
Chloride of ammonium (NH, CI). The chief impurity of this salt as met with in commerce is chloride of iron ; this may be readily separated by precipitation, a few drops of sulphydrate of
ammonium (NH, HS) being added to a solution of chloride of ammonium, and any black precipitate of sulphide of iron produced being filtered off ; hydrochloric acid is then added in quantity just sufficient to decompose any excess of sulphide of ammonium, and the liquid boiled till all odour of the sulphuretted hydrogen has left it: when the trifling excess of hydrochloric acid has been saturated with ammonia, the solution is evaporated and the salt crystallized. 1 part should be dissolved in about 8 parts of water.
Molybdate of ammonium (NH, M00,). In the preparation of this salt molybdic acid is at first prepared; this is done by roasting native sulphide of molybdenum (MoS) in a platinum crucible at a low red heat, with constant stirring, as long as sulphurous acid (S0,) is evolved. The impure molybdic acid thus obtained is then dissolved with the aid of heat in ammonia. Much of its impurity is left undissolved, while more separates when the solution is evaporated to crystallize. By crystallization the bimolybdate is formed ([NH,), Mo, 0,).
Acetate of ammonium (NH, C,H,O, or NH, Ā). This salt may be best prepared by neutralizing acetic acid with carbonate of ammonium.
Hydrate of ammonium (NH, HO). Commercial ammonia may be advantageously used in analysis ; or it may be prepared by taking equal weights of freshly burnt lime (Ca, 0) and powdered chloride of ammonium (NH, CI), introducing them into a flask A, and adding a little water: the evolved gas is passed through a small quantity of water con
tained in the first wash-bottle, represented in the annexed figure by B; it is then conducted into a bottle, b, or a series of bottles similarly fitted with tubes, and half-full of distilled water. The following equation represents the change:-
NH, Cl+CHO=NH, HO+CaCl; but although NH, HO represents the hydrate of ammonium corresponding