Some salts, nevertheless, generally those which contain the elementary acid-radicals, but also a few others, as the cyanide and the carbonate, resist this method of decomposition. All potassium salts, however, when so heated, fuse, and, with but few exceptions, sink into the charcoal. They impart also a violet tinge to the blowpipe flame playing over them,—the cause of this is the volatilization of a small portion of the salt, and its subsequent decomposition by the carbonaceous constituents of the flame, with separation of potassium. Potassium, it will be remembered, when combining with oxygen, inflames, burning with a violet light; to the trace of potassium-vapour produced in this experiment, and its immediate reoxidation on contact with the highly-heated air surrounding the flame, the reaction in this case is due. A good method of performing this experiment is to dip a loop of platinum wire (perfectly clean and imparting by itself no colour to the blowpipe flame) into a solution of the potassium salt to be tested: the blowpipe flame should be as blue as possible, with no white streaks which would interfere with the observation of the colour: the woodcut shows the arrangement. If a potassium salt be mixed with good alcohol, and the

Fig. 8.

mixture repeatedly stirred, upon setting it on fire the characteristic violet flame will be produced.

Almost the only insoluble salts which potassium forms, and by means of which it may be recognized, are the following:-the chloroplatinate, perchlorate, carbazotate, and acid tartrate.

The Chloroplatinate is produced by the action of hydrochloroplatinic acid (HPtCl,) [the so-called bichloride of platinum (PtCl), with 1 equivalent of hydrochloric acid (HCl)] on solutions of potassium salts: it is a yellow crystalline precipitate; the crystals are octahedral, and belong to the regular system. The test, as is the case with most other liquid tests, is applied by simply mixing it with the solution to be tested; and if the

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precipitation of the insoluble salt does not occur immediately, it is well to agitate the liquid by stirring, or by shaking the test-tube containing it. In the present instance, to hasten the change, a few drops of hydrochloric acid should be added at the same time as the test-liquid; the presence of alcohol also renders the precipitate more insoluble. If so small a quantity of potassium should be present as to give no precipitate under these circumstances, the chloroplatinate of potassium may yet be obtained by evaporating the solution to which the reagent has been added just to dryness, and then digesting it with alcohol. The chloroplatinate being far more insoluble in that menstruum than in water, remains as a yellow residue if but a minute trace only of potassium be present.

The formula of the precipitate is KPtCl,, in which the K is the basic and the PtCl, the acid radical. The double decomposition which produces it is as follows::

1 part of this salt dissolves in 144 parts of cold water, but it is more soluble in hot water: 1 part requires about 3775 parts of rectified spirit for its solution.

THE PERCHLORATE is produced by the action of perchloric acid (HCIO) in solutions of potassium salts, if the latter are not too dilute it is a white crystalline precipitate; the crystals are right rhombic prisms. Its formula is KC1O,. 1 part dissolves in 65 parts of water at 15° C., and in a less quantity of boiling water. It is quite insoluble in alcohol.

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THE CARBAZOTATE is produced by the action of carbazotic acid (HC.H2X,O) on solutions of potassium salts: it is a yellow precipitate which is crystalline, the crystals belonging to the rhombic system. Its formula is KC,H,X,0. 1 part requires 260 parts of water at 15° C. for its solution. It is insoluble in alcohol.

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The Acid Tartrate is produced by excess of tartaric acid

* X=NO2, a radical which, in the nitro-substitution compounds, is found to replace H.

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(H,C, H ̧O, or H ̧T) added to solutions of potassium salts: it is then precipitated rapidly, particularly if they be shaken or stirred. The delicacy of this test is much increased if a solution of the acid tartrate of sodium is added, instead of tartaric acid, to a neutral solution of the potassium salt. It is a white crystalline precipitate, the form of the crystals being that of the oblique prism with a rhombic base, and belonging to the monoclinic system. The formula of the precipitate is KHC, H,O, or KHT; the reaction producing it is as follows:

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It is important that excess of tartaric acid should be present, and not excess of the potassium salt, as in the latter case the neutral tartrate, KT, would be formed, and this is a perfectly soluble salt. The neutral tartrate is also produced by dissolving the acid tartrate in potash (KHO) solution, thus,—

KHT+KHO=KKT+H20.

1 part of acid tartrate of potassium dissolves in 240 parts of water at 10° C.: it is insoluble in alcohol. It dissolves in strong acids.

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THE SILICOFLUORIDE is produced by the action of hydrofluosilicic acid (H, Si, F,); a very slight gelatinous film at first appears in the liquid, but gradually thickens to a precipitate. Its formula is K, Si, F. It dissolves very sparingly in cold, but more readily in hot water.

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THE ACID METANTIMONIATE is soluble.

The tests most commonly employed for the detection of potassium are, the violet colour of the blowpipe flame, and the formation of the chloroplatinate and the acid tartrate.

SALTS OF SODIUM.

Solution for the reactions:-chloride of sodium (NaCl) in water.

The metal sodium resembles potassium very closely. It forms two compounds with oxygen, a protoxide (Na,O), and a peroxide (Na,O,); but among the numerous salts which this metal

forms with other acid-radicals, none are found corresponding to the peroxide. The salts of sodium are therefore all regarded as formed after the types protochloride (MCI), and protoxide (M, O), and so are called proto-salts. They are white, unless associated with a coloured acid-radical; they possess in general no poisonous properties; one indeed, the chloride (NaCl), appears to be quite necessary to the well-being of the animal organism.

When heated on charcoal before the blowpipe, sodium salts, like potassium salts, fuse and sink into the charcoal, and suffer similar decompositions. They impart to the blowpipe flame a brilliant yellow tinge, which is much more intense than the violet colour produced by potassium salts, the latter being less volatile than those of sodium. Thus, in a mixture of salts tested in this manner, the potassium flame is entirely masked by the sodium, unless the salt of the former metal is present in greatly preponderating quantity: if the sodium salt is mixed with even 20 or 30 times its weight of the potassium compound, the yellow colour, though weakened, is still distinctly perceptible. The same colour is observed when alcohol is inflamed after digestion with sodium salts.

The only insoluble salt by means of which sodium can be recognized, is the acid metantimoniate; but the test is not delicate, and is difficult of application.

THE CHLOROPLATINATE and the PERCHLORATE are soluble.

THE CARBAZOTATE is soluble; but a slight precipitate occurs in very concentrated solutions.

THE ACID TARTRATE is soluble; but a slight precipitate occurs in very strong solutions.

THE SILICOFLUORIDE is much more soluble than the corresponding potassium compound.

The Acid Metantimoniate is produced by the action of metantimoniate of potassium (K, H, Sb, O,+6aq). This reagent is liable to decomposition* by keeping its solution; it should

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* Metantimoniate of potassium (K2 H2 Sb2 07) passes readily into antimoniate of potassium (KSBO,) by losing H2O; thus

K2 H2 Sb2 0,=2KSbO,+H2O.

therefore be dissolved only just before applying the test. The precipitate separates in crystalline grains, but does not appear immediately if the solution be dilute: the deposition is facilitated by agitation.

Its formula is Na, H, Sb,O,+6aq.

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It is almost insoluble in cold water, but slightly soluble in boiling water. It is quite insoluble in alcohol. In carbonate of potassium solution it dissolves, but not in other potassium salts.

The tests employed in practice for the detection of sodium are, the colour of the blowpipe flame, and the formation of the acid metantimoniate: the former of these is very delicate; the latter, however, is little used.

SALTS OF LITHIUM.

Solution for the reactions:-chloride of lithium (LiCl) in water. Lithium differs in many respects from the two preceding metals; it forms two combinations with oxygen, but the salts of lithium are all proto-salts; of these we may take the chloride (LiCl) and the oxide (Li,O) as types. The lithium salts are white when the acid-radical is colourless.

They are more fusible than the potassium or sodium salts, and impart a very distinct carmine colour to the blowpipe flame; the presence of a large quantity of a potassium salt does not materially interfere with this reaction, but a small quantity of a sodium compound gives rise to a yellow flame.

The insoluble salts by which lithium may be recognized are these:---the carbazotate, the carbonate, the phosphate, and the silicofluoride.

THE CHLOROPLATINATE and the PERCHLORATE are soluble.

THE CARBAZOTATE is produced by the action of carbazotic acid.

The Carbonate is produced by the action of a very soluble neutral carbonate, e.g. carbonate of potassium (K, CO,), of sodium (Na, CO), or of ammonium ([NH4], CO,), upon rather concentrated solutions of salts of lithium: it is crystalline.

Its formula is Li2 CO. 1 part requires 100 parts of cold water for its solution, but less of boiling water. By most acids it is decomposed and dissolved: it is insoluble in alcohol.

THE ACID TARTRATE is soluble.

The Phosphate is produced by the action of phosphate of sodium (Na2 HPO) on solutions of lithium salts: the precipitation takes place especially on ebullition. If the solution is then evaporated to dryness,