Synthetical Reactions.—To a small quantity of a strong solution of carbonate of potassium add acid tartrate of potassium so long as effervescence occurs; the resulting liquid is solution of normal tartrate of potassium (Potassæ Tartras, B. P.) (K,T), crystals of which may be obtained on evaporation.

This is a common method of converting an acid salt of a bivalent acidulous radical into a normal salt. The carbonate added need not be a carbonate of the same, but may be of a different metal; compounds like Rochelle salt (KNaT) are then obtained. Thus:

To a strong solution of carbonate of sodium add acid tartrate of potassium until effervescence ceases; the resulting liquid is solution of tartrate of potassium and sodium. This is the officinal process (Soda Tartarata, B. P.) (KNaT, 4H,O).

Analytical Reactions.

First Analytical Reaction.—To solution of tartaric acid, or any tartrate carefully made neutral by solution of soda, add solution of chloride of calcium; a white precipitate, tartrate of calcium, falls. Collect the precipitate on a filter, wash, place a small quantity in a test-tube, and add solution of potash; on stirring the mixture the precipitate dissolves. Heat the solution; the tartrate of calcium is again precipitated.

The solubility of tartrate of calcium in cold potash solution enables the analyst to distinguish between tartrates and citrates, otherwise a difficult matter. Citrate of calcium is not soluble in the alkali. The absence of much ammoniacal salt must be ensured in both cases, the precipitates being soluble in such liquids.

Second Analytical Reaction.—Acidulate a solution of a tartrate with acetic acid, add acetate of potassium, and well stir the mixture; a crystalline precipitate of acid tartrate of potassium slowly

separates. The precipitate being insoluble in alcohol, the addition of a little spirit of wine renders the test more delicate.

This reaction is not applicable in testing for very small quantities of tartrates, the acid tartrate of potassium being not altogether insoluble.

Third Analytical Reaction.-To a neutral solution of a tartrate add solution of nitrate of silver; a white precipitate of tartrate of silver, Ag ̧CHO,, falls. Boil the mixture; it turns black, owing to the reduction of the salt to metallic silver.

Other Reactions.-Tartrates heated with strong sulphuric acid char immediately.- -Tartaric acid and the soluble tartrates prevent the precipitation of ferric and other oxides by alkalies, soluble double tartrates being formed, which on evaporation yield liquids that do not crystallize, but, spread on sheets of glass, dry up to thin transparent plates or scales. The potassio-tartrate of iron (Ferrum Tartaratum, B. P.) is a preparation of this kind. Tartrates decompose when heated, carbonates being formed and carbon set free, the gaseous products having a peculiar, more or less characteristic odour, resembling that of burnt sugar.

CITRIC ACID.

Formula H,CH,O,, H2O or H ̧CiAq.

Molecular weight 210.

Citric acid exists in the juice of many of our common garden fruits; but it is from the lemon or lime that the acid of commerce is usually obtained. The British Pharmacopoeia directs that the hot lemon-juice be saturated by chalk, the resulting citrate of calcium collected on a filter, washed with hot water till the liquor passes from it colourless, then mixed with cold water, decomposed by diluted sulphuric acid, and the filtered solution evaporated to the crystallizing point.

The elements represented by the formula CHO, are those characteristic of citrates. They form a trivalent grouping; hence three classes of salts may exist-one, two, or three atoms of the basylous hydrogen in the acid, H,CH,O,, being displaced by equivalent proportions of other basylous radicals. Citric acid itself is the only citric compound of much direct importance to the pharmaceutist.

ANALYTICAL REACTIONS.

First Analytical Reaction. -To solution of citric acid, or any citrate carefully neutralized by alkali, add solution of chloride of calcium; a white precipitate, citrate of calcium (Ca,C12), falls. Treat the precipitate for tartrate of calcium; it is not dissolved by the potash.

A mixture of citrates and tartrates can be separated by this reaction. They are precipitated as calcium salts, and the washed precipitate mixed with solution of potash, diluted and filtered; the filtrate contains the tartrate, which is shown to be present by reprecipitation on boiling. The precipitate still on the filter is washed, dissolved in solution of chloride of ammonium, and the solution boiled; the citrate of calcium is reprecipitated. The presence of much sugar interferes with this reaction.

Second Analytical Reaction.—To a neutral solution of a citrate add solution of nitrate of silver; a white precipitate of citrate of silver (Ag,Ci) falls. Boil the mixture; the precipitate does not turn black as tartrate of silver does.

Other Analytical Reactions.-Citric acid forms no precipitate corresponding with the acid tartrate of potassium.—Limewater gives no precipitate with citric acid or citrates, unless the solution is boiled; it usually precipitates tartrates in the cold.

-Citrates, when heated with strong sulphuric acid, do not char immediately.—Citric acid and citrates prevent the pre

cipitation of oxide of iron by alkalies, soluble double compounds being formed. The Ferri et Ammonia Citras, B. P., is a preparation of this kind.-Citrates decompose when heated, carbonates being formed and carbon set free: the odour of the gaseous products is not so characteristic as that of tartrates.

PHOSPHORIC ACID.

Formula H,PO. Molecular weight 98.

The source of the ordinary phosphates and of phosphorus itself (Phosphorus, B. P.) is the tricalcic phosphate (Ca,2PO). It is the chief constituent of the bones of animals, being derived from the plants on which they feed, plants again obtaining it from the soil. Phosphorus is obtained from bones by the following processes:-The bones are burnt to remove all traces of animal matter. The resulting bone-earth is treated with sulphuric acid, by which an acid phosphate (CaH2PO), often called superphosphate of lime, is produced:

Ca 2PO2H,SO1 = CaH2PO2CaSO,.

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The acid phosphate is mixed with charcoal and strongly heated in a retort, when it splits up into tricalcic phosphate and phosphoric acid

the phosphoric acid being reduced by the charcoal to phosphorus and hydrogen, and carbonic oxide gas liberated :—

H‚PO̟ ̧ + C1 = P + H1 + 4CO.

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The chief use of phosphorus in pharmacy is in the formation of a dilute solution of phosphoric acid. Phosphorus is boiled with nitric acid until dissolved, the operation being conducted in a retort in order that any nitric acid which would otherwise be lost may be condensed and again poured on to the phosphorus. A flask, in the neck of which a funnel is inserted and a second funnel inverted so that its mouth rests within the mouth of the first, is even more efficient and convenient for this operation than a retort. The solution evaporated to a low bulk to remove nitrous compounds, and rediluted so as to contain nearly 14 (13·8) per cent. of acid (H.PO,), constitutes the Acidum Phosphoricum

Dilutum, B. P. If the necessary appliances are at hand, the student should make four or five ounces of this preparation.

Some phosphorous acid (HPHO) is also formed in the first stages of the reaction.

The elements represented by the formula PO, are those characteristic of phosphates. The grouping is trivalent; hence there may exist trimetallic or normal phosphates (M',PO), dimetallic acid phosphates (MHPO,), monometallic acid phosphates (M'H ̧PO), and lastly, trihydric phosphate (H ̧PO), common phosphoric acid, itself. These are the ordinary phosphates met with in nature or used in pharmacy; the rarer phosphates and metaphosphates will be mentioned subsequently.

Analytical Reactions.

First Analytical Reaction.—To an aqueous solution of a phosphate (e. g. Na HPO,) add solution of sulphate of magnesium with which chloride of ammonium and ammonia has been mixed; a white crystalline precipitate of ammonio-magnesian phosphate falls (MgAmPO).

Chloride of ammonium is added to prevent the precipitation of hydrate of magnesium. Arseniates, from their close analogy to phosphates, give a similar precipitate with the magnesian reagent.

Second Analytical Reaction.—To an aqueous solution of a phosphate add solution of nitrate of silver; light-yellow phosphate of silver (Ag,PO) is precipitated. To a portion of the precipitate add ammonia; it dissolves. To another portion add nitric acid; it dissolves.

A phosphate may be distinguished from an arseniate by this reaction, arseniate of silver being of a chocolate colour.

Third Analytical Reaction.-To a solution (in a few drops of acid) of a phosphate insoluble in water (e. g. Ca ̧2PO) add an