As in the case of chlorine-water, the water intended for the absorption of the sulphur dioxide should be kept cold, so as to avoid the loss of gas, and the finished solution must be preserved in small, completely filled, glass-stoppered vials in a cool, dark place, as the sulphurous acid rapidly absorbs oxygen and is converted into sulphuric acid when carelessly exposed, thus losing all its valuable medicinal properties. The precautions regarding fracture of the generating flask, already stated under chlorine-water, should also be observed in the case of this solution.

The pharmacop eial test with lead acetate paper depends upon the reaction between sulphur dioxide and nascent hydrogen (generated from zine with hydrochloric acid), resulting in the formation of hydrogen sulphide, thus SO, HHS 2H,O. Slight traces of sulphuric acid are unavoidable, except in freshly made solutions; hence the official limit test.

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The strength of sulphurous acid solutions is officially determined, volumetrically, with iodine as an oxidizing agent, the following reac tion taking place: H2SO,(SO, + H2O) + I, + H2O = 2HI + HSO, 2 atoms of iodine converting 1 molecule of sulphurous acid into sulphuric acid. Each Cc. of iodine solution, containing 0.012653 Gm. iodine, therefore corresponds to 0.003195 Gm. SO2, and 2 Gm. of the official acid must require at least 40 Cc., for 6.4 per cent. of 2 is 0.128, and 0.128 divided by 0.003195 yields 40. In order to obtain accurate results in the titration of sulphurous acid solutions, the official directions should be modified so as to insure an excess of iodine during the process. Instead of adding the decinormal iodine solution gradually to the diluted acid liquid, the latter should be added all at once to a definite volume of the iodine solution, known to be in excess. After addition of a little starch solution the excess of iodine is titrated with decinormal sodium thiosulphate solution and subtracted from the volume first added, to obtain the exact quantity required by the given weight of sulphurous acid solution. The reason for this modification is that the sulphuric acid formed is gradually reduced by the hydriodic acid to sulphurous acid, with the liberation of iodine, and thus the results are constantly vitiated.

CHAPTER XLI.

THE COMPOUNDS OF POTASSIUM.

THE Pharmacopoeia recognizes 17 salts of potassium, besides 7 preparations of salts, including 3 liquids, for which working formulas are given; the following comprise the list:

Potassa. KOH.-This compound, better known as caustie potash, is, chemically speaking, potassium hydroxide or hydrate, obtained by decomposing a solution of potassium carbonate with milk of lime, evaporating the clear filtrate in perfectly clean iron or silver vessels until a small quantity of the liquid congeals upon cooling, and then pouring it into cylindrical moulds, whence the sticks are removed while still warm.

The purity of the product obtained depends upon the quality of the potassium carbonate employed, and if made from the bicar bonate it is of much better quality. White caustic potash in sticks, labelled potassa by lime, is the kind generally used for pharmaceutical purposes, and should contain not over 5 or 6 per cent. of moisture; commercial caustic potash is sometimes found to contain as much as 20 or 25 per cent. of water. For chemical

purposes potassa is purified by means of alcohol or baryta, being then known as potassa by alcohol or potassa by baryta.

Potassa is a powerful caustic, very deliquescent, and rapidly absorbs carbon dioxide from the air; it must therefore be handled carefully and preserved in tightly stoppered bottles.

The Pharmacopoeia requires that official potassa shall contain at least 90 per cent. of absolute potassium hydroxide, which is ascertained by titration with normal acid, each Cc. of which requires 0.05599 Gm. KOH for neutralization. The official assay, requir ing 9 Cc. of H2SO, for 0.56 Gm. of potassa, is absolutely accurate only in the absence of soda, as the latter, having a lower molecular weight, requires a relatively larger quantity of acid for saturation ; the small amount of soda permitted will not, however, materially affect the result, and may well be ignored.

With a few exceptions, the limits of impurities allowed by the Pharmacopoeia, in this and other compounds of potassium, rarely exceed 0.5 per cent., and are usually determined volumetrically. Since potassa readily absorbs carbon dioxide, as much as 1.38 per cent. of potassium carbonate is allowed in the official article, as shown by the test with lime-water; 5 Cc. of lime-water, containing about 0.148 per cent., or 0.0074 Gm. of Ca(OH)2, are capable of precipitating 0.0138 Gm. of K,CO, as CaCO3, which is equal to 1.38 per cent. of 1 Gm. of the sample.

Besides slight traces of potassium silicate and nitrate, 1.5 per cent. of soda is also permitted in the official potassa, which is indicated by the quantity of normal potassium hydroxide solution necessary to cause an alkaline reaction in the filtrate obtained after precipitating all KOH present in 0.56 Gm. of the sample as acid potassium tartrate, by means of tartaric acid. Any soda present in the potassa will also have been converted into an acid tartrate, but will remain in solution, and, upon the addition of sufficient KOH solution, be converted into normal double tartrate; 0.2 Cc. KOH solution corresponds exactly to 0.008 Gm. NaOH, which is 1.5 per cent. of 0.56 Gm., and the first drop added beyond this point should cause a permanent pink color, using phenolphtalein as indicator, thus showing an excess of

alkali.

Potassa with Lime. This preparation is a simple mechanical mixture of equal parts of potassa and lime, intended as a milder application than potassa alone. The object of mixing the ingredients in a warm mortar is to prevent the absorption of moisture, and, as the powder rapidly deteriorates upon exposure to air, it must be kept in tightly stoppered vials. Potassa with lime is known also as Vienna Caustic. It is rarely used.

Sulphurated Potassa, or liver of sulphur, has been known for nearly 500 years, and for over 100 years has been made in the same

manner as now officially prescribed. When potassium carbonate and sulphur are heated together, carbon dioxide is evolved and the sulphur unites with the potassium, forming polysulphides, a portion of which is oxidized to thiosulphate by the oxygen of the carbonate in excess over that passing off as carbon dioxide. Small quantities of potassium sulphate are also possibly formed, and, since high heat favors such a change, the temperature should be so regulated that the mass at no time shall assume a thin fluid condition, and that as little sulphur as possible be consumed. If the preparation is carefully made, the following reaction is likely to occur: 3K,CO,S = 2KS, † K2S2O, † 3CO2; but with a higher heat potassium sulphate is formed from the thiosulphate.

Sulphurated potassa is not a definite chemical compound, its composition being variable and depending upon the care used in its manufacture. It must be protected against air and moisture, to avoid further oxidation, which is indicated by a change in color from liver-brown to green and finally gray.

The medicinal virtues of sulphurated potassa reside chiefly in the potassium sulphides present, the Pharmacopoeia demanding at least 12.85 per cent. of sulphur in such combination, which may be determined by treatment with crystallized cupric sulphate. The following equation, CuSO,5H2O + K2S ̧ CuS SK„SO, 5H,O, shows that 248.8 parts of crystallized cupric sulphate require 31.98 parts of sulphur for complete precipitation of the copper; hence 1 Gm., as prescribed in the official test, will require 0.1285 Gm. of sulphur, which is equivalent to 12.85 per cent, of the weight of sulphurated potassa used.

Potassium Acetate. KC,H,O, or CH,COOK.—This salt is prepared by neutralizing acetic acid with potassium carbonate or bicarbonate, the latter being preferable on account of its greater purity, evaporating the resulting solution to dryness, fusing the residue, and allowing the salt to solidify. The product, being very deliquescent, must be bottled while still warm, and should be well protected against air.

The salt absorbs moisture very quickly when in contact with air, which it is impossible to prevent while weighing, hence only 98 per cent. of acetate is officially demanded.

In order to determine the quality of organic salts of potassium volumetrically, it is necessary that they first be converted into carbonate by thorough ignition, the oxygen of the atmosphere aiding in the change. In the case of potassium acetate the following reaction occurs: 2KC,H,O,+0,- K.CO, 3H,O - 3CO,, two molecules, or 196 parts, of acetate furnishing one molecule, or 138 parts, of carbonate. Each Cc. of HSO, required in the official test to neutralize the carbonate resulting from 1 Gm. of potassium acetate, represents 0.098 Gm., or 9.8 per cent., of acetate; for 138:196 :: 0.069 :0.098.

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Potassium Bicarbonate. KHCO,.-When carbon dioxide is passed into a concentrated solution of potassium carbonate, chemical inion takes place, potassium bicarbonate or acid carbonate being formed according to the equation K,CO, HO CO2 = 2KHCO3. The solution is afterward decanted from any separated silica, and crystallized. Potassium bicarbonate is permanent in the air, any hygroscopic tendency indicating contamination with carbonate; this can be verified by adding to a solution of the salt barium chloride or magnesium sulphate, which are not precipitated by the pure bicarbonate. The Pharmacopoeia permits slight traces of carbonate and chloride, also of iron.

Potassium Bichromate, more properly Dichromate. K,Cr,O,. -Although the official title of bichromate has been retained in the Pharmacopoeia, this is not in conformity with the chemical composition of the salt. The term bichromate, according to accepted usage, would indicate a monobasic acid salt, requiring the formula KHCrO,, a salt not known, whereas the official salt has the composition K.Cr.O,, showing it to be a compound of dichromic acid, HCrO. This acid may be looked upon as obtained by the union of two molecules of chromic acid, with the elimination of water; thus, HCrO, HCrO, H.Cr2O, HO; or it may be assumed + that chromic anhydride is capable of forming both chromic and dichromic acids; thus, CrO,H,O HCrO, and 2CrO, + H2O =HCrO. Dichromic acid may be said to be chromic acid holding chromic trioxide in solution, and is analogous to disulphurie, or fuming sulphuric, acid.

Potassium dichromate is obtained by treating a solution of the chromate with sulphuric acid-thus, 2K,CrO, H.SO, K,Cr.O, - K2SO, + H2O—and separating the resulting salts by crystallization. The chromate is obtained direct from chrome-iron ore, FeOCO, by roasting the same, in reverberatory furnaces, with potassium carbonate and chalk, the latter simply preventing fusion of the mixture, which is finally treated with water and strained to remove the iron.

Potassium Bitartrate. KHC,H,O, or (CHOH)COOHCOOK. -Acid potassium tartrate, or cream of tartar, as it is more familiarly known, is prepared for medicinal use by treating purified tartar with diluted hydrochloric acid for the purpose of removing the calcium tartrate present as chloride; the mixture is heated and constantly agitated while cooling. Some tartaric acid and potassium bitartrate remain finally in the mother-liquors, which are utilized in the manufacture of tartaric acid.

Crude tartar, or argol, is obtained as a natural deposit in wine casks during the fermentation of grape-juice, and is purified by repeated treatment with water, clay, and animal charcoal, to remove