1G6 ALUM.— Precipitation.
the crystallized state ; but in practice this quantitydiminishes according to the less saturated state ofthe lie. Since potassium chloride is more soluble inboiling water than potassium sulphate, it is preferablefor precipitating the alum flour; the chloride is sol-uble in 2 parts of boiling and in 3 parts of cold water,and hence the mixture may be made in the cold.
In the instance cited above the 33'5 parts ofaluminium sulphate would require 14-5 parts ofpotassium chloride for precipitation, which would betaken up by 26'2 parts of boiling water, and produce96 - 5 parts of alum.
The mixture being made and the precipitate sub-sided, 12 parts of the alum would be held in solution,so that only 84'5 parts of flour are thrown down inthe solid form. If the same quantity of potassiumchloride be dissolved in cold water, 46 parts at50° Fahr. (10° C.) will be required, which, whenadded to the 23 - 5 parts of aluminium sulphate insolution, will yield 84 parts of alum as a precipitate,while 13'8 remain dissolved: in both cases the dif-ference is immaterial, whether the precipitant bedissolved in hot or cold water.
Another precipitant which is used with muchadvantage on account of its great solubility, isammonium sulphate: this is soluble in 2 parts ofwater, but the amount of alum obtained is less thanwhen the potassa salts are employed, in the propor-tion of 100: 95'6 ; the reason being that the atomicweight of these bodies differ, the equivalent of am-monium hydrate, (NII 4 ).,, used being 36, while thatof potassium is 39'1.
To recur to the foregoing example, the 33'5 partsof aluminium sulphate will take 15'8 parts of am-monium sulphate to form with them 87'7 parts ofalum, of which 77 parts are precipitated and only10'C retained in the solution ; thus, the disadvantagearising from the lesser equivalent of ammonia alumis counterbalanced by the greater solubility of theammoniacal salt as a precipitating agent.
The potassium sulphate, for the use of alum-makers, is obtained as a secondary product in themanufacture of nitric acid, and in the purification ofpotassium hydrate and the crude carbonate of thatalkali: thus produced it is an acid sulphate, com-monly called bisulphate of potash, the compositionof which is expressed by the symbols KIIS0 4 , andcannot be employed for alum .making till the excessof acid is neutralized by fresh additions of the causticor carbonated alkali to form a neutral or bipotassicsulphate of the composition, K 2 S0 4 .
It is only under the aboye circumstances thatpotassium hydrate or carbonate can be employed inthe preparation of the alum flour ; for if no free acidis present it throws down a basic double sulphate ofaluminium and potassium which is insoluble in water,though redissolved by a further addition of bisuL phate of potash. A greater or less quantity of thiscompound is always formed during the evaporationof the liquors, in consequence of the iron presentabstracting some of the acid, but, as in the preced-ing, a slight addition of the bisulphate of potashrodissolves it.
Knapp recommends that the purification of crudepotassium hydrate and carbonate should be con-ducted on the same premises with the alum works,for the purpose of turning to use the bisulphates andchloride formed in this operation as precipitants ofthe alum. With regard to the latter salt, he says itis of greater importance than the sulphate, inasmuchas it effects the decomposition of the sulphates ofiron, giving rise to potassium sulphate and chloridesof iron, which are highly deliquescent salts, and fromwhich the alum flour can be removed with greaterease than from the sulphates of iron. But thedecided advantage attending the use of the chloride iswhen a ferric sulphate is present in the lie, as this lattersalt, in combination with potassium sulphate, formsa difficultly soluble double salt (basic iron alum),which, precipitates in the form of a yellow powder,and contaminates the alum flour. As an excess ofpotassium chloride would convert the aluminiumsulphate into aluminium chloride, a result whichwould entail a loss, he recommends that the lieshould be examined before each operation, with aview to determine the proper amount of chloride tobo added, as it happens that the quantity requiredto precipitate the whole of the aluminium as alumseldom or never meets with sufficient ferrous sul-phate in the lie to be converted into potassiumsulphate. This examination is conducted as follows:—A saturated solution of the precipitating salt isadded from a burette, or other graduated vessel, toa measured quantity of the liquor, the mixture beingbriskly agitated at each addition ; successive portionsof the reagent are poured in as long as the flourthrown down increases in quantity, and when nofurther precipitation is observed the number ofmeasures is read off, which, with sufficient accuracyon the large scale, will stand for the amount of solu-tion required to precipitate a certain amount of theliquor; by this means the tedious process of weigh-ing is avoided. If potassium sulphate has been .employed, from its difficult solubility in the cold itmay precipitate during the cooling of the liquor,apparently increasing the bulk of the precipitate, andgiving rise to an error in the calculation, To avoidthis source of inaccuracy it is best to employ, as thetesting liquor, a saturated solution of ammoniumsulphate, which, being yery soluble both in hot andcold water, is not precipitated, One part of theammoniacal salt is equivalent to 1'32 parts of potas-sium sulphate, or 1-13 of potassium chloride.
Potassium chloride for alum making is obtainedin large quantities from the waste liquor of the soapworks, from saltpetre refineries, and from the glass-houses. Wood ashes should never be used, onaccount of the lime which they invariably contain.
The ammoniacal reagent is obtained from theliquor from gas works; this is converted into am-monium sulphate either directly by the addition ofsulphuric acid, or by treatment with sulphate oflime or iron. The crude ammoniacal liquor obtainedfrom the manufacture of ammonium chloride, by thedestructive distillation of animal matters, may beemployed with equal advantage for the same purpose.