POT

POT

( 241 )

of the latter cryftals is fo different from the reftthat they cannot be eafily miftaken. It lliouldalways be remembered in thefe operations thatpotafli is extremely greedy of carbonic acid,which it abforbs from the air veryfpeedily, fothatall evaporations fhould be made as much as poffi-ble in narrow-mouthed veffels, and at a boilingheat that the fteam which is conftantly pallingoff may preferve the liquor from the contaft ofair. All flow evaporation in the open air istherefore entirely inadmiffible, as it would allowthe whole of the alkali to be converted into acarbonat.

M. Lowitz of Peterfburgh, has given verygood directions for obtaining pure cryftals'ofpotalh by fimple evaporation of the caultic ley,and Berthollet has explained with his ufualaccuracy the procefs by alcohol. That ofLowitz is the following : b make a ftrong leywith carbonat of potafh and lime in the ufualway, {train it through cloth and evaporate it ata boiling heat in a veffel of very clean iron tilla ftrong pellicle appears on the lurface. Coverthe veffel, and let the liquor cool, by whichmoft of the neutral falts and carbonat of potafhcryftallize. Pour off the clear folution andevaporate it further, during which fcum offfrom the furface the faline pellicle as long as itforms. When this has ceafed, remove it fromthe fire and ftir it inceffantly till cold. Thenrediffolve it in twice its weight of cold water,filter the folution and evaporate it in a' glafs re-tort (or what is better, a clofe veffel of filver orfilver-plate) till it begins to depofit regularcryftals which are now pure potafh. When allthe cryftals have depofited, feparate them fromthe liquor (which is always brown) let themdrain in a clofe veffel, rediffolve them and againcryftallize them as before till they form withwater a perfectly tranfparent folution. Theliquor by further evaporation will furnifh more®f the cryftals of potafli till it is almoft ex-haufted. Some nicety of management is re-quired in the laft procefs, for if the folution betoo much concentrated before it cools, thewhole congeals into a folid white mats, whichmull be a little diiuted, and again brought to aboiling heat in order to cryftallize.

The cryftals of potalh formed in this wayaffume the form of very thin broad platescrolfing each other in every direction, and focompleatly {hutting in the mother liquor, thata bottle full of the mixture of cryftals andfolution may often be inverted without fpillinga drop of the liquor. When mejted in. a red

* An. China.- tom, %% %

heat in a filver veffel, thefe cryftals run into amafs of a fnowy whitenefs. .

The other method of obtaining pure potafli isby means of alcohol. Of all the fubftancescontained in common potafh, namely potafh,carbonat of potafli, alumine, filex, fulphat andmuriat of potafh, only the firft is to any notabledegree foluble in well redftilied alcohol, fo thatthe purification of potalh would appear to beeffected by this means with perfect accuracyand limplicity, and is fo to a great degree,though the curious vefearches of Berthollet onthis i'ubjeft fhew that there are fome decompo-fitions th it -take place, which might not be atfirft fufpetted. The experiments of this ex-cellent chemift are the following. 11 A caufticley was prepared in the ufual way from commoncarbonat of. potalh and lime, and was boileddown in an iron veffel till it began to thicken.It was then put into a retort and alcohol waspoured on it, and boiled for a while till part ofthe fpirit had diftilled over. When cold, theretort contained two diftindt liquors, the loweftwas of a dark colour, and at the bottom of itwere a number of faline cryftals, (which provedto be carbonat of potalh) mixed with a blackearthy matter: the upper liquor was lefs colour-ed, and confifted chiefly of the alcohol holdingthe potalh in folution. The lower liquor wasa faturated folution of carbonat of potafli inwater, and ftrongiy effervefced with acids, andthe earthy impurities were either obtained fromthe lime, or originally contained in the carbonatof potafli employed. The alcoholic folutiondid not effervefee with acids nor precipitatelime water, but it llightly troubled muriat ofbarytes and gave a fmall quantity of eftervefeingprecipitate, which Ihewed that fome carbonatof potafli was ftill contained. This folutionwas then boiled in a retort till part of thealcohol had diftilled off, and on cooling itfeparated into two liquors immifcible by fhaking,the upper part of which appeared like a yellowoil, and gave no longer any indication of car-bonic acid, and was now a folution of as purepotafli as could be obtained. By evaporation,and cooling at a certain point of concentration,clear cryftals of potafli feparated, but the motherliquor gradually affumed a darker colour. Thelower of thefe two liquors alfo contained con-Iklerably pure potafli, which cryftallized byevaporation, but it was mixed with fome car-bonat of potafli, and the folvent was chieflywater with a very little alcohol. If the entirealcoholic fplution of potalh is diftilled to drynefs

« Mem. de 1 ’Acad. for 1783.

2 H

VOL. II.