POT

POT

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architecture for columns, See. and are muchefteemed.

PORZELLANERDE . See Clay {Porcelain.)

POTASH of Commerce. See Carhop at ofPotafh.

POTASH, Vegetable Fixed Alkali, Gewachs-c’hili. Germ.

The term potafh, which originally fignifiedone fpecies of impure carbonat of potafh, andis Hill applied to it in commerce, is reflriftedin the modern chemical nomenclature to thepure vegetable fixed alkali free from carbonicacid or any other admixture.

The fources of this alkali have been mention-ed when deferibing its carbonat, and it isprocured entirely from the vegetable kingdom,blit Klaproth has difeovered that it forms abouta fifth part of the compofition of leucite andiepidolite, and probably will be detected inmany other minerals now that analyfts areaware of its exiftence in the mineral kingdom,fo that HriCtly fpeaking, the term vegetable asapplied exclufively to this alkali is incorreCt.

Potafh is eafily prepared in tolerable purity,but with difficulty abfolutely fo. For moltpurpofes of chemical analyfis the following wayis fufficient. a Select a pure carbonat of potafh,(made either by refining common pearlafh, orby calcining cream of tartar, or deflagrating itwith equal parts of nitre) boil it with fufficientwater in a polifhed iron kettle with about halfits weight of well burnt lime from white marblefor a few minutes, Hrain the liquor throughlinen, wafh the bulky mafs of Hacked andcarbonated lime once with hot water to extractthe adhering alkali, add it to the firft liquor,and without waiting to filter it, reduce thewhole by boiling till it is of a thick fyrupyconfidence; after which pafs it once morethrough linen and fet it by in a glafs bottle.In a day or two the liquor becomes perfectlyclear, and may then be poured off from thefediment, and kept in a well Hopped glafsbottle. Liquid potafh thus prepared, if a verypure carbonat of potafh be fele&ed, will befufficiently pure for mod experiments. Theule of the lime is to abforb the carbonic acid,and the precife quantity required for this pur-pofe depends on the nature of the lime, and itsperfect calcination. When too little lime hasbeen ufed, the alkaline liquor Hill retains aportion of carbonic acid, and will thereforebecome turbid with lime or barytic water,/which turbidnefs will difappear on fuperfa-turation by any acid, whereby the precipitate

is diftinguifhed from that formed L» the prefenceof a fulphat). When too much lime has beenemployed, the ley contains lime in folution aswell as perfectly cauftic alkali, and will becometurbid by adding carbonic acid or Amply byblowing through it from the mouth for a time,or by a fliort expofure to the air. For theanalyfis of earths therefore it is better to havea fmall excels of carbonic acid, which effectu-ally excludes lime. The lime fliould be ofCarrara marble or oyfler fliells, which con-tain no alumine, as the latter earth is readilydiflolved by the alkali, and is with difficulty gotrid of. The actual quantity of alkali in a givenfolution is determined by evaporating it todrynefs in a filver crucible, and weighing thewhite folid alkali that is left. Liquid alkaliprepared in this way is known to be pure (thatis, free from earth,) by fuper-faturating a littleof it with pure nitric or muriatic acid, thenadding carbonat of ammonia till it is fenfiblyammoniacal, and boiling for a few minutes.If no earth be prefent no turbidnefs will enfueafter Handing for feveral hours.

Potafh as commonly prepared however, oftencontains, befides the pure alkali, carbonic acid,fulphat and muriat of potafh, manganefe, filex,alumine, and oxyd of iron. Of thefe, thethree firH if not in any confiderable quantityare often of no importance in the analyfis ofminerals, but it is neceflary that the chemillfhould know that they exiH, and when requiredfhould be able to free the alkali from them.The way of detecting the prefence of earths hasjuH been mentioned; the fulphuric acid maybe fhewn by firH faturating the alkali by per-fectly pure nitric or muriatic acid, and thenapplying a barytic tefl: and the muriatic acidmay be difeovered by firH faturating with thenitric, and then precipitating the fulphuric acid(if any) by nitrat or acetite of barytes, andlaHly teHing with nitrat of filver. The way ofpreparing potafli perfectly pure is by cryHalli-zation of the pure alkali either from the wateryfolution or from the folution in alcohol. Thereare however fome flight objections to eachmode, though both of them anfwer fufficientlywell for all purpofes when made with care.

When a folution of potaffi with the aboveimpurities is evaporated gradually, as foon asit has reached a certain point of concentration,the fulphat and muriat of potafli begin to fepa-rate, being much lefs foluble than the alkaliitfelf, afterwards the carbonat of potafli cryflal-lizes, and laflly die pure alkali, and the form

* Klapr.atli.