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whole of this fub-pruffiat before it can be de-pended on as a teft.
The proportions of the conffcituent parts ofpure pruffiat of potafh are not abfolutely con-ftarit, but in general they may be Rated to bethofe in which the alkali is perfectly faturatedwith the pruffic acid, and contains the fmalleftpoffible quantity of oxyd of iron confident withthe nature of the fait. The latter is dill veryconfiderable, being from 22 to about 30 percent, of the weight of the entire fait. It maybe found in the following way: take a knownquantity (too grains for example) calcine it tomoderate rednefs in a filver crucible, by whichthe pruffic acid will burn off, and the potafhand oxyd of iron be left. Wafh the refiduethoroughly with hot water to diffolve the alkali,filter, and colleft the oxyd of iron left. Butas this is in the lowed date of oxydation in thepruffiat, calcine it for a fhort time in a coveredcrucible with wax, .till it becomes magnetic,and then weigh it. When this exceeds about30 per cent, of the pruffiat employed, the lattergenerally depofits Pruffian blue by the affufionof acids, and confequently is impure.
Another fubdance that is always found inimpure pruffiat is fulphat of potafh, for it isfcarcely poffible to obtain any potafh free fromit without unufual care, and befides, the Pruffianblue itfelf contains a little fulphuric acid ob-tained from the alum with which it is prepared.This acid may lead to error where the abfenceof a metal in any dilution is to be inferred fromthe want of a precipitate on adding the pruffiat,for if the latter contains any fulphuric acid itwill decompofe and precipitate the falts ofbarytes, and drontian, and (if in quantity) thofeof lime. The fulphuric acid mud thereforebe previoully feparated by a folution of barytes.It may be here remarked that the delicacy ofthis ted is fo great as to dete£I fulphuric acidin the mod minute quantity ; and as mod ofthe common pruffic dilutions contain this acid,a precipitate is generally formed when a baryticdilution is added, which was erroneoufly fup-pofed to arife from the union of pruffic acidwith this earth, and this was brought in con-firmation of the hypothefis of the metallic natureof barytes. This midake was firft detected byMr. Meyer of Stettin, ™ which is of importance,as it allows us to ufe barytes to feparate thefulphuric acid from the pruffiat, without dangerof decompofing the pruffiat itfelf, unlefs toomuch be employed.
Pruffiat of potafh may be prepared in thefollowing way :
Procure a dilution of very pure carbonat ofpotafh, make it caudic or nearly fo, by lime(in the way mentioned under the article Potajh)put it into a glafs flafk, heat it nearly to boiling,and projedf in it fome pruffian blue finelypowdered. The blue colour will be takenaway alinod immediately, and turned to brown,after which continue projecting more of theblue, till the portion lad added no longer lofesits colour. The lixivium is to be boiled for aquarter of an hour, and is then as much faturatedas it can be in this way, and is to be poured offclear from the fediment, and the latter edulco-rated. Then collect all the clear liquors, heatthem and add a little lulphuric acid moderatelydiluted, and continue a moderate heat " for fometime, after which a copious precipitate will beformed of pruffian blue. Separate this by fil-tration, and affay a fmall portion of the liquorby fulphuric acid, and if any blue precipitateappears, add more of the acid to the wholeliquor, which mud again be heated for a time.When by allaying a portion no more blue pre-cipitate is yielded, evaporate the liquor con-fiderably in a gentle heat, and fet it by to cryf-tallize., In a day or two there w r ill be foundtwo forts of crydals ; one of them hard, pointed,and colourlefs, which is fulphat of potafh : theother fofter, yellowiffi, and generally cubical,which is pruffiat of potadi. Pick out thele cryf-tals (more of which may be obtained by furtherevaporation of the mother water) and rediffovethem in cold water, and add to the folutiongradually fome pure barytes diffolved in warmwater, or acetited barytes, as long as any preci-pitate appears, and no longer. The folutionalfo ffiould be fomewhat dilute, otherwife theprecipitation will not dop at the point whenonly the fulphat of barytes ceafes to beformed, but pruffiat of barytes will then beproduced, and being not very foluble, it willprecipitate. Then again evaporate the filteredfolution, and the pruffiat of potafh will nowcrydallize perfectly pure, and of a pale yellowcolour. The purity of thefe crydals is knownby their not affuming a blue colour whenfprinkled with muriatic acid. If this ted fhouldfail they mud be again diffolved, treated withacid, and the folution evaporated as before. 0. Mr. Henry gives a procefs fomewhat dif-ferent, by which through the intermede ofcopper, the pruffiat is dill more effedtualJy
“ Phil Jourr.. 4to. vol iv. p. 30.
* Henry's Epitome, 4to edit,