TAR

TAR

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the leaft repetition of the alternate folutions andevaporations. M. Lowitz d finds that it isbetter to ufe a flight excefs of fulphuric acidin the decompofition of the tartrite of lime toenfure the total feparation of the tartareousacid ; for if too little fulphuric acid were added,the mixture would confift of tartareous acid, offulphat of lime, and of fome undecompofedtartrite of lime, which latter is readily folublein an excefs of its own acid, fo that the clearliquor would not be pure tartareous acid, buta very acid folution of tartrite of lime, fromwhich the pure acid would cryftallize with dif-ficulty. On the other hand, if there was anyconfiderable excefs of fulphuric acid it wouldproduce equal inconvenience, both by difl'olvingfome of the fulphat of lime, and alfo, when theacid liquor was much concentrated, by partiallydecompofing and colouring the tartareous acidand leflening the product.

To afcertain whether the fulphuric acid hasbeen added in due proportion, take out a littleof the clear liquor remaining after the tartriteof lime has been duly digefted with the fulphu-ric acid, and having diluted it, drop in fome fo-lution of acetited lead as long as any precipitatefalls down, which wi'l therefore confift eitherentirely of tartrite of lead, or of a mixture of thisfait with fulphat of lead, according as there isor is not any excefs of fulphuric acid in the li-quor. Then add fome pure and moderatelyftrong nitric acid, and, if the precipitate con-fifts merely of tartrite of lead, it will be redif-folved totally and immediately, and the folutionwill remain clear for fome hours. If the pre-cipitate contains much fulphat of lead mixedwith the tartrite it will not be wholly redif-folved: but if only a fmall quantity of fulphatof lead be mixed with the tartrite it will firft betotally rediflolved, and in a few minutes afterwill become fomewhat turbid. This latter isthe exa£t point to be defired, as it indicates aflight excefs of fulphuric acid, and the liquormay be brought to this ftate by adding eithermore fulphuric acid, or fome referved tartriteof lime, according to circumftances.

In the above mentioned mode of obtainingtartareous acid, chalk, or carbonat of lime, isufed to decompofe the cream of tartar, whichit does merely by engaging the excefs of acidand leaving the remainder of the fait in theftate of tartrite of potafh. But if quick-limebe fubftituted to. the chalk, the whole of thecream of tartar is decompofed, a much largerquantity of tartrite of lime, and confequently of* An, Chim. Tom. 34,

tartareous acid, is obtained, and the fuperna-tant liquor is a folution of cauftic potalh. Ithas been found however by V^uquelin, that thepotafh retains a fmall quantity of tartrite oflime in folution, fo that when the alkaline li-quor is evaporated nearly to drynefs it gelati-nizes by cooling, owing to the feparation of thiscalcareous fait. It may be decompofed by car-bonat of potafh or foda, which produces car-bonat of lime and tartrite of the alkali em-ployed : or the tartareous acid may be deftroyedby calcination, and the lime, carbonated in theprocefs, will remain.

In ufing lime to decompofe cream of tartar,the quantity to be added is not fo readily afcer-tained as with chalk, where the ceflation of theefFervefcence is the fign of the faturation of theacid. Where the lime is well burnt and of uni-form purity, a little experience will guide theoperator with fufficient certainty. Calculatingfrom the obferved proportions of acid in thetartar, and of chalk required in the firft-men-tioned procefs, and of pure lime in chalk, wemay eftimate that all the acid in ioo parts ofcream of tartar (which Thenard c reckons at57 per cent.) will require full 42 parts of purelime for its faturation, and fomewhat more limefhould perhaps be added to enfure the completedecompofition of the tartar. The lime fhouldbe previoufly flacked and mixed with fufficientwater to bring it to the confiftence of pafte.

Lowitz has propofed another method whichis perhaps preferable in every refpecl, exceptthat it is fomewhat more expenfive, and that nocauftic alkali is obtained. 1 It confifts firft indecompofing the cream of tartar by chalk inthe ufual way, added as long as any effervef-cence takes place; and then pouring into thefiltered fupernatant liquor muriat of lime aslong as any precipitate falls down. By thismeans the tartrite of potafh in the liquor is to-tally decompofed, muriat of potalh remains infolution, and the precipitated tartrite of lime isadded to that produced by tire chalk; and bothare afterwards decompofed by fulphuric acid inthe ufual way. The fame chemift alfo advifes toadd to the folution of tartareous acid in the laftpart of the procefs a quantity of Charcoal powder,the depurating power of which has been men-tioned under that article. This, however, is cer-tainly not eilential to the obtaining a perfectlyfine colourlefs cryftallized acid, and, we believe,is feldom if ever ufed.

Tartareous acid has a ftrong and Amply acidtafte, and is foluble in five or fix parts of cold* An. Chim, Tom. 41. p. JI.