FERRO-TARTRATE OF POTASH.
555
The London Pharmacopoeia professes to follow Soubeiran’s process,hut the formula which is given is much more complex; it is as fol-lows :—Mix three ounces of sesquioxide of iron with half a pint ofhydrochloric acid, and digest for two hours in a sand bath. Add tothese two gallons of water, and set aside for an hour; then pour off thesupernatant liquid. Four pints and a half, or as much as may besufficient, of solution of potash being added, wash what is precipitatedfrequently with water, and, while moist, boil it with eleven ounces and ahalf of bitartrate of potash, previously mixed with a gallon of water.If the liquor should be acid when tried by litmus, drop into it solution ofsesquicarbonate of ammonia until it is saturated. Lastly, strain theliquor, and with a gentle heat let it evaporate, so that the salt mayremain dry.
The theory of this process is as follows:—By the reaction of sesqui-oxide of iron and hydrochloric acid we obtain water and sesquichlorideof iron [vide p. 541). On the addition of caustic potash, the sesqui-chloride is decomposed, hydrated sesquioxide of iron is precipitated, andchloride of potassium is left solution. These changes are illustrated bythe following diagram:—
REAGENTS. RESULTS.
« * - ... f 3 eq. Potassium. 120-*-— -_ - 3 eq. Chloride Potassium 228
3 eq. Potash .... 144 { 3 0xygen . 24~- > _ <; --
2eq. Sesquichloride $3 eq. Chlorine.108—' —■—
Iron ..164 ^2 eq. Iron .56----2eq.Sesquiox-'l Hydrated S«-
iaelron. 80 quioxide of
■Water- J Iron,
When the hydrated sesquioxide of iron is boiled with bitartrate ofpotash, one equivalent or 40 parts of sesquioxide combine with oneequivalent or 66 parts of tartaric acid of the bitartrate of potash, andform an equivalent or 106 parts of tartrate of sesquioxide of iron, whichcombine with an equivalent or 114 parts of tartrate of potash, to formone equivalent or 220 parts of ferrotartrate of potash.
REAGENTS.
RESULTS.
1 eq. Sesquioxide of Iron. 40)
1 eq. Bitartrate 5 1 Tartaric Acid 66$Potash.... 180 1 1 eq. Tartrate Potash 114—
1 eq. Tartrate of Sesqtiiox. Iron 106
}
1 eq. Ferro-tartrate ofPotash, 220
The processes of the Dublin and Edinburgh Colleges are muchinferior to the above, and need not be minutely described. A mixtureof iron, bitartrate of potash, and water, is exposed to the air, by whichthe iron is converted into sesquioxide, and combines with the bitartrate.
Properties. —It is an olive-brown inodorous powder, with a stypticinky taste. It reacts on vegetable colours, mildly alkaline. It is slightlydeliquescent, probably from the tartrate of potash which it contains. Itdissolves in about four times its weight of water, and slightly in alcohol.
Characteristics. —Ferrocyanide of potassium does not occasion anyblue colour with it, unless a few drops of acid be added. Potash, soda, andtheir carbonates, do not decompose it at ordinary temperatures, nor doesammonia or its carbonate even by the aid of heat. Tincture of nutgallscauses a dark-coloured precipitate. Sulphuric, nitric, or hydrochloricacid, throws down the sesquioxide of iron from a solution of this salt;an excess of acid redissolves it: the solution has then a very astringenttaste. Tartaric acid causes the formation of crystals of tartar. Heatedin a covered crucible, ferrotartrate of potash yields charcoal, carbonateof potash, and protoxide of iron.