35G FERROCYANIDES 



a salt of the metal with ferrocyauide of potassium. Thus, in the preparation of tho 

 reddish or purple ferrocyanide of copper, 



2(CuO,SO s ) + K 2 Cfy =Cu 2 Cfy + 2(KO,SO). 

 The above equation written in full becomes : 



2(CuO,S0 8 ) + K 2 C 8 N 8 Fe = Cu 2 C 6 N 8 Fo + 2(KO,S0 3 ). 

 2(CuS0 4 ) + X 4 C G N 6 Pe = 2(X 2 SO 4 ) + Cu 2 C 6 N 8 re. 



Fcrrocyanide of potassium is much used as a test for various metals, in consequence 

 of the characteristic colours of the precipitates formed with many of them. The prin- 

 cipal ferrocyanides, with their colours and modes of preparation, will be found in the 

 following list : 



Ferrocyanide of aluminium. An instable compound formed by digesting hydrate of 

 alumina with ferroprussic acid. 



Ferrocyanides of antimony and arsenic. Neither of these salts is known in a state 

 of purity. 



Ferrocyanide of barium. This salt may bo prepared by boiling prussian blue in 

 slight excess with baryta-water, and evaporating to crystallisation. 



Ferrocyanide of bismuth. When a solution of ferrocyanide of potassium is added 

 to a solution of a .salt of bismuth, a yellow precipitate is obtained. It becomes of a 

 greenish tint on keeping for some time. 



Ferrocyanide of cadmium may be obtained as a white precipitate on adding a solution 

 of ferrocyanide of potassium to a soluble salt of cadmium. 



Fcrrocyanide of calcium maybe prepared in the same manner as that of barium. 

 but, owing to the sparing solubility of lime in water, we must substitute cream of 

 lime for baryta-water. 



Ferrocyanide of cerium is a white salt only slightly soluble in water. Its properties 

 are very imperfectly known. 



Ferrocyanide of chromium. The protochloride of chromium gives a yellow pre- 

 cipitate with ferrocyanide of potassium. 



Ferrocyanide of cobalt. Salts of cobalt give a pale blue precipitate with ferrocyanide 

 of potassium. It appears to decompose on keeping, as its colour becomes altered. 



Ferrocyanide of copper. When ferrocyanide of potassium is added to a solution of 

 subchloride of copper, a white precipitate appears, which, on exposure, becomes con- 

 verted into a purplish red substance, apparently identical with the ordinary ferro- 

 cyanide of copper which falls down on the admixture of salts of the protoxide of 

 copper with solutions of ferrocyanide of potassium. 



Ferrocyanide of glucinum may be obtained, according to Berzelius, under the form 

 of an amorphous varnish, by decomposing ferrocyanide of lead with a solution of sub- 

 sulphate of glucina. 



Ferrocyanide of hydrogen constitutes ferroprussic acid. 



Ferrocyanide of iron, or prussian blue. This salt exists in several conditions, ac- 

 cording to the mode of preparation. The ordinary salt is formed by adding a solution 

 of ferrocyanide of potassium to a solution of a persalt of iron. 



Ferrocyanide of lead is procured as a white precipitate by adding a solution of 

 ferrocyanide of potassium to a salt of lead. 



Ferrocyanide of magnesium is probably best prepared by neutralising ferroprussic 

 acid with magnesia or its carbonate. It forms a pale yellow salt. 



Ferrocyanide of manganese may be obtained as a white precipitate, on adding ferro- 

 cyanide of potassium to a solution of pure protochlorido or protosulphate of man- 

 ganese. 



Ferrocyanide of mercury. This compound cannot be obtained in a state of purity 

 by precipitation. It has not been sufficiently examined. 



Ferrocyanides of 'molybdenum. Molybdinous salts give, with forrocyanide of po- 

 tassium, a dark brown precipitate, soluble in excess of the precipitant. 



Ferrocyanide of nickel is obtained under the form of a pale apple-green precipitate, 

 on addition of prussiate of potash to a salt of nickel. 



Ferrocyanide of silver. Ferrocyanido of potassium gives a white precipitate with 

 silver salts. 



Ferrocyanide of sodium may be formed by the action of caustic soda on prussian 

 blue. 



Ferrocyanide of strontium can be procured precisely in tho same manner as iL. 

 corresponding barium salt. 



Ferrocyanide of tantalum has probably never been obtained pure. Wollaston found 

 that tantalic acid (dissolved in binoxalate of potash) gave a yellow precipitate with 

 prussiate of potash. 



