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XII. On the Combination of Chlorine with the Prussiate of Potash, 

 and the presence of such a compound as an impurity in 

 Prussian Blue. By JAMES F. W. JOHNSTON, A. M. 



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(Read January 7. 1828J 



AT has been long known that the Prussian blue of commerce 

 contains an admixture, in greater or less quantity, of alumina, 

 sulphate of potash, and common alum, one or all of them being 

 easily detected in every specimen. The sulphate of potash and 

 the alum may be separated by frequent boiling in water, but they 

 are seldom in such quantity as to render this process necessary. 

 The alumina may be removed by digestion in muriatic acid, and 

 the washing consequent upon this mode of treatment will free 

 it from all the soluble impurities. 



When the alkalies or earths are digested with Prussian blue, 

 in order to form the common Prussiates, and the yellow solution 

 is evaporated, it almost uniformly happens that after the first or 

 second crop of crystals is separated, there remains a dark brownish- 

 red liquid, which either does not crystallize at all, or gives crys- 

 tals of the required prussiate of a dirty brown colour, and mixed 

 with a greater or less portion of a red matter, either massive, or 

 in small, red, four-sided needles and prisms. This may be ob- 

 served in preparing the prussiates of lime or soda by the com- 

 mon process, but has been more frequently taken notice of in 

 forming the cyanide of mercury ; because the least colouring 

 matter in this salt is at once perceptible, and because in the pre- 

 paration of it, a partial loss is of greater consequence. To the 

 presence of a portion of this red salt, particularly in extempo- 

 raneous prussiates, I attribute those differences in the colour of 



