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XXV. On the Action of a mixt^ire of Red Prussiate of Potash 

 and Caustic Alkali upon Colouring Matters. By John 

 Mercer, Esq.^ 



A BOUT ten years since I discovered and used extensively 

 -^^ in calico-printing the oxidizing properties of a mixture 

 of red prussiate of potash and caustic alkali. For many 

 years I have been in the habit of communicating to my friends 

 several applications of this interesting reaction, among whom 

 I may mention Mr. Crura of Glasgow and Dr. Lyon Playfair. 

 Since then Boudault t has directed attention to the oxidizing 

 power of the same mixture, as far as relates to metallic oxides, 

 but has not shown any important practical application of the 

 knowledge thus acquired. 



There are but few processes known in the arts for bleach- 

 ing indigo, the principal of these being that in which chromic 

 acid liberated from the bichromate of potash by means of an 

 acid is used. In certain cases this process is attended with 

 various disadvantages, and the cloth requires to be subjected to 

 a clearing process to remove the oxide of chromium. The 

 topical application of a mixture of red prussiate of potash and 

 an alkali at once effects the same pm*pose, and in a most com- 

 plete manner, leaving a brilliant white on the spot where the 

 colour is discharged without rendering any injury to the 

 fabric. The manner of applying this discharge may be ar- 

 ranged to suit the conditions of the calico-printer. As a class 

 experiment for a lecture-table it is convenient to impregnate 

 the indigo-blue calico with a solution of prussiate of potash, 

 and then dip it into a weak solution of alkali. 



This action is a beautiful illustration of those double affi- 

 nities which we frequently find at play in combinations or 

 decompositions. Thus, though neither chlorine nor charcoal 

 can decompose alumina per se, the same gas passed over a 

 mixture of alumina and charcoal combines with the metallic 

 radical ; the charcoal in this case having aided the combina- 

 tion by withdrawing the oxygen. It is the same kind of 

 action in the case under consideration. Red prussiate of 

 potash, Feg Cyg 3K, differs from the yellow prussiate, Fcg 

 Cyg 4K, by containing one atom less potassium. When pot- 

 ash is presented to the former, this deficient atom of potas- 

 sium is supplied, but the affinity is not strong enough to 

 liberate the oxygen. When however a second body having 

 an attraction for oxygen, such as litharge or indigo, is pre- 

 sented to the potash and red prussiate, this second affinity 



* Communicated by the Chemical Society; having been read Feb. J, 

 1847. 



t Journal de Pharmacie. [Phil. Mag., vol. xxvii. p.307.] 



