Sciencemadness Discussion Board

Help with Potassium cyanide

Steve_hi - 15-4-2013 at 08:45

Someone today gave me an old tobaco can of Potassium cyanide which was used for case hardening steel we used to have some in school machine shop you get the steel red hot and put the cyanide on it. I was wondering if this could be recrystalized and purified. safely and if so how it is a greyish colour.

Magpie - 15-4-2013 at 13:29

KCN is easily oxidized to KCNO and is very soluble in water, IIRC.

Before attempting purification I would assay it to determine its purity. For this I would recommend the Liebig method, as I have used it before with apparent success. I found it in "Quantitative Analysis," by Pierce and Haenisch, 3rd ed. You may also be able to find it by searching Google Books.

Liebig Method for Cyanides
To the dissolved sample, which should contain 6-8 mmol of cyanide ion, add 30-40 mmol of ammonium hydroxide and 0.1-0.2g KI, and dilute to a volume of 200mL. Titrate with standard 0.1M AgNO3 until the addition of 1 drop of reagent causes the appearance of a permanent turbidity in the solution.

Prior to the stoichiometric point, a soluble silver cyanide complex ion is formed:

Ag+ + 2CN- -----> Ag(CN)2-