01 gram of potassium
cyanide, it also corresponds to .1 per cent. of this salt counted on the
10 c.c. of cyanide solution taken. The titration should be performed in
a fairly good uniform light. The learner should practice on a fairly
pure solution of potassium cyanide at first, and this may conveniently
have a strength of about 1 per cent. For practice with solutions
containing zinc make a solution containing 1.1 gram of crystallised zinc
sulphate in 100 c.c. and slowly add measured quantities of from 1 to 5
c.c. of this to the 10 c.c. of cyanide liquor before diluting for the
titration.
If a cyanide solution blackens on the addition of the silver nitrate it
contains sulphide. In this case, shake up a considerable bulk of the
liquor with a few grams of lead carbonate, allow to settle and make the
assay on 10 c.c. of the clear liquor.
If the cyanide liquor be suspected to contain free prussic acid, take 10
c.c. for the assay as usual; but, before titrating, add .1 or .2 gram of
sodium carbonate. On no condition must caustic soda or ammonia be added.
The difference between the results, with and without the addition of
carbonate of soda, is supposed to measure the quantity of free prussic
acid. If this has to be reported it is best done as "prussic acid
equivalent to ... per cent. of potassium cyanide.
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