Table of Contents
The most reliable method of determining ferrocyanide in a cyanide solution is to determine the total iron and calculate to ferrocyanide.
Volumetric Method
Procedure
To 200 to 500 cc solution, depending upon the quality of ferrocyanide thought to be present, add 10 cc HCl and 5 cc HNO3, and evaporate to about 50 cc. Add 8 cc H2SO4, and evaporate to strong fumes and almost dryness. Cool, add 5 cc water and 5 cc H2SO4, and again evaporate almost to dryness. Cool, add 50 cc water and 5 cc HCl, and heat to dissolve the soluble salts.
Add 3 grams pure zinc (20 mesh). If arsenic is thought to be present, add 3 cc of 4 per cent solution of copper sulphate (CuSO4 to 5H2O). The copper will deposit on the zinc and the arsenic on the copper as copper arsenide. When the zinc is almost dissolved add 50 cc water and very carefully add 5 cc H2SO4. Filter into a 400-cc beaker after all evolution has ceased, wash well with cold water, and dilute the filtrate to 300 cc.
Titrate immediately with 0.10 N potassium permangante, KMnO4 solution to a faint permanent pink coloration.
0.10N KMnO4 Solution. (See “Determination of Reducing Power.”)
1 cc 0.10N KMnO4 = 0.0056 gram iron, Fe
= 0.0369 gram potassium ferrocyanide, K4Fe(CN)6
Colorimetric Method
Treat a 100-cc sample of solution as in Method 1, to and including the second evaporation of H2SO4.
Cool, add 50 cc water, and heat to dissolve the soluble salts. Filter, and wash with hot water.
To the filtrate, the volume of which should be about 50 cc, add ammonium hydroxide, NH4OH, until alkaline and a slight excess. Heat for a few minutes to coagulate the precipitate, then filter, and wash with hot water.
Dissolve the precipitate on the filter with warm 5 per cent H2SO4 solution, and wash with hot water. Add 0.10N KMnO4 solution to a faint pink, avoiding an excess. The pink color will disappear on standing.
Transfer the solution to a Nessler tube, add 10 cc of a 10 per cent solution of potassium thiocyanate, KCNS, and dilute to 100-cc mark with water.
To the second Nessler tube add 75 cc of 5 per cent H2SO4 solution and 10 cc of the KCNS solution. Then add from a burette chop by drop the standard iron solution until the colors match, then” add sufficient water to bring the solution to the even 100-cc mark.
STANDARD IRON SOLUTION
To 200 cc of 5 per cent solution of H2SO4 add 7.021 grams of ferrous ammonium sulphate [FeSO4 (NH4)2SO4 to 6H2O]. Add 0.10N KMnO4 to a faint pink coloration, then add 40 cc H2SO4, and dilute to 1000 cc.
1 cc FeSO4(NH4)2SO4:6H2O solution = 0.001 gram Fe
Determination of Reducing Power
To 5 to 25 cc of solution, depending upon the amount of reducing agents present, add sufficient water to bring the volume to 200 cc. Then add 25 cc of 25 per cent H2SO4 solution and titrate with 0.10N potassium permanganate (KMnO4) solution to the first faint pink coloration, which remains permanent for 2 min.
The result is reported in cubic centimeters of 0.10N KMnO4 solution per 1000 cc of cyanide solution.
The amount of cyanide solution taken for this determination should be so adjusted as to require 5 to 10 cc 0.10N KMnO4 solution. Larger titrations generally result in fading end points.
0. 10N KMnO4 Solution. Dissolve 3.16 grams of potassium permanganate in water, and dilute to 1000 cc. This solution should be kept in a dark bottle.