Because of the distinctive colors of the precipitates formed, various cations have been proposed for the qualitative detection of the ferrocyanide ion. Since interference frequently occurs, some care is usually necessary. As an illustration, the following condensed procedure serves for the detection of the ferrocyanide ion in the presence of ferricyanide and thiocyanate:
A dilute solution of the alkali salts of the anions is acidified with a slight excess of hydrochloric acid and is treated with an excess of ammonium molybdate solution. Brown molybdenum ferrocyanide precipitates. The liquid is shaken with a little fine asbestos and filtered. The residue is washed with cold, dilute acid, and then dissolved in ammonium hydroxide. The solution is saturated with hydrogen sulfide and then acidified with sulfuric acid. After removal of the precipitated sulfide by filtration, the filtrate is boiled to drive off the hydrogen sulfide. A ferric chloride solution is added; the formation of Prussian Blue indicates ferrocyanide.
A simple test in which most anions do not interfere may be carried out by the addition of one drop of a 0.5 percent solution of N-chlorosuccinimide in 95 percent ethanol to a neutral or slightly acid ferrocyanide test solution on a black spot-test plate. Colorless, thin prismatic crystals form with concentrations as low as 8 γ of potassium ferrocyanide in 0.05 ml. of solution.
Numerous quantitative methods have been proposed for the determination of ferrocyanide, but many of these are usable only in the absence of interfering substances. Since many anions and cations do interfere, no one general method of analysis can be given. Most of the following procedures, however, are of rather broad scope. Precipitation of the lead, zinc, and silver salts can be utilized, preferably with excess potassium ion present in the case of zinc.
An excellent procedure using ceric sulfate has been devised. The titration is run either potentiometrically or using an indicator such as the 0-phenanthroline-ferrous complex. This procedure, of course, gives high results if another reducing agent is present.
Oxidation with permanganate serves satisfactorily, while the use of bromine or chlorine requires caution because of some instability of their solutions. Oxidation with iodine in the presence of phosphate and fluoride to stabilize the oxidation products has been reported. Sodium vanadate in acid solution also has been mentioned as the oxidizing agent. In most of these cases the excess oxidizing agent is titrated with sodium thiosulfate. Provision is made in one procedure for the presence of other reducing substances. It is probable that procedures suitable for specific analytical problems can be devised by modification of these or other methods described in the literature.
The procedure outlined below is based upon that presently used by American Cyanamid Company for the quantitative determination of ferrocyanide ion in a substantially pure sample of sodium ferrocyanide. It is, therefore, essentially an assay method. It involves the precipitation of zinc potassium ferrocyanide, 3Zn2Fe(CN)6·K4Fe(CN)6, with zinc chloride in the presence of potassium chloride, using uranyl nitrate as an outside indicator. The zinc chloride reagent is checked against potassium ferrocyanide, which has been standardized with potassium permanganate. The permanganate, in turn, is standardized with very pure sodium oxalate.
Where the amount of free moisture in the crystalline salt is desired, it may be obtained by careful drying of the salt at 105 °C. to determine the total water present. The water of crystallization, which corresponds to the ferrocyanide in question, must be subtracted.
Standard Potassium Permanganate (0.1 N). In accordance with the usual precautions, an aqueous solution of potassium permanganate containing 3.2 g. per liter is prepared and standardized with sodium oxalate.
Standard Potassium Ferrocyanide Solution. Forty-two grams of reagent- grade potassium ferrocyanide trihydrate is dissolved in 500 ml. of distilled water, the solution is filtered and diluted to one liter. For standardization, 25 ml. is diluted with water to 500 ml., and 5 ml. of concentrated sulfuric acid is added. This solution is titrated with the standard potassium permanganate to a faint pink end point.
Standard Zinc Chloride Solution. Ten grams of pure metallic zinc is dissolved in a mixture of 150 ml. of concentrated hydrochloric acid and 300 ml. of water. Two hundred grams of ammonium chloride is dissolved in the solution, which is filtered and diluted to one liter. For standardization, 25 ml. of this solution and 10 ml. of the 10 percent potassium chloride reagent are added to ensure the formation of a precipitate of definite composition. This solution is heated to 90 °C. (without boiling) and is titrated with the standard potassium ferrocyanide until a drop of uranyl nitrate solution as an outside indicator on a spot plate is turned faintly brown. (See “Procedure” below).
Potassium Chloride Solution (10 percent). One hundred grams of potassium chloride is dissolved in 600 ml. of distilled water. The solution is filtered and diluted to one liter.
Uranyl Nitrate Solution. Three grams of uranyl nitrate, UO2(NO3)2·6H2O, is dissolved in 50 ml. of distilled water.
Ferrocyanide Assay Procedure
A sample of the crystalline salt or of a solution of ferrocyanide equivalent to about 30 g. of sodium ferrocyanide, measured accurately, is dissolved in distilled water to a volume of one liter. Into a 400-ml. beaker are pipetted exactly 25 ml. of the standard zinc chloride solution, 10 ml. of the 10 percent potassium chloride reagent, and 165 ml. of distilled water. This solution is heated to 90 °C. (without boiling) and titrated with the ferrocyanide solution to be analyzed, uranyl nitrate being used as an outside indicator on a spot plate. The ferrocyanide solution is added rapidly with constant stirring until within 1-4 ml. of the end point. (The zinc chloride reagent, bluish at first, turns almost white at the end point.) The addition is continued using 0.5 ml. portions with fifteen seconds’ stirring before the spot test is made. When the end point has been passed, 0.5 ml. of standard zinc chloride solution is added, after which the titration is continued with the dropwise addition of ferrocyanide and a test after each drop.
Since in each case 25.5 ml. of the standard zinc chloride solution is titrated, the quantity of ferrocyanide ion is the same for both the standard and the “unknown” solutions. Thus, the percentage of ferrocyanide ion may be calculated as follows:
% Ferrocyanide =
100 x (ml. standard required) x (molarity of standard) x 0.212/(ml. of “unknown” soln.) x (g. of solid in sample).