2086
Anal. Chem. 1983, 55, 2086-2089
Effect of Thiocyanate on the Pyridine-Pyrazolone Method for the Spectrophotometric Determination of Cyanide Shigeru Nagashima Department of Chemistry, Tokyo Metropolitan Industrial Technical Institute, Nishigaoka, Kita-ku, Tokyo 115, Japan
The rate of reaction between thiocyanate and chloramine-T varied with the pH of the solution, which gave a complicated pH dependence, i.e., it was rapid in the acidic and weak alkaline regions (around pH 5 and 8) and was slow in the neutral and alkaline regions (around pH 7 and 9). The effect of thlocyanate on the pyridine-pyraroione method was Interpreted on the basis of the results and pH conditions at the determination of cyanide. The reaction between cyanide and chloramine-T was fast and independent of the pH of the solution over a wide pH range. The difference between both reaction rates was applied to the simultaneous determination of cyanide and thlocyanate.
It is well-known that thiocyanate causes a significant positive error in the spectrophotometric determination of , as the pyricyanide based on the Konig reaction ( l ) such dine-pyrazolone method (2), the pyridine-barbituric acid method (3), the pyridine-benzidine method ( 4 ) , and the pyridine-p-phenylenediamine method (5). Epstein (2),who proposed the pyridinepyrazolone method, observed the thiocyanate error and found that the reaction between thiocyanate and chloramine-T was slow and that it was accelerated by adding iron(II1) chloride. Asmus and Garschagen (3)determined thiocyanate with iron(II1) chloride solution, chloramine-T solution, and pyridine-barbituric acid reagent. However, Ludzack et al. (6),who studied the determination of cyanide by using pyridine-benzidine reagent and pyridine-pyrazolone reagent, stated that the effect of thiocyanate on the pyridine-pyrazolone method did not present a consistent pattern, although they also used iron(II1). Recently, spectrophotometric methods for the determination of cyanide by use of barbituric acid and pyrazolone reagents without pyridine were studied by the present author (7-9). In the study (9),it was pointed out that the methods, including techniques by use of reagents containing pyridine, were divided into two types in the extent of thiocyanate error, i.e., one type giving a large positive error, the pyridine-barbituric acid method (3), the isonicotinic acid-barbituric acid method (8, 9), the pyridine-benzidine method ( 4 ) ,and the pyridine-p-phenylenediamine method (5),and the other giving a less positive error, the pyridine-pyrazolone method (2) and the isonicotinic acid-pyrazolone method (9). (Of these, the benzidine method and the p-phenylenediamine method are not discussed in this paper, since bromine water and not chloramine-T solution is used for oxidation of cyanide.) The reason the effect of thiocyanate on the methods was studied was because it was found that the rate of reaction between thiocyanate and chloramine-T varied greatly with the pH of the solution (i.e., an acid-base catalyzed reaction). On the other hand, the reaction between cyanide and chloramine-?' was fast and independent of the pH of the solution over a wide pH range. The difference between both reaction rates was applied to the simultaneous determination of cyanide and thiocyanate. The procedure adopted to monitor the reactions was similar to that given in ref 10 and 11. In this
paper, the results obtained with pyridine-pyrazolone reagent are described.
EXPERIMENTAL SECTION Reagents. Standard Cyanide Solution. Dissolve 2.51 g of KCN in water and dilute to 1 L (1000 mg of CN-/L). Prepare working solutions by dilution with water. Standard Thiocyanate Solution. Dissolve 1.68 g of KSCN in water and dilute to 1 L (1000 mg of SCN-/L). Prepare working solutions by dilution with water. Buffer Solutions. Prepare them by mixing 0.10 M KHzP04 and 0.10 M Na2HP04in various ratios. Pyridine-Pyrazolone Reagent. Dissolve 300 mg of pyrazolone and 25 mg of bis(pyrazo1one) in 25 mL of pyridine and dilute to 150 mL with water (12). Prepare daily. Chloramine-T Solution (1% w / v ) . Prepare daily. General Procedure. In a dried volumetric flask (50 mL), place 20.0 mL of sample solution containing cyanide and/or thiocyanate. Add 4.0 mL of buffer solution and mix thoroughly. Add 0.5 mL of chloramine-?' solution, stopper the flask, and shake gently. Keep the solution at 25 A 0.1 "C for a fixed time (1-30 min). Add 10.0 mL of pyridine-pyrazolone reagent, dilute to 50 mL with water, stopper the flask again, and mix. Keep the mixture at 25 "C for 30-40 min. Measure the absorbance at 620 nm against a reagent blank with a 10-mm cell. RESULTS AND DISCUSSION Effect of pH on the Reactions of Cyanide and Thiocyanate with Chloramine-T. The overall reactions producing cyanogen chloride may be written as follows:
-+ -+
CH3C6H4S02NC1Na+ CN- + 2H20 CH3CsH4S02NH2+ CNCl
Na+ + 20H-
4CH3C6H4S02NC1Na+ SCN- + 4H20 4CH3C6H4SO2NH2+ CNCl + 4Na+
SO4'-
+ 3C1-
In the pyridine-pyrazolone method, the measured absorbance corresponds to the amount of cyanogen chloride produced in the reactions. The effect of pH was examined by varying the buffer solution (0.10 M phosphate solution) and by varying the standing time after the addition of chloramine-?' solution, where the pH was denoted as the mixed solution containing the sample and the buffer solution. (The pH hardly changed by addition of the chloramine-?' solution.) Results in Figure 1 were obtained for 16.0 wg of SCN-/20 mL. The rate of reaction between thiocyanate and chloramine-?' varied greatly with the pH of the solution. The plot of absorbance vs. pH as for the data in Figure 1 revealed a diagram having a minimum and a maximum (Figure 2); where the pH of the final solutions was 7.1-8.0, as shown by numerals in parentheses in Figure 2. On the other hand, the reaction between cyanide and chloramine-T was fast and independent of the pH of the solution over the range of pH 4.7-9.1 (Figure 2).
Effect of Concentration of the Phosphate Buffer Solution. The reaction of thiocyanate with chloramine-T was much dependent on the pH of the solution, and the effect of concentration of the buffer solution was then examined
0003-2700/83/0355-2086$01.50/0 0 1983 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
2087
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