ANALYST, JANUARY 1986, VOL. 111 11 Indirect Determination of Cyanide in Waste Waters by Formaldehyde Chronopotentiometry Jesus Rodriguez Procopio, Jose-Maria Pinilla Macias and Lucas Hernandez Hernandez* Facultad de Ciencias, Departamento de Quimica Analitica, Universidad Autonoma de Madrid, 28049 Madrid, Spain An indirect method for the determination of cyanide at concentrations between 0.01 and 0.1 pg ml-1 is described. The method is based on the displacement observed in the quarter-wave potential of the chronopotentiometric graph of formaldehyde, in 0.1 M NaOH using a gold electrode, on increasing the concentration of cyanide. The method exhibits a relative error of 2% and a relative standard deviation of 1%. It has been applied to the determination of cyanide in waste waters.Keywords: Cyanide determination; chronopotentiometry; waste waters Previously1 we have studied the chronopotentiometry of formaldehyde with a gold electrode in a basic medium and the effects on the potential- time graph of species that are adsorbed on gold, including cyanide, bromide and tetraethyl- ammonium. It was observed that increasing the concentration of cyanide, produced a displacement of the chronopoten- tiometric graph of formaldehyde to a less negative potential. Van Effen and Evans2 examined this effect in studies of the cyclic voltammetry of benzaldehyde with a gold electrode, observing a displacement of peak potential and a decrease in peak height with increasing the concentration of cyanide. This peak disappeared when the cyanide concentration reached 10-4 M.The concentration of cyanide present in solution necessary to cause a displacement of the potential - time graph was very small. The aim of this work was to develop an indirect method for the determination of cyanide at concentrations less than 0.1 pg ml-l using the chronopotentiometric behaviour of formaldehyde. The optimum formaldehyde concentration and pH have been studied and a calibration graph was constructed and the precision and accuracy of method were established. The method was applied to the determination of cyanide in waste waters. Experimental Apparatus A constant-current coulostat (Amel 381) was used as the current supplier and the chronopotentiograms were recorded on a potentiometric recorder (Mettler DKlO/GA13). A gold electrode with a projected area of 0.785 cm2 was inlaid in a PTFE tube, which provided a shield around the electrode surface with an upward orientation for obtaining a greater stability of id/C (where i is the applied current, t the transition time and C the bulk concentration).The contact was established by pushing a copper wire, protected by a Pyrex tube, against the rear side of the gold disc. This contact was replaced periodically. The counter electrode was a platinum wire introduced into a compartment separated from the main cell by a sintered-glass disc. This compartment was filled with a portion of the solution under investigation. The reference electrode was a saturated calomel electrode. A coulostat (Metrohm E 524) was used to supply a constant potential of -1.4 V in order to obtain an electrode with an oxide-free surface.The temperature was fixed at 25 rt 0.1 "C with a Tamson Tee 3/150 thermostat. A suitable distillation apparatus, des- cribed previously,3 was used to isolate the cyanide ion from the sample. ~~ ~~~ ~ * To whom correspondence should be addressed. Reagents All chemicals were of analytical-reagent grade. Formaldehyde solutions were prepared from formaldehyde solution (Merck) without further purification and titrated by the Tollens method.4 The presence of 10% of methanol as a stabiliser in commercial formaldehyde did not interfere in the measure- ments. Cyanide working standard solutions were prepared by accurate dilution of a 100 pg ml-1 NaCN stock solution standardised by a silver titration. Procedure From 500 ml of sample, made free of sulphide by the addition of a cadmium salt, cyanide was distilled into 50 ml of 1.0 M NaOH solution.375 The contents of the alkaline trap were transferred into a 100-ml calibrated flask and diluted to the mark.Next, 20 ml of this solution and 5 ml of 0.01 M formaldehyde solution were pipetted into a 100-ml calibrated flask and made up to the mark with cyanide-free doubly distilled water. (If the total cyanide content was expected to be between 0.1 and 1.0 mg ml-1 only 2 ml of cyanide solution were pipetted and 9 ml of 1.0 M NaOH solution were added to the calibrated flask before addition of the formaldehyde.) The solution obtained was transferred into the cell and a constant potential of -1.4 V vs. S.C.E. was applied to the working electrode for 5 min and at the same time a stream of nitrogen was passed through the solution.The potential was then removed and the nitrogen was passed through the shield of the electrode for 1 min. Between 1 and 2 rnin were allowed for the solution to become quiescent and then a constant current was applied to the working electrode in order to obtain a transition time of 30 s. The chronopotentiogram was recorded and the quarter-wave potential was determined. The measurements were made in duplicate. For the preparation of the calibration graph, a 0.0-10.0 ml volume of a cyanide working standard solution (1 pg ml-I), 10 ml of 1.0 M NaOH solution and 5 ml of 0.01 M formaldehyde solution were pipetted into a 100-ml calibrated flask and the resulting solution was made up to the mark with distilled water.The procedure was continued as for a sample, measurements being taken in duplicate. A new calibration graph was plotted periodically, because it is dependent on the condition of the electrode surface. Results and Discussion Variation of the Quarter-wave Potential with Cyanide Concen- tration A study of quarter-wave potential variation, ET,4, as a function of cyanide concentration was made. The results are shown in12 ANALYST, JANUARY 1986, VOL. 111 . -0.4 - u! s d $ -0.3 - up -0.2 } \ 0.2 0.4 0.6 0.8 Cyanide concentration/pg ml-1 I -0.4 4 c! v) -0.2 d 5 0.0 0.2 10 s - Fig. 1. Variation of uarter-wave otential with cyanide concentra- tion in 0.54 mM formjdehyde and l.l M NaOH solution Time - -0.6 4 c! m 2 -0.4 a u; -0.2 I I I I 12 12.5 13 13.5 PH Fig.2. Variation of Na2C03 - NaH2P04 bufqer and 0.54 mM formaldehyde solution uarter-wave potential with pH in 0.1 M Table 1. Accuracy and precision for the determination of cyanide. Each result is the average of ten determinations Cyanide concentratiodyg ml-1 ~ ~~ Taken Found 0.019 0.020 0.037 0.039 0.056 0.054 0.075 0.073 0.093 0.094 Relative Relative standard error, YO deviation, % 1.61 0.81 1.80 1.05 1.25 1 .oo 0.46 0.89 0.61 0.42 Fig. 1. Two zones were observed for which ET,4 was linear with cyanide concentration, the first at concentrations between 0.01 and 0.1 pg ml-1 and the second at concentrations between 0.1 and 0.5 pg ml-1. The more sensitive first zone provides a method for the determination of cyanide at concentrations of less than 0.1 pg ml-1.Effect of Formaldehyde Concentration To determine the optimum formaldehyde concentration, a study of the minimum cyanide concentration that produces an observable displacement of potential - time graphs for several formaldehyde concentrations was made. From the results, it was concluded that the optimum formaldehyde concentration was between 0.4 and 0.6 mM. At this concentration it was Fig. 3. Typical chronopotentio rams of standards of NaCN in 0.1 M NaOH and 0.54 mM formaldehyfie solution. [NaCN]: (1) 0; (2) 0.019; (3) 0.037; (4) 0.056; (5) 0.075 and (6) 0.093 pg ml-1 Table 2. Comparison of the results of the determination of cyanide by a direct spectrophotometric method and indirectly by formaldehyde chronopotentiometry (IDFC) Cyanide found/pg ml-'* Sample Spectrophotometry IDFC I t 2 t 3$ 4$ 5$ 0.02 0.03 0.16 0.05 0.11 * Each result is the average of ten determinations. t River water (polluted by industrial effluents).$ Precious metal refining waste water. 0.01 0.03 0.17 0.05 0.11 possible to determine cyanide at concentrations down to 0.01 pg ml-1. A pre-treatment time of 5 min was chosen to obtain a constant potential of -1.4 V vs. S.C.E., in order to produce an oxide-free electrode surface, which gives a better repro- ducibility in transition times and quarter-wave potentials, as was observed in the chronopotentiometric study. 1 Also a greater constancy for EZI4 was attained on using a current intensity that produced a transition time of about 30 s, decreasing the double-layer capacity and convection prob- lems.Effect of pH on the Sensitivity A greater sensitivity was observed when the quarter-wave potential of formaldehyde alone was more negative. Fig. 2 shows the variation of quarter-wave potential with pH, in 0.1 M Na2C03 - NaH2P04 buffer solution. On increasing the pH, the quarter-wave potential was shifted to more negative potential. These results indicate a greater sensitivity at a pH greater than 13. Also a study of the reproducibility of Et/4 was performed at different pH values. It was observed that a greater reproduci- bility can be obtained at a pH higher than 13. From the results a 0.1 M NaOH solution was selected as the supporting electrolyte.ANALYST, JANUARY 1986, VOL. 111 13 Calibration Graph, Accuracy and Precision A calibration graph of quarter-wave potential versus cyanide concentration was obtained for concentrations between 0.01 and 0.1 pg ml-1 (the chronopotentiograms obtained are shown in Fig.3). A series of ten solutions of cyanide concentrations of 0.014.1 pg ml-1 were prepared and their concentrations were determined using the calibration graph. The results obtained are given in Table 1. The results indicate that the proposed method is accurate, with a relative error of less than 2% and a precision, expressed as the relative standard deviation, of less than or equal to 1%. Determination of Cyanide in the Presence of Interferents Determinations of cyanide in the presence of different amounts of chloride, sulphide, sulphate, carbonate and phosphate were made. The cyanide concentration was 0.05 pg ml-l.The maximum tolerable concentrations of these interferents were 7 pg ml-1 and 0.02 pg mi-1 for chloride and sulphide, respectively, and 0.1 M for the others. At higher concentrations, these substances caused changes in the shape of the potential - time graph, producing positive errors. Determination of Cyanide in Waste Waters The proposed indirect method was applied to the determi- nation of cyanide in waste waters. Spectrophotometry is widely applied for the determination of cyanide in waste waters and is usually based on the reaction of cyanide with chloramine-T to form cyanogen chloride, which combines with pyridine and a cyclic amine to form a dye.375>6 Potentiometry with a cyanide-selective electrode is also used.5.6 Both methods have a similar sensitivity and precision with a relative error and relative standard deviation of less than 2%.”10 For the proposed method the sample is made highly acidic with sulphuric acid, and heated under reflux while bubbling air through the solution.The hydrogen cyanide evolved is absorbed in a 1.0 M NaOH solution; this separates cyanide from any interfering substances.3.5 Under these conditions only sulphide interferes. This can be eliminated by adding a cadmium salt before cyanide separation . 3 3 The results obtained for the determination of the cyanide concentration in several waste waters are given in Table 2. These values are in agreement with those obtained by the benzidine - pyridine method,3 having similar precision, ranging between 2 and 5% (owing to the recovery efficiency).In this way the determination of cyanide using the chrono- potentiometric method gives results comparable to those obtained by the standard method. The method is simple and the analysis time is approximately the same as that of the spectrophotometric and cyanide-selective electrode methods. However, there is no need to prepare and store several reagent solutions as in the spectrophotometric method because the proposed method only requires inexpensive 10-2 M formaldehyde solution. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. References Procopio, J. R., PhD Thesis, Facultad de Ciencias, Universi- dad Autdnoma de Madrid, 1985. Van Effen, R. M., and Evans, D. H., J. Electroanal. Chem., 1980, 107, 405. American Public Health Association, American Water Works Association and Water Pollution Control Federation, “Stan- dard Methods for the Examination of Water and Waste- waters,” Fifteenth Edition, American Public Health Associa- tion, New York, 1980. Kolthoff, I. M., and Elving, P. J., “Treatise on Analytical Chemistry, Part 11,” Volume 13, Interscience, New York, 1966. Csikai, N. J., and Barnard, A. J., Anal. Chem., 1983,55,1677. Pohlandt, C . , Jones, E. A., and Lee, A. F., J. S. Afr. Znst. Min. Metall., 1983, 83, 11. Aldridge, W. N., Analyst, 1945, 70, 474. Nagashima, S., Anal. Chim. Acta, 1978, 99, 197. Murty, G. V. L. N., andviswanathan, T. S.,Anal. Chim. Acta, 1961,25, 293. Cussbert, P. J., Anal. Chim. Acta, 1976, 87, 429. Paper A51215 Received June 17th, 1985 Accepted August 12th, 1985