ANALYST, APRIL 1988, VOL. 113 573

Kinetic - Spectrophotometric Determination of Low Cyanide Concentrations in the Presence of Large Amounts of Thiocyanate

Francisco Javier Manuel de Villena Rueda, Luis Maria Polo Diez* and Rafael Perez Perez Department of Analytical Chemistry, Faculty of Chemistry, University Complutense -of Madrid, 28040- Madrid, Spain

A kinetic - spectrophotometric method to determine cyanide based on the reaction between CN- and Mn(CN)5N0*- ions with measurement of the absorbance at 384 nm is proposed. The optimum pH is 10.0. The effect of temperature is not critical, the activation energy of the reaction being 12.9 kcal mot-'. Using the method of tangents the range of linearity was 0.1-1.3 mg 1-1 of CN- with a coefficient of variation of 1.8% for n = 7 and 0.8 mg 1-1 of CN- and a detection limit of 0.06 mg 1-1 of CN-. By applying the fixed time method at 15 min, the coefficient of variation and detection limit were 1.0% and 0.04 mg 1-1, respectively. The thiocyanate ion does not interfere. Other important interferents in Aldridge's method, such as bromide, iodide and nitrite ions, are minimised. Good results were obtained for the determination of cyanide impurities in analytical-reagent grade potassium thiocyanate reagent, the mean recovery being about 98%. Keywords: Cyanide determination; kinetics; spectrophotometry

The methods most often used for the determination of smal 18-769 2 6 ~ ~ ~ ~ c 20.54 and N 28.59% [the theoretical =a1 amounts of cyanide are the modified Aldridge spectropho percentages for the KzMn(CN)sNO salt are: Mn 18.74, K K tometric methods. The original method was based on thc 26.67, 20.48 and N 28.66%1. A 0.5 M NH4CI - NH3 buffer er formation of cyanogen bromide and its subsequent reactioi at pH was also used. with benzidine in the presence of pyridine to form a highly coloured compound.1 The most important modifications of Sample this method involve the use of reagents such as pyrazolone,2 barbituric acid ,3 1,3-dimethyIbarbituric acid4 or p-phenylene- Analytical-reagent grade potassium thiocyanate (Prolabo) diamine5 instead of benzidine. These methods have been with a specified cyanide content lower than 0.010% was recommended as standards.69 but in all instances the time at analysed. A mass of 1.4232 g was dissolved in distilled water which colour is measured is fairly critical. Some of these and diluted to 50 ml to prepare the analytical solution (about methods have been automated.1@--'2 2.8 mg 1-1 of CN-).

The thiocyanate ion also undergoes Aldridge's reaction, being responsible for serious interferences (0.02 mg 1-1 of Procedure SCN- interferes in the determination of 0.2 mg 1-1 of CN-).4 Other important interferences are due to Br-, I-, S2O32-, In 25-ml flasks containing cyanide were placed 5 ml of the 1.66 S2-, BOTq-, NO2- and citrate ions..' Hence, previous X 10-3 M manganate(I1) complex stock solution and 5 ml of separation of cyanide, usually by distillation, is required, the ammonia buffer sohtion, making the volume up to the making the procedure very tedious. mark with distilled water. The final concentration of CN-

In this paper we propose a simple kinetic - spectrophoto- covered the range 0.1-1.3 mg 1-1. The absorbance at 384 nm, metric method to determine small amounts of cyanide in the against water as reference, was measured with time. Either the presence of large amounts of thiocyanate which avoids the initial rate Of the fixed time method was applied to the need for the distillation of cyanide. Other interferences such resulting kinetic curves. All solutions and cuvettes were as Br-, I-, NOz- and citrate are also minimised. thermostated at 25 "C.

Experimental Apparatus A Pye Unicam SP 8-200 spectrophotometer was used with 1 cm lidded and thermostated cuvettes. A Metrohm E 510 pH meter, equipped with a combined Metrohm AG 9100 elec- trode, was also used.

Reagents All chemicals used were of analytical-reagent grade.

An aqueous 2.10 x 10-3 M cyanide solution (54.7 mg 1-1 of CN-) was prepared from potassium cyanide (Merck) and standardised bv comdexometric titration .13 Less concen-

Results and Discussion Spectrophotometric Study of the Reaction Between Mn(CN)a02- and CN- The absorption spectrum of an aqueous Mn(CN)5N02- solution shows a characteristic band at 384 nm (E = 1.63 x lO31mol-1 cm-1) as shown in Fig. 1. In the presence of cyanide the intensity of this band diminishes with time. The resulting spectrum corresponds to that of the reduced form, Mn(CN)5N03-, showing two weak bands at 540 and 344 nm (E = 23 and 115 1 mol-1 cm-1) (Fig. 1).

To explain this transformation the following redox reaction was proposed:

trated solutiork weri prepared by suitable dilution. An aqueous 1.66 X 10-3 M pentacyanonitrosylmanganate(I1) stock solution was prepared from the potassium salt, which was synthesised by the method of Cotton et al. 14 An analysis of the product obtained gave the following composition: Mn

Mn(CN)5N02- + CN- + Mn(CN)SN03- + B(CN), which was confirmed by codometric titration.15 The reaction is Slow and this allowed a kinetic - spectropho- tometric method for the determination Of cyanide to be developed on the basis of the band at 384 nm. Both complexes also show bands in the UV region, but these were not sitable for analytical purposes. * To whom correspondence should be addressed.

*

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574 ANALYST, APRIL 1988, VOL. 113

Table 1. Analytical characteristics

Method

Fig. 1. plexes . Peaks:

1 0

8 + $

m

P

a 0.5

550 450 350 Wavelengthlnm

Absorption spectra of manganese nitrosyl - cyanide com- pH = 10.0 (NHZ - NH,); Mn(CN)5N02- = 8.70 x l o - 4 ~ . 1 = Mn(CN)5N02-; 2 = Mn(CN),N03-

8 0.50 m C e 51 9

I I 1 I 1

0 2 4 6 8 Time/min

Fig. 2. Kinetic curves of the CN- - Mn(CN)5N02- reaction. h = 384 nm; pH = 10.0 (NHZ - NH3); Mn(CN)5N02- = 3.32 x M. CN-: 1,O.ll; 2,0.33; 3, 0.55; 4, 0.88; 5, 1.09; and 6, 1.31 mgl-l

Thermodynamically the above reaction is favoured by increasing the pH up to about 10. This is because there is a decrease in the potential of the (CN)2 - CN- system up to pH 10, above which it remains constant. The potential of the Mn(CN)5N0*- - Mn(CN)5N03- system, in contrast, does not depend on pH.

Experimentally we have observed that the kinetics of the reaction are favoured at higher pH, the reaction rate being significant only in alkaline media. However, the rate of decomposition of the Mn" complex becomes appreciable above pH 11. Therefore, pH 10.0 was chosen as the optimum for the determination proposed under Experimental.

Kinetic Determination of Cyanide

Fixed time/min

Tangents 5 10 15 20 Rangeof linearity/mg 1-1 0.1-1.3 0.1-1.3 0.1-1.3 0.1-1.3 0.1-1.3 Detectionlimit*/mg 1-1 0.06 0.06 0.05 0.04 0.03 Coefficient of

variation?, Yo . . . . 1.8 1.9 1.5 1.0 0.8 * 3sdm, m being the slope of the calibration graph. ? For 0.80 mg 1-1 of CN- and n = 7.

Table 2. Recovery studies. Initial CN- concentration, 0.47 mg 1-1

CN-/mg 1-1

Added Found s,, YO Recovery, YO 0.30 0.75* 1 7 97 4 0.40 0.84 1.8 96.6 0.50 0.99 1.6 102.1 0.60 1.04 1.7 97.2 0.70 1.12 1.6 95.7

* Mean for n = 5.

k versus the reciprocal of the absolute temperature, the activation energy for the reaction was calculated to be 12.9 f 0.1 kcal mol-1. The reaction between the Mn" complex and cyanide obeys Arrhenius' law. The analytical charcteristics of the method are summarised in Table 1.

The fixed time method was also applied; its analytical characteristics are also summarised in Table 1. The range of linearity was practically the same for the two methods, even when different measurement times were used in the fixed time method. Regarding the sensitivity, the detection limits of the method of tangents and the fixed time method at 5min are similar. As expected, the detection limit was lower for increasing times of measurement in the fixed time method. The coefficient of variation for the fixed time method follows the same behaviour, the precision of the method of tangents being similar to that given by the fixed time method for shorter times.

On the other hand, when using the F test for (x = 0.05, no significant differences were found between Aldridge's stan- dard method and the kinetic method at 20 min as recommen- ded in the Aldridge method, for n = 7 and 0.80 mg 1-1 of CN-.

The effect of different species on the determination of 0.80 mg 1-1 of CN- was studied. Kirkbright's cnterionlh of +2s (equivalent to 0.015mgl-1 of CN- for n = 7) was applied. For the maximum molar ratio tested, viz., X : CN- = 500 : 1, the following species do not interfere: C032-, B033-,

Br-, tartrate, citrate and acetate ions. It must be emphasised that Br- interferes drastically in the modified Aldridge method,4 restricting the Br- : CN- molar ratio to lower than 0.2: 1. On the other hand, the proposed method allows the species I- and NOz- in ratios of 100: 1, in contrast to the modified Aldridge method,4 in which I- and NO2- ratios

C*O4'-, P043-, AS043-, As0y3-, SO42-, SCN-, F-, C1-,

order with respect 'to cyanide. The apparent reaction rate constant, k , was determined from the intercept on the ordinate axis, which gave a value of 1.86 x 10-2 min-1 mg-1 1

Determination of Cyanide in a Commercial Potassium Thio- 'yanate Reagent

(25 "C). This constas was also determined at other temperat- ures within the range 21-37 "C. From the Arrhenius plot of In

The procedure specified under Experimental using aqueous cyanide solutions to prepare the calibration graph and the

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ANALYST, APRIL 1988, VOL. 113 575

method of tangents was applied to a sample containing a thiocyanate : cyanide molar ratio of about 2700 : 1. From the results obtained for five determinations the average value was 0.47 mg 1-1 of CN-, equivalent to 0.0083% in the sample, the coefficient of variation being 1.8%. The accuracy was evaluated from recovery studies and the results are sum- marised in Table 2. The mean recovery was 97.8%, which indicates the validity of the proposed method for the determination of CN- in the presence of large amounts of SCN-.

Conclusions The proposed method allows the determination of low cyanide concentrations in the presence of large amounts of thiocyan- ate, which causes the most serious interference in Aldridge’s method. Moreover, other interferences are minimised.

The results obtained for the determination of cyanide under experimental conditions in which the SCN- : CN- ratio is very high show the usefulness of the proposed method.

1. 2. 3.

4.

References Aldridge, W. N., Analyst, 1944,69,262; 1945,70,474. Epstein, J., Anal. Chem., 1947, 19,272. Murty, G. V. L. N., and Viswanathan, T. S . , Anal. Chim. Acta, 1961,25,293. “Sistemas Modernos de Anilisis,” E. Merck, Darmstadt, 1984, p. 13.

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Bark, L. S., and Higson, H. G., Talanta, 1964, 11,471; 1964, 11, 621. Joint Committee of the Association of British Chemical Manufacturers and the Society for Analytical Chemistry, “Recommended Methods for the Analysis of Trade Effluents,” Heffers, Cambridge, for the Society for Analytical Chemistry, London, 1958. American Public Health Association, “Standard Methods for the Examination of Water and Wastewater,” Thirteenth Edition, Washington, DC, 1971, p. 397; Fifteenth Edition, Washington, DC, 1980, p. 312. Hanson, N, W., Editor, “Official Standardised and Recom- mended Methods of Analysis,” Second Edition, Society for Analytical Chemistry, London, 1973. American Society for Testing and Materials, “Annual Book ASTM Standards,” D2036-74, American Society for Testing and Materials, Philadelphia, PA, 1974, p.500. Casapieri, P., Scott, R., and Simpson, E. A., Anal. Chim. Acta, 1970,49, 188. Royer, J. L., Twichell, J. E., and Muir, S . M., Anal. Lett., 1973,6,619. Rios, A., Luque de Castro, M. D., and Valcarcel, M., Talanta, 1984,31,673. De Sousa, A., In. Quim. Anal., 1%1,15,61; 1962, 16, 126. Cotton, F. A., Monchamp, R. R., Henry, R. J. M., and Young, R. C., J. Inorg. Nucl. Chem., 1959, 10, 28. Manuel de Villena, F. J., PhD Thesis, Faculty of Chemistry, University Complutense, Madrid, 1987. Kirkbright, G. F., Talanta, 1966,13, 1.

Paper A7131 7 Received July 23rd, 1987

Accepted September 28th, 1987

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