Analyst, April, 1968, Vol. 93, j5p. 219-223 219 The Polarographic Determination of Some Dithiocarbamates and their Heavy Metal Complexes BY D. J. HALLS, A. TOWNSHEND AND P. ZUMAN* (Department of Chemistry, University of Birmingham, P.O. Box 363, Birmingham 16) Procedures for the determination of monoalkyl- and dialkyldithio- carbamates and some of their metal complexes that are used as pesticides are described. The methods are based on a detailed investigation of the polarographic behaviour of these compounds, and are suitable for the analysis of pesticide preparations and residues. DITHIOCARBAMATES, I, 11, have many uses; they are widely used as pesticides, especially as their metal complexes, and are applied in the rubber industry as vulcanisation accelerators and anti-oxidants. This latter property also makes them useful additives for oils and greases.In each instance, accurate determination of the dithiocarbamates is essential. Most of the analytical methods in general use are based on the Clarke method,l in which the dithiocarbamate is destroyed in acidic solution to give carbon disulphide. The latter is absorbed in methanolic potash, and the potassium methyl xanthate so formed is titrated iodimetrically. The conditions for this determination appear to be quite critical, especially for some ethylenebisdithiocarbamate complexes, but standard methods are based on this pr~cedure.~ s3 Dithiocarbamate pesticide residues, however, have been analysed* by determining the released carbon disulphide spectrophotometrically. Paper chromatography and infrared spectroscopy7 have also been suggested for the determination of dithiocarba- mates.Polarography has proved to be useful in pesticide analysis, both for the analysis of pesticide preparations and for the determination of residuesQ Furthermore, it has been applied to the determination of the alkali dithiocarbamates13~~~ and to the dithiocarb- amate pesticides.15Js Nangniot16,17sl* used the hanging mercury drop to improve the sensitivity of the determination. The polarography of dithiocarbamates has also been used for the determination of carbon disulphide19 and amino-acids.20 The present study is part of a more general investigation of the polarographic behaviour of dithiocarbamates. This paper describes the polarographic properties of simple mono- alkyldithiocarbamates, I, dialkyldithiocarbamates , 11, and ethylenebisdithiocarbamates, 111, that are relevant to their polarographic determination.A more detailed report of their general polarographic behaviour is given elsewhere.21 On the basis of these results, methods are devised for the polarographic determination of these compounds and some of the metal complexes that are used as pesticides viz.- NABAM (disodium ethylenebisdithiocarbamate) ZINEB (zinc ethylenebisdithiocarbamate) MANEB (manganese ethylenebisdithiocarbamate) POLAROGRAPHIC STUDIES- All simple dithiocarbamates give anodic waves. It was originally suggested22 that these were caused by a reversible couple with the corresponding disulphides, but it was later re~ognised~~ that they arose from mercury compound formation.Recent papers on the polarographic determination of diethyldithio~arbamate~~,~~ reported irregularities in the shape and concentration dependence of the anodic waves, but gave no explanation for this. * On leave from J. Heyrovsk? Institute of Polarography, Czechoslovak Academy of Sciences, Prague. 0 SAC and the authors.220 HALLS, TOWNSHEND AND ZUMAN : POLAROGRAPHIC DETERMINATION [Analyst, Vol. 93 R S R S \ / / \ N-C H S- Ia R=CH, Ib R = C2Hs IC R = C,H, \N4f R/ 's- IIa R = C2H6 IIb R = CH2C,Hs S S / \ \CLNH--CH2-CH,--NH-C # -s I11 S- At lower pH values monoalkyldithiocarbamates, I, show two waves. They merge in alkaline media. The waves are accompanied by an adsorption pre-wave at the lower pH values. The total wave height over the whole pH range studied (pH 1 to 13), and the height of the single, well developed wave in 0-1 N sodium hydroxide solution are linearly proportional to the concentration of the monoalkyl dithiocarbamates, I, and are suitable for analytical applications. The waves of the dialkyldithiocarbamates, 11, above pH 6.8 were complicated by an increase in the limiting current caused by adsorption of a film of the mercury compound, and by its desorption accompanied by streaming of the solution.21 These cause a round- shaped increase in the limiting current and a non-linear dependence of the total wave height on concentration.The height increases with increasing concentration of the dithiocarbamate more than is expected for a linear wave height - concentration relationship.Addition of gelatin or the use of 60 per cent. v/v ethanol - water, 0.1 N in sodium hydroxide, eliminated this complication and a linear wave height - concentration plot was found under such con- ditions. The anodic wave of diethyldithiocarbamate was accompanied by the usual adsorption pre-wave. The polarographic behaviour of disodium ethylenebisdithiocarbamate (111, NABAM) in aqueous solutions was complicated by adsorption phenomena that caused the wave to be ill-separated and difficult to measure. Well developed waves , suitable for analytical purposes were found in 0-1 N sodium hydroxide solution in 90 per cent. ethanol - water, or 50 per cent. dimethylformamide - water. Even under these conditions the wave was accompanied by an adsorption pre-wave, but the total wave height was proportional to the concentration of NABAM.TABLE I COMPOSITION OF DITHIOCARBAMATE COMPLEXES IN 0.1 N SODIUM HYDROXIDE Molar ratio of metal to dithiocarbamate S // \ H S- C2H6 S- // \ / \ C2Hs S S CH,-NH-C S- S caH\ // N-C H2-NH-4 S- / ' 1 \N-c// CdII 1:2* 1:2 1:l and2:l PbII 1:2* 1:2 l : l a n d 2 : 1 HgII 1:2* 1:2 1 : 1 and 2 : 1 No complex * Formation of 1 : 1 complex possible; formation of cadmium sulphide observed. 2::; E;} The addition of metal ions to the solutions of dithiocarbamates and recording of the anodic wave of dithiocarbamates and the cathodic waves of the metal ions and soluble metal complex make it possible to determine the stoicheiometric ratio in which metal ions reactApril, 19681 OF SOME DITHIOCARBAMATES AND THEIR HEAVY METAL COMPLEXES 221 with dithiocarbamates, and to elucidate the composition of the compounds formed.These are summarised in Table I for 0-1 N sodium hydroxide solution. The compounds formed by the reaction of mono- and dialkyldithiocarbamates are insoluble, hence in these instances only a decrease of the anodic wave of the dithiocarbamates was observed. However, if the metal ion formed soluble anionic species (e.g., plumbate and zincate) in alkaline media, an excess of the metal ion would give a cathodic reduction wave. In those instances where no reaction was observed (CoII, MnII, Zn, Ni) the anodic wave was unchanged. The reaction of lead with diethyldithiocarbamate gave the expected PbL, complex, but this was partly soluble and gave a cathodic wave at -0-80 volt veysus S.C.E.The complexes formed between 1 mole each of NABAM, 111, and metal ion in dilute sodium hydroxide solutions were soluble. Thus the decrease in the anodic wave of NABAM was accompanied by a rise in the cathodic wave of the complex. However, the metal- (NABAM) , complexes were hardly soluble, so that this cathodic wave decreased on the addition of more ligand. Manganese and zinc did not complex with NABAM in 0-1 N sodium hydroxide solution. Analytical methods for MANEB and ZINEB were based on these observations. For analytical purposes, 0.1 N sodium hydroxide solution containing 0 to 90 per cent. of ethanol, depending on the compound studied, proved to be the best supporting electrolyte. Under these conditions, the dithiocarbamate anion is liberated and the metal ion is trans- formed into a soluble complex or an insoluble oxide or hydroxide.Hence an addition of MANEB to the alkaline solution gave an anodic dithiocarbamate wave that was identical in height, shape and half-wave potential with a NABAM wave obtained under identical conditions, and manganese dioxide was precipitated. An analytical method for MANEB based on these observations would not suffer from the uncertain stoicheiometry it shows in the Clarke method. Furthermore, it was established that the sensitivity of the polarographic method was just sufficient to allow the determination of dithiocarbamate residues, particularly MANEB. For greater sensitivity, the hanging drop method could be used.16 Finally, polarography can be used in the study of the kinetics of acid decomposition of dithiocarbamates by measuring the height of the anodic dithiocarbamate wave.It was found25 that the decomposition is that of the conjugate acid form, IV, of the dithiocarbamate, and the elimination - addition reaction is accompanied by two acid - base equilibria- R1 S R1 S R1 S ‘NAY / \ R’ S- w H+ IV R1 ‘NH + CS2 / R d R’ R’ K2 \H + H+ .. .. .. (3) \ / NH,+ / a 7 Ra RP In acidic media equilibrium (1) is shifted to the left and cleavage with rate constant k, takes place. Because equilibrium (3) is shifted to the left-hand side, the reaction with rate constant k-, cannot take place. Hence in acidic media at pH 5 (pK, + l), irreversible cleavage of dithiocarbamates takes place and condensation does not occur. In alkaline media, equilibrium (3) is shifted to the right-hand side.Formation of dithiocarbamates from carbon disulphide and amine with constant k-, takes place. Because equilibrium (1) is shifted to the right-hand side, the protonised form, which is the primary product, is transformed into an inactive conjugate base (dithiocarbamate anion). Hence in alkaline media at pH 2 (pK, - l), irreversible condensation of amines with carbon disulphide222 HALLS, TOWNSHEND AND ZUMAN: POLAROGRAPHIC DETERMINATION [AfidJW!, VOl. 93 takes place and the dithiocarbamate anions are stable and do not undergo cleavage. Thus, the determination of carbon disulphide or simple amines based on the formation of dithio- carbamates is best carried out under these conditions. In this way, it is possible to explain the greater stability of monoalkyldithiocarbamates in acidic solutions when compared with dialkyldithiocarbamates ; this is predominantly caused by the fact that the acidity of the protonated form, IV, is greater for mono- than for dialkyldithiocarbamates.By a suitable choice of pH of about 3-5 to 5, monoalkyldithio- carbamates can be determined in samples that originally contained dialkyldithiocarbamates because of the rapid decomposition of the latter under these conditions. Conversely, dialkyl- dithiocarbarnates are, unlike the monothiocarbomates, resistant to alkaline decomposition, so that after heating a mixture of these compounds in 2 N sodium hydroxide solution for 2 to 3 hours only the dithiocarbamate remains. Pre- liminary experiments indicated two successive reactions.Moreover, the carbon disulphide formed in this reaction can be determined polarographically. 1. DETERMINATION OF A MONOALKYLDITHIOCARBAMATE- in the range 5 x anodic wave height. Evaluate the results from a calibration curve. measurements. It would be possible to follow the acid cleavage of NABAM in a similar manner. RECOMMENDED ANALYTICAL PROCEDURES Dissolve the sample in 0-1 N sodium hydroxide solution to obtain a final concentration to 1 0 - S ~ . Record the polarographic wave, and measure the total NOTE-The solution should not stand for more than 30 minutes before carrying out the polarographic 2. DETERMINATION OF A DIALKYLDITHIOCARBAMATE- Dissolve the sample in 0.1 N sodium hydroxide solution, either 60 per cent.in ethanol or containing 0.02 per cent. of gelatin. The final dithiocarbamate concentration should be in the range 5 x 1 O A 5 ~ to 8 x 1 0 d 4 ~ . Record the polarographic curve, and measure the total anodic wave height. Evaluate the results from a calibration curve. 3. DETERMINATION OF A MONOALKYLDITHIOCARBAMATE IN THE PRESENCE OF A SIMPLE Record the anodic waves after this solution has stood for 20 to 30 minutes (in which time the dialkyl derivative will have decomposed completely) and the solution has been purged with oxygen-free nitrogen. Complete the determination as in 1. 4. DETERMINATION OF A DIALKYLDITHIOCARBAMATE IN THE PRESENCE OF A SIMPLE MONO- Dissolve the sample in 2 N sodium hydroxide solution, and heat the solution for 2 to 3 hours at 70" C (so that the monoalkyl derivative is completely decomposed).Purge the solution with oxygen-free nitrogen. This removes isothiocyanate formed by the decom- position of the monoalkyldithiocarbamate that is capable of forming monoalkylmonothio- carbamate, which gives anodic waves similar to the dithiocarbamates. It also prevents oxidation. Cool the solution, and complete the determination as in 2. A calibration curve should be prepared by taking the standards through the whole procedure. DIALKYLDITHIOCARBAMATE- Dissolve the sample in a buffer solution of pH 3.5 to 5. ALKYLDITHIOCARBAMATE- 5. DETERMINATION OF DISODIUM ETHYLENEBISDITHIOCARBAMATE (NABAM)- Dissolve the sample in 0.1 N sodium hydroxide solution in 90 per cent. ethanol to give a final NABAM concentration of 1 x M to 5 x M, or in an aqueous 0.1 N sodium hydroxide solution if the final NABAM concentration is to be higher. Record the anodic waves and measure the total wave height.Evaluate by using a calibration curve obtained from experiments in the same medium. 6. DETERMINATION OF ZINC ETHYLENEBISDITHIOCARBAMATE (2INEB)- waves. from NABAM or ZINEB. Dissolve the sample in aqueous 1-0 N sodium hydroxide solution and record the anodic Measure the total wave height, and evaluate from a calibration curve preparedApril, 19681 OF SOME DITHIOCARBAMATES AND THEIR HEAVY METAL COMPLEXES 223 For the determination of ZINEB residues (0-3 to 3 p.p.m. of wet weight of vegetable matter), the dithiocarbamate should be extracted from the host material (1OOg) by homo- genising with 10 ml of N sodium hydroxide solution. A calibration curve should be prepared by adding standard amounts of NABAM or ZINEB to the host material.7. DETERMINATION OF MANGANESE ETHYLENEBISDITHIOCARBAMATE (MANEB)- The procedure is the same as that for ZINEB; 3 x M to 3 x lo-* M MANEB can be determined. All the anodic waves appear in the potential range between -1.0 and -0.2 volt, and were recorded from negative to positive potentials. The reproducibility was that usual for polarographic determinations, i e . , +2 to 4 per cent. in pure solutions and +5 to 15 per cent. in the presence of biological material. We thank Professor R. Belcher for his interest and encouragement, and Mr. A. Stevenson and Mr. M. J. V. Wayman of Robinson Brothers Ltd., West Bromwich, for samples of NABAM, MANEB and ZINEB.D. J. H. also thanks Professor M. Stacey for the provision of a research grant, and P. 2. thanks the Science Research Council for a Senior Visiting Fellow- ship and the provision of a polarograph. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 13. 14. 15. 16. 17. 18. 19. 20. 21. 32. 23. 24. 25. REFERENCES Clarke, D. G., Baum, H., Stanley, E. L., and Hester, W. F., Analyt. Claem., 1951, 23, 1842. Stevenson, A., J . Sci. Fd Agric., 1964, 15, 509. Bontogan, W. R., J . Ass. 08. Agric. Chem., 1965, 48, 562. Cullen, T. E., Anajyt. Chem., 1964, 36, 221. Henermann, R. F., J . Ass. Ojf. Agric. Chem., 1957, 40, 264. McKinley, W. 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C., and Tyler, W. P., J . Amev. Chem. Soc., 1960, 72, 4561. Zuman, P., Zumanovb, R., and SouEek, B., Chemickd Listy, 1953, 47, 1522. Stricks, W., and Chakravarti, S. K., Analyt. Chem., 1962, 34, 508. Zahradnik, R., and Zuman, P., Colln Czech. Chem. Commun., 1959, 24, 1132. -, Ibid., 1964, 9, 39. -, Ibid., 1960, 28, 373. -, Ibid., 1960, 28, 381. Received October 12th, 1967