JULY, 1968 THE ANALYST Vol. 93, No. I108 Amperometric Titration of Copper and Cadmium Presence of Zinc, Cobalt and Nickel with Sodium Die th yldit hiocar bamate in the BY A. BROOKES AND A. TOWNSHEND (Chemistry Departmewt, Birmingham University, P.O. Box 363, Birmingham 15) The reaction of some metal ions with diethyldithiocarbamate in 0.1 M sodium hydroxide solution is investigated polarographically. Methods are presented for the amperometric titration of cadmium in the presence of zinc and cobalt, and of cadmium and copper in admixture. MANY metals, including copper, cadmium and zinc, have been titrated amperometrically with sodium diethyldithiocarbamate.1 However, because numerous metal ions react with this ligand, the use of masking agents or pH control is necessary to achieve selectivity.For instance, copper(I1) can be titrated in the presence of zinc, nickel and iron when EDTA is present2 or in the presence of manganese(I1) and chromium(II1) when tartrate is added.3 Copper can be titrated in the presence of zinc in ammoniacal solution at pH 11, and the zinc can subsequently be titrated after acidification to pH 6 with acetic acid.* Other examples are also kn0wn.l It has been shown5 that zinc, manganese(II), nickel and cobalt do not complex with disodium ethylene-l,2-bisdithiocarbamate in 0.1 N sodium hydroxide solution, whereas copper(II), cadmium and lead do form complexes. Thus, under these conditions, it would be possible to titrate copper or cadmium amperometrically in the presence of the non- complexing metals by using sodium ethylene-l,2-bisdithiocarbamate without adding a masking agent.However, aqueous solutions of this reagent are not stable for more than a few hours,6 so sodium diethyldithiocarbamate was investigated as an alternative titrant. Solutions of this compound in 0-01 M sodium hydroxide solution are stable for at least 3 days6 M) did not complex with diethyldithiocarbamate in 0.05 M sodium hydroxide solution. Addition of titrant to the metal-ion solution immediately produced an anodic wave of the ligand. Copper(I1) gave the expected 2 : 1 ligand-to-metal complex. No anodic wave appeared until this ratio had exceeded 2: 1. M) was titrated, the first increment of titrant produced a small anodic wave, but the height of this wave hardly increased until the ligand-to-metal ratio was 2: 1.A smaller concentration of nickel (lo-* M), however, showed no evidence of complexing with diethyldithiocarbamate in 0.1 M sodium hydroxide solution. When ligand was added to a cadmium solution, the anodic wave of the ligand appeared when the ligand-to-metal ratio was greater than 1.0, whereas in the reverse titration, the ligand wave had disappeared when the ratio was 2.0. Addition of ligand to cadmium until it was in appreciable excess over cadmium gave the titration graph shown in Fig. 1. It shows that the 1: 1 species is converted into the 2: 1 complex when a large amount of ligand is present. Moreover, the anodic wave of a 1-fold excess of uncomplexed ligand in the presence of the 1 : 1 complex decreased to half- height after 30 minutes but showed little change thereafter.Polarographic measurements showed that zinc, manganese(I1) and cobalt(I1) (6 x When nickel (5 x Cadmium showed rather more complicated behaviour. 0 SAC and the authors. 426;426 BROOKES 0.15 a 0.IC J 0 L 3 -0 3 0.05 W 6 U 0 4ND TOWNSHEND : AMPEROMETRIC TITRATION [Ana&St, VOl. 93 0 0.2 0.4 0.6 0.8 1.0 1.2 10-3~ Diethyldith iocarbamate solution added, mi Fig. 1. Amperometric titration of 4 x lo-' moles of cadmium(I1) with 1 0 - 3 ~ sodium diethyldithiocarbamate solution The results can be interpreted on the basis of competition for the metal ions between M(0€1)2 + 2L- + ML, + 2 OH- (M = Co(II), Mn(II), Cd or Cu) (1) For cobalt(II), manganese(I1) and zinc, the equilibria lie almost completely towards the left- hand side of the equations, whereas for copper(II), it is completely to the right-hand side; nickel is an intermediate example, and small amounts of ligand are insufficient to send the equilibrium towards the right-hand side.These results enable limits for the values of the solubility products of these metal dithiocarbamates to be calculated, on the basis of the diethyldithiocarbamate (L-) and hydroxide- . . or 2110,~- + 2L- + 2H20 + ZnL, + 4 OH- . . .. .. . . (2). TABLE I SOLUBILITY PRODUCTS OF METAL DITHIOCARBAMATES Metal Cobalt (11) Manganese (11) Zinc Nickel Copper (11) log KgO (M(OH),)' . . .. - 15 - 13 - 17 - 15 - 20 log KBO (ML,) by Hulanickis . . - - log K B O (ML,) * * .. .. >-16 >-14 >-19 -19.7 f0.5 <-28 - 17 - 23 - 30 solubility products of the metal hydroxides.These results do not take into account the effect of supersaturation phenomena and possible slow reactions between diethyldithiocarbamate and the metal hydroxides. These values are given in Table I. The reaction of cadmium can best be described as follows- rapid slow Cd(OH), + L- 7 Cd(0H)L + OH- . . * - (3) Cd(0H)L + L- 7 CdL, + OH-. . .. . . (4). Equilibrium in equation (3) is displaced markedly and rapidly to the right-hand side, but equilibrium in equation (4) needs an excess of ligand to drive it to this side. The latter reaction is also slow, and is responsible for the slow decrease in ligand wave-height in the presence of the 1 : 1 complex. This slow decrease could not have arisen from decomposition of either free or complexed ligand to give sulphide ions because addition of further ligand still gave the 2: 1 end-point on extrapolation of the steep part of the titration graph.Complexes of the type Cd(0H)L have also been postulated' when L represents C1-, POg3-, P,OIo5- or nit rilo t riacet ate. The investigation indicated that it should be possible to titrate amperometrically cadmium or copper in the presence of zinc, cobalt or manganese, in a hydroxide solution.July, 19681 OF COPPER AND CADMIUM WITH SODIUM DIETHYLDITHIOCARBAMATE 427 Table I1 shows that the titration of 2 x 10-4 M cadmium was satisfactory in the presence of a 50-fold excess of cobalt(I1) and a 5-fold excess of zinc. A 50-fold excess of zinc or man- ganese( 11) , however, gave low results, presumably because of co-precipitation of cadmium with manganese hydroxide or as cadmium zincate.It was also possible to titrate copper in the presence of similar amounts of nickel (Table 11). Care should be taken, however, not to spatter nickel hydroxide at the top of the polarographic cell during de-aeration, because the precipitate is difficult to dislodge from the glass, and it is likely to entrain copper. TABLE I1 TITRATION OF CADMIUM AND COPPER Other ions added, moles x lo7 * moles x lo7 Other ions found, Cadmium added, moles x lo7 Cadmium found, moles x lo7 2.0 2.0 2.0 2.0 2.0 2.0 1.0 3.0 - - - - - Zn 100 Zn 10 Mn 100 Mn 10 co 100 cu 1.0 cu 1.0 cu 4.5 Ni 0.5 c u 5.0 Ni 5.0 cu 5.0 Ni 10.0 * Poor graph. Non-linear graph, - 1.9 1.5 2.0 0*1* - --t 2.1 1.0 c u 3.0 c u - cu - - - - - cu - c u therefore no intercept.The formation of the two cadmium complexes enabled a method to be devised for the determination of cadmium and copper in admixture. The sum of the metals is first deter- mined by adding increments of titrant in the range appreciably in excess of that required to form CdL, and CuL,. The titration is then repeated, with lesser amounts of titrant so that the inflection for the CdL complex can be detected, as in Fig. 1. This inflection occurs when CuL, and CdL have been formed. EXPERIMENTAL- The polarographic curves were recorded in a Kalousek vessel with a standard calomel reference electrode and a Cambridge pen-recording polarograph. The titrant was a loh3 M solution of analytical-reagent grade sodium diethyldithiocarbamate in water, freshly made up every 3 days.The total anodic current of the diethyldithiocarbamate was measured at -0.3 volt versus S.C.E., except when the concentration of free ligand was sufficiently large M) for the adsorption wave to be separate from the main wave,6 when measure- ments should then be made at -0.2 volt, so that the total wave-height is measured. PROCEDURE DETERMINATION OF CADMIUM, OR COPPER, IN THE PRESENCE OF ZINC AND COBALT- M cadmium solution to 8 ml of 0.1 M sodium hydroxide solution in the polarographic cell. De-oxygenate with nitrogen, and titrate with diethyldithiocarbamate in 0.2-ml increments. Correct the wave-height for dilution effects, and plot the corrected wave-height vewus volume of titrant ; extrapolate the straight line to zero wave-height to obtain the end-point.DETERMINATION OF COPPER AND CADMIUM IN ADMIXTURE- Dilute a volume of the unknown solution, containing about 10-7 moles of cadmium and copper, to 10 ml with 0.1 M sodium hydroxide solution, and proceed as above. The end-point obtained gives the end-point for the formation of CuL, and CdL,. Repeat the titration with five times the volume of unknown solution, by adding the titrant in O-l-ml increments. The plot of corrected wave-height versus volume of titrant (as in Fig. 1) gives Add 2 ml of about428 BROOKES AND TOWNSHEND the end-point for the formation of CuL, and CdL at the point of first appearance of the anodic current. The concentration of copper and cadmium can readily be calculated. If the approximate concentration of copper plus cadmium is known, the first titration can be omitted, and the two end-points obtained from the second graph. In this instance, the titration should be carried out rapidly, to avoid the slow changes in wave-height with time noted above. The authors thank Professor R . Belcher and Dr. P. Zuman for their interest, Cambridge Instruments Ltd. for the loan of a polarograph and the S.R.C. for provision of a maintenance grant for A.B. REFERENCES 1. 2. 3. 4. 6. 6. 7. 8. Stock, J . T., “Amperometric Titrations,” Interscience Publishers, New York, 1965, p. 441. Usatenko, Yu. I., and Tulyupa, F. M., Izv. Vysh. Ucheb. Zavedenii Khim. i. Khim. Tekhn.oZ., 1958, -,- , Zav. Lab., 1959, 25, 280. Stricks, W., and Chakravarti, S. K., Analyt. Chewz., 1962, 34, 508. Halls, D. J., Townshend, A., and Zuman, P., Analytica Chim. Acta, 1968, 41, 63. -,-,- , Ibid., 1968, 41, 61. Sill&, L. G., and Martell, A. E., Editors, “Stability Constants of Metal-ion Complexes,” The Hulanicki, A., Acta Chim. Hung., 1961, 27, 41. Received January 21st, 1968 No. 3, 56; Chem. Abstr., 1959, 53, 1991d. Chemical Society, London, 1964.