134 An,alyst, February, 1978, Vol. 103, $p. 134-139 Determination of Nickel, Cobalt and Copper by Direct Photometric Titration with Cyanide M. A. Leonard and R. Murphy Department of Analytical Chemistry, Queen’s University (f Belfast, Belfast, BT9 5AG Defiartment of Analytical Chemistry, Queen’s University of Belfast, Belfast, BT9 5AG, and Messrs. Gallaher Ltd., 138 York Street, Belfast, BT15 1JE Nickel(II), cobalt(I1) and copper(I1) at the 0.1 M level are titrated photo- metrically with cyanide in ammoniacal solution. The nickel - cyanide reaction shows reproducible 1:4 stoicheiometry and can be used for titration. The reaction with cobalt shows firm 1:5 stoicheiometry but is complicated by formation of oxygen-containing species. The reaction with copper shows uncertain stoicheiometry of about 1:4.In the nickel titration zinc does not interfere but cobalt and copper add on, both showing 1 :4 metal - cyanide stoicheiometry . Keywords : Nickel determination ; cobalt determination ; copper determination ; photometric determination ; cyanide Nickel(I1) and cobalt(II), in the presence of excess of cyanide, stoicheiometrically form the complex ions [Ni(CN) J2- and [Co(CN),(H20)I3-. Before the introduction of EDTA, nickel and cobalt were determined by reaction with a known excess amount of cyanide in mildly ammoniacal solution followed by back-titration of the remaining cyanide with standard silver nitrate solution. Visual turbidimetric or potentiometric equivalence point indication was used. When ammoniacal solutions of nickel, copper and cobalt are titrated with cyanide, marked colour changes occur and it was decided to see if such reactions could form the basis of useful photometric titrations of these metals.The use of cyanide as titrant would introduce selec- tivity towards “B type” and “right-hand side transition” metal ions, an advantage in com- parison with the broad-spectrum titrant EDTA. Use of dilute ammonia solution as solvent would introduce further selectivity. Experimental Reagents Potassium cyanide solution, 0.4 M. Prepared from fresh solid reagent and standardised by potentiometric titration against standard silver nitrate using silver and mercury(1) sulphate electrodes. Nickel sulphate, copper(II) sulfihate and cobalt(I1) nitrate solutions, 0.1 M. The solutions were standardised against standard EDTA solution.Instrumentation Solution absorption spectra were produced on a Pye-Unicam SP8000 spectrophotometer. Titrations were performed on an EEL photometric titrator using Ilford standard spectrum filters, a 40-ml cell and a 10-ml microburette. Results Titration of Nickel Upon addition of cyanide a smooth transition to the yellow tetracyano complex occurs as shown by the regular change in absorption spectrum (Fig. 1). Fig. 2 shows nickel - cyanide titrations at 575 nm and using various over-all concentrations of ammonia. A C, of at least 1 M is required in order to avoid precipitation of insoluble nickel complexes. The hexaamminenickel( 11) complex [Ni(NH3),J2+ is coloured blue with A,,,. 582 nm.LEONARD AND MURPHY 135 0.8 (u m 0.6 e en 2 0.4 0.2 I 400 500 600 700 80 0 0 W avel engthh m Fig.1. addition of cyanide. 40 mm. and E, 1:4. Change in the absorption spectrum of [Ni(NH3),]a+ upon the CNi = 0.0402 M ; CNns = 1.0 M ; cell path length = Molar ratios of Ni to CN: curve A, 1: 0; B, 1: 1; C, 1 : Z ; D, 1: 3; A small, but distinct and reproducible, break occurs at the 1 : 2 nickel to cyanide molar ratio and the final end-point break seems always to occur about 0.7% low, based on 1 : 4 stoicheio- metry. 0 4 8 12 16 Amount of 0.3944 M CN-/ml Fig. 2. Photometric titration of nickel with cyanide in ammoniacal solution. Filter 606 ( hmax.T = 575 nm). Taken, 14.0 ml of 0.100 4 M NiSO, solution plus n ml of concentrated NH, solution. Starting volume, 26 ml. Curve A, n = 1 ml; B, 12 = 2 ml; C, n = 4 ml; and D, n = 8 ml. Theoreti- cal end-point for Ni to CN ratio 1 : 4 = 14.25 ml.It is unexpected that the initial absorbance decreases with increase in ammonia concentra- For tion but the final end-point is independent of this variable. log p4 = 31.3 and for Ni2+ + 4 CN- + [Ni(CN),I2- Ni2+ + 6 NH, e [Ni(NH,),12+136 LEONARD AND MURPHY: DETERMINATION OF NICKEL, COBALT AND log p6 = 8.49,1 where ,8 is the over-all formation constant. readiness with which cyanide can displace ammonia in nickel complexes. Analyst, VoZ. 103 This difference illustrates the Titration of Cobalt The reaction of hexaamminecobalt (11) with oxygen : 2 [Co(NH3)6I2+ + 0 2 [(NH3)5Co(02)Co(NH3)~14+ + 2NH3 causes complications. occurs with the cyanocobaltate(I1) complex in the presence of oxygen: The colour change is froin pink to dark brown.A similar reaction 2[Co(CN)5(H@)J3- + 0 2 + [(C~)5C0(02)C0(CN),16- the colour change being from yellow-green to brown. Because of these reactions all cobalt titrations were carried out under nitrogen. The absorption spectra of [CO(NH,),]~+ and [Co(CN),H20I3- are shown in Fig. 3. The high absorbance of [Co(CN),(H20)I3- at 800 nm is rapidly and completely destroyed by oxygen. 400 500 600 700 800 Wave1 engthhm Fig. 3. Absorption spectra of [Co(CN),(H,0)l3- and [Co(NH,),12+. For the cyanide complex Cc0 = 0.008 M ; Cm3 = 1.4 M ; and Cm = 0.048 M. For the ammine, Cco = 0.008 M ; and CNH3 = 3.7 M. Cell path length = 20 mm. Both solutions kept under nitrogen. Fig. 4 shows the titration of hexaamminecobalt(I1) with cyanide at 435 nm; a set of curves increasing in absorbance is evident, as would be expected from the absorption spectra. An over-all ammonia concentration greater than 2 M is required in order to avoid precipitation.For a range of ammonia concentrations distinct breaks occur at 1 : 5 stoicheiometry; the reaction is [CO(NH,)6]2+ + 5CN- + [Co(CN)5(H20)I3- + 6NH3 or Precipitation is presumably due to the formation of uncharged species such as [Co(NH,) 4(CN)2]0 or mixed hydroxy complexes. Titration of cobalt at 550nm, as would be anticipated from the spectra, gives a slowly decreasing absorbance but this fall is terminated by an upward step at 1 : 5 stoicheiometry. At wavelengths above 640 nm (filter 608) a titration curve similar to that obtained at 435 nm is found.Again an acceleration of absorbance increase is evident as 1 : 5 stoicheiometry is approached. [Co2(CN),016- Such precipitates readily dissolve in excess of cyanide. Titration of Copper colourless transition are shown in Fig. 5. The absorbance decrease at regular with cyanide addition. molar ratio, caused presumably by the insolubility of [Cu(NH,),(CN),]O. The absorption spectra of tetraamminecopper( 11) -cyanide complexes illustrating the blue to 605 nm is not Precipitation tends to occur at the 1 : 2 copper to cyanideFebruary, 1978 COPPER BY DIRECT PHOTOMETRIC TITRATION WITH CYANIDE 0.8 0.6 a S + 2 0.4 2 0.2 t- 137 0 1 .o 2.0 3.0 Amount of 0.394 4 M CN-/mi Fig. 4. Photometric titration of cobalt with cyanide in ammoniacal solution under nitrogen.Filter 601 ( h m a x . ~ = 435 nm). Taken, 2.0 ml of 0.100 2 M Co(NO,), solution plus n ml of concentra- ted NH, solution. Starting volume, 30 ml. Curve A, n = 4 ml; B, n = 6 ml; and C, n = 10 ml. Theo- retical end-point for Co to CN ratio 1 : 5 = 2.54 ml. Copper - cyanide titration curves are shown in Fig. 6. The end-points, as determined by extrapolation of the linear portions, vary somewhat with over-all ammonia concentration but the general stoicheiometry is 1 : 4, ie., the reaction is basically [CU(NH,),]~+ + 4CN- + e + [CU(CN),]~- + 4NH3 For Cu+ + 4CN- + Cu(CN)i- log f14 = 30.3 and for Cu2+ + 4NH3 + Cu(NH3)2,+ log fl, = 12.6. These values show the ability of cyanide to displace ammonia but it will be seen from Fig. 6 that end-point curvature increases with increasing ammonia concentration , thus illustrating the decrease in the conditional formation constants of copper - cyanide species with increase in ammonia concentration.1 .o A 0.8 a 0.6 e B a 0.4 0.2 0 Tendency to / ' I precipitate \ \\ 500 600 700 WavelengWnm 800 Fig. 5. Changes in the absorption spectrum of [Cu(NH,),I2+ upon the addi- tion of cyanide. CcU = 0.004 M; C,, = 0.30 M ; cell path length = 40 mm. Molar ratios of Cu to CN: curve A, 1 : O ; B, 1: 1; C, 1:2; D, 1 : 3 ; and E, 1:4.138 LEONARD AND MURPHY: DETERMINATION OF NICKEL, COBALT AND Analyst, VoZ. 203 1 .o 5 0.8 d 0.6 0.4 6.2 0, % a 0 0.4 0.8 1.2 1.6 2.0 2.4 Amount of 0.394 4 M KCN/ml Fig. 6. Photometric titration of copper with cyanide in ammoniacal solution. Filter 607 (Amax.* = 600 nm).Taken, 2.0 ml of 0.100 1 M CuSO, plus n ml of concentrated NH, solution. Starting volume, 35 ml. Curve A, n = 1 ml; B, n = 2 ml; C, n = 4ml; D, n = 8 ml; and E, n = 16 ml. Theoretical end-point for Cu to CN ratio 1 : 4 = 2.03 ml. Titration of Mixtures The only really feasible determination in this system is that of nickel, and we therefore The interfering ions chosen examined the effect of interferences only on the nickel titration. were the soluble ammine formers zinc, cobalt and copper. Titration of Nickel and Zinc in Fig. 2, although with a less obvious break a t 1 : 2 Ni: CN stoicheiometry. log p4 = 16.7 and for log f14 = 9.1. This result is reasonable given the low log p4 difference and high ammonia concentration existing in the titration. Zinc in a 1 : 1 molar ratio with nickel causes no interference.The titration curve appears as For Zn2+ + 4CN- + Zn(CN)2,- Zn2+ + 4NH3 + Zn(NH3)2,+ Titration of Nickel and Cobalt adds on to the nickel titre exactly. ratios of 1 : 2 and 1 : 4. Under the conditions shown in Fig. 7(a), cobalt appears to react with 1 :4 stoicheiometry and Small but distinct breaks occur at nickel to cyanide molar The nickel is titrated first. Titration of Nickel and Copper Fig. 7(b) shows that the end-point is equivalent to the sum of nickel plus copper. The titration curve is an interesting shape and shows that nickel is titrated before copper; there is some indication of completion of nickel complexation. log fi4 [Ni(CN)i-]-log Is, [Ni(NH,)i+] = 22.8 log 194 [Cu(CN)S,-]-log f l 4 [Cu(NH,);+] = 17.7 These relationships explain why nickel is titrated first.Discussion The titration system described could be useful for the determination of nickel in the presence of ions that do not complex strongly with cyanide or that precipitate in dilute ammonia solution. Also, in a different sphere of interest, photometric titrations of this type are reallyFebwary, 1978 COPPER BY DIRECT PHOTOMETRIC TITRATION WITH CYANIDE 139 continuous Yoe Q) m 2 0.6 -2 2 n Q 0.4 0.2 c 0 2 4 6 8 1 0 1 2 0 2 4 6 8 1 0 1 2 Amount of 0.394 4 M KCN/ml Amount of 0.394 4 M KCN/ml Fig. 7. (a), Photometric titration of a mixture of nickel and cobalt with cyanide in ammoniacal solution under nitrogen. Filter 606. Curve A: taken, 8.0 ml of 0.1004 M NiSO, solution, 1.0 ml of 0.1002 M Co(NO,), solution and 4 ml of concentrated NH, solution. Starting volume, 26 ml. Curve B, titration of 1.0 ml of 0.100 2 M cobalt only under the same conditions. Theoretical end-point for Ni to CN ratio 1 : 4 and Co to CN ratio 1:4 = 9.17 ml; for Ni to CN ratio 1:4 and Co to CN ratio 1: 5 = 9.42 ml. ( b ) , Photometric titration of a mixture of nickel and copper with cyanide in ammoniacal solution. Filter 606. Curve A, 1.0 ml of 0.1001 M CuSO, solution and 4 ml of concentrated NH, solution. Curve B, 8.0 ml of 0.1004 M NiSO, solution and 4ml of concentrated NH, solution. Curve C, 1.0 ml of 0.100 1 M CuSO, solution, 8.0 ml of 0.1004 M NiSO, solution and 4 ml of concentrated NH, solution. All starting volumes 26 ml. Theoretical end-point for nickel only = 8.15 ml : for Ni to CN ratio 1 : 4 plus Cu to CN ratio 1 : 4 = 9.16 ml. Jones molar ratio plots and can be valuable aids to the understanding of complexation reactions. Reference 1. Ringborn, A., “Complexation in Analytical Chemistry,” Interscience, New York and London, 1963. Received August 1st 1977 Accepted August 22nd, 1977