990
Anal. Chem. 1983, 55, 990-992
Electrogravimetric Determination of Cobalt, Nickel, and Copper in the Presence of Chloride Ion Judith F. Owen, C. Stuart Patterson,* and Gregory S. Rice Department of Chemistry, Furman University, Greenville, South Carolina 296 13
Electrogravimetryis recommended as the quickest and one of the most precise methods for determination of Co, Ni, and Cu in aqueous solutions of their divalent salts. In the course of isopiestic vapor pressure studies on various salts of these cations, we have attempted to use the standard literature methods ( I , 2). Our requirements are for analyses that yield results that are consistently better than 0.1 % relative standard deviation. The recommended electrolytic bath solutions contain sulfate ion and we have found the standard procedures to be equally satisfactory when perchlorate is the counterion. However, we have invariably obtained poor cathodic deposits and erratic results when chloride ion is present. The poorly adhering deposits are apparently related to the erratic results which are variously attributed to the deposition of oxides with the metal, to the deposition of oxidized species on the anode, or to the codeposition on the cathode of platinum species produced by attack of chlorine atoms on the anode ( 3 , 4 ) . Some of these problems are said to plague the cobalt and nickel procedures even in the absence of chloride because of the high ammonia concentration. Our anodes did lose weight when electrolyzed in chloride solutions and the results were high (based on analysis for chloride and assumed 1:2 stoichiometry). Two approaches have been recommended for ameliorating the adverse effects of chloride-masking and removal. 1. Masking. Some standard procedures call for the addition of a small amount of an anodic depolarizer such as hydrazine or hydroxylamine even when no chloride is present if the plating bath is ammoniacal. Larger amounts are recommended when chloride is present. These compounds either are oxidized at a lower potential than chloride or else react with the nascent chlorine atoms thereby preventing their attack on the anode. 2. Removal. Obviously if chloride can simply be removed the problem is removed with it. However simple this may appear in principle, it can be rather difficult in practice since not only must removal be complete but the remaining sample must be retained quantitatively for the subsequent electroanalysis. Some procedures call for preliminary electrodeposition of the metal followed by its dissolution in HN03, heating with H2S04to drive off the HN03, and finally redeposition of the metal. Others suggest quantitative precipitation of the metal, e.g., by addition of carbonate followed by filtration and dissolution of the precipitate in H2SO4 to form the electrolytic bath. We first attempted to use the masking approach and found that these agents did significantly reduce the weight loss by our anodes but the results remained unacceptably erratic and the deposits poor. Of seven analyses attempted (three on a CuClzsolution and two each on solutions of NiClz and CoCl2), five gave precision poorer than 0.1% relative standard deviation and a sixth had to be aborted because of an obviously nonadherent deposit. Because of the complexity and time-consuming nature of the various separation methods, we set about development of an alternative approach for quantitative removal of chloride prior to electrolysis which avoids the necessity for quantitative transfers of precipitates, filtrates, or deposits. This report
presents procedures for such an alternative method for preparing electrolytic bath solutions for Co, Ni, and Cu starting with their divalent chlorides. The objectives of the new procedure were to remove chloride completely and to produce a resulting solution containing only electrolytes compatible with the optimum electrolytic bath solutions. It was also desirable to effect this exchange as quickly and easily as possible with a minimum of transfer operations which threaten recovery. It is often recommended that excess HN03be removed from electrolytic baths by fuming (5), and it has long been known that similar exhaustive heating with nonvolatile acids will drive out excess C1- as HC1 (6, 7). However, no procedure seems to have been published which makes use of this reaction to eliminate chloride interference in the analyses of interest. The method we have devised involves addition of a measured amount of concentrated HzS04directly to the weighed sample (in our case, an aqueous solution of the divalent metal chloride) in the electrolytic beaker, volatilization of chloride as HC1 gas by careful heating, and the addition of the components required to form bath solutions of the composition recommended in the literature. The electrogravimetric analyses then proceed by the standard methods ( I ) . The method is stoichiometrically straightforward since the recommended baths are comprised of aqueous mixtures of the metal sulfate, H2S04,(NH4)2S04,and NH40H in varying proportions for the different metals (excess acid for copper; excess ammonia for cobalt and nickel). The relative concentration of each component and the total electrolyte concentration, which is critical for control of the current-voltage relationship, are predetermined by selection of the sample size and the quantity of HzS04used in displacing the chloride. Procedures for Removal of Chloride. Before attempting quantitative determinations, we carried out qualitative tests to establish the minimum treatment required for quantitative removal of chloride. It was also established that acceptable deposits could be formed on electrolysis of the resulting solutions. The procedures developed on the basis of these testa are given below. 1. Procedure for Electrogravimetric Determination of Cu in Aqueous CuClZ. A sample containing approximately 0.3 g of Cu is weighed from a weight buret (to 0.1 mg) into a 100-mL tall-form (electrolytic) beaker and 2.3 mL of concentrated H2S04is added slowly to avoid spattering due to overheating. Upon addition of HzS04,brown and white precipitates form with the evolution of fumes. The mixture is placed on a hot plate and heated to near boiling or until vapor condensation can be seen on the beaker. The internal condensation is then continuallyremoved by flaming the sides of the beaker with a Bunsen burner to prevent return of chloride to the beaker by reflux. When the mixture is initially heated the brown and white solids dissolve leaving a green solution. After about 2 min, the solution turns a pale green. (Although all the chloride can be removed without the addition of HN03, the addition of 1 mL of HN03 while the solution is pale green helps remove HCl as an azeotrope, boils off any nitrites, and improves plating.) In subsequent transitions, the solution turns from pale green to blue green, blue green to light blue, and finally to light blue with a white solid present.
0003-2700/83/0355-0990$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
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Table I. Quantitative Tests of Procedures Cu(I1) in CuCl, Solution 12 replicate analyses 1.2240 mmol of Cu(I1) av concn = g of solution re1 std dev = 0.06%
sample no. 1 2 3 4 5 6 7 8 9 10 11 12
g of
solution 3.5’96 7 4.9628 3.1324 4.0!377 3.3 08 3 3.3444 3.27’10 3.3756 3.7 7 2 5 3.5496 3.4687 3.11.91
g of
mmol of Cu(II)/ g of solution
cuo
1.2229 1.2239 1.2229 1.2247 1.2244 1.2248 1.2244 1.2247 1.2244 1.2236 1.2236 1.2230
0.2795 0.3852 0.2473 0.3189 0.2574 0.2603 0.2545 0.2627 0.2396 0.2762 0.2697 0.2424
Ni(I1) in NiC1, Solution 10 replicate analyses
av concn =
1.9149 mmol of NilII) \-
I
g of solution
re1 std dev = 0.04% sample
g of
no.
solution
g of Nio
mmol of Ni(II)/ g of solution
1 2 3 4 5 6 7 8 9
2.3625 2.6052 2.6204 2.8191 2.4832 2.7186 2.3413 2.4821 2.4626 2.6911
0.2657 0.2929 0.2983 0.3168 0.2792 0.3056 0.2632 0.2791 0.2767 0.3024
1.9159 1.9153 1.9133 1.9 144 1.9154 1.9150 1.9161 1.9156 1.9142 1.93.43
10
Co(I1) in CoCl, Solution 4 replicate analyses 1.8531 mmol of Co(II\ g of solution re1 std dev = 0.05% -
av concn =
sample
g; of
no.
solution
1 2 3 4
2.4673 2. !57 7 9 2.5320 2.5 46 8
g of
coo
0.2690 0.2817 0.2764 0.2780
1
,
mmol of Co(II)/ g of solution 1.8538 1.8542 1.8523 1.8522
Upon the formation of white solid (enough to loosely cover the bottom of the beaker) most of the C1- is removed and heating should be continued for 5-10 min to assure the complete removal of chloride. When removal is complete, the solution is diluted by addition of 100 mL, of distilled water and the electrolysis is carried out by the standard procedure (8). 2. Procedure for Electrogravimetric Determination of Ni in Aqueous NiC1,. A sample equivalent to -0.3 g of Ni is weighed to 0.1 mg into a 100-mL electrolytic beaker from a weight buret and 4.1 mL of concentrated H2S04is added slowly down the sides of the beaker. Upon the addition of the concentrated H2S04,fumes are evolved and a yellow precipitate forms. Then the mixture is placed on a hot plate and heated to boilling or near boiling until the solid changes from yellow to white, and finally to light green. Any condensation is continiually removed from the sides of the beaker
991
by flaming the sides with a Bunsen burner. Heating should be continued for 5 min after the indicated color changes have taken place. After chloride is removed, the solution is diluted with 65 mL of distilled water; then 35 mL of concentrated NH,QH is added, the solution is heated to -70 OC, and the electrolysis is carried out by the standard procedure (9) using a copperplated platinum cathode. (Some standard procedures call for addition of hydrazine but none was used in these nickel analyses.) 3. Procedure for Electrogravimetric Determination of Co in Aqueous CoCl,. A sample containing -0.3 g of cobalt is weighed to 0.1 mg into an electrolytic beaker from a weight buret and 4.1 mL of concentrated H2S04is poured slowly down the sides of the beaker. This addition should be made very carefully since a violent reaction can result. Fumes are evolved and the solution changes from red to blue and blue to purple or blue with a pink solid. The mixture is placed on a hot plate and heated to near boiling. Any condensate in the beaker should be continuously removed by flaming the sides of the beaker with a Bunsen burner. After being heated, the mixture changes from a light purple solution with a pink solid to a dark, bluer solution with a pink solid. The solution is heated for 10 min or until no more fumes are visible and the boiling appears to have ended. When the C1- is removed, the mixture is diluted with 65 mL of distilled water, and 36 mL of concentrated NHIOH and 0.5 mL of 85% hydrazine hydrate are added. The solution is heated to -70 *@and the electrolysis carried out by the standard procedure (IO).
RESULTS Multiple replicate analyses were carried out on a solution of each of the three metal chlorides using the procedures given above. Twenty-six determinations were made; the results of these analyses are tabulated in Table I to demonstrate the consistency of the methods. It was intended to run 12 replicates of each solution but two nickel samples were lost to a known mechanical error while the initial samples ofthe cobalt solution gave poor deposits and somewhat poorer precision (although still superior to the “standard” masking procedure) than the four samples reported. It was discovered that these poor deposits were evidently caused by the presence of residual traces of chloride. The procedure used for the four samples reported is that given above and differed from the earlier version in the addition of the prescribed 0.5 mL of hydrazine and extra ammonia when deposition seemed abnormally slow as indicated by the solution’s color. We conclude from these results that the depolarizer (hydrazine) works quite well for traces of chloride although, as reported above, it is ineffective when stoichiometric concentrations of chloride ion are present. It may be that residual chloride is not the problem here at all and that the depolarizes serves only to prevent anodic corrosion by the NH, solution. Hydrazine is often a recommended component of the conventional baths for both nickel and cobalt analyses (but not copper which is plated from acidic medium) although our results seem to indicate that it is not necessary in the case of nickel if chloride is absent. That the precision obtained is a reasonable indication of accuracy is supported by the following data: chloride analyses (gravimetric as AgC1) gave chloride-to-metalratios of 2.O0Ob and 2.002, for the NiCl, and CuCl, solutions, respectively. A similar analysis of the CoCl, solution gave a chloride-to-cobalt ratio of 1.993j. We do not believe that this less satisfactory result is a consequence of the chloride problem but rather simply confirms that the standard electrogravimetric method
@92 ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
tends to give slightly high results for cobalt unless the current density is kept very low (11). Apparently this problem can be overcome by addition of a measured amount of nickel to be codeposited with the cobalt (12) but we discovered this fact too late to conduct an adequate test of that approach. Preliminary results seem to confirm this effect. CONCLUSIONS The results reported justify the conclusion that the method presented shows six distinct advantages over the literature methods. I. Time. Prior to the beginning of the electrodeposition no procedure requires more than 20 min of preliminary treatment of the bath solutions. 2. Chloride Removal Is Complete. After the electrolytic deposition of the metal, each bath was tested for the presence of chloride, and the tests were either entirely negative or, at most, resulted in a trace indication of chloride. None of the deposits formed from solutions prepared by the procedures prescribed displayed any of the undesirable characteristics that result from the presence of chloride. 3. Ease of Bath Preparation. During pretreatment, no extraneous solutes or solvents are needed which must later be removed. Thus, the bath is made directly without the use of any separation processes such as deposition, redissolution, filtration, or elution. 4. Minimum Sample Handling Required. Transfer operations invariably threaten recovery, but since the sample is weighed directly into the beaker and the entire procedure carried out in the same container, this source of error is removed. 5. Flexibility. Other acids of low volatility may be substituted for the HzS04if the anion is a more desirable counterion than S042- (e.g., HC104 or H,PO,) for other metals.
6. Self-Indicating. An unanticipated series of diagnostic color changes occurred upon the addition of concentrated H2S04and during the subsequent heating and gradual removal of chloride. Consequently, the procedure is self-indicating so that after brief experience,easily obtained by qualitative trial runs, the pretreatment can be carried out with little concern for careful timing or temperature control. Finally, the ultimate success of the procedure is indicated by the precision of the 26 analyses of the three solutions which yielded consistent precision with a pooled relative standard deviation of 0.05%. In addition, the experimental ratios of anion to cation indicate that this precision is a reliable indicator of the accuracy of the results.
Registry No. CuCl,, 7447-39-4; NiClZ,7718-54-9; CoC12, 7646-79-9;Co, 7440-48-4;Ni, 7440-02-0;Cu, 7440-50-8;sulfuric acid, 7664-93-9. LITERATURE CITED Erdey, Laszlo "Gravlmetric Analysis", Part 11, Belcher, R., Gordon, L., Eds.; Pergamon: London, 1965; Internatlonal Series of Monographs on Analytlcal Chemistry, Vol. 7. Bassett, J.; Denney, R . C.; Jeffery, G. H.; and Mendham, J.; "Vogel's Textbook of Quantitative Inorganic Analysis", 4th Ed.; Longman: New York, 1978. Reference 1, p 76. Reference 2, p 533. Reference 2, p 537. Ephraim, F. "Inorganic Chemistry", 5th ed.;Thorne, P. C. L., Roberts, E. R., Eds.; Interscience: New York, 1948. Reference 1, p 77. Reference 1, p 77. Reference 1, p 390. Reference 1, p 406. Reference 1, p 407. Reference 1, p 391.
RECEIVED for review August 30, 1982. Resubmitted January 19, 1983. Accepted February 17, 1983.
