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LITERATURE CITED (11 . . N. E. Good. G. D. Wimet. W. Winter. T. N. Connollv. S. Izawa. and R. M. M. Singh, Biochernkk, 5 , 467 (1966). (2) R. G. Bates, C. A. Vega, and D. R. White, Jr., Anal. Chem., 50, 1295 (1978). (3) C. A. Vega and R . G. Bates, Anal. Chern., 48, 1293 (1976). (4) H. B. Hetzer, R. G.Bates, and R. A, Robinson, J. Phys. Chem., 70, 2869 (1966). (5) k. G. Bates, "Determination of pH", 2nd ed.,Wiley, New Ywk, N.Y., 1973, Chap. 4 and appendix, table 1. (6) H. B. Hetzer, R. A. Robinson, and R. G. Bates, J. Phys. Chem., 66, 1423 (1962).
(7) H. S. Harned and R. A. Robinson, Trans. Faraday SOC..36. 973 (1940). (8) N. W. Please, Biochem. J . , 56, 196 (1954).
RECEIVED for review July 21,1978. Accepted August 14, 1978. Work supported in part by the National Science Foundation under Grant CHE76 24556. M.S. thanks the following organizations in the Republic of South Africa for financial assistance: the C*S.l.R*,Ernest Oppenheimer and Sentrachem Ltd.
Instrumental Approach to Potentiometric Stripping Analysis of Some Heavy Metals Daniel Jagner Department of Analytical Chemistry, University of Gothenburg, Fack, S-402 20 Goteborg, Sweden
The method is based on the potentiostatic reduction and amalgamation of analytes and the subsequent registration of the potential-time curve when the reduced metals are reoxidized by means of mercury(I1) ions. Potentiostatic reduction Is achieved by means of a simple battery-operated potentiostat, equipped with a timerhwitcher unit facilitating semiautomatic analysis. The time-potential curves are registered by means of a pH-meter and/or an x-f recorder. Analytical procedures for bismuth, cadmium, thallium, lead, copper, and zinc ions in different media are reported. The relative precision is approximately 0.06.
The increasing need for trace metal analysis, in particular for the determination of lead and cadmium, is well documented. Consequently, there is also a need for simple analytical instrumentation suitable for this purpose. The hitherto simplest instrumental approach to trace metal analysis is that of ion-selective electrodes, a general purpose p H meter being the main instrumentation. Unfortunately ion-selective electrodes are not selective and sensitive enough for metal analysis in the concentration range below lo4 M ( I ) , which is that of most interest. The majority of the most important trace metals do, however, dissolve readily in mercury. This has been utilized in a large number of analytical procedures based on anodic stripping voltammetry ( 2 ) . Advanced instrumentation, e.g., for differential pulse anodic stripping voltammetry, has been designed for this purpose (3). Such dedicated instrumentation is, however, normally expensive since it does not consist of equipment usually available in the chemical laboratory. This paper describes an instrumental technique, which combines the simplicity of the ion-selective electrode instrumentation with the selectivity and sensitivity of anodic stripping voltammetry.
PRINCIPLE If a working electrode, consisting of material insoluble in mercury and with a high hydrogen overvoltage, is held potentiostatically a t a sufficiently reductive potential in a deaerated sample containing Hg(I1) ions and mercury soluble metal ions M(n) the reactions 0003-2700/78/0350-1924$01 .OO/O
+ 2eM(n) + ne-
Hg(I1)
-
--
Hg(1)
M(Hg)
will occur simultaneously. If the hydrodynamic conditions at the working electrode surface are kept constant, e.g., by rotating the electrode or by stirring the sample, the rate of reduction of M(n) ions will be constant until a significant fraction of these ions has been reduced. By keeping the electrode surface small the decrease in M(n) concentration will occur only very slowly. If, a t unchanged hydrodynamic conditions, the potentiostatic circuitry is disconnected and at the same time the working and the reference electrodes remain connected to a high input impedance voltmeter (e.g., a p H meter) the potential changes due to the reactions
+ iHg(I1)
+
M(n) + nHg(1) 2
can be observed. The experimental curve thus registered, potential vs. time, consists of a normal redox titration curve superimposed on a capacitance background. If the background contribution is accounted for, the time elapse between two consecutive equivalence points will be proportional to the amount of the particular metal in the mercury phase. Analogous to anodic stripping voltammetry, the amount of amalgamated metal is proportional to the concentration of metal ions M(n) in the sample and to the time of pre-electrolysis (plating). Consequently. the potential-time curve can be made the basis for quantitative analysis, either by the use of a calibration plot, a n internal standard, or by using a standard addition procedure. The principle of potentiometric stripping analysis has been described previously by the author in connection with more complicated instrumentation ( 4 , 5 ) .
EXPERIMENTAL Instrumentation. The only instrumentation required for potentiometric stripping analysis is a potentistatic circuitry and a pH meter, the latter being used both for the adjustment of an adequate plating potential and for the registration of the stripping curves. In all experiments, the same potentiostat was used but three different ways of registering the potentiometric stripping analysis curves were investigated. In the first method, which is G 1978 American Chemical Society
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Table I. Optimum Experimental Conditions for the Determination of Six Different Ions b y Potentiometric Stripping Analysis scanning optimum plating potential, E , , , vs. optimum potential, E vs. element pH range SCE, V SCE, V remarks Bi(II1) 0-0.5 - 0.8-- 1.0 -0.15 HCl suitable medium. 0- 3 -0.7--0.95 - 0.25 Zn(I1) interference possible when plating CU(I1) below - 0.95 V. Acidity with HC1. 0- 3 -0.8-- 1 . 2 -0.50 Hydrogen evolution may commence at Pb(I1) -1.3 V when pH < 2 . Acidity with HCl. 0-3 -0.9--1.2 0.65 Hydrogen evolution may commence a t Tl(III), Tl(1) -1.3 V when pH
