(in absence of O2 and 0;) in separate (11) experiments, is reported in Figure 5 . Table I summarizes the way to treat the polarographic data for the quantitative resolution of systems containing together OH-, 0 2 - , 0 2 , and HzO. All these species are certainly present whenever moisture and oxygen are in contact with molten solutions containing oxides, see Equations 1-4 and 11. Sometimes, of course, one or more of them can be voltammetrically undetectable; this is, for instance, the case of superoxide ion in the experiment presented in Figure 4. In this case the absence of detectable traces of 0 2 - (even after prolonged oxygen flow) indicates that Equilibrium 11 lies mainly to the right. In effect, successive potentiometric findings have confirmed (17) this qualitative finding; an
equilibrium constant K = 1 X lov3 Kg1l2 could be calculated for Reaction 11 at 503 OK. Then the timedependent decreasing of superoxide concentration, apparent from the curves reported in Figure 2 doesn’t seem due (or at least, not entirely due) to the slow rate with which oxygen, produced by Reaction 11, leaves the solution. Slow chemical steps must be involved in the forward process of Reaction 11. Preliminary kinetic findings have confirmed this supposition. A study (made possible by the analytical tool presented in this work) about the influence of reagents and products on the rate of Process 11 and on its reaction mechanism is in progress in our laboratory.
(17) P. G. Zambonin, J. Electroanal. Chem., in press.
RECEIVED for review March 5,1971. Accepted May 26,1971.
Analytical Evaluation of a Cyanide-Ion Selective Membrane Electrode under Flow-Stream Conditions Bernard Fleet and Henning von Storp Department of Chemistry, Imperial College, London, S . W.7., England Evaluation of a silver sulfide-silver iodide membrane electrode as a continuous monitor for cyanide ions has shown that it is well suited for the measurement of cyanide in the concentration range to 5.10-5M. Serious anion interferences are encountered only from iodide and sulfide ions, and a detailed study of the values obtained for the interference from iodide indicates that the potential-determining mechanism is more complex than the simple replacement reaction; Agl 2CNAg(CN)zIand involves a contribution from the direct displacement reaction AgCN IAgl CN-
Ion-selective electrodes are ideally suited for continuous monitoring, for example, in effluent analysis or in the computer control of industrial chemical processes. The use of these devices in continuous analysis has been reviewed by Light (6). At the present time, however, one of the major limitations to the routine use of these electrodes is the lack of sufficient reliable data on their performance and interference characteristics. The aim of the present work was to evaluate the Orion cyanide member electrode (Model 94-06) under continuous analysis conditions. Selectivity ratios for a range of interfering ions have also been measured.
THE RANGE of solid membrane electrodes originally introduced by Ross et al. ( I ) have found increasingly wide application both for the direct potentiometric determination of anions and cations and also as end-point sensors. Cyanide monitoring is currently of importance in several fields (2), atmospheric and water pollution control, analysis of plating baths and cyanide oxidation systems, and in biological measurement of cyanide-containing plant glycosides (3). Two main types of cyanide electrode are at present commercially available. One of these is manufactured by Orion ( 4 ) and consists of a solid membrane of mixed silver sulfide and silver iodide. The electrode developed by Pungor and coworkers ( 5 ) consists of a silver iodide impregnated silicone rubber membrane.
EXPERIMENTAL
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(1) M. S. Frant and J. W. Ross, Science, 154, 1553 (1966). (2) L. S. Bark and H. G. Higson, Analyst, 88, 751 (1963). (3) B. Gyorgy, L. L. Andre, L. Stehli, and E. Pungor in “Proceedings of the International Measurement Confederation on Electrochemical Sensors,” Veszprem, Hungary, 1968, p 111. (4) Orion Research Inc., Bulletin 94-06, 11 Blackstone Street, Cambridge, Mass. ( 5 ) K. Toth and E. Pungor, “Proceedings of the International Measurement Confederation on Electrochemical Sensors,” Veszprem, Hungary, 1968, p 35.
Chemicals. All chemicals used were of analytical reagent grade. Stock solutions of 0.1M NaOH and 0.1M NaCN were stored in polythene bottles. Standard cyanide solutions were standardized argentometrically using Liebig’s method. Apparatus. The Technicon AutoAnalyzer was used, the main modules required being a pump I1 and a sampler 11. An Orion Model 801 digital pH meter was used for potentiometric measurements. The output from the potentiometer was displayed on a Servoscribe recorder (Model R E 511 A, Smiths Industries, London). A backing-off circuit was incorporated to increase sensitivity. The electrode (Model 94-06) was obtained from Orion Research Inc., (Cambridge, Mass.) and was modified for continuous analysis by fitting a flow-through cap on to the membrane face (Figure 1). The flow-through cap also contained an agar junction connected to the saturated calomel reference electrode. Procedure. The flow system is shown in Figure 2. Solutions of adjusted ionic strength buffer and sample are fed as
(6) T. S. Light in “Proceedings of Symposium on Ion Selective Electrodes,” Nut. Bur. Stand. (US.), Spec. Publ., 314, 1969, p 349.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
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(1) The dissociation constant and solubility product of the reactions involved are as follows (8):
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