Polarography with controlled current density or chronopotentiometry with current density sweep at a dropping mercury electrode

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279

Polarography with controlled current density or chronopotentiometry with current density sweep at a dropping mercury electrode From its inception, polarograph~c procedure has consisted of an essentially discontinuous, but ill practice continuous, change of potential of a dropping mercury electrode and the measurement of the corresponding current. In the fifties, the reverse procedure was tried1, 2. This was followed by criticism 3 and more detailed exposition 4,~. The further lack of interest may be ascribed to a serious drawback inherent in the proposed method, viz. the high current density during the first stage of drop life. This results in the starting of a secondary electrolysis process at a more negative electrode potential followed b y a reverse reaction in the case of rapid (reversible) systems, because of the increase of electrode potential. The interpretation and quantitative evaluation of the graph suffer from this secondary effect.

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Fig. I. (a), Drop volume; (b), current density; (c), current intensity as functions of time in arbitrary units (schematized).

The use of a circuit with controlled current density overcomes this difficulty. Automation as in normal polarography brings about a slowly increasing current density in such a way that during drop life it may be considered as constant. The impressed current intensity requires a special form and synchronization with the drop fall by means of a drop-life timer (Fig. I). The procedure may be considered essentially as chronopotentiometry at a mercury drop expanding at the normal rate, an idea not yet elaborated in the literature but only perhaps suggested 8. The special feature lies in the application of a slow current density sweep. When the apparatus has just been started, the transition time is too long to be contained within the drop life, and consequently only a small part of the chronopotentiogram is covered by the recorder. The increasing current density involves a J. Electroanal. Chem., 16 (1968) z79-28I

2~0

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lowering of the transition time and a moment comes at which drop time and transition time coincide. From this time on, the electrode potential undergoes a considerable shift in the negative direction during the last period of the drop life. This indicates that a second process (reduction of a second component or decomposition of the supporting electrolyte) starts during this period. In contrast to the constant current intensity method, where the left branch of the envelope of the recorded diagram resembles the classical polarogram, here it is the right branch (Fig. 2). seconds

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Fig. 2. P o l a r o g r a m for a 5" lO-4 M Cd ~+ soln. in o.I M KC1.

By analogy with the term "limiting current" in ordinary polarography, "limiting time" may be used for the moment when a considerable change of the course of the envelope is observed. It is also possible to apply the same construction for the determination of its value. The reproducibility appears to be very good. The results obtained thus far indicate that the limiting current is proportional to the concentration (this holds also for a second component), to the two-thirds power of the head of mercury, and to the one-sixth power of the drop-time. The limiting time is inversely proportional to the gradient of the current density sweep. Compared with classical polarography, this procedure allows the ohmic voltage drop in the circuit to be more easily eliminated. The apparently additional advantage of the absence of a maximum in the time vs. potential curve loses much of its interest because the value of the limiting time is strongly affected b y the absence of a capillary-active substance. This is in conformity with the swirling around the mercury drop observed in the constant intensity method 8. It will be possible to reduce, by electronic means, the recorded diagram to the neighbourhood of the envelope and thus to reduce the paper consumption. It is hoped in the future to analyse the theoretical basis of the proposed method and give more detailed results.

Laboratory of Analytical Chemistry, Technological University, Delft (Netherlands) J. Electroe~nal. Chem., 16 (1968) 279-281

H. L. KIES

SHORT COMMUNICATIONS

:28I

i M. ISHIBASI-IIAND T. FUJINAGA,Anal, Chim. Acea, 18 (1958) 112. 2 M. SENDA, T. KAMBARAAND Y. TAKEMORI,J. Phys. Chem., 61 (1957) 965. ] I. M. KOLTHOFFAND Y. OKINAKA,J. Am. Chem. Soc., 8o (1958) 4452. 4 T. FUJINAGA,Progress in Polarography, Vol. i, edited by P. ZIJMANAND I. M. KOLTHOFF, Interscience l°ublishers Inc., New York, 1962, p. 2Ol. .5 T. FUJINAGAAND I~. IZUTSU,J. Etectroanal. Chem., 4 (1962) 287. 6 R. W. MURRAY,Anal. Chem., 35 (1963) 1785.

Received August 9th, I:967 j. Electroanal. Chem., 16 (1968) 279-281

BOOK

REVIEWS

Transactions of the Third International Vacuum Congress, Stuttgart, Germany, ~965, Vol. 2, Parts I, I I & III, edited b y H. ADAM, Pergamon Press, Oxford, 1967, xxx + 776 pages, £20.

These 3 handsome books, totalling 800 pages and 3.7 kg weight contain the 122 papers that were presented at the 3rd International Vacuum Congress held in Stuttgart, Germany, in June 1965 . They constitute Volume 2; Volume I, containing the plenary lectures, was published earlier (reviewed in J. Electroanal. Chem., 14 (r967) 371). The Congress attracted 750 participants and the immediate impression one gains (apart from the explosive growth of science, to which we are now attuned) is that vacuum science and technology has come to reach into a surprising number of subjects. Only a few years ago a vacuum system would mean that basic surface phenomena were being investigated in the absence of most of the air. Now we are sputtering, gettering, cryopumping and measuring routinely to the lO -l° tort range; we are studying gas flow forms and vacuum metallurgy, producing microcircuits and testing space vehicles. At a world forum of this nature one might reasonably expect a cross-section of world effort. Certainly, a nice balance is struck between industry on the one hand and government and university laboratories on the other, and 19 countries are ostensibly represented. There are, however, disappointments. Only one Russian laboratory is represented and there is still nothing from China, where a sophisticated vacuum science and technology exists (when will they burst upon us?). Again, there are 31 papers from the United States, but the General Electric Research Laboratory with its continued eminence from LANGMUIRand DUSHMAN to the present, has made no contribution; I.B.M. and Edwards High Vacuum each have 6. The shadow of Professor ALPERT (the Bayard-Alpert gauge, the first ultrahigh vacuum tap) rightly falls across m a n y of the pages but his active laboratory at the University of Illinois is only responsible for one paper via Spain. The United States and Germany (host country) together account for exactly half of all the papers. The official languages were English, French and German but English is the popular choice---7 o of the papers, including 3 from Germany, are in English. Only i i are in French. J. Electroanal. Chem., 16 (1968) 281-282