PROPERIIES
of the electrolytic junction formed between an anion-exchange (AM) and a cation-exchange (CM) membrane have recently been a subject of increased attention.l-U This increasing interest is partially due to a nearly complete analogy between ion-exchange membranes and semiconductors. Also, there is an attractive possibility to apply ion-exchange membrane systems as models for the explanation of certain physiological phenomena. Most of the previous papers deal with the rectification effect at the AM/CM junction. The impedance properties of the junction have not yet been measured extensively; it is to be expected that such measurements can give information on the structure of the junction. We have undertaken such investigations, including the determination of the various factors (frequency, potential difference on membranes, characteristics of membranes etc) that influence the impedance.
We have found a very interesting dependence of the impedance and its components -ohmic and capacitive-on the pd at the membrane junction and on the frequency of the ac signal. Measurements were done in a specially constructed apparatus with an ac measuring bridge and steady state po1arization.S The characteristic dependence on pd is shown in Fig. . The curve representing the imaginary component, COC, of the admittance shows a break at cu -0.4 V, and after passing a minimum toward positive values of pd, rises steeply to finish with a nearly constant value. The dependence of admittance, Y, on pd does not show a minimum, but there are two linear parts in the middle of the curve, which diverge from linearity only at extreme values of pd. The linear parts of the log Y/pd curve intersect at approximately -0.4 V, and this is the value that originates spontaneously (without polarization) in such a membrane system.4 Accordingly, linear parts to the left and right from -0.4 V correspond to polarization in reverse and forward direction, respectively. The linearity of log Y/pd dependence, especially the linear rise of the right hand branch of the curve, is similar in shape to theoretical expressions known in the theory of p-n junction of semiconductors. This conGrms the anaolgy of AM/CM junction to p-n junction, supporting the idea that the membrane junction also contains a space charge, the thickness of which changes with pd. Such an idea is also confirmed by some of our results on the frequency *