The Use of Membrane Electrodes in the Study of Soils

T~HE glass electrode has rendered such outstandA ing service to soil science in facilitating hydrogen-ion determinations that similar electrodes sensitive to metallic cations have long seemed desirable. The prospect of a direct electrometric determination of calcium, magnesium, potassium, or sodium ions is highly alluring and the importance of such determinations in agronomy and in plant physiology can hardly be overemphasized. This immediately opens up the possibility of denning with precision the ionic environment of the plant root in the soil. Some progress is now being made towards this end. In this brief report we shall discuss the results already obtained and the applications which lie immediately ahead. There is also another way in which our knowledge of membrane behavior impinges upon the subject of plant nutrition. The root hair itself is believed to function through one or more membrane layers. Thus, the general theory of membrane permeability is of the greatest importance. The physical chemistry of inanimate membranes may well provide the clues which are needed to interpret, at least qualitatively, the relationship of living organisms to their chemical environment. Such reasoning was undoubtedly present in the minds of Michaelis and his coworkers (11) who extensively studied the properties of dried collodion membranes. More recently, highly significant advances in theory were made simultaneously by Teorell (12) and by Meyer and Sievers (10). Their work marks the beginning of a quantitative comprehension of selective permeability. For the first time we are now able to see how the electrochemical properties of membranes are related to their selectivity and to compare membranes quantitatively with one another. Thus, the element of empiricism which led earlier workers to search for minerals which would function as calcium electrodes has largely been replaced by a systematic study of membranes combining the following properties: (a) A high effective charge per unit volume, equivalent to a high base exchange capacity; (b) mechanical and chemical stability; (c) ready means of preparation, including the possibility of controlling the electrochemical properties of the product. In this laboratory the work started with the examination of the zeolites and zeolite like minerals (3). These fulfilled condition (a) admirably, but they were deficient in chemical stability and were exceedingly difficult to prepare, since perfect crystals had to be ground down to very thin plates. The next step was the examination of clay films using the high base exchange clays and the method of preparation devised by Hauser and le Beau (2). Here the mechanical stability in aqueous solutions was unsatisfactory. This disadvantage was remedied by the use of preliminary heat treatments ranging to 600 °C. Up to a certain point these heat treatments had the favorable effect of increasing the charge per unit volume and in some cases of affording a gradation in ionic selectivity. So far only the two clay minerals montmorillonite and beidellite have been investigated as to their suitability for membranes. Others would undoubtedly repay study.