“In Situ” Use of the Ion Selective Electrode, the Selectrode^tm, in Studies of Soil-Plant Relationships

Embedded in soils or immersed in a soil/solution system solid state ion selective electrodes, the Selectrodes, were used in continuous measurements of the activities of hydrogen, chloride and copper (III) ions of the solution phase. The total copper (II) concentration of the solution phase was assayed by the standard addition technique. Cation exchange isotherms were directly obtained by means of two pH-Selectrodes, one of which being charged with the exchanger material. The potentialities of “in situ” use of Selectrodes in soil-plant relationship work are emphazised. Great interest have recently been focused on the application of ion selective electrodes for “in situ” measurements in aqueous systems. Thus, fluoride electrodes are currently being used for monitoring the fluoride level in fluoridated water supplies, and at the Woods Hole Oceanographic Institution a device for continuous measurement of marine fluoride/chlorinity profiles have been constructed1. Although of inherent importance, no attempts to perform “in situ” measurements in soils have been reported. This might probably be ascribed to limitations in the existing electrode types. Recently, however, a new type of solid state ion selective electrode, the Selectrode, was developed in this laboratory2,3. By avoiding a membrane construction, it has been possible to vary the shape, size and design of the Selectrode to such a degree that in addition to the conventional “dip” type Selectrode, “flow-through”, “button” type and micro Selectrodes, (sample volume 5 μl) successfully have been produced. Because of its simplicity of construction, robustness, and the capacity of shaping the electrode body according to need, the Selectrode was thought to be ideally suited for “in situ” measurements in soils or sea waters. A Selectrode of the conventional “dip” type is constructed as a Teflon tube sealed at one end with a pressed Teflon-graphite matrix, into which is inserted a stainless steel contact. The solid electroactive material is either mixed into the Teflon-graphite powder before the matrix is pressed or rubbed as a superficial layer into the matrix surface facing the sample solution. The latter approach, with an electroactive compound adsorbed on the surface of an electrochemically inert, but conducting, material has the advantage that the surface can easily be renewed, or doped with a different electroactive material, thus readily making the Selectrode sensitive to another ionic specie. A number of Selectrodes activated with various water insolube compounds have thus far been prepared, e.g. Selectrodes capable of measuring: redox, and mercury (III), silver (I), cadmium(II), lead (II), potassium, iodide, bromide, chloride, copper (II) and hydrogen ions. Of these the three latter Selectrodes and a calomel reference Selectrode were used for the experiments reported in this communication, the copper (II) Selectrode being of the “dip” type and the others being “button” type Selectrodes.