An automated titration system has been assembled which consists of ion selective electrodes (ISEs), automatic buret, interface hardware, minicomputer, cathode-ray tube (CRT) display, light-pen, teletypewriter, and a special function panel. Each of the various parts of the system was designed to perform the task for which it is best suited, with provisions for powerful scientist-computer interactive tech-
that excessive anodic polarization did not occur during generation, the solution was stirred as fast as possible by manual rotation of the electrode, and particular attention paid to monitoring its potential. As the end point, corresponding to complete oxidation of the available Yb(II), was approached, current densities at the tungsten anode were reduced from an initial value of ca. 2 mA cm-2 to ca. 0.2 mA cm-2 and the length of time required for potential stabilization after each generation was observed to increase. However, a smooth S-shaped curve (Figure 1), consistent with normal potentiometric titration behavior, was obtained for the dependence of potential on total charge passed. The lower portion of this corresponded to the oxidation of Yb(II) to Yb(III) and the upper portion to the oxidation of tungsten metal. Good agreement was noted between the position of the point of inflection in this curve and the theoretical end point for the first process. Any indications of the liberation of chlorine, as manifest in a constant potential of the order of +0.3 volt with respect to the platinum reference, were entirely absent.By re-plotting potentials corresponding to points in the upper portion of the curve vs. log (qq-md where q is the total charge passed, and qmt is the charge passed up to the point of inflection, a Nernst plot for the oxidation potential of tungsten was obtained (Figure 2). By carrying out the same procedure with an electrode of platinum foil in place of the tungsten electrode, the position of the end point of the Yb(II) oxidation could be checked, and a Nernst plot for the oxidation potential of platinum obtained (Figure 3).Nernst plots for the anodic oxidation of ytterbium could also be obtained from the first portion of each curve and were in agreement with data obtained by reduction of Yb(III) (Figure 4). It is of interest that both titration curves have the predicted unsymmetrical shape for processes with different numbers of electrons.Although only a limited number of points appear in Figure 2, the linearity is excellent and the slope corresponds to a twoelectron process at 450 °C. The formal potential for the W(II)-W(0) couple [-0.585 V vs. 1.0m Pt(II)-Pt(0)] is also such that Eu(III) should oxidize tungsten metal. Further work is in progress to establish the mechanism(s) of the processes taking place on tungsten in the solvent. The demonstration of the validity of the reference potential (Figure 3) is encouraging.
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