Mixed-potential mechanism for the potentiometric response of the sodium tungsten bronze electrode to dissolved oxygen and in chelometric titrations

Abstract

tions of complexing agents, the solution conditions are not constant, so that the chemical reaction, whose rate may depend on the structure of the electrode-solution interface, may cause different potential shifts at different concentrations of complexing agents. Consequently, the results of such studies should be treated with some caution. It is possible to obtain unambiguous results by using concentrations low enough so that the chemical reaction does not exert a significant effect on the half-wave potential. This requires concentrations of 10~6M or less, so that differential pulse polarography must be used. It is important to demonstrate that the half-wave (or peak) potential is independent of the concentration in order to be certain that the chemical reactions do not cause changes in the potential of the wave.The second major factor in the determination of the half-wave potential, that of coordination with the buffer anions as well as hydroxide ions, is entirely predictable (14). Assume that there are three ions in the solution which form stable complexes with the electroactive species. If the three ions are represented by X, Y, and Z and the respective formation constants by Kx, Kv, and Kz, then the half-wave potential may be written as follows:It is assumed that the diffusion coefficients for the oxidized, reduced, and complexed species are equal, and that the activity coefficients are constant. In addition, the concentrations of the complexing agents must be at least 100 times larger than the concentrations of methylmercury in order to minimize changes in concentration at the electrode surface during electrolysis. In a real experiment, X, Y, and Z might represent the two buffer anions and the hydroxyl ion. Use of a supporting electrolyte with coordinating tendencies toward methylmercury would add a fourth variable to the equation. It is clear that the coordinating properties of all the ions in the solution must be considered when evaluating the significance of the halfwave potential. The fact that methylmercury forms complexes with most common buffer anions to about the same degree as hydroxide (28)(29)(30)(31) attests to the futility of attempting pH studies without accounting for this behavior.The differential pulse polarographic peak current vs. concentration curve for the first wave is linear from 10-7 to 10"4M methylmercury. The extreme length of this linear range suggests that this technique may have analytical utility. Two improvements over previous procedures may be realized by this technique. First, sensitivities of 10~7M or 20 Mg/l. in the solution polarographed can be achieved. If a small degree of concentration can be effected during the required extraction procedures, it should be possible to accurately measure methylmercury concentrations less than 1 Mg/l. in the original sample. Second, one may construct synthetic systems and study coordination chemistry at concentrations less than 10~6M. It is not possible to do this type of study with most of the normally used techniques. The main drawback...

Microfabricated Liquid Junction Ag/AgCl Reference Electrode and Its Application to a One-Chip Potentiometric Sensor

“…Although it has been found that an AgCl film spontaneously precipitates upon simply immersing silver in 1−2 M KCl solution, no films are formed in saturated KCl solution . The resulting potential will be a mixed potential, which is observed when two or more simultaneously proceeding oxidation and reduction reactions are involved in exhibiting a potential at the electrode/electrolyte interface. ,, Because the current and potential of each contribution are related with the Butler−Volmer equation, the electrode potential settles at a potential under the zero total current condition. It is evident that it is not an equilibrium state.…”

Active Potentiometry for Dissolved Oxygen Monitoring with Platinum Electrodes

“…Additionally, a potentiometric sensor provides a direct measurement of the oxygen concentration, independent of the diffusion situation inside the sensor and the analyte. Different potentiometric sensors for dissolved oxygen have been realized based on sodium tungsten bronzes [ 6 , 7 ], antimony [ 8 ], copper [ 9 , 10 ], cobalt [ 11 , 12 ], and ruthenium oxide [ 13 ] as material for the indicator electrode. Among them, most promising is the usage of copper and cobalt electrodes because of their applicability in neutral pH media and up to air-saturated oxygen concentrations as well as the availability of these metals in microfabrication technology.…”

Development of silicon-based potentiometric sensors: Towards a miniaturized electronic tongue

“…Lipophilic ionic additives were not added to membranes to avoid the mixed potential response [15,16].…”