Using Platinum Electrodes to Indicate Redox Potential

Platinum electrodes are not inert as originally thought to be. The reactivity of platinum electrodes can explain their erratic behavior in many electrochemical measurements of metallurgical interest, e.g., in flotation systems, streaming potential measurements, contact-angle measurements, and in leaching systems.

Experimental Procedure

A rotating platinum electrode was used in many of the measurements to study the effect of rotation on measured Eh values. The electrode made by the Pine Instrument Co., Grove City, Pennsylvania, consisted of a stainless steel rod with a platinum disc soldered to the end. It was covered with a teflon insulation along the sides, so that only the circular tip of the electrode was exposed to the solution. The platinum surface was brightened prior to its use on a metallurgical polishing wheel using alumina as an abrasive unless specified otherwise. The electrode was rotated with a Sargent synchronous motor at 350 rpm. The contact of the electrode with the external circuit was made by filling a notch at the top of the stainless steel shaft with mercury and by dipping a copper wire into the mercury pool. The performance of the rotating platinum electrode was compared with the performances of a Beckman type-Eh electrode and a platinum wire electrode. All the potentials were measured with respect to a saturated calomel electrode. A saturated KCl-agar bridge was used to minimize the liquid junction potential. A Beckman Zeromatic pH meter together with a Beckman 97200 electrode switch was used to measure both the Eh and pH.

Experimental Results

The following summarizes some of the experimental observations.

1. Chemical treatment of a platinum-wire electrode or a rotating platinum electrode with sulfuric, nitric, and chromic acids resulted in an enhanced Eh compared to a freshly polished electrode in solutions of manganous sulfate, ferrous sulfate, and distilled water. In one experiment the pre-treated electrode indicated a potential of 0.702 V in distilled water, whereas the freshly polished electrode showed a lower value of 0.502 V. The enhanced value of the pre-treated electrode gradually decreased to a steady new value of 0.540 V.
2. Platinum-wire electrodes anodized in an electrolyte of a mixture of sulfuric and nitric acids showed Eh values at pH 0 as high as 1.0 V; cathodized electrodes recorded a lower value of 0.750 V at pH 0 for the H2O – O2 couple. An anodic cycle followed by a cathodic cycle of equal length produced an intermediate result. The potential measured was high immediately after anodization and a gradual decrease occurred if kept as such in the cell.
3. An electrode which remained stationary in an electrolyte for hours attained a steady value of potential. If reinserted after cleaning in. a chromic acid solution, it indicated an enhanced initial potential.
4. The lower steady potential, observed in a solution flushed with nitrogen, showed a marked increase when the electrode was taken out and reinserted after chemical or anodic treatment in acid solutions.
5. An electrode kept stationary in a solution, but prior to reaching a steady potential, indicated a difference of 30-40 mV in the measured potential on rotation. Stirring of the solution, too, had a similar influence on the Eh. In all cases, steady-state Eh values were obtained in shorter periods with a rotating electrode and the results were reproducible.
6. The steady-state potentials observed with all three types of stationary electrodes used, namely, platinum-disc type electrode, platinum-wire electrode, and Beckman type-Eh electrode, were identical.
7. When a redox electrode pair were connected to the measuring circuit, the meter did not indicate a steady reading. To keep in touch with the variation in Eh towards a steady state at a constant pH of the solution, it was necessary to keep the electrode pair connected to the circuit at all times. Even after the meter reading had been stabilized, once the electrodes were disconnected from the circuit and reconnected afterwards, one no’longer observed the previous steady reading. Again the Eh drifted from a lower to a higher value, or vice versa, and reached a steady-state reading corresponding to the former meter reading. This observation was true, whatever type of pre-treatment had been given to the platinum electrode. A similar puzzling behavior upon grounding or disconnecting platinum electrodes is also mentioned by Henry.

In a system consisting of ferric and ferrous ions, the measured Eh is dependent on the ratio in the solution. According to Latimer the standard potential of the ferric-ferrous couple is given as

Fe++ = Fe+++ + e E° = -0.771 V

Since the experimentally observed Eh is influenced by the hydrolysis of ferric ion and the concentration of the different ionic species present in the solution, the lack of precise data on the activities together with the problems associated with the electrode behavior, makes interpretation of the results difficult.

For the concentration ratio of ferric to ferrous at unity, the observed Eh was 0.742 V. From a knowledge of the ionic strength of the solution, activity coefficients of ferric and ferrous ions in the solution were calculated. Though the solutions were mixed to give a ferric-ferrous ratio of unity in terms of concentration, the actual ratio of the activities was not unity. The Eh calculated from the activity values was 0.748 V which is in close agreement with the observed value of 0.742 V. The observed Eh value also agrees well with reported values in the literature.

Eh measurements during precipitation of iron by aeration from leach solutions of manganese ores have been proved to be useful. In the light of the past work, some laboratory studies were made in aqueous solutions of artificial mixtures of ferrous sulfate and manganous sulfate. Upon aeration of each solution the oxidation potential followed the oxidation reaction of iron in the solution and the stabilized readings corresponded to the completion of precipitation of iron from the solution. The final Eh readings of the present investigation are plotted in Figure 8 together with those reported previously in the combined Eh-pH- diagram for iron and manganese. All the experimental points are located in the area where Mn++ and Fe(OH)3 are stable and limited within the equilibrium hydrogen peroxide-oxygen line and the experimental oxygenated water line. On prolonged aeration all the points would presumably have reached the experimental oxygenated water line as shown in Figures 2 and 5.

In summary what one observes in such cases is a mixed potential having the Eh value at pH 0 lying between 0.84 to 0.98 V, and a general expression for the Eh-pH relationship may be written as

Eh = 0.90 – 0.059 pH

The anomalous behavior of platinum has been a problem in different fields of study. Kolthoff and Kameda attributed the irregularities in the measurement of pH with a hydrogen electrode to adsorption of NaOH on platinized platinum. In zeta-potential measurements of mineral grains, the asymmetry potential as well as the potential drift commonly associated with the streaming potential measurements using a pair of platinum electrodes may be explained in the light of the present investigation. Under the experimental conditions commonly used in flotation research, the platinum electrodes can develop polarity through adsorption or chemical reaction not only with oxygen but also with inorganic and organic additives.

The potential of the ferric-ferrous couple has been determined from equilibrium data as well as from direct potential measurements. In either case, unless corrections are made for the hydrolysis of ferric ion and for the activities, however, the results cannot be interpreted satisfactorily. Popoff and Kunz studied the influence of the variation of iron concentration and acid concentration on measured potentials, from which the redox potential was determined to be 0.7477 V. Bray and Hershey corrected for the hydrolysis and complex chloride formation and reported the electrode potential to be 0.772 V. The redox potentials for the various ferric-ferrous couples observed in the present study agree with the reported values when proper corrections are made.

Most of the potentials measured in hydrometallurgical systems are formal potentials. Further, it may not be possible to interpret the measured potential in terms of one redox couple, quantitatively. In leaching systems containing different species what one measures often will be a “mixed potential.” But Eh respond readily to changes in ferric-ferrous ratios. The regions of precipitations, complexations, and solubility can easily be identified in an Eh-pH diagram. The extent of the complexation and the characteristics of a freshly formed precipitate will influence the observed potential.

The following equilibrium relationships, which were studied experimentally, were in fair agreement with theoretical deductions.

Fe++ + 3H2O = Fe(OH]3 + 3H+ + 3e

Eh = 1.056 – 0.177 pH – 0.059 log [Fe++]

Fe (OH)2 + H2O = Fe(OH)3 + H+ + e

Eh = 0.262 – 0.059 pH

In addition to the change in the relative concentration of ferrous to ferric ions, oxygen can set up its own potential separately. The coincidence of the observed water-oxygen line with that of the oxygenated ferrous and ferric sulfate solution substantiates a common relationship for oxygenated aqueous systems. Since such systems are often involved in hydrometallurgical and flotation studies, this observation is of specific interest.

In many hydrometallurgical operations Eh has been thought of as a convenient indicator. In the acid leaching of uranium ores, for example, the tetravalent uranium in uraninite, pitchblende, and coffinite is essentially insoluble in sulfuric acid solutions of pH near 1 in the absence of an oxidizing agent. Ferric ion is most commonly used for this purpose, and the ratio of ferric to ferrous ion in solution is maintained in excess of 1:1 through the use of manganese dioxide or sodium chlorate.