970 Leaching

Using Platinum Electrodes to Indicate Redox Potential

Platinum electrodes will not be inert as initially thought to be. The reactivity of platinum electrodes can clarify their erratic conduct in many electrochemical measurements of metallurgical curiosity, e.g., in flotation techniques, streaming potential measurements, contact-angle measurements, and in leaching methods.

Experimental Procedure

A rotating platinum electrode was used in most of the measurements to research the impact of rotation on measured Eh values. The electrode made by the Pine Instrument Co., Grove Metropolis, Pennsylvania, consisted of a stainless-steel rod with a platinum disc soldered to the top. It was coated with a teflon insulation alongside the edges, in order that solely the circular tip of the electrode was uncovered to the answer. The platinum floor was brightened prior to its use on a metallurgical sprucing wheel utilizing alumina as an abrasive until specified in any other case. 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 prime of the chrome steel shaft with mercury and by dipping a copper wire into the mercury pool. The efficiency of the rotating platinum electrode was compared with the performances of a Beckman type-Eh electrode and a platinum wire electrode. All of the potentials have been measured with respect to a saturated calomel electrode. A saturated KCl-agar bridge was used to reduce the liquid junction potential. A Beckman Zeromatic pH meter along with a Beckman 97200 electrode change was used to measure both the Eh and pH.

Experimental Results

The following summarizes a few of the experimental observations.

  1. Chemical remedy 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 possible of Zero.702 V in distilled water, whereas the freshly polished electrode confirmed a lower value of 0.502 V. The enhanced value of the pre-treated electrode progressively decreased to a gentle new value of Zero.540 V.
  2. Platinum-wire electrodes anodized in an electrolyte of a mix of sulfuric and nitric acids showed Eh values at pH 0 as high as 1.Zero V; cathodized electrodes recorded a decrease worth of 0.750 V at pH Zero for the H2O – O2 couple. An anodic cycle adopted by a cathodic cycle of equal size produced an intermediate outcome. The potential measured was high instantly after anodization and a gradual lower occurred if stored as such in the cell.
  3. An electrode which remained stationary in an electrolyte for hours attained a gentle worth of potential. If reinserted after cleansing in. a chromic acid answer, it indicated an enhanced preliminary potential.
  4. The decrease regular potential, observed in a solution flushed with nitrogen, confirmed a marked improve when the electrode was taken out and reinserted after chemical or anodic remedy in acid options.
  5. An electrode stored stationary in an answer, however prior to reaching a gentle potential, indicated a distinction of 30-40 mV in the measured potential on rotation. Stirring of the answer, too, had an analogous influence on the Eh. In all instances, steady-state Eh values have been obtained in shorter durations with a rotating electrode and the outcomes have been reproducible.
  6. The steady-state potentials observed with all three kinds of stationary electrodes used, specifically, platinum-disc sort electrode, platinum-wire electrode, and Beckman type-Eh electrode, have been similar.
  7. When a redox electrode pair have been related to the measuring circuit, the meter did not point out a gentle reading. To communicate with the variation in Eh in the direction of a gentle state at a continuing pH of the answer, it was crucial to hold the electrode pair related to the circuit at all times. Even after the meter reading had been stabilized, as soon as the electrodes have been disconnected from the circuit and reconnected afterwards, one no’longer observed the previous regular reading. Once more the Eh drifted from a decrease to a better worth, or vice versa, and reached a steady-state studying corresponding to the previous meter studying. This remark was true, whatever sort of pre-treatment had been given to the platinum electrode. An analogous puzzling conduct upon grounding or disconnecting platinum electrodes can also be talked about by Henry.

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

Fe++ = Fe+++ + e E° = -Zero.771 V

Because the experimentally observed Eh is influenced by the hydrolysis of ferric ion and the concentration of the totally different ionic species current within the answer, the shortage of exact knowledge on the actions along with the issues related to the electrode conduct, makes interpretation of the results troublesome.

For the concentration ratio of ferric to ferrous at unity, the noticed Eh was 0.742 V. From a information of the ionic power of the answer, activity coefficients of ferric and ferrous ions in the answer have been calculated. Although the options have been combined to give a ferric-ferrous ratio of unity when it comes to focus, the actual ratio of the activities was not unity. The Eh calculated from the activity values was Zero.748 V which is in close settlement with the noticed worth of 0.742 V. The noticed Eh value also agrees nicely with reported values in the literature.

Eh measurements throughout precipitation of iron by aeration from leach options of manganese ores have been proved to be useful. Within the mild of the previous work, some laboratory research have been made in aqueous solutions of artificial mixtures of ferrous sulfate and manganous sulfate. Upon aeration of every answer the oxidation potential adopted the oxidation reaction of iron in the answer and the stabilized readings corresponded to the completion of precipitation of iron from the solution. The ultimate Eh readings of the present investigation are plotted in Determine 8 together with these reported previously in the combined Eh-pH- diagram for iron and manganese. All of the experimental factors are situated in the area the place Mn++ and Fe(OH)three are secure and restricted inside the equilibrium hydrogen peroxide-oxygen line and the experimental oxygenated water line. On extended aeration all the factors would presumably have reached the experimental oxygenated water line as shown in Figures 2 and 5.

In summary what one observes in such instances is a combined potential having the Eh worth at pH 0 mendacity between 0.84 to Zero.98 V, and a basic expression for the Eh-pH relationship may be written as

Eh = Zero.90 – 0.059 pH

The anomalous conduct of platinum has been an issue in several fields of research. Kolthoff and Kameda attributed the irregularities within 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 in addition to the potential drift generally related to the streaming potential measurements using a pair of platinum electrodes may be explained in the mild of the present investigation. Underneath the experimental circumstances commonly used in flotation analysis, the platinum electrodes can develop polarity by way of adsorption or chemical reaction not solely with oxygen but in addition with inorganic and organic components.

The potential of the ferric-ferrous couple has been decided from equilibrium knowledge as well as from direct potential measurements. In both case, until corrections are made for the hydrolysis of ferric ion and for the actions, nevertheless, the outcomes can’t be interpreted satisfactorily. Popoff and Kunz studied the affect of the variation of iron focus and acid focus on measured potentials, from which the redox potential was determined to be 0.7477 V. Bray and Hershey corrected for the hydrolysis and sophisticated chloride formation and reported the electrode potential to be Zero.772 V. The redox potentials for the varied ferric-ferrous couples noticed in the current research agree with the reported values when correct corrections are made.

A lot of the potentials measured in hydrometallurgical techniques are formal potentials. Further, it is probably not potential to interpret the measured potential when it comes to one redox couple, quantitatively. In leaching techniques containing totally different species what one measures typically might be a “mixed potential.” However Eh respond readily to modifications in ferric-ferrous ratios. The regions of precipitations, complexations, and solubility can simply be recognized in an Eh-pH diagram. The extent of the complexation and the traits of a freshly shaped precipitate will affect the observed potential.

The following equilibrium relationships, which have been studied experimentally, have been in truthful settlement with theoretical deductions.

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

Eh = 1.056 – Zero.177 pH – 0.059 log [Fe++]

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

Eh = 0.262 – 0.059 pH

In addition to the change in the relative focus of ferrous to ferric ions, oxygen can set up its personal potential separately. The coincidence of the noticed water-oxygen line with that of the oxygenated ferrous and ferric sulfate answer substantiates a standard relationship for oxygenated aqueous methods. Since such methods are sometimes involved in hydrometallurgical and flotation studies, this remark is of particular interest.

In lots of hydrometallurgical operations Eh has been regarded as a convenient indicator. In the acid leaching of uranium ores, for instance, the tetravalent uranium in uraninite, pitchblende, and coffinite is actually insoluble in sulfuric acid options of pH near 1 in the absence of an oxidizing agent. Ferric ion is most commonly used for this function, and the ratio of ferric to ferrous ion in answer is maintained in excess of 1:1 by means of using manganese dioxide or sodium chlorate.

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