Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved. Evaluation of Platinum Electrodes and Three Electrode Potential Standards to Determine Electrode Quality P. R. Owens,* L. P. Wilding, L. M. Lee, and B. E. Herbert ABSTRACT buffer, quinhydrone in pH 4 buffer (Patrick et al., 1996), and ferrous–ferric sulfate (Light, 1972) are three com- The pre-installation evaluation of Pt electrodes in electrode poten- mon laboratory standards that are used to check the tial (E H ) standards is crucial for reliable field interpretations. During proper functioning of E H electrodes before use. a routine check of electrode quality in three different E H standards before field installation, inconsistent and variable results were re- corded. Due to the preliminary observational results, the objectives BACKGROUND OF THE PROBLEM of this study were to: (i) elucidate why E H readings in the same buffer Twenty 50-cm Pt E H electrodes were procured from a non- were different before and after soaking in water, (ii) identify the commercial source. The electrodes were constructed by cut- most rigorous standard for detecting poor electrode function, and (iii) ting 1.0-mm diam. (18 gauge) Pt wire into 1.3-cm segments determine causation for disparity in the electrode performance in (Fig. 1). The Pt segments were soaked in a 1:1 mixture of E H standards when using constructed E H electrodes. Electrodes were concentrated nitric and hydrochloric acids for about 4 h to evaluated in quinhydrone in pH 4 and pH 7 buffers and ferrous–ferric remove contamination of other metals from the surface. The sulfate solutions. Additionally, an experiment was developed to inten- segments were soaked in distilled water overnight. Three milli- tionally expose Cu in E H electrodes with decreasing area of the metal meters of the Pt wire segments were inserted in the end of a exposed. Finally, aqueous macroscopic electrochemical and Fourier drilled 2.6-mm diam. (10 gauge) solid Cu wire. The Cu wire Transform Infrared spectroscopic evaluations of Cu in the three stan- was crimped to make electrical connection and to secure the dards were conducted to characterize and quantify the reduction– Pt wire segment. Exposed Cu was covered with a clear water- oxidation reactions of Cu. This study found that quinhydrone in pH 7 proof epoxy, purchased at a local hardware store (brand name buffer was not a rigorous standard for determining improper function was not provided), so the Pt was the only metal exposed to of Pt electrodes constructed utilizing Cu wire. Due to the interrelation- the redox reaction. In the last step, heat shrink tubing was ship between the reduction–oxidation potential of Cu and pH, stan- used to cover the epoxy. This type of E H electrode construction dards with low pH proved to be a more rigorous standard. is modified from the design described by Faulkner et al. (1989) and Muellar et al. (1985) and typically inserted directly into the soil. Before shipment these electrodes were validated in T he measurement of E H is a standard procedure for quinhydrone buffered at pH 7. The data were shipped with wet soils investigations. Reduction–oxidation po- the electrodes. Upon receiving the electrodes they were soaked in water tentials are indicators of nutrient availability, mobility overnight and rechecked with quinhydrone in pH 4 buffer and of heavy metals, and are also important in the develop- quinhydrone in pH 7 buffer solutions. After checking, only 4 ment of pedogenic properties such as soil color, iron of the 20 electrodes were within the acceptable range listed depletions, and concentrations (Sigg, 2000). The reduc- in Methods of Soil Analysis (Patrick et al., 1996) for quin- tion–oxidation potential is a measure of the propensity hydrone in pH 4 after soaking in water overnight. However, of redox specific chemical reactions to occur. Since soils 19 of the 20 electrodes were within the range with quinhydrone are heterogeneous and most commonly are not at ther- in pH 7 buffer after soaking in water overnight. Due to the modynamic equilibrium, an E H measurement is com- conflicting data of the electrodes checked in quinhydrone in posed of mixed potentials and has no quantitative utility. pH 4 and pH 7 standards, several experiments were conducted Given the problems with interpretation of E H and E H with the following objectives. determination in the field, this parameter is still used. The E H measurements are quick, simple to obtain, and OBJECTIVES infer useful information regarding biogeochemical reac- 1. Elucidate why E H readings in the same buffer were differ- tions that are possible in soils and sediments. Also, given ent before and after soaking in water? the numerous theoretical problems with E H measure- 2. Identify the most rigorous standard for detecting poor ment in mixed potential environments, it is extremely electrode function. important to ensure the electrodes are functioning prop- 3. Determine causation for disparity in the electrode per- formance in E H standards when using constructed E H erly before installing in the soil. Quinhydrone in pH 7 electrodes. P.R. Owens, Purdue Univ., Dep. of Agronomy, Lily Hall of Life MATERIALS AND METHODS Sciences, 915 W. State St., West Lafayette, IN 47906; L.P. Wilding, Soil and Crop Sciences Dep., Texas A&M Univ., College Station, TX 77845; L.M. Lee and B.E. Herbert, Dep. of Geosciences, Texas Preliminary Experiment on the Procured Electrodes A&M Univ., College Station, TX 77845. Received 6 Aug. 2003. *Cor- responding author (prowens@purdue.edu). After the initial checks, ten of the procured 50-cm elec- trodes were randomly selected from the batch of 20. Five Published in Soil Sci. Soc. Am. J. 69:1541–1550 (2005). electrodes were left unaltered and five were re-epoxied. On Wetland Soils doi:10.2136/sssaj2003.0205  Soil Science Society of America Abbreviations: E H , electrode potential; FTIR, Fourier Transform In- frared Reflectance; SEM, scanning electron microscope. 677 S. Segoe Rd., Madison, WI 53711 USA 1541 Published online August 4, 2005