Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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3Cl2 + 2OCN- + 4OH- = 6Cl- + 2CO2 + N2 + 2H2 O In industrial plants ORP is rarely applied to nice clean reactions where poten- tials can be estimated easily. In waste treatment or sewage plants, for instance, solu- tions contain a host of constituents that the reagent oxidizes and reduces simul- taneously. ORP relates to the concentrations and activities of all participating reactions. It frequently becomes necessary to determine the control points exper- imentally. If day-to-day relative potential values are to be compared, the pH meter must be standardized to the same starting point. Short the meter glass and reference inputs and adjust the standardization control until zero millivolt is displayed using the "absolute millivolt" mode to set the potential to some arbitrary value when the electrodes are reading the potential in a repeatable standard solution. Because ORP is a characteristic measure of redox equilibrium it should not require standardization or calibration. The measured potential is absolute in a sense. Yet, frequently, it is desirable to check systems for proper operation and electrode poisoning. Solutions of known potential can be developed by saturating buffer solutions with quinhydrone. The reaction is such that the measured poten- tial will vary only along with the solution pH and temperature. The procedure is as follows: (1) Saturate the buffer with quinhydrone, made up fresh for each test. Quinhydrone is not readily soluble so a few crystals stirred into the buffer are suf- ficient. The solution will be amber colored. (2) Clean the platinum electrode. (3) Place the platinum and reference electrodes in a quinhydrone-buffer solution and mea- sure the potential and temperature. Measured potential will generally be within ±10 mV of theoretical value. The ORP values of quinhydrone-buffer solution can be seen in Table III. 510

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