Metal Finishing Guide Book

2012-2013

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Table III. ORP Values of Quinhydrone-Buffer Standards pH 4 pH 7 20OC 25OC 30OC 20OC 25OC 30OC Ag/AgCl +268 +263 +258 +92 +86 +79 Calomel +233 +218 +213 +47 +41 +34 Reference Cr2O72- + 2H+ + 3H2SO3 = 2Cr3+ + 4H2O + 3S042In the second part of the process the waste liquors are neutralized to a pH level of 7 to 8. At this pH the chromic ion precipitates as a sludge and is sent to clarifiers for ultimate disposal or recovery. All these reactions will take place in a definite pH and in a specific millivolt range. Although applications for ORP measurement are not as widespread as for pH one of the most important applications in the metal-finishing industry is the oxidation of cyanide wastes. Oxidation converts the toxic cyanides to harmless compounds. Typically, chlorine gas or sodium hypochlorite are the oxidants. In the first step caustic or lime is added to make the cyanide-bearing waste alkaline to a pH of 9 to 10. An acid solution would release deadly cyanide gas; therefore, the system generally incorporates pH control. The first stage reactions are Cl2 + CN- + 2OH- = 2Cl- + OCN- + H2O The second step in cyanide oxidation takes place at a controlled pH in the 7 to 8 range. The reaction is: 3Cl + 2OCN- + 4OH- = 6Cl- + 2CO + N + 2H O 2 2 2 2 In industrial plants ORP is rarely applied to nice clean reactions where potentials can be estimated easily. In waste treatment or sewage plants, for instance, solutions contain a host of constituents that the reagent oxidizes and reduces simultaneously. ORP relates to the concentrations and activities of all participating reactions. It frequently becomes necessary to determine the control points experimentally. 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 potential 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 sufficient. The solution will be amber colored. (2) Clean the platinum electrode. (3) Place the platinum and reference electrodes in a quinhydrone-buffer solution and measure 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. 569

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