Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table III. ORP Values of Quinhydrone-Buffer Standards pH 4 25O 20O Reference Ag/AgCl Calomel C +268 +233 C +263 +218 30O C +258 +213 20O C +92 +47 pH 7 25O C +86 +41 30O C +79 +34 dized and a reduced state must be present. The first type of metallic electrode to be considered consists of a metal contact with a solution of its own ion. The metallic electrode is in a reduced state and its ions are in an oxidized state. An example of this type is silver in a silver nitrate solu- tion. It is used mainly on the analytical field. The second type of metallic electrode consists of a metal coated with a sparingly soluble salt of this metal in a solution of soluble salt with the same anion (e.g., sil- ver-silver chloride in a solution of potassium chloride). The third type of metallic electrode consists of an inert metal in contact with a solution containing both the reduced and oxidized state of an oxidation-reduction system. An example would be platinum in contact with ferric-ferrous ions. Platinum and gold are the most com- mon ORP electrodes. The nature of the test solution and the method to be used will determine the choice of the electrode. The reference electrodes can be identical, but a noble met- al electrode replaces the glass pH electrode. The signal from the ORP electrodes must be fed into an amplifier with high-input resistance. If one or both reactions pair hydrogen ions the ORP measurement becomes pH dependent. Consider the following reaction, which occurs in the reduction of hexa- valent chromium: Cr2O7 2- + 14H+ + 6e- = 2Cr3+ + 7H2 O The reaction depends on solution pH. Potential changes measured by the ORP electrode will continue to vary with the redox ratio, but the absolute potential will also vary with pH. In the first step the pH is lowered to 2 to 2.5. Sulfur dioxide or sodium sulfite solution is used as the reductant. The overall reaction is Cr2O7 2- + 2H+ + 3H2SO3 = 2Cr3+ + 4H2 O + 3S04 2- In the second part of the process the waste liquors are neutralized to a pH lev- el of 7 to 8. At this pH the chromic ion precipitates as a sludge and is sent to clar- ifiers for ultimate disposal or recovery. All these reactions will take place in a def- inite 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 oxi- dation 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-+ H2 O The second step in cyanide oxidation takes place at a controlled pH in the 7 to 8 range. The reaction is: 509

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