Metal Finishing Guide Book

2012-2013

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Table II. Standard Potential (E0) Electrode Reaction E0 (V) Na+ + e- = Na -2.714 CNO- + H2O + 2e- = CN- + 2OH- -0.97 Zn2+ + 2e- = Zn 2H+ + 2e- = H -0.763 Ag/AgCl electrode, 4N KCl +0.199 Calomel electrode, sat KCl (Cr2O7)2- + 14H+ + 6e- = 2Cr3+ + 7H2O +0.244 2 0.000 +1.33 the sample. When the glass (pH) and reference electrodes are paired, or built together as a combination electrode, and connected to a pH meter, the voltage developed at the electrode pair is amplified and displayed on a meter or a digital readout. The meter is standardized with a known solution called a buffer. A buffer is a specially prepared solution that resists changes in pH and has a specific pH value at a specific temperature. In Table I the component of the most common buffers is shown. When highly accurate pH measurements are required, temperature, meter standardization, and electrode technique should be considered. Temperature has two effects on the accuracy of pH readings. The first effect is on the electrodes, which is corrected by either automatic temperature compensation (ATC) or by manually adjusting the temperature control on the pH meter. Temperature also effects the pH value of the buffer (which is used for standardization). Because samples do not change pH with temperature in the same way, the relationship between temperature and pH must be determined experimentally. Most buffer bottles have a chart on their labels that gives the pH as a function of temperature. The pH meter should be standardized with accurate buffers, which have pH values close to the pH values of the samples. Use of the slope control corrects for nonideal electrode behavior associated with aging of the electrodes. Proper storage of the electrodes increases their life and provides faster response and less drift when making pH measurements. The electrodes should be carefully rinsed between each measurement to prevent contamination. Do not wipe the glass electrode, which could transfer a charge to the glass bulb resulting in a long stabilization time. The rinsed glass electrode should be stored in distilled water. Finally, the consistency of the measuring technique is important, whether the samples are measured in static or stirred condition; however, the standardization buffer and the sample solution should be treated the same. OXIDATION-REDUCTION POTENTIAL Oxidation-reduction potential (ORP) measurements are used to monitor chemical reactions, quantify ion activity, or determine the oxidizing or reducing properties of solutions. Although ORP measurements are somewhat similar to those of pH, the potential value must be interpreted carefully for useful results. An ORP measurement is made using the millivolt mode of the pH meter. Consequently, by substituting a metallic electrode for the pH glass electrode, many other ions besides the hydrogen ion can be detected with the same pH meter. In many chemical reactions electrons are transferred from one substance to another. By definition a substance gains electrons in a reduction reaction and loses electrons in an oxidation reaction. Oxidation and reduction reactions occur 567

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