Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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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 sam- ples 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 chem- ical reactions, quantify ion activity, or determine the oxidizing or reducing prop- erties 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 los- es electrons in an oxidation reaction. Oxidation and reduction reactions occur together. The available electrons from an oxidized substance are taken up by the reduced substance until an equilibrium condition is reached. Since it is impossible to measure absolute potentials, an arbitrary standard, the hydrogen electrode, is chosen. ORPs are defined relative to this standard. The elec- trode reaction 2H+ + 2e-= H2 is assigned a potential of 0.000 V when the hydrogen activity is 1 M (concen- tration of 1 mole/L), and the partial pressure of hydrogen is 1 atm. When reactions are written as oxidation (e.g., Na = Na+ gases are at 1 atm pressure. Table II shows the standard potential E0 various reactions. C, ion activity is unity, and associated with + e-), potentials have the opposite polarity. The standard potential E0 of any oxidation-reduction reaction is referenced to the standard hydrogen electrode and refers to the condition of the oxidation-reduction reaction where temperature is 25O The ORP is characteristic of reactions involving both oxidation and reduction and varies as a function of (1) the standard potential, (2) relative ion concentration, (3) temperature, and (4) the number of electrons transferred in the reactions. ORPs are usually displayed as millivolts (mV). When measured with a pH meter (set to read in mV), this ORP is generally the electromotive force (EMF) difference developed between the ORP electrode and a constant voltage reference elec- trode (saturated calomel, instead of a normal hydrogen electrode) that is immersed in the solution. Any one of three different types of metallic electrodes may be used. There are three types of metallic electrodes used in ORP measure- ments that differ in construction but are based on the same principle that an oxi- 508

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