Metal Finishing Guide Book


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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 electrode reaction 2H+ + 2e- = H2 is assigned a potential of 0.000 V when the hydrogen activity is 1 M (concentration of 1 mole/L), and the partial pressure of hydrogen is 1 atm. When reactions are written as oxidation (e.g., Na = Na+ + 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 25OC, ion activity is unity, and gases are at 1 atm pressure. Table II shows the standard potential E0 associated with various reactions. 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 electrode (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 measurements that differ in construction but are based on the same principle that an oxidized 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 solution. 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., silver-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 common 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 metal 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 hexavalent chromium: Cr O 2- + 14H+ + 6e- = 2Cr3+ + 7H O 2 7 2 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 568

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