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For example, if the saturated a c lomet electrode For an electrode of the second type, the general A (SCE) Is used as the reference electrode, and the equation becomes measured voltage E Is 400 mv, Eh = 400 + 244 = E - EO — f log (A—)Y at 250C 644 my at 25°C (voltage of saturated calomel h n electrode Table 5-4 Is 244 3 my at 25°C) It the where measured value E is —544 mV, the Eh = A— = the concentration (activity) of the anions ' —544 + 244 = --300 mV at 250C The El; of an being measured must correspond to the anion of oxidation-reduction system will always differ from the the sparingly soluble salt coating the metallic measured value E by the voltage difference between electrode in this case, the activity of the oxidant the reference electrode being used and a normal hydrogen electrode Both the Eh value and the (sparingly soluble salt) is 1 by definition The term +tog (Ox)!(Red) becomes +log [7 1(A—) Y)which measured value E of any particular oxidation- is --log (A—)Y reduction system will depend on the values of E0, n (the number of electrons reacting), and the molar Electrodes of this type can be used for the direct concentrations (activities to be strictly accurate) of determination of potenUometric titration of a solution the oxidant and the reductant Therefore, the observed containing the anions of the sparingly soluble salt ' potential can be expressed as a combination of formingthe coatis on the electrode equations (1)and (2) The third type of metallic electrode consists of an 59 2 (Oxidant) inert metal in contact with a solution containing both E = EO `- E' + log (>ieductant)at 25°C (3) the oxidized and reduced states of an oxidation- n ' reduction system Making ORP Measurements Ideally, the inert metal electrode in a well-mixed oxidation-reduction system serves only to acquire the ' What Electrode to Use electrochemical potential of electrons, depending on the prevailing redox equilibrium in solution The metal There are three types of metallic electrodes used In donates and accepts electrons and otherwise does ORP measurements which differ in construction, but not participate in the oxidation-reduction reaction The ' all are based on the same principle that an oxidized particular metal used is relatively unimportant as long and a reduced state must always be present as it is sufficiently noble Platinum and gold are the The first type of metallic electrode to be considered most common ORP electrodes ' consists of a metal in contact with a solution of its An example would be platinum in contact with a own Ions The metal electrode is in the reduced state solution of ferric-ferrous Eons Electrodes of this and Its Ions are in the oxidized state type are used for polentiometric oxidation-reduction ' When an electrode of the first type is used, the lilrations, and for the direct measurement of the general form of the Nernst equation can be further oxidizing or reducing intensity of solutions simplified to ' Eh = EO + .2log Mn+ at 25°C Equipment Necessary for Measurement Equipment for ORP measurement compares closely where to that for pH measurement The reference electrodes Mn+ = the concentration (activity) of the cations can be Identical, but a noble-metal electrode replaces ' corresponding to the metal electrode being used In the glass pH electrode The potential is measured in this case, the activity of the reductant (the pure millivolts Temperature compensation is not used The metal electrode) is always i by definition, and signal from the ORP electrodes must be fed Into an / ' therefore the reductant term drops out amplifier with high-input resistance M example of this type Is sliver In a silver nitrate The sleeve junction calomel reference electrode is solution It Is used for the direct measurement and recommended for many applications since it provides ' potentfometric titratlon of solutions of the cations of an easily cleaned function with fast response, the particular metal electrode being used If chloride Ion interferes with the oxidation-reduction The second type of metallic electrode which may system, the quartz unction calomel electrode with sail be used consists of a metal coated with a sparingly bridge and chloride-free filling solution should be used ' soluble salt of this metal in contact with a solution of a as the reference soluble salt with the same anion, for example, silver- silver chloride in a solution of potassium chloride The half-cell reaction for this type of electrode can be written for the metal (M) and the ancon (A) as follows MxAy + ne— = x M + + yA — c ;?7