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r. ' In most industrial reactions the oxidation reduction reaction is usually characterized by a large change In potential will be controlled at a point that ensures the Eh (or E) of the solution at the equivalence point excess of one reactant This, In turn, ensures that the Becausp of this, potenhometric oxidation-reduction ' reaction will go to completion The ORP measured by titrations are among the most accurate methods of an Instrument can be estimated by the following rules chemical analyses Some examples are titration of 1 Write out the reactions iodine with thiosulfate, titration of manganous ions ' 2 Determine the equivalence potential with permanganate, litration of cyanide ions with 3 Subtract the reference potential hypochlorite and titrallon of ferrous ions with ceric The actual magnitude of the measured value (E) of solutions Other important uses of this electrode are in any particular oxidation-reduction system will depend the manufacture of bleach, treatment of industrial t on all the constants of that system (E0, n, E' the wastes and in studying biological systems voltage of the reference electrode being used to complete the cell) and on the concentrations of the Tdrations ' oxidant and the reductant Therefore, in any reversible Suppose ferrous iron (Fe2+) is litrated with ceric oxidation-reduction system, the measured value, E, or ammonium nitrate solution the calculated value, Eh, is a function of the concen- Ce4+ + e- = Ce3+ Ej= 1 61V trations of the oxidant and the reductant Regardless Fe3+ + e_ = Fel+ E?=0 77V of the initial magnitude of the measured oxidation reduction potential, E, or the calculated potential, Eh, Ce4+ + Fel+ = Ce3+ + Fe3+ E� -E�_ +0$4 both E and Eh of a given system will become more posi- At the start of the titration no ceric ions are in ' tive when the concentration of the oxidant relative to solution The potential of a platinum sensing electrode the reductant increases, i e oxidizing intensity (referenced to that of hydrogen at 25'C) is becomes greater Conversely, the values of E and Eh Fe3+ will become more negative when the concentration of Eh2 = E + 59 2 fog (Fe2+) (6) ' the reductant increases relative to the oxidant,i e reducing intensity becomes greater When ceric ions are added to the solution, the ratio The actual potential measured is the difference of ferric to ferrous iron increases Equation 6 indicates '• between that of the noble-metal electrode and the that the potential sensed by the electrode will Increase reference electrode Potentials referenced to the by 59 2 millivolts at 250C for every decade increase in hydrogen electrode, are commonly designated Eh redox ratio until the equivalence point Is reached ' Instrument mantuacturers, however, normally At this point, the titration Involves both the ferrous- supply the sliver-silver chloride or calomel electrodes ferric ion equilibrium and the ceric-cerous fon for ORP measurement (see Table 5-1) Then the OAP equilibrium The potential of the latter equilibrium is, at potential, measured by the amplifier is 25°C, ' E (ORP) = Eh - E'(reference) Ehi - E° + 59 2 log (Ce (7) Instrument makers have traditionally used the Euro- ' pean convention for polarity of oxidation-reduction At the equivalence point Eh1 = Eh2 = Eeq and potentials since this corresponds with the measured (Ce3+) - (Fe3+) (Ce4 +) _ (Fe2+) potentials Thermodynamically, this means that the oxidation-reduction reaction is written as a reduction Adding Equations 6 and 7 ' For example, consider ORP measurement of a Ce3+)(Ce4 standard solution of chlorine in water using Ag/AgCI 2Eeq = Ei� f E� 4 59 2 log 3+) reference electrode (Fe2+ (Ce Ej + E� ' ORP = En-E'(reference) Eeq = —2 = EO(chlorine) --E'(reference) This relationship shows that the equivalence = +1 35v - (+0 199v) potential is a function only of the Eo values of the half- The meter will read +1 151 V reactions involved it the addillon of ceric ion In the above example is continued, the potential will Increase When to Use ORP toward E�, reaching it when the ratio(Ce4+)1(Ce3+) ' Use of an "electrode of the third type" (noble metal equals one Near the equivalence point there Is a large In oxidation-reducilon system) is essential for the change in potential for small increments of added detection of the equivalence point in numerous lilrant, thus, an ORF'titration curve is similar to an acid- potentiometric oxidation-reduction titrations This base titration curve Some examples of redox tilrations ' method of analysis involves the determination of a are given in Table 5-3 reducing agent by thralron with an oxidizing agent of ' known concentration or vice versa This type of 29