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How to Make oxidation I= edLIC110fl I 'OP1111A Measurements with Metallic; 1=1( x : io ol, Oxidation-reduction potential (ORP) measurements When reactions arc, wnllen as oxidations (for ' are used to monitor chemical reactions, quantity ion example, Na = Na 1 I P ) polontials have (tie activity or determine the oxidizing or reducing opposite polarity properties of solutions While ORP measurements are The standard potential, EO of any oxidation- somewhat similar to those of pH, the potential value reduction reaction is referenced to the standard must be Interpreted carefully for meaningful results hydrogen electrode and refers to conditions of the The following discusses the principles and electrode oxidation-red uclion reaction where temperature is types involved, how measurements are made, and 25°C, ion activities are unity and gases are at 1 alm typical applications pressure Table 5-1 shows the standard potential, EO An ORP measurement is made using the millivolt associated with various reactions mode of a pH meter Thus by substituting a metallic The oxidation-reduction potential (ORP) is charac- electrode for the pH glass electrode, many other ions terfslic of reactions involving both oxidation and besides the hydrogen ion can be detected with the reduction ORP varies as a function of (a) the same pH meter standard potenlial EO associated with each rection (1)) iefalive ion conr,entralions (L.) lemperature, and ' Understanding Oxidation-Reduction (d) the number of elections lian%lerred in the In many chemical reactions, electrons are Trans- leacliorrs ferrel from one substance to another By definition a substance gains electrons In a reduction reaction a Common ORP Potential Values substance loses electrons In an oxidation reaction Oxidation ieduction potentials are usually displayed "'qOxldation and reduction reactions occur together The as millivolts (mV) When measured with a pH meter ravailable electrons from an oxidized substance are (winch is set to read in millivolts) this oxidation- taken up by the reduced substance until an reduction potential is generally the EMF difference equilibrium condition Is reached developed between the ORP metallic indicating 't'The relative tendency of different subst }» ances to electrode and a constant voltage reference ., �r electrons (relative reductlofl potentials) varies electrode, (for example, saturated cdlomel instead oF- -- . depending`on the number of electrons in the outer a normal hydrrt.ogen electrode) that is immersed in the F4�.�v shelf and on the size of the atom or Ion Accordingly test solution Any one of three different types of ' these substances can be tabulated in descending metallic electrodes may be used The nature of the order with those substances which gain electrons test solution and the method to be used will most easily at the top determine the choice of the electrode This is discussed later tDefining OAP Regardless of which of these electrodes is used, Since It is impossible to measure absolute polen- the potential can be expressed by a general form of 'i tlals, an arbitrary standard, the hydrogen electrode, is the Nernst equation ' ,,chosen Oxidation-reduction potentials are defined Eh 4 EO + 59 2 log o Oxidant al 25°C (1) �+ relative to this standard The electrode reaction n g (Reductant) 2H+ + 2e- = H2 where ' Is assigned a potential of 0 00 volts when the Eh = the voltage difference between the hydrogen Ion activity is 1 0 M and the partial pressure oxidation-reduction electrode and the normal of hydrogen gas is one atmosphere hydrogen electrode mV EO = a constant characteristic of the system in Electrode Reaction E°Volts question, mV Na ' + e` = Na -2714 714' Na 4' + H ` ` - _2 = Eh when the activities of the oxidant and the 2O + 2e CN + QOH 97 reductant are equal, (i e , the ratio of (Ox)t Zn+ + + 2e` = Zn -0763 + (Red) = 1, and since the logarithm o( 1 is 211 + 2e-" H2 00 t) SO= ++ 4H+ + 2e'- = H2S03 + H2U + p i 7 equal to 0, E must equal Eli) ' Ag1AgCl electrode, 4N KCI 1 0 19J n the number of electrons reacting Calomel electrode sal KCI + 0244 Since the normal hydrogen electrode Is rarely used Ag+ + e- = Ag roil as the reference electrode in actual measurements, Cr20 7 + 1411+ + se- = 2Cr3 i , 7 neo the measured potential (E) will not be equal to Eh Eh ' C12 + 2e` = 2cl- _ + i 35_ can be calculated by adding algebraically the Table 5 1 Slandard oxidation Reduction Polenliais measured voltage, E, to the constant voltage, E', of �F the constant voltage reference electrode used C