Redox Theory
THEORETICAL BACKGROUND
Redox indicators measure the ratio of oxidised species to reduced species in a solution containing both, according to the Nernst equation:E=E0 + kT log (activity of oxidised form) n (activity of reduced form)
where E is the potential assumed by the electrode, E0 is a reference potential, k is a constant, T is the absolute temperature (toC + 273.2), and n is the number of electrons involved in the equilibrium between the oxidised and reduced species. This equilibrium can be represented as a chemical equation by(oxidised form) +ne (reduced form)
where e is an electron.
The indicator electrode measures electron movement, and to do this it must be
chemically inert and be an electron conductor. Platinum and gold fulfil this function in most environments, and so are used as redox indicator electrodes.
REDOX MEASUREMENT
As with pH measurement, the electrochemical potential developed by a redox indicator electrode is measured by comparing it to a stable reference potential, as provided by a reference electrode. Thus the physical and electrical form of a redox "cell" is essentially similar to a pH cell, and it is used in the same way, normally with a pH meter to measure the millivolts output of the cell in zero (or negligible) current conditions.
There are two important aspects that are worth noting. Firstly, the Nernst equation shows that as the ratio of oxidised to reduce species (the "redox ratio") increases, the indicator electrode becomes more positive, whereas with a glass electrode, as the pH increases the glass becomes more negative. The practical result of this is that with most pH meters the redox indicator electrode is connected to the reference electrode terminal (or a switch provided on the meter to reverse the connection). Secondly, whereas the pH measurements a standardising procedure is used to calibrate the cell, by employment of buffer solutions of known pH, with redox measurements this is not customary. This follows from the fact that it is the actual magnitude of the millivolts developed by the cell that is of relevance, so no standardisation is called for, given a knowledge of the electrochemical potential contribution by the reference electrode.
Electrochemical measurements are ultimately referred to the so-called hydrogen scale, the convention for which is that the electrochemical potential of a hydrogen electrode in contact with hydrogen gas at one atmosphere partial pressure and a solution containing hydrogen ions at unit activity is zero at all temperatures. The standard SENTEK reference electrode used in SENTEK combination electrodes is the 3MPK1 silver chloride type, and this exhibits potentials on the hydrogen scale of:
E @ 15oC = +120 mV + 2 mV Ag, AgCl, 3MKCl 20oC = +206 mV + 2 mV 25oC = + 203 mv + 2 mV
Thus, to refer a reduction-oxidation potential value measured with a SENTEK combination system to the hydrogen scale, the appropriate value above should be added to the measured value. Where a redox indicator electrode is used with a SENTEK saturated KCl calomel reference electrode, the relevant corrections are:E @ 15oC = +251 mV + 2 mV cal, satd KCl 20oC = +248 mV + 2 mV 25oC = +245 mV + 2mV
Because as the solution condition becomes more oxidised the indicator electrode becomes more positive, connections should be made to the measuring instrument accordingly, bearing in mind that with pH meters the glass input terminal is the negative input. With some digital instruments this is simplified through positive and negative mV readings being indicated on the display with respect to the indicator input connection.
A common procedure for all types of measuring instrument is initially to arrange that no back-off potential is applied between the electrode terminals, either by setting the offset to zero, or by shorting between the terminals and then setting the reading obtained to zero. This then ensures that subsequent measurements give the true redox cell emf values. With meter movement instruments it is sometimes necessary to apply an offset in order to obtain a reading on the meter scale, and if this is done then of course this must be allowed for when readings are recorded. With all measurements it is essential to refer to the pH/mV instrument instruction manual to ensure the correct measuring procedures are being followed.
REDOX "STANDARDS"
Occasionally there is a need or desire to check the performance of a redox system to ensure that it is behaving satisfactorily, and for this purpose "standard" redox solution can be used. A typical circumstance may be where it is suspected that the redox indicator electrode is poisoned; in which case it may be found that after standardising with one solution to the known mV value, the reading with a second standard is not at the known value. If the reference electrode has been poisoned, it is usually found that the reading obtained with one standard solution is different from the known value, although the difference in values between readings in two known standard solution redox values. (Note: the temperature compensation system available on most pH/mV measuring instruments for pH measurements should not in general be used for redox measurements. Where standardising solutions are used, these should be at the same temperature as the test solutions).
Suitable standard solutions and the emf values given by SENTEK combination electrodes with them are:
1. a) 0.1M K4 Fe (CN) 6 = 4.22g)
in 100mls solution
0.05M K3 Fe (CN) 6 = 1.65g)
E20 = +192mV + 2mV @ 20oC
b) 0.01M K4 Fe (CN) 6 = 0.42g)
in 100mls solution
0.05M K3 Fe (CN) 6 = 1.65g)
0.36M KF = 3.39g)
E20 = +258mV + 2mV @ 20oC
2. The quinhydrone system, formerly used as a pH indicator, can be employed. A pinch of quinhydrone is added to a buffer solution of known pH (not higher than 7), and the mV value given by a redox cell stood in this mixture is measured.
Using 0.05M potassium hydrogen phthalate (the primary British Standard pH buffer) = 10.21g/litre solution:
E15 = -268 mV +2 mV
E20 = -264 mV +2 mV
E25 = -260 mV +2 mV
b) Using a 0.025M Na2HPO4 = 3.55g) per litre solution
0.025M KH2PO4 = 3.40g)
E15 = -102mV +2 mV
E20 = -97 mV +2 mV
E25 = -90mV + 2mV
Where the reference electrode is used as a saturated KCl calomel rather that the 3M KCl silver-silver chloride type for which the values above are quoted, the following corrections should be applied:? E15 = 41mV
? E20 = 42mV
? E25 = 42mV
These values should be subtracted from the values given above, which means that values(1) will be numerically smaller, but still positive, and values
(2) will be numerically larger, and still negative.
ELECTRODE POISONING
Redox indicator electrodes are susceptible to physical poisoning by precipitate, grease or oil deposition on their surfaces in the same way as pH glass indicator electrodes are, and the same care and if necessary, cleaning procedures can be employed as with pH systems. This also applies to any reference electrode used with the indicator electrode. However, in some circumstances a platinum electrode may be poisoned in a different way, either by formation of a molecular layer of oxide on the metal surface, or by absorption of organic or other contaminants. The former tends to occur more in very high pH conditions, and the latter in very low ones (although not, it must be emphasised, exclusively so). Specific chemical attack can also occur in very aggressively oxidising conditions, e.g. aqua regia.
Gold shows a slightly lesser tendency to form oxide films, and can occasionally be employed advantageously instead of platinum where this occurs.
Where is molecular level of poisoning is suspected, e.g. by drifting readings or sluggish response, then after simple cleaning of the type that may be used for a glass electrode has been tried and proven to be ineffective, then reductive desorption can be tried, by cathodic cleaning. This consists of making the indicator electrode cathodic in a very dilute sulphuric acid solution (pH 3-4), with another inert material, e.g. platinum or carbon, as anode, by connection to a 4.5 volt battery supply. This will cause hydrogen to be evolved at the redox electrode surface, and should be continued for about 10 minutes. After such cleaning it will be found that the electrode will be hyper-active, and 1-2 hours subsequent immersion in a redox solution may be needed to restore it to a normal operating condition.Familiar applications include the following redox titrations, involving the addition of an oxidising (or reducing) reagent to a reducing (or oxidising) solution, and the determination of redox conditions in biochemical systems.
REDOX POTENTIAL MEASUREMENT
A redox potential measurement is made in exactly the same manner as a pH measurement, using an indicator electrode-reference combination. It is not customary to employ standard reference solutions, but as a check it is possible to use the following solution to identify that the platinum electrode is functioning:39.21 g/l ferrous ammonium sulphate, Fe (NH4)2SO4.6H20
48.22 g/l ferric ammonium sulphate FENH4 (SO4)2. 12H20
56.2 ml/l concentrated sulphuric acid H2SO4
According to T.S. Light, Analytical Chemistry, 44, 1039 (1972), this solution will give a cell emf of +430 mV at a platinum electrode when used with a saturated potassium chloride reference at 25oC. With the standard SENTEK 3M KCl silver-silver chloride reference system (as used in the combination electrodes), the cell emf at 25oC is +468mV. However, it should be appreciated that the cell emf does depend on the condition of the reference electrode and on the temperature of measurement, and deviations from these quoted values do not necessarily indicate that the platinum electrode is malfunctioning.
An alternative checked method is to use two known buffer solutions e.g. pH 4 and one of the standard solution of approximately pH7. A pinch of quinhydrone is added to each, and the difference value should, if a pH meter is used, show the difference in pH between the two buffer solutions, taking due account of the temperature of measurement. (This method eliminates uncertainties due to the reference electrode).
In general, although the system is of low electrical impedance, conventional pH meters having a millivolts scale or digital presentation are used with redox electrode cells. As with other potentiometric, including pH measurements, it is essential to measure the emf in open circuit conditions. If the leakage current across the electrodes allowed is greater than approximately 10-10 amp., polarisation of the system may occur, giving rise to false and drifting readings.
TEMPERATURE EFFECTS
The same temperature characteristics of a slope factor and a temperature dependent zero emf apply as in pH measurement. Although compensation can be applied instrumentally, it is not normally employed. This is mainly because redox potential measurements are mostly made to follow reactions rather than for their own sake. The completion of a redox reaction is normally accompanied by a sharp change in the redox millivolts reading, and this change is usually much larger than the errors induced by neglect and temperature side effects.