Electrode saturation

Finally, calomel electrodes (and more especially hydrogen electrodes) are not suitable for field measurements because they are not sufficiently robust. The calomel electrodes are however essential for calibrating the field reference electrodes. Saturated KCI calomel electrodes are the most suitable because there is then no doubt about the reference potential of the calibrating electrode. Lack of adequate calibration is a common cause of cathodic protection system mismanagement. 

In practice, it is very often necessary to determine the potential of a test (indicator) electrode connected in a cell with a well defined second electrode. This reference electrode is usually a suitable electrode of the second kind, as described in Section 3.2.2. The potentials of these electrodes are tabulated, so that Eq. (3.1.66) can be used to determine the potential of the test electrode from the measured EMF. The standard hydrogen electrode is a hydrogen electrode saturated with gaseous hydrogen with a partial pressure equal to the standard pressure and immersed in a solution with unit hydrogen ion activity. Its potential is set equal to zero by convention. Because of the relative difficulty involved in preparing this electrode and various other complications (see Section 3.2.1), it is not used as a reference electrode in practice. 

The hydrogen electrode is one of the electrodes suitable for pH measurements. If a hydrogen electrode saturated at unit pressure is combined with a reference electrode, then the EMF of this cell (where the reference electrode is written on the left-hand side in the scheme of the cell) is given by the equation... 

Potentials were measured at room temperature relative to an aqueous calomel electrode saturated with NaCl, using a rotating platinum electrode data for R = H complexes from reference 13, all other data from references 5 and 9. 

The electrode potential is the EMF of a cell in which the electrode on the right-hand side in the scheme is the test electrode, while the electrode placed on the left-hand side is a standard hydrogen electrode (i.e. a hydrogen electrode saturated with hydrogen under standard pressure Po = 1 -013 x 10 Pa and placed in a solution with an activity of hydronium ions equal to one). 

Potentials measured in DMF solutions vs. Ag/AgCl electrode in an aqueous 0.10 M LiCl solution. b Potentials measured in acetonitrile solutions at room temperature tn. an aqueous calomel electrode saturated with Nad. c The difference in potentials measured with techniques (a) and (b) was estimated as -0.03 V, i.e. potentials in DMF may be estimated by adding 0.03 V to the corresponding values measured in acetonitrile. d Potentials measured in CH2C12 us. saturated calomel electrode. 

The left half-cell, connected to the negative terminal of the potentiometer, is called the standard hydrogen electrode (S.H.E.). It consists of a catalytic Pt surface in contact with an acidic solution in which JAH, = 1. A stream of H2(g) bubbled through the electrode saturates the solution with H2(aq). The activity of H2(g) is unity if the pressure of H2(g) is 1 bar. The reaction that comes to equilibrium at the surface of the Pt electrode is... 

The standard potential (E°) for this reaction is +0.268 V. If the cell is saturated with KCI at 25°C, the potential is +0.241 V. A calomel electrode saturated with KCI is called a saturated calomel electrode, abbreviated S.C.E. The advantage in using saturated KCI is that [Cl ] does not change if some liquid evaporates. 

Predict the value of E for a calomel electrode saturated with KC1, given that E° for the calomel electrode is 0.268 V. (Your answer will not be exactly the value 0.241 used in this book.)... 

S.C.E., saturated calomel electrode A calomel electrode saturated with KCI. The electrode half-reaction is Hg2Cl2(.v) + 2e 2Hg(/) + 2C1-. 

For most potentiometric measurements, either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few mV. The silver/silver chloride electrode also finds application in non-aqueous solutions, although some solvents cause the silver chloride film to become soluble. Some experiments have utilised reference electrodes in non-aqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for non-aqueous systems as are any of the prototypes that have been developed to date. When there is a need to exclude water rigorously, double-salt bridges (aqueous/non-aqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the non-aqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence, the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause dramatic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

Silver/silver chloride electrode saturated with KC1 served as reference and a glassy carbon rod served as auxiliary electrode. 

The following table gives the standard potential (in V) of the silver chloride electrode (saturated KC1) at different temperatures (in CC).L2 The uncertainty is 0.05 mV. 

An example of a gas concentration cell is a system of two hydrogen electrodes saturated with hydrogen at different pressure (p1 > p2), both dipping into the same solution with the hydrogen ions activity aH+ ... 

Reference electrode Saturated calomel electrode (SCE) Neikam and Desmond, 1964 Neikam el al., 1964 - Peover and White, 1967 8 Reference electrode Ag/Ag+ (0.01 M)... 

Potentiometric study. pH measurements were effectuated using pH stat TITRINO titrimeter with a combined glass electrode saturated with KCl. 

The unknown soda ash is titrated with standard HCl using a potentiometric (pH) end point measured with a pH meter using a pH glass electrode-saturated calomel reference electrode combination. The end-point breaks are compared with indicator color changes. 

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