Potential asymmetry

As already explained, a glass electrode has an asymmetry potential which... 

It has become fairly common to adopt the manufacture of combinations of internal reference electrode and its inner electrolyte such that the (inner) potential at the glass electrode lead matches the (outer) potential at the external reference electrode if the glass electrode has been placed in an aqueous solution of pH 7. In fact, each pH glass electrode (single or combined) has its own iso-pH value or isotherm intersection point ideally it equals 0 mV at pH 7 0.5 according to a DIN standard, as is shown in Fig. 2.11 the asymmetry potential can be easily eliminated by calibration with a pH 7.00 0.02 (at 25° C) buffer solution. 

The brackets indicate the molar concentrations of the various molecular species. The empirical quantity a is defined by pH = —log a. In sea water, pH measurements do not yield a thermodynamic hydrogen ion activity due to liquid junction and asymmetry potentials a only approximates the hydrogen activity an+. For sea water of 33%>o salinity at 20 °C and at pH 8, 87% of the inorganic phosphate exist as HPO4-, 12% as PO4-, and 1% as H2POj. Of the PO - species 99.6% is complexed with cations other than Na+. The equilibrium relationship for the system is shown in Fig. 15. 

The const in (6.44) points to the importance of the construction aspects of ion sensors. Even if the glass membrane were placed between two identical solutions, the Em would not be zero. This is due to the fact that the membrane develops an asymmetry potential, which arises from the different degrees of mechanical stress at the interior and exterior interfaces of the glass. This affects the exchange current densities. We return later to this point, in the discussion of ion sensors with asymmetric membrane. 

Asymmetry potential — In case of any membrane it happens that the potential drop between the solution and either inner side of the - membrane is not completely identical so that a nonzero net potential drop arises across the entire membrane. This is best known for - glass electrodes and other - ion-selective electrodes. The reasons of asymmetry potentials are chemical or physical differences between each side of a membrane, in particular an inhomogeneous membrane structure resulting from fabrication conditions and/or curvature. Asymmetry p. can change in the course of membrane ageing. To measure asymmetry p. one should use a symmetrical cell with identical solutions and -> reference electrodes on each side of the membrane. 

In practice, the value of k is never obtained as such, because the meter is adjusted so that the standard reads the correct value for its pX, the scale being Nernstian. As k contains in addition to the reference electrode potentials, a liquid-junction potential and an asymmetry potential, frequent standardization of the system is necessary. The uncertainty in the value of the junction potential, even when a salt bridge is used, is of the order of 0.5 mV. Consequently the absolute uncertainty in the measurement of pX is always at least 0.001/(0.059// ) or 0.02 if n = I, i.e. a relative precision of about 2% at best. For the most precise work a standard addition technique (p. 32) and close temperature control are desirable. All measurements should be made at constant ionic strength because of its effect on activities. Likewise,... 

The E.M.F. of this cell should be zero, but in practice the value is found to be of the order of rt 2 millivolts, for a good electrode. This small difference is called the asymmetry potential of the glass electrode it is probably due to differences in the strain of the inner and outer surfaces of the glass membrane. It is necessary, therefore, to standardize each glass electrode by means of a series of buffer solutions of known pH in this way the value of in equation (10) for the particular electrode is found. 

This cell closely resembles the original Sorensen cell, with a glass electrode substituted for the hydrogen electrode. Since glass electrodes are subject to an unpredictable asymmetry potential, pH measurements are made in practice by substituting a standard buffer for the sample solution and then comparing the pH of the sample with the pH of the standard, pH, according to the equation... 

The sources of the asymmetry potential are obscure but undoubtedly include such causes as differences in strain on the two surfaces of the membrane imparted during manufacture, mechanical abrasion on the outer surface during use, and chemical etching of the outer surface. To eliminate the bias caused by the asymmetry potential, all membrane electrodes must be calibrated again.st one or more. standard analyte solutions. Such calibrations should be carried out at least daily, and more often when the electrode receives heavy use. 

---