2.4. Ion Selective Electrodes and Potentiometry

2.4.1. Definition of pH

Definition of pH Acids may be defined as proton donors and bases as proton acceptors:

The dissociation of weak acids and bases are equilibrium processes, and the equilibrium law can be applied to them:

where K is the dissociation constant. For water,

and

Kw for Water Since only a small fraction of water is dissociated, [H20] may be considered constant:

Conductivity measurements have shown that Kw is 1x10-14 mol2/L2. 'The acidity or alkalinity of a solution can be measured by its hydrogen ion concentration but it is more convenient to use pH defined by:

Therefore, from Eq. (31),

When pH = pOH = 7, the solution is said to be in 'neutral' pH.

Ka of Weak Acid. From Eq. (29), pH of a weak acid can be described as:

Therefore, if Ka of an acid is known and pH is measured, the concentration of A- ion can be calculated when the concentration of HA is known.

2.4.2. Measurement of pH

Glass type pH Electrode. Usually, pH is measured by a glass electrode type pH sensor. The glass electrode consists of a glass membrane which is selectively permeable only to H+ and an internal Ag/AgCl electrode immersed in 0.1 M KCl solution (Fig. 2.15). When this glass electrode is immersed in a test solution of unknown pH and the voltage is measured against a standard electrode (such as a Calomel electrode; Fig. 2.16), the measured voltage follows Nernst equation:

where Em is the difference in potential between the inside (yi) and outside (yo) of the membrane measured by the sensor, R is the gas law constant (8.31 J/mol-K), T is the absolute temperature, z is the valance of the ion, F is Faraday constant (96,500 coul/mol; note that 1 coul = 1 J/V), and Ci and Co are the concentrations of H+ ions inside and outside of the membrane, respectively. The C's should be activities but for dilute solutions, molar concentration can be used. In recent pH sensors, the glass electrode and the reference electrode are combined in one body (Fig. 2.16b). Note that the voltage output (Em) is proportional to the log of the concentration (log Co), not the concentration itself (Co) (see Fig. 2.17).

Measurement Circuit. From Eq. (35), a decade change in Co will give Em of 58.1 mV for H+ at 20oC. This voltage output has to be amplified if one desires to take data, for example, with a computer. A circuit shown in Fig. 2.18 can be used for such an amplification. It uses one operational amplifier in a non-inverting mode. The operational amplifier has to be a FET-input type that has an input resistance on the order of 1012 ohms. This is because the resistance of the glass pH electrode is close to 5x109 ohms. To make voltage measurement from such a high resistance source, the input impedance of the amplifier has to be much greater. The other point is that only the non-inverting configuration (of the operational amplifier)

Fig. 2.15. Glass membrane electrode for pH measurement.

Fig. 2.16. Measurement setup for pH: (a) separate glass electrode and reference electrode; (b) a combination electrode.

Fig. 2.17. Response of potentiometric sensor to variations in ion concentration.

Fig. 2.18. Signal conditioning circuit for potentiometry.

has to be used to take full advantage of the input resistance of the operational amplifier.

2.4.3. Other Ion-Selective Electrodes

pH measuring glass electrode is just one example of ion-selective electrodes (ISE). There are many different ISE's but it would be convenient to classify them in terms of the membrane used for partitioning.

Glass Membrane. These originate with the hydrogen-ion selective electrode having well-behaving glass membrane that has high mobility for H+ ion. Subsequently, electrodes have been developed for other cations such as Na+, K+, and NH4+

Inorganic Salt Membrane. These electrodes are based on inorganic halides and sulphides, for example, silver salts, lanthanum fluoride and heavy-metal sulphides. These membranes have been produced from preparations ranging from whole crystals to dispersions in an inert matrix, such as polythene or silicon rubber. They are targeted at ions such as halides, CN-, S--, Ag+, Cu++, and Pb++.

Organic Membrane. The electrodes based on neutral carriers generally have the highest selectivity in this class. However, cation exchangers or complexing agents or anion exchangers have been successfully employed in electrodes with liquid or solid membranes, selective to cations or anions, respectively.

Gas Sensing Electrode. These electrodes are an extension of ion-selective measurements to detection of gaseous analytes. Gas-sensing probes are complete electrochemical cells, incorporating both the ion-selective electrode and a reference electrode within the sensor. Assay of the target gaseous sample is not performed directly, but is related to a changing parameter (usually pH) which can be monitored by an ion-selective electrode.