Modern potentiometry

Abstract

For most chemists, potentiometry with ion-selective electrodes (ISEs) primarily means pH measurements with a glass electrode. Those interested in clinical analysis might know that ISEs, routinely used for the determination of blood electrolytes, have a market size comparable to that of glass electrodes. It is even less well known that potentiometry went through a silent revolution during the past decade. The lower detection limit and the discrimination of interfering ions (the selectivity coefficients) have been improved in many cases by factors up to 10(6) and 10(10), respectively, thus allowing their application in fields such as environmental trace analysis and potentiometric biosensing. The determination of complex formation constants for lipophilic hosts and ionic guests is also covered in this Minireview.

Figure 1

Determination of unbiased selectivity coefficients for a Ag⁺-selective polymer membrane electrode.[24] According to Eq (4), the large potential difference between the Ag⁺ and Na⁺ calibration curves translates into a selectivity coefficient of log $K_{\text{AgNa}}^{\text{pot}}=-8.7$. The data were obtained with a membrane that was not exposed to Ag⁺ before recording the calibration curves for Na⁺and K⁺.[24]

Figure 2

Calibration curve of a Ag⁺-selective polymer membrane electrode, exhibiting a subnanomolar detection limit.[21] Inset: Responses upon repeated exposure to 1 and 10 nanomolar levels of silver nitrate.

Figure 3

Picture of the 3-μL measuring cell. A Ca²⁺-ISE (left, indicator electrode) and a Na⁺-ISE reference electrode (right) are inserted into a 1-mm i.d. silicone tubing and put in contact with the aqueous sample plug of 3 μL).[20]

Figure 4

Potentiometric detection of 300 amol of Ca²⁺ (10⁻¹⁰ M in 3 μL) at a constant background of 10⁻⁶ M NaNO₃. A miniaturized Na⁺-ISE was used as reference electrode.[20]

Figure 5

Schematic representation of a solid-contact ISE. The measuring current (in the order of fA) is transported by ions in the solutions and the ISE membrane and by electrons in the metal. The two processes are coupled in the redox layer (a conducting polymer or a redox-active self-assembled monolayer). If the redox layer is absent or not lipophilic enough, a water film may form at the inner surface of the membrane, which leads to potential instabilities and deteriorates the lower detection limit.

Figure 6

Potentiometric determination of the fraction of uncomplexed Pb²⁺ as a function of pH in a drinking water sample spiked with 10 ppb of Pb²⁺. Dashed line: calculated free Pb²⁺ activity for a total carbonate concentration of 4.14 mM.[71]

Figure 7

Comparison of Pb²⁺ activity values of environmental samples obtained by potentiometry at pH 4.0 with those by ICPMS.[71]

Figure 8

Sandwich immunoassay with potentiometric detection: a) The antibody is immobilized on gold by self-assembly, b) anti-mouse IgG antigen is bound to the antibody, c) a second antibody with Au nanoparticle labels is bound to the antigen, d) Ag is deposited on Au nanoparticles, and e) dissolved Ag⁺ is detected with an Ag⁺-ISE.

Figure 9

Calibration curve of the Ag⁺-ISE response to IgG.