The Basics of the Routine Analysis with ISEs
It is not possible to measure a single electrode potential. Therefore, all electrode potentials are measured against some reference electrode (RE), and the respective EMFs are used for further processing.
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The Basics of the Routine Analysis with ISEs
9.1 Reference Electrodes It is not possible to measure a single electrode potential. Therefore, all electrode potentials are measured against some reference electrode (RE), and the respective EMFs are used for further processing. The primary RE which is supposed to have zero potential at all temperatures is the so-called standard hydrogen electrode. This is a piece of platinum with rough surface, immersed into acidic solution, and gaseous hydrogen is bubbling through the solution. Platinum works as catalyst for hydrogen oxidation reaction which takes place on the platinum surface: H2 Pt $ 2Hþ;aq þ 2e The respective potential depends on the pH and also on the hydrogen partial pressure: u ¼ u0
RT RT pH ln pH2 F 2F
ð9:1Þ
Thus, the primary reference for all potentiometric measurements is the potential of the hydrogen gas electrode immersed into solution with pH = 0, at pressure of 1 atm. In practice, this electrode is inconvenient and therefore replaced in routine measurements with a suitable secondary RE. Earlier, calomel electrode, Hg/ Hg2Cl2 in saturated KCl, was often in use. However, given its toxicity, mercury must be avoided, if possible. Therefore, most frequently used is silver chloride electrode (Ag/AgCl) immersed into 3 M KCl solution. This solution is connected with sample or calibrator via salt bridge. In the so-called single-junction REs, the bridge is filled with the same solution as that in the RE, for example, 3 M KCl. If the contamination of the sample with K+ and/or Cl- ions is of no importance for the results, a single-junction RE schematically shown in Fig. 9.1 is a suitable choice. In many cases, the contamination of samples with K+ and Cl- ions from the salt bridge may bias the results. Then use of the so-called double-junction RE is preferable. A scheme of a double-junction RE is shown in Fig. 9.2. This kind of
K. N. Mikhelson, Ion-Selective Electrodes, Lecture Notes in Chemistry 81, DOI: 10.1007/978-3-642-36886-8_9, Ó Springer-Verlag Berlin Heidelberg 2013
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9 The Basics of the Routine Analysis with ISEs Cable
Body (chamber)
Ag/AgCl electrode
Reference solution (3 M KCl)
Salt bridge Liquid junction
Fig. 9.1 Schematic design of a single-junction reference electrode (left) and Thermo-Fisher sure-flow reference electrode (right)
REs consist of two bodies one inside another one, making two chambers. The inner chamber is equipped with an electrode, most commonly—Ag/AgCl and filled with 3 M KCl. The inner reference solution is connected with the outer chamber via inner salt bridge made of a suitable porous material. The outer chamber is filled with electrolyte consisting of ions different from those to be measured. In this way, it is possible to avoid the errors caused by the contamination of sample or calibrator with ions leaking from the reference chamber. The most common choice for the electrolyte in the outer chamber is 1 M LiCH3COO. Thus, there are two liquid junctions between KCl and LiOAc and between LiOAc and sample soluti
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