Potentiometric cells

FIGURE 5-18 Flow-through potentiometric cell-cap design. A, reference electrode B, iodide electrode C, flow-through cap D, inlet E, outlet. (Reproduced with permission from reference 49.)... 

Fig. 18a.l. Schematic diagram of a potentiometric cell with an ion-selective electrode (ISE) as the indicator electrode. EM is the electrical potential of the sensing membrane and IFS the internal filling solution. 

The electrochemical potential in a potentiometric cell is inevitably measured with respect to a standard electrode. Several types of electrodes are commonly used. The standard hydrogen electrode (SHE) is a hydrogen half-cell in which the cell reaction is as follows ... 

The EMF across the entire potentiometric cell, shown in Fig. 18a.l, is the sum of the individual potentials that include the reference electrode potential and other sample-independent potentials (Econst), the liquid junction potential (Ej) and the membrane potential (EM) ... 

What are the essential parts of an electrochemical potentiometric cell and what is the main operational characteristic of a potentiometric measurement ... 

These systems involve galvanic cells (p. 229) and are based on measurement of the potential (voltage) difference between two electrodes in solution when no net current flows between them no net electrochemical reaction occurs and measurements are made under equilibrium conditions. These systems include methods for measuring pH, ions, and gases such as CO2 and NH3. A typical potentiometric cell is shown in Fig. 34.2. It contains two electrodes ... 

The potentiometric cell is constructed so that the half-cell potentials of the two reference electrodes and the composition of the internal electrolyte are constant. The sample solution is maintained at a high constant ionic strength so that Eij is effectively constant. This allows [Eq. (3)] to be simplified to yield ... 

For reliable high-temperature potentiometric studies, the measured open-circuit potential should be stable and reproducible within a few millivolts or less. Similar to the low-temperature electrochemical measurements, both the indicator and reference electrodes should be used in devising a high-temperature potentiometric cell. Both the electrochemical probes should be resistant to chemical degradation and pressure (mechanical) stress in high-temperature aqueous environment. The development of suitable indicator and reference electrodes that can reliably... 

The Ag/AgCl electrode is used both as an internal reference element in potentiometric ISEs, and as an external reference electrode half-cell of constant potential, required to complete a potentiometric cell (see Figure 4-1). In both cases, the Ag/AgCl electrode must be in equilibrium with a solution of constant chloride ion activity. 

Potentiometric ion-selective electrodes are passive probes, which in contrast to voltammetric sensors do not convert the analyte in the sample. The response of an ISE depends linearly on the logarithm of the activity (concentration) of a potential determining ion (primary ion) in the presence of other ions. The schematic layout of a complete potentiometric cell including an ion-selective electrode is shown in Figure 2. The electrochemical notation of the cell assembly is given as ... 

The effect of an ambiguous pumping current characteristic at 7<1 can be avoided by exposing the anode to a reference atmosphere, favorably to ambient air (Fig. 17.15.9) like a potentiometric cell generating a superimposed Nernst voltage,... 

Oxoacidic properties of oxo-compounds of chromium molybdenum in molten equimolar KN03-NaN03 mixture were studied by Kust [115], who used the standard potentiometric cell with a liquid junction. The measurement routine consisted in certain additions of neutral chromate or molybdate to the solution of the corresponding salt of composition Na2R207. Determination of the equilibrium O2- concentrations in these buffer solutions allowed the authors to estimate the equilibrium constants as... 

K2Cr207 in KNO3 is put down at a constant rate into a melt containing a Lux base. The e.m.f. values of the potentiometric cell are registered and printed using a recorder. This method may be employed to analyse ionic melts basicity in industrial molten systems since it allows one to estimate the concentration of bases in the melts studied, to an accuracy of the order of 3.5%. However, the thermodynamic parameters cannot be estimated on the basis of the e.m.f.-time plots obtained, since the e.m.f. recorded is not referred to the equilibrium state. 

Combes and Tremillon [154] studied the oxoacidic properties of tungsten(VI) oxide and the solubility of calcium tungstate in a molten equimolar KCl-NaCl mixture at 1000 K. A potentiometric cell with the membrane indicator electrode Ni,NiOlYSZ was used for the detection of the equilibrium oxide ion concentration. Investigation of the equilibria taking place in CaW04 solutions in KCl-NaCl allowed them to determine the solubility of CaO in the said melt at 1000 K as 0.084 mol%. The solubility of Scheelite (CaW04) was determined to be 10-3 5 mol kg-1, and the equilibrium constant of reaction (1.2.38) was estimated as 1010. 

Banik [166, 167]. The acidic properties of Na2B407 were studied by the potentiometric titration method using NaOH and Na2C03 as base-titrants, and the equilibrium oxide ion molality was detected by the potentiometric cells with the membrane oxygen electrode Pt(02)lYSZ. The process of titration was performed both from the acid to the base and from the base to the acid. The most characteristic potentiometric curves and the dependencies of the ligand number on the titrant molality are presented in Fig. 1.2.13. 

Rybkin and Seredenko reported the construction of empirical scales of oxoacidity (acidity rows) in molten KC1 at 800 °C and Csl at 650 °C [62, 63]. Estimation of the oxoacidic properties was performed for buffer solutions obtained by the addition of equimolar quantities of conjugated acid and base in the melt. E.m.f. (pO) measurements were performed in the potentiometric cell with the use of a membrane oxygen electrode Pt(02)lYSZ. 

The second process is the transfer of the formed oxide ion through the solid electrolyte membrane. The potentiometric cells with membrane oxygen electrodes should be characterized by the slope of the E-pO plot equal to 1.15RT/F if the electrode processes correspond to equations (2.4.3) and (2.4.8). The transport number of O2- ions is an important criterion of the membrane electrode reversibility—for good solid electrolytes it is close to unity (1). 

Lux acid-base reactions require data on the dependence of e.m.f. of the cell used on the equilibrium oxide-ion activity (concentration) in the melt. The calibration of the potentiometric cell is performed as described below. 

Potassium and sodium hydroxides and sodium peroxide are conventionally used as strong Lux bases for the calibration of the potentiometric cells. 

Fig. 2.4.2. The principle of the construction of a potentiometric cell with oxygen electrode 1, cell with the cover (2) 3, crucible-container with the melt studied 4, an oxygen electrode 5, a reference electrode 6, tube for adding weights of Lux acids or bases 7, tube for passing the melt of inert gas (N2, Ar, if necessary).

Characteristic features of the construction of the reference electrodes mentioned consist in the materials serving as current taps. Platinum is used with silver reference electrodes, but cannot be used for lead electrodes since the lead forms alloys with Pt and the current taps are destroyed by such interactions. This requires the use of iron or tungsten wires as intermediate current-tap materials. The potentials of the mentioned reference electrodes are highly reproducible, and they were extensively and thoroughly studied over a wide range of temperatures and concentrations in potentiometric cells of the following kinds ... 

The work of some of us (Cherginets, Banik and Rebrova) is devoted to studies of the reversibility of various oxygen electrodes such as Pt(02)lYSZ and NilNiO, in the KCl-LiCl eutectic melt at 400, 500, 600 and 700 °C. The potentiometric cell for the measurements at 400 and 500 °C consisted of the lead reference electrode (PblPbCl2, 0.05 mol kg-1) and one of the indicator oxygen electrodes [232] ... 

Fig. 2.4.8. Calibration -pO plot for potentiometric cell (2.4.28) with a membrane oxygen electrode in the KCl-LiCl eutectic melt at 700 °C 1, the section where the effect of oxygen admixtures is predominant 2, section with 3, section with z = 1.

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