Potentiometric measuring system

MODULAR POTENTIOMETRIC MEASURING SYSTEM FOR THE DEVELOPMENT AND COMFORTABLE TESTING OF MINIATURIZED ION SENSORS... 

Sources of Error. pH electrodes are subject to fewer iaterfereaces and other types of error than most potentiometric ionic-activity sensors, ie, ion-selective electrodes (see Electro analytical techniques). However, pH electrodes must be used with an awareness of their particular response characteristics, as weU as the potential sources of error that may affect other components of the measurement system, especially the reference electrode. Several common causes of measurement problems are electrode iaterferences and/or fouling of the pH sensor, sample matrix effects, reference electrode iastabiHty, and improper caHbration of the measurement system (12). 

To further corroborate these potentiometric results, we again used optical second harmonic generation (SHG). The SHG measurement system was essentially the same as used in the study described in Section II, except the laser beam was first reflected by a mirror and then totally reflected by the liquid-liquid interface. 

Bacarella, A. L. Grunwald, E. Marshall, H. R Purlee, E. L., The potentiometric measurement of acid dissociation constants and pH in the system methanol-water. pKa values for carboxylic acids and anilinium ions, j. Org. Chem. 20, 747-762 (1955). 

Thermodynamics describes the behaviour of systems in terms of quantities and functions of state, but cannot express these quantities in terms of model concepts and assumptions on the structure of the system, inter-molecular forces, etc. This is also true of the activity coefficients thermodynamics defines these quantities and gives their dependence on the temperature, pressure and composition, but cannot interpret them from the point of view of intermolecular interactions. Every theoretical expression of the activity coefficients as a function of the composition of the solution is necessarily based on extrathermodynamic, mainly statistical concepts. This approach makes it possible to elaborate quantitatively the theory of individual activity coefficients. Their values are of paramount importance, for example, for operational definition of the pH and its potentiometric determination (Section 3.3.2), for potentiometric measurement with ion-selective electrodes (Section 6.3), in general for all the systems where liquid junctions appear (Section 2.5.3), etc. 

In addition to their use as reference electrodes in routine potentiometric measurements, electrodes of the second kind with a saturated KC1 (or, in some cases, with sodium chloride or, preferentially, formate) solution as electrolyte have important applications as potential probes. If an electric current passes through the electrolyte solution or the two electrolyte solutions are separated by an electrochemical membrane (see Section 6.1), then it becomes important to determine the electrical potential difference between two points in the solution (e.g. between the solution on both sides of the membrane). Two silver chloride or saturated calomel electrodes are placed in the test system so that the tips of the liquid bridges lie at the required points in the system. The value of the electrical potential difference between the two points is equal to that between the two probes. Similar potential probes on a microscale are used in electrophysiology (the tips of the salt bridges are usually several micrometres in size). They are termed micropipettes (Fig. 3.8D.)... 

Electrode Systems. Direct Potentiometric Measurements. Potentiometric Titrations. Null -point Potentiometry. Applications of Potentiometry. 

The sensitivity of instruments using low resistance circuits is determined primarily by the sensitivity of the galvanometer (Figure 4.5). Electrode systems that have a high resistance, e.g. glass electrodes, require a high impedance voltmeter, which converts the potential generated into current which can be amplified and measured. Such instruments are commonly known as pH meters but may be used for many potentiometric measurements other than pH. 

Because potentiometric measurements reflect the activity of an ion rather than its concentration, it is usually necessary to calibrate the system with standard solutions of known activity. It is possible to calculate the concentration of an... 

Ideal potentiometric measurements, especially in analytical chemistry, would require that the potential of the reference electrode be fixed and known, and that the composition of the studied solution affect only the potential of the indicator electrode. This would occur only if the liquid-junction potential could be completely neglected. In practice this situation can be attained only if the whole system contains an indifferent electrolyte in a much larger concentration than that of the other electrolytes, so that the concentration of a particular component in the analysed solution, which is not present in the reference electrode solution, has only a negligible effect on the liquid-junction potential Such a situation rarely occurs, so that it is necessary to know or at least fix the liquid junction potential... 

Figure 4.17 — (A) Exploded view of a tubular flow-cell integrated microconduit system. I Ag/AgCl inner reference electrode M sensitive membrane S internal reference solution. (B) Detail of the integrated microconduit shown within the dotted lines in C. (C) Integrated-microconduit FI manifold for potentiometric measurements C carrier stream R reference electrode solution P pump V injection valve I indicator electrode R reference electrode I pulse inhibitor G ground W waste. (Reproduced from [140] with permission of Pergamon Press).

These results suggest that interactions between silicate species and surfactant micelles are weak in the precursor solution. The absence of any organization in the system prior to precipitation seems to indicate that the most important step in the process is the formation of siliceous prepolymers. The interaction of these prepolymers with surfactants could be responsible for micelle growth and subsequent reorganization of the silica/micelle complexes into ordered mesoporous structures. Such a hypothesis might be confirmed by preliminary potentiometric measurements using a bromide ion-specific electrode the amount of free bromide anion increasing at pH around 11 when the polymerization of silica starts. 

Before considering instrumentation in some detail in later chapters, it will be helpful to outline the kinds of experiments that we wish to implement electronically. It is useful to characterize electroanalytical techniques as either static or dynamic. Static methods are philosophically akin to the passive observation mentioned earlier. They entail measurements of potential difference at zero current such that the system defined by the solid-solution interphase is not disturbed and Nernstian equilibrium is maintained. Although such potentiometric measurements (e.g., pH, pM) are of great practical importance, our focus here will be on the dynamic techniques, in which a system is intentionally disturbed from equilibrium by excitation signals consisting of a wide variety of potential and current programs. 

These conditions are fulfilled for the reduction of phenacylsulpho-nium salts at low pH values cleavage of the C—S bond occurs in the first step and proton transfer is involved only in the consecutive steps. The observed shift of half-wave potentials (74) with pH follows the plot predicted by equation (27). The intersection of the two linear parts indicates that phenacylsulphonium salts are moderately strong acids with pK values between 7 and 8 (Fig. 20). Potentiometrically measured pK values, used for verification, are in good agreement with the approximate pK values obtained from polarographic data. The system involved in the first step can be described by scheme (28) ... 

For most potentiometric measurements, either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few mV. The silver/silver chloride electrode also finds application in non-aqueous solutions, although some solvents cause the silver chloride film to become soluble. Some experiments have utilised reference electrodes in non-aqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for non-aqueous systems as are any of the prototypes that have been developed to date. When there is a need to exclude water rigorously, double-salt bridges (aqueous/non-aqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the non-aqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence, the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause dramatic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

The GECE sensors were used for lead determination in real water samples suspected to be contaminated with lead obtained from water suppliers. The same samples were previously measured by three other methods a potentiometric FIA system with a lead ion-selective-electrode as detector (Pb-ISE) graphite furnace atomic absorption spectrophotometry (AAS) inductively coupled plasma spectroscopy (ICP). The results obtained for lead determination are presented in Table 7.1. The accumulation times are given for each measured sample in the case of DPASV. Calibration plots were used to determine the lead concentration. GEC electrode results were compared with each of the above methods by using paired -Test. The results obtained show that the differences between the results of GECE compared to other methods were not significant. The improvement of the reproducibility of the methods is one of the most important issues in the future research of these materials. 

In biological systems, most of the elements are present as complexes, rather than as free ions, hence direct potentiometric measurements provide little information except for major electrolytes such as Fl+, Na+, K+, Cl- and possibly Ca2+, Mg2+. The introduction of ion-selective microelectrodes (e.g. tips < 1 jtm diameter) has allowed these major electrolyte ions to be determined in single cells. 

Direct potentiometric measurement an Agl membrane electrode with a double junction reference electrode system must be used to quantify CN-. 

The indicator electrodes for potentiometric measurements traditionally have been categorized into three separate classes. First-class electrodes consist of a metal immersed in a solution that contains the metal ion. These electrode systems provide a direct response to the ion or species to be measured ... 

A number of the most common potentiometric electrode systems and their applications are summarized in Table 2.2. Additional information is available in the biennial reviews of Analytical Chemistry within the section on potentiometric measurements. 

In general a necessary part of a potentiometric measurement is the coupling of a reference electrode to the indicating electrode. The ideal reference electrode has a number of important characteristics (1) a reproducible potential, (2) a low-temperature coefficient, (3) the capacity to remain unpolarized when small currents are drawn, and (4) inertness to the sample solution. If the reference electrode must be prepared in the laboratory, a convenient and reproducible system is desirable. 

Although all potentiometric measurements (except those involving membrane electrodes) ultimately are based on a redox couple, the method can be applied to oxidation-reduction processes, acid-base processes, precipitation processes, and metal ion complexation processes. Measurements that involve a component of a redox couple require that either the oxidized or reduced conjugate of the species to be measured be maintained at a constant and known activity at the electrode. If the goal is to measure the activity of silver ion in a solution, then a silver wire coupled to the appropriate reference electrodes makes an ideal potentiometric system. Likewise, if the goal is to monitor iron(UI) concentrations with a platinum electrode, a known concentration of... 

Potentiometry has found extensive application over the past half-century as a means to evaluate various thermodynamic parameters. Although this is not the major application of the technique today, it still provides one of the most convenient and reliable approaches to the evaluation of thermodynamic quantities. In particular, the activity coefficients of electroactive species can be evaluated directly through the use of the Nemst equation (for species that give a reversible electrochemical response). Thus, if an electrochemical system is used without a junction potential and with a reference electrode that has a well-established potential, then potentiometric measurement of the constituent species at a known concentration provides a direct measure of its activity. This provides a direct means for evaluation of the activity coefficient (assuming that the standard potential is known accurately for the constituent half-reaction). If the standard half-reaction potential is not available, it must be evaluated under conditions where the activity coefficient can be determined by the Debye-Hiickel equation. 

The number of reversible metal-metal ion electrodes is limited so that the accurate direct potentiometric measurement of the activity of a metal ion with an electrode of the same metal usually is not feasible, except perhaps with the Ag/Ag,(OH2)4 system. However, a number of metal ion-metal half-reactions are sufficiently reversible to give a satisfactory potentiometric titration with a precipitation ion or complexing agent. These couples include Cuu(OH2>6+/Cu, Pbn(OH2>4+/Pb, Cdu(OH2)l+/Cd, and Znn(OH2)i+/Zn. However, all these metals can be determined by EDTA titration and the mercury electrode that is described in the preceding section. 

The ( >p pjj values needed for the computation of the Vp/Op values associated with the linear polyion, DxS, were obtained by interpolation of the ( )p versus a/b plots accessible for ionic polysaccharides (Fig. 12) where the b values were calculated with Eq. (16). Tlie log Kc and the log K, values determined for the commercially available DxS sample (Pharmacia), with a DS value of ca. 2, were also used at under various NaCl concentration levels. The Kf- values were determined in complex-ation studies facilitated by potentiometric measurements, whereas the Kj values due to Mattai and Kwak [46], were obtained by a dye method. In spite of the difference in valence of the counterions and the binding constants, the Vp/np values resolved for both systems are consistent with... 

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