Potentiometric determination

Potentiometric determinations of the pK of thiazole and essentially its alkyl derivatives are summarized in Table T50. The most reliable values are given by Phan-Tan-Luu et al. (321), who realized a critical study of the classical Henderson method for the determination of pK. ... 

The potentiometric determination of an analyte s concentration is one of the most common quantitative analytical techniques. Perhaps the most frequently employed, routine quantitative measurement is the potentiometric determination of a solution s pH, a technique considered in more detail in the following discussion. Other areas in which potentiometric applications are important include clinical chemistry, environmental chemistry, and potentiometric titrations. Before considering these applications, however, we must first examine more closely the relationship between cell potential and the analyte s concentration, as well as methods for standardizing potentiometric measurements. 

Measurement of pH With the availability of inexpensive glass pH electrodes and pH meters, the determination of pH has become one of the most frequent quantitative analytical measurements. The potentiometric determination of pH, however, is not without complications, several of which are discussed in this section. 

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

Radio, N. Komijenovic, J. Potentiometric Determination of an Overall formation Gonstant Using an Ion-Selective Membrane Electrode, /. Chem. Educ. 1993, 70, 509-511. 

DIRECT POTENTIOMETRIC DETERMINATION OF SULFATE CONTENT IN BIOLOGICAL FLUIDS (URINE)... 

The study of these electrodes has proved primary and secondary characteristics favorable to their practical utilization in potentiometric titrations and in direct potentiometric determinations. 

The electrodes have a wide linear response range to CrP" and Ni " ions concentration. For this reason, they are adequate for the potentiometric determinations of copper and nickel ions in diluted solutions (dilutions may go down to 10 M) as well as in checking the industrial waters. 

Methods of sulfate determination used in common practice suffer from essential disadvantages (time consumption, laboriousness, low sensitivity, etc.). For this reason, creation of sulfate-selective electrode suitable for direct potentiometric determination of this ion, has been attempted for a long time. Nowadays, nearly a dozen of sulfate-selective electrodes based on charged or neutral carriers of different stmcture have been described. Flowever, no known receptors for sulfate ions are available commercially. 

The aim of this work is the optimization of distillation process using H SO for fluoride separation and potentiometric determination in anhydrite samples by means of chemometric tools. 

STUDIES ON A Pb -SELECTIVE ELECTRODE WITH MACROCYCLIC LIQUID MEMBRANE. POTENTIOMETRIC DETERMINATION OF Pb + IONS... 

In all cases some sensitivity to hydrogen ions remains in any potentiometric determination with these modified glass electrodes the hydrogen ion concentration of the solution must be reduced so as to be not more than 1 per cent of the concentration of the ion being determined, and in a solution containing more than one kind of alkali metal cation, some interference will be encountered. 

FIGURE 5-17 Flow injection potentiometric determination of potassium in serum. (Reproduced with permission from reference 47.)... 

In aqueous solutions at pH 7, there is little evidence of complex formation between [MesSnflV)] and Gly. Potentiometric determination of the formation constants for L-Cys, DL-Ala, and L-His with the same cation indicates that L-Cys binds more strongly than other two amino acids (pKi ca. 10,6, or 5, respectively). Equilibrium and spectroscopic studies on L-Cys and its derivatives (S-methyl-cystein (S-Me-Cys), N-Ac-Cys) and the [Et2Sn(IV)] system showed that these ligands coordinate the metal ion via carboxylic O and the thiolic 5 donor atoms in acidic media. In the case of S-Me-Cys, the formation of a protonated complex MLH was also detected, due to the stabilizing effect of additional thioether coordination. ... 

As an example, consider the potentiometric determination of concentration of ions in a solution which is to be accomplished with the aid of an electrode of metal N. Using a simple cell with the reference electrode M /E, ... 

Liposome-water partition potentiometric determinations, 25°C, 0.15 M KCl [8, 71]. Liposomes were made of large (phosphatidylcholine) unilamellar vesicles. 

A continuous potentiometric determination of sulphate in a differential flow system160 consisted of a flow cell with two Pb2+-selective electrodes in series. All solutions contained 75% of methanol and were adjusted to pH 4 a standard solution of Pb(II) passes the first sensor and, after being mixed with the sulphate sample stream, yielding a PbS04 precipitate in addition to excess of Pb(II), it passes the second sensor from the potential difference between the sensors the sulphate content of the sample can then be derived. 

Figure 4.5 Potentiometrically determined [162] lipophilicity profiles of a dipeptide, showing the effect of background salt concentrations. The unfilled symbols [282] and the filled symbols [371] are based on shake-flask measurements. [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science...

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. 

It would appear from Eq. (3.2.8) that the pH, i.e. the activity of a single type of ion, can be measured exactly. This is not, in reality, true even if the liquid junction potential is eliminated the value of Eref must be known. This value is always determined by assuming that the activity coefficients depend only on the overall ionic strength and not on the ionic species. Thus the mean activities and mean activity coefficients of the electrolyte must be employed. The use of this assumption in the determination of the value of Eref will, of course, also affect the pH value found from Eq. (3.2.8). Thus, the potentiometric determination of the pH is more difficult than would appear at first glance and will be considered in the special Section 3.3.2. 

R.J. Mortimer, P.J.S. Barbeira, A.F.B. Sene, and N.R. Stradiotto, Potentiometric determination of potassium cations using a nickel(II) hexacyanoferrate-modified electrode. Talanta 49, 271-275 (1999). 

Jagner [28] has also described a semi-automatic titration for high-precision determination of chlorine in seawater, where it has been used for the potentiometric determination of total halides (silver electrode) and alkalinity (glass electrode), and for the photometric titration of total alkaline-earth metals. Several titrations can be effected simultaneously. 

Ke and Regier [71] have described a direct potentiometric determination of fluoride in seawater after extraction with 8-hydroxyquinoline. This procedure was applied to samples of seawater, fluoridated tap-water, well-water, and effluent from a phosphate reduction plant. Interfering metals, e.g., calcium, magnesium, iron, and aluminium were removed by extraction into a solution of 8-hydroxyquinoline in 2-butoxyethanol-chloroform after addition of glycine-sodium hydroxide buffer solution (pH 10.5 to 10.8). A buffer solution (sodium nitrate-l,2-diamino-cyclohexane-N,N,N. AT-tetra-acetic acid-acetic acid pH 5.5) was then added to adjust the total ionic strength and the fluoride ions were determined by means of a solid membrane fluoride-selective electrode (Orion, model 94-09). Results were in close agreement with and more reproducible than those obtained after distillation [72]. Omission of the extraction led to lower results. Four determinations can be made in one hour. 

Ion-selective electrodes have been used for the potentiometric determination of the total cupric ion content of seawater [284], Down to 2 xg/l cupric copper could be determined by this procedure. 

The experimental stability ratio (W), the potentiometrically-determined surface charge, and the electro-kinetic mobility of 70 nm particles over the pH range from 3 to 11 are shown. The drawn-out line in Fig. c summarizes experiments obtained with I = 0.05 - 0.1. (Modified from Liang and Morgan, 1990.)... 

Diloxanide Furoate 0.3 g Potentiometric determination Each ml of 0.1 N Tetrabutylammonium hydroxide = 0.03282 g of C14HnCl2N04... 

Potentiometric Determination of Lead (II) Ion using 2-[(4-Chloro-Phenylimino)-Methyl]-Phenol as an electroactive Material... 

Our method at present is not based upon theoretical models or departures from ideal behavior. It consists in the use of potentiometric determinations and literature values of activity coefficients, starting with HCl-HC10 electrolyte mixtures and with the assumption that HClOi, is completely dissociated since the association constant pK = 7 is extremely small in this case. 

Z. Koudela and B. Matyska XIII. Potentiometric determination of certain cations by means of complcxone III" solutions. Collect. Czech, chem. Commun. 16, 80 (1951). 

Often, the potentiometric determination of concentration requires a preferred pH range. If pH is also important, then the ionic strength adjuster can conveniently function additionally as a pH buffer. Such tablets are called total ionic strength adjustment buffers (or TISABs). ... 

Figure 4.2 Plot of the variable (emf — she) against the volume of ceric ion soiution during a potentiometric determination of [Fe ]. The end point is clearly shown by a sharp transition from the standard electrode potential of the analyte couple, p 2+. to that of the titrant couple, 3 (cf. equation (4.1)).

Acid-base equilibria are quite sensitive to the presence of any organic solvent, but since the interpretation of measured pH values is limited to pure water solution, it has been both necessary to define a proton activity (termed pan) for any mixture and to use conveniently modified electrodes to carry out the potentiometric determinations of pan in the range of normal and subzero temperatures. 

The potentiometric determinations have been achieved by the use of a glass-calomel electrode assembly described elsewhere (Lanoque et ai, 1976). The potential of a glass electrode, reversible with respect to H, immersed in a solution whose proton activity is ah is given by... 

The results presented above show that with conveniently modified electrodes, potentiometric determinations of pan can be performed in aqueous-organic solvents at normal and subzero temperatures. In particular, it must be emphasized that these modified electrodes actually demonstrate the pH response theoretically expected. 

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