Direct potentiometry concentration standard

Most measurements include the determination of ions in aqueous solution, but electrodes that employ selective membranes also allow the determination of molecules. The sensitivity is high for certain ions. When specificity causes a problem, more precise complexometric or titri-metric measurements must replace direct potentiometry. According to the Nernst equation, the measured potential difference is a measure of the activity (rather than concentration) of certain ions. Since the concentration is related to the activity through an appropriate activity coefficient, calibration of the electrode with known solution(s) should be carried out under conditions of reasonable agreement of ionic strengths. For quantitation, the standard addition method is used. 

The technique is generally unaffected by the state (ionic, imdissociated, sometimes complexed) of the analyte to be titrated. For example, the direct potentiometric determination of pH in a solution of a weak acid reports only the hydrogen ion concentration. Since the major portion of the acid is present in the undissociated form, direct potentiometry can not provide data yielding the total acid concentration. Potentiometric titration involves titrating the acid solution with a standard base, determining the equivalence point volume of standard base solution used, and calculating the total weak acid concentration from the stoichiometric data. 

Fluoride samples of high concentration can be titrated potentiometrically [60] with lanthanum nitrate or thorium nitrate reagent. The potentiometric standard addition technique with NaF standard solution [61] also was found applicable however, direct potentiometry using calibration curves is most often relied on in water analysis. 

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. 

Potentiometry also is a direct means to evaluate the standard potential for half reactions (E°) and has been applied for appropriate reversible systems. Such measurements require corrections for activity coefficients or extrapolation of the data to infinite dilution. Again, direct measurements in which equal molar concentrations of the oxidized or reduced species are introduced into the system provide a simple approach to such evaluations and are as precise as those obtained by less direct methods. However, E° values also can be extracted from potentiometric titration data. For example, in the titration of Fe (OH2)g+ ion with Ce (OH2) + ion, theFei (OH2)6+ ion concentration equals the Fe (OH2)5" concentration... 

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