Potentiometry precision

Perhaps the most precise, reHable, accurate, convenient, selective, inexpensive, and commercially successful electroanalytical techniques are the passive techniques, which include only potentiometry and use of ion-selective electrodes, either direcdy or in potentiometric titrations. Whereas these techniques receive only cursory or no treatment in electrochemistry textbooks, the subject is regularly reviewed and treated (19—22). Reference 22 is especially recommended for novices in the field. Additionally, there is a journal, Ion-Selective Electrode Reviews, devoted solely to the use of ion-selective electrodes. 

It may happen that AH is not available for the buffer substance used in the kinetic studies moreover the thermodynamic quantity A//° is not precisely the correct quantity to use in Eq. (6-37) because it does not apply to the experimental solvent composition. Then the experimentalist can determine AH. The most direct method is to measure AH calorimetrically however, few laboratories Eire equipped for this measurement. An alternative approach is to measure K, under the kinetic conditions of temperature and solvent this can be done potentiometrically or by potentiometry combined with spectrophotometry. Then, from the slope of the plot of log K a against l/T, AH is calculated. Although this value is not thermodynamically defined (since it is based on the assumption that AH is temperature independent), it will be valid for the present purpose over the temperature range studied. 

Horvai, G., and Pungor, E., Precision of the Double Known Addition Method in Ion-Selective Electrode Potentiometry, Anal. Chem. 55, 1983, 1988-1990. 

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. 

Another galvanic cell of highly practical and theoretical importance is the so-called standard cell (see Section 2.2.2), use of which has to be made as a calibration standard in non-faradaic potentiometry. For this purpose, the saturated Weston cell is the most accepted as its emf is reproducible, precisely known, only slightly temperature dependent in the region around 25° C (1.01832 V) and insensitive to unexpected current flows, if any. 

An appropriate ion-specific electrode was found to provide a convenient, precise and relatively inexpensive method for potentiometry of copper(II) ion in copper-complex azo or formazan dyes. Copper(II) ion in copper phthalocyanine dyes can be quantified after anion exchange. Twelve commercial premetallised dyes evaluated using this technique contained copper(II) ion concentrations in the range 0.007 to 0.2%. Thus many copper-complex direct or reactive dyes are likely to contribute low but possibly significant amounts of ionic copper to textile dyeing effluents [52]. 

The differences between potentiometry and amperometry are summarized in Table 1.1. It will be seen that amperometric measurements are generally more precise and more versatile than those made by using potentiometry, so the majority of this book will therefore be concerned with amperometric measurements. 

With very few exceptions, quantitative epoxide assay techniques currently in use are derived from the reeotion of ethylene oxides with halogen adds, notably hydrochloric acid and hydrobromio add, in a variety of solvents. Acid uptake may be determined by any of several reliable procedures. These include titration with standard base8 nr back-titration with standard acid.744 The end-point may be detected visually in the presence of suitable acid-base indicators, or by the more precise technique of potontionaetry.447.4 -470 A useful alternative, applicable in the presence of easily hydrolysed substances or of amines that buffer the end-point, is the technique of argentiometry. In this procedure excess of halide ion is titrated with silver nitrate in tV presence of ferric thiocyanate indicator,470 1884 or potentiometri-cally.188 ... 

Winefordner, J. D., and M. Tin Separation of Trace Quantities of Bromide from Large Amounts of Chloride by a Distillation Method and Measurement of the Bromide by Precision Null-Point Potentiometry. Anal. Chem. 35, 382 (1963). 

Differential Electrolytic Potentiometry. II. Precision and Accuracy of Application to Redox Titrimetry. Analyst 83, 212 (1958). 

Malmstadt, H. V., and J. G. Winefordner Precision Null-Point Potentiometry. A Simple, Rapid and Accurate Method for Low Concentration Chloride Determination. Anal. Chim. Acta 20, 283 (1959). 

The simplest method of measurement with ion-selective electrodes is direct potentiometry by use of the Nemst equation. However, this makes extreme demands on the reproducibility of the junction potential, and there is the problem of variation of activity with ionic strength. Concentration-cell techniques have proved to be very precise, especially in terms of null-point potentiometi... 

Ascorbic acid and Na ascorbate can be determined in an automated constant current coulometric system. Under optimal conditions, an excellent precision of 0.3% was achieved, with 95% probability . Ca ascorbate can be determined by potentiometry (using Ag as indicator electrode) and constant current coulometric methods. Automatic coulom-etry possesses the advantage of speed and, with its satisfactory precision, is well suited to routine pharmaceutical analysis . 

More than brief discussion of the numerous ways in which end points can be taken other than by visual methods is beyond our scope. For example, end-point techniques may involve photometry, potentiometry, amperometry, conductometry, and thermal methods. In principle, many physical properties can be used to follow the course of a titration in acid-base titrations, use of the pH meter is common. In terms of speed and cost, visual indicators are usually preferred to instrumental methods when they give adequate precision and accuracy for the purposes at hand. Selected instrumental methods may be used when a suitable indicator is not available, when higher accuracy under unfavorable equilibrium conditions is required, or for the routine analysis of large numbers of samples. 

In contrast to direct potentiometry, the potentiometric titration technique offers the advantage of high accuracy and precision, although at the cost of increased time and increased consumption of titrants. Another advantage is that the potential break at the titration endpoint must be well defined, but the slope of the sensing electrode response need be neither reproducible nor Nernstian, and the actual potential values at the endpoint are of secondary interest. In many cases, this allows for the use of simplified sensors. 

Potentiometry. Potentiometric methods rely on the logarithmic relationship between measured potential and analyte concentration. The most common involves an instrument called a pH-Stat , in which a glass (pH) electrode follows reactions that either consume or produce protons. Since pH changes cause changes in enzyme activity, the pH is maintained at a constant value by the addition of acid or base. The rate of titrant addition is then proportional to the rate of the enzymatic reaction. Precise measurements using the pH-Stat require low buffer concentrations in the enzymatic assay mixture. 

The acidity of the coordinated water in organometallic aquocomplexes varies over a large range. In aqueous solutions pH potentiometry is one of the most precise methods of determining the acid dissociation constants, K3, for the organome-tal ions. Several stable oxidation states of the transition metal ions in organometallic compounds can exist in water. Cyclic voltammetry gives information on the reversibility of the oxidation/reduction processes. 

Potentiometry can be used even for dilute solutions of sparingly soluble substances if unusual care is taken e.g. 0.1 mM adenine, titrated in the presence of 8 /jlM cupric ions (in water), readily gave a precise constant (Albert and Serjeant, 1960). Solvents other than water should never be used if results of biological significance are required. Mixtures of water and an organic solvent give particularly misleading results (Albert and Serjeant, 1984). 

The precision of the differential potentiometric method can be improved by using null-point potentiometry [15—19]. The ion concentration is measured, not from a single potential reading, but by adjusting the composition of one of the half-cell solutions to matdi the other until a potential (the null, or bias poten-... 

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