Potentiometry advantages

One of the most fruitful uses of potentiometry in analytical chemistry is its application to titrimetry. Prior to this application, most titrations were carried out using colour-change indicators to signal the titration endpoint. A potentiometric titration (or indirect potentiometry) involves measurement of the potential of a suitable indicator electrode as a function of titrant volume. The information provided by a potentiometric titration is not the same as that obtained from a direct potentiometric measurement. As pointed out by Dick [473], there are advantages to potentiometric titration over direct potentiometry, despite the fact that the two techniques very often use the same type of electrodes. Potentiometric titrations provide data that are more reliable than data from titrations that use chemical indicators, but potentiometric titrations are more time-consuming. 

With a low constant current -1 (see Fig. 3.71) one obtains the same type of curve but its position is slightly higher and the potential falls just beyond the equivalence point (see Fig. 3.72, anodic curve -1). In order to minimize the aforementioned deviations from the equivalence point, I should be taken as low as possible. Now, it will be clear that the zero current line (abscissa) in Fig. 3.71 yields the well known non-faradaic potentiometric titration curve (B B in Fig. 2.22) with the correct equivalence point at 1.107 V this means that, when two electroactive redox systems are involved, there is no real need for constant-current potentiometry, whereas this technique becomes of major advantage... 

A technique utilizing the advantages of both differential measurement and titration is null-point potentiometry [37, 38, 83]. The same arrangement is used as described under (Z ), but concentration c is not constant and is varied until Cj = c = c. Then,... 

The main advantages that should gradually promote potentiometry with ISEs to routine use in clinical laboratories are simplicity of instrumentation, the possibility of decreasing the sample volume (especially important in pediatry), and the possibility of avoiding tedious centrifugation necessary for preparation of plasma and serum (significant under intensive care conditions). On the other hand, work with ISEs requires experience and skill... 

Analytical determinations with the fluoride ion-selective electrode These are based either on direct potentiometry of fluorides [37, 84, 85, 88, 430] or on titration determinations of either fluorides or of other ions and also on titrations with fluoride ions as indicator. The advantages of potentiometry with an ISE over other analytical methods for determining fluorides were pointed out by Crosby etai [67], Further comparison studies [42, 56, 191, 433] came to the same conclusions, confirmed also by a study of 16 methods [365]. Fluoride ions are titrated either with La (for concentrations greater than 1 mM) or Th (in the concentration range 0.2-1 mM F ) [13, 102, 103, 113,233, 234]. Titration with fluoride ions can be used for the determination of Al with formation of the AIF4 complex up to nanomolar concentrations, especially in ethanol-water mixtures [25] (see also [267,384]). Precipitation titrations can also be used to determine La, Th and UOJ [241, 384] as well as Li in... 

Ion solvation has been studied extensively by potentiometry [28, 31]. Among the potentiometric indicator electrodes used as sensors for ion solvation are metal and metal amalgam electrodes for the relevant metal ions, pH glass electrodes and pH-ISFETs for H+ (see Fig. 6.8), univalent cation-sensitive glass electrodes for alkali metal ions, a CuS solid-membrane electrode for Cu2+, an LaF3-based fluoride electrode for l , and some other ISEs. So far, method (2) has been employed most often. The advantage of potentiometry is that the number and the variety of target ions increase by the use of ISEs. 

For monitoring catalytic (enzymatic) products, various techniques, such as spectrophotometry [32], potentiometry [33,34], coulometry [35,36] and amperometry [37,38], have been proposed. An advantage of these sensors is their high selectivity. However, time and thermal instability of the enzyme, the need of a substrate use and indirect determination of urea (logarithmic dependence of a signal upon concentration while measuring pH) cause difficulties in the use and storage of sensors. 

An important advantage of these techniques is that the measurements of electrical potential can be very accurate, which allows monitoring until almost complete conversion, or until equilibrium in a reversible process. In fact, potentiometry is extremely powerful for obtaining equilibrium constants [25]. However, there are also restrictions and limitations (a) the solution must be conducting (b) the response time of the electrode can be relatively long, so there is a limit to the speed of a reaction which can be monitored and (c) there can be appreciable interference from impurities, or intermediates and products. 

Details of the application of polarography and various elaborations of this technique to the determination of stabihty constants are described by Heyrovsky and Kuta, Nancollas, Hartley etal. Beck and Nagypal, and Cukrowski. A major advantage of polarography is its usefiilness as a complement to potentiometry for determining stabihty constants. ... 

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 . 

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. 

Log D profiles may be obtained by performing the filter probe experiment over a range of pH values. The critical part of the experiment is to discover whether the compound of interest has an isobestic point in its UV spectrum. If it does not, the experiment cannot be performed if it does, both log P and pKa values may be determined. This method is similar to the potentiometric method described earlier except that the amount of un-partitioned substance is determined by spectroscopy rather than by potentiometry. The advantage of this method is that only one experiment needs to be performed to yield log P, log P pp, and pKaS, but not all compounds have an isobestic point. Sample concentrations down to 4 mg/400 mL may be determined (approx. 0.000025 M). 

There are advantages to potentiometric titration over direct potentiometry, despite the fact that the two techniques very often use the same types of electrodes. 

The potentiometric titration method possesses some advantages characteristic of potentiometry. The measured e.m.f. values are dependent on the logarithm of the activity (concentration) of the potential-defining ion and this considerably widens the range of concentrations which can be detected. The experimental techniques and routine are rather simple and the obtained results are well reproducible. Owing to the stable activity coefficients of metal cations the measurements can be performed in sufficiently concentrated solutions of the corresponding metal halides. Performing the measurement does not imply any interference in the acid-base processes in the melt studied and the experiment is faster than the isothermal saturation technique discussed above. 

Potentiometric and refractive index detection are not affected by volume but are relatively insensitive in the nanolitre to picolitre range compared to amperometric detection (micro surface area) and fluorimetric detection (micro amount of material). At 1 pL, limits of detection are similar for potentiometry, amperometry and fluorescence. On-chip LC is very compatible with mass spectrometry due to the low volumes and flow rates required. Battery-operated ion trap MS has been reported but miniaturisation of MS offers no sensitivity or selectivity advantages. Electrospray ionisation (ESI) has been successfully integrated into a chip format allowing for many ESI nozzles on one chip. Arrays make pattern recognition possible. 

There are a number of different experimental methods to determine the stoichiometry and equilibrium constants for Reaction (V11.2). cf. Section Vll.3.4. The advantages and disadvantages of the most frequently used methods, potentiometry, liquid-liquid extraction and solubility, are briefly discussed in the following sections for more details see the book by Rossotti and Rossotti [1961ROS/ROS]. 

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