Potentiometry, automation

In this present book, we will look at the analytical use of two fundamentally different types of electrochemical technique, namely potentiometry and amper-ometry. The distinctions between the two are outlined in some detail in Chapter 2. For now, we will anticipate and say that a potentiometric technique determines the potential of electrochemical cells - usually at zero current. The potential of the electrode of interest responds (with respect to a standard reference electrode) to changes in the concentration of the species under study. The most common potentiometric methods used by the analyst employ voltmeters, potentiometers or pH meters. Such measurements are generally relatively cheap to perform, but can be slow and tedious unless automated. 

Laboratory investigations play an essential role in medicine. Laboratory results are taken into consideration in about two thirds of all medical decisions in medical systems of industrialized countries today. The vast majority of clinical chemistry analyses are based on few analytical principles including photometry, ligand binding assays and potentiometry. For these standard methods complete automation has been achieved and multi-channel, random access analyzers realize several hundred analyses per instrument and hour on a very high level of user-friendliness. Consequently, clinical chemistry is very cost efficient today typically clinical chemistry analyses contribute less than 5 % of all costs of tertiary care hospitals. 

Potentiometry. The dissociation constants of 4-alkylamido-2-hydroxy-benzoic acids in the presence of Brij 35 micelles were measured at 25°C and 0.10 M ionic strength (NaNO ). The ligand (0.002 M) was titrated with 1 M NaOH using a 655-Multi-Dosimat automated titrator (Metrohm), equipped with a 605-pH-meter and a 614-Impulsomat unit. 

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 . 

Vytas, K. Kalous, L Jekova, J. Automated potentiometry as an ecologic alternative to two-phase titrations of surfactants. Egypt. J. Anal. Chem. 1997, 6, 107-123. 

Several methods can be used for the determination of chloride in water [2], The argentometric and mercuric nitrate methods are based on the titration of chlorine in the presence of an indicator. Experimental procedures are easy, but many substances may interfere with the results. There are also other methods such as potentiometry, capillary electrophoresis and other automated methods (ferricyanide method or flow injection analysis). 

Several types of electrochemical techniques have been used in automated systems (see Table 24.1). At first glance, their use in instrument systems appears straightforward, since each transducer converts chemical information directly into an electrical signal. Unfortunately, few applications are found for those methods involving net current flow (e.g., amperometry) because the rate of mass transfer (and hence the current) depends on the sample flow-rate, which may vary, and on how clean the electrode surface is. This discussion will therefore be restricted to potentiometry, a zero-current technique. 

The glass electrode used for measuring pH is one of the most successful examples of potentiometry in automated instruments. Modern glass electrodes are highly reliable they give selective, sensitive, and stable response to acidity over a very wide range of pH and have been widely applied in industrial monitoring and control. 

In practice, electrochemistry not only provides a means of elemental and molecular analysis, but also can be used to acquire information about equilibria, kinetics, and reaction mechanisms from research using polarography, amperometry, conductometric analysis, and potentiometry. The analytical calculation is usually based on the determination of current or voltage or on the resistance developed in a cell under conditions such that these are dependent on the concentration of the species under study. Electrochemical measurements are easy to automate because they are electrical signals. The equipment is often far less expensive than spectroscopy instrumentation. Electrochemical techniques are also commonly used as detectors for LC, as discussed in Chapter 13. 

Use of potentiometry for pH titration allows analyses to be carried out in colored or turbid solutions. Also, it solves the problem of selecting the correct indicator for a particular acid-base titration. The endpoint can be determined more accurately by using a first or second differential curve as described earlier. It also permits pH titrations in nonaqueous solvents for the determination of organic acids and bases as described subsequently. In addition, it can be readily automated for unattended operation. 

This article provides some general remarks on detection requirements for FIA and related techniques and outlines the basic features of the most commonly used detection principles, including optical methods (namely, ultraviolet (UV)-visible spectrophotometry, spectrofluorimetry, chemiluminescence (CL), infrared (IR) spectroscopy, and atomic absorption/emission spectrometry) and electrochemical techniques such as potentiometry, amperometry, voltammetry, and stripping analysis methods. Very few flowing stream applications involve other detection techniques. In this respect, measurement of physical properties such as the refractive index, surface tension, and optical rotation, as well as the a-, //-, or y-emission of radionuclides, should be underlined. Piezoelectric quartz crystal detectors, thermal lens spectroscopy, photoacoustic spectroscopy, surface-enhanced Raman spectroscopy, and conductometric detection have also been coupled to flow systems, with notable advantages in terms of automation, precision, and sampling rate in comparison with the manual counterparts. 

D. Jagner, L. Renman, and S.H. Stefansdottir. Determination of iron(III) and titanium(IV) as their solochrome violet RS complexes by constant-current stripping potentiometry. Part 1. Automated single-point calibration method for iron(III). Analytica Chimica Acta 281 305-312,1993. J.M. Laporte, J.P. Truchot, F. Ribeyre, and A. Boudou. Combined effects of water pH and salinity on the bioaccumulation of inorganic mercury and methylmercury in the shore crab Carcinus maenas. Marine Pollution Bulletin 34 880-893,1997. 

In routine blood analysis of electrolytes, where ion-selective electrodes are used nearly universally, very small concentration changes are sometimes determined with direct potentiometry. This requires potential stabilities and reproducibihties on the order of 10-100 pV, which is achieved in temperature controlled flow-through cells and with frequent, automated recalibrations between measurements/ In batch mode benchtop analyses with ISEs and in environmental monitoring applications, such a high precision is often not achieved. Precision and accuracy is mainly limited by variations in the liquid junction potential between the calibration and sample phases and by interferences from other sample ions, temperature fluctuations, and, if concentrations rather than activities are desired, variations in activity coefficients. 

Vytras K, Kalous J, Jezkova J (1997) Automated potentiometry as an ecologic alterative to two-phase titration of surfactants. Egypt J Anal Chem 6 107-128... 

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