Potentiometric measurement

By convention, the electrode on the left is considered to be the anode, where oxidation occurs 

by convention, potentiometric electrochemical cells are defined such that the indicator electrode is the cathode (right half-cell) and the reference electrode is the anode (left half-cell). 

The double vertical slash ( ) indicates the salt bridge, the contents of which are normally not indicated. Note that the double vertical slash implies that there is a potential difference between the salt bridge and each half-cell. 

What are the anodic, cathodic, and overall reactions responsible for the potential in the electrochemical cell shown here Write the shorthand notation for the electrochemical cell. 

The oxidation of Ag to Ag+ occurs at the anode (the left-hand cell). Since the solution contains a source of Cb, the anodic reaction is 

The measured potential E is described by Nemst s equation as follows  

Eq - constant standard electrode potential ( connection potential ). 

For monovalent ions the slope of the electrode characteristic (2.303 RTri F ) is theoretically equal to 59.1 mV for 10 concentration units each at 25° C. 

The ion activity represents the probability with which an ion interacts with another ion. The ion activity depends on the temperature and on the amount and kind of the other ions that are present. It is not affected by non-ionic substances such as lipids, erythrocytes, etc. The ion concentration states the absolute number of ions in solution and does not change if other ions are dissolved. A relation between the activity a and the concentration c can be established with the help of the activity coefficient yaccording to the following equation  

The activity coefficient is always of the same magnitude for each of the ions in a specific solution however, it changes with the solvent concentration. 

the galvanometer is connected to the unknown voltage and the slide wire again adjusted for balance with a resistance value R2 The defining equations are 

The first equation is not significant. The current / must remain constant and may be set at some convenient initial value by adjusting the rheostat A/ . The working battery must be chosen so that the current drain upon it during operation does not reduce its terminal voltage. From the last two equations, 

Thus is determined when the ratio R2/R1) and the standard cell emf are known. 

A direct-reading instrument can be made by normalizing Ri to IQ and 1 to 1 V then (volts) = R2 (ohms calibrated as volts). The slide wire is calibrated directly in voltage units. The rheostat AR can be used to zero-adjust the potentiometer for the required normalization. Many commercially available potentiometers are direct reading. Provision exists for adjusting initially to different standard cell emfs. A typical value for is 1.019 V abs, when an unsaturated cell is used. 

The liquid junction (diffusion) potential should always be a concern until a cell without transfer is tested. Note that the terms liquid junction potential and diffusion potential are used interchangeably in the literature. Measurement, calculation, and minimization of the liquid junction potential formed at the interface of two solutions are some of the topics to be addressed in this chapter. 

A variety of commercial electrometers can be used for potentiometric measurements. However, the internal resistance of the electrometer should be above 10 Q. At such internal resistance, the cnrrent passing through the electrodes of a cell potential of about 1 V will be 10 A, which is a negligible valne. 

The Ag/AgCl reference electrode, shown in Fignre 4.2, is one of the most popular and reliable electrodes with the following electrochemical half-reaction  

The commercial Ag/AgCl electrode can work at temperatures up to about 80°C, and the electrode potentials at elevated temperatures can be calculated quite accurately. It is important to know how the potential of the reference electrode changes with temperature. When analyzing the Nernst equation for the Ag/ AgCl electrode. Equation 5.1, the variables that depend on temperature are the standard electrode potential, Agci(syAg(s),ci-(aq) Cl-(aq) activity, aci, aq)- The 

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Potentiometric measurements are made using a potentiometer to determine the difference in potential between a working or, indicator, electrode and a counter electrode (see Figure 11.2). Since no significant current flows in potentiometry, the role of the counter electrode is reduced to that of supplying a reference potential thus, the counter electrode is usually called the reference electrode. In this section we introduce the conventions used in describing potentiometric electrochemical cells and the relationship between the measured potential and concentration. 

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. 

Precision The precision of a potentiometric measurement is limited by variations in temperature and the sensitivity of the potentiometer. Under most conditions, and with simple, general-purpose potentiometers, the potential can be measured with a repeatability of +0.1 mV. From Table 11.7 this result corresponds to an uncertainty of +0.4% for monovalent analytes, and +0.8% for divalent analytes. The reproducibility of potentiometric measurements is about a factor of 10 poorer. 

Harris, T. M. Potentiometric Measurement in a freshwater Aquarium, /. Chem. Educ. 1993, 70, 340-341. 

Ga.s Eeeders. Chlorine gas is usually fed from a chlorine cylinder equipped with a pressure gauge, reducing valve, regulating valve, feed-rate indicator, and aspirator-type injector for dissolving the chlorine gas in water. The feeder can be manually, or more desirably automatically, controlled utili2ing continuous amperometric or potentiometric measurement of the free chlorine residual. The chlorine solution is normally introduced into the return line to the filter. 

In case b) a potentiometrically measured calibration curve for ion M+ would show a sigmoidal form, starting with a sub-Nernstian slope leading to a super-Nemstian... 

Standard potentials are determined with full consideration of activity effects, and are really limiting values. They are rarely, if ever, observed directly in a potentiometric measurement. In practice, measured potentials determined under defined concentration conditions (formal potentials) are very useful for predicting the possibilities of redox processes. Further details are given in Section 10.90. 

Standard potentials Ee are evaluated with full regard to activity effects and with all ions present in simple form they are really limiting or ideal values and are rarely observed in a potentiometric measurement. In practice, the solutions may be quite concentrated and frequently contain other electrolytes under these conditions the activities of the pertinent species are much smaller than the concentrations, and consequently the use of the latter may lead to unreliable conclusions. Also, the actual active species present (see example below) may differ from those to which the ideal standard potentials apply. For these reasons formal potentials have been proposed to supplement standard potentials. The formal potential is the potential observed experimentally in a solution containing one mole each of the oxidised and reduced substances together with other specified substances at specified concentrations. It is found that formal potentials vary appreciably, for example, with the nature and concentration of the acid that is present. The formal potential incorporates in one value the effects resulting from variation of activity coefficients with ionic strength, acid-base dissociation, complexation, liquid-junction potentials, etc., and thus has a real practical value. Formal potentials do not have the theoretical significance of standard potentials, but they are observed values in actual potentiometric measurements. In dilute solutions they usually obey the Nernst equation fairly closely in the form ... 

In view of the problems referred to above in connection with direct potentiometry, much attention has been directed to the procedure of potentio-metric titration as an analytical method. As the name implies, it is a titrimetric procedure in which potentiometric measurements are carried out in order to fix the end point. In this procedure we are concerned with changes in electrode potential rather than in an accurate value for the electrode potential with a given solution, and under these circumstances the effect of the liquid junction potential may be ignored. In such a titration, the change in cell e.m.f. occurs most rapidly in the neighbourhood of the end point, and as will be explained later (Section 15.18), various methods can be used to ascertain the point at which the rate of potential change is at a maximum this is at the end point of the titration. 

The equipment required for direct potentiometric measurements includes an ion-selective electrode (ISE), a reference electrode, and a potential-measuring device (a pH/millivolt meter that can read 0.2mV or better) (Figure 5-1). Conventional voltmeters cannot be used because only very small currents are allowed to be drawn. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species. Such electrodes exhibit a fast response and a wide linear range, are not affected by color or turbidity, are not... 

FIGURE 5-1 Schematic diagram of an electrochemical cell for potentiometric measurements. 

Explain clearly (using equations) why a highly selective ISE is not always sufficient for accurate potentiometric measurements. 

Discuss the major sources of errors in potentiometric measurements. 

Immunoassays have been based on the potentiometric measurement of marker ions such tetrapentylammonium ion (TPA ) that are loaded in phospholipid liposomes Complement mediated immunolysis of these loaded vesicles is caused... 

In potentiometric measurements of emf, it is necessary to have a cell where emf is constant with time and is exactly known. For this purpose, a standard Weston cell is commonly employed. A diagram of a simplified potentiometer is shown in Figure 6.9. It consists of a... 

Electrodes which only respond to certain free (not bound) measured ions are called ion-selective electrodes (ISE). The term is now usually applied to all potentiometric measuring electrodes that are capable of providing data concerning the concentration or activity of... 

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. 

Applications Potentiometry finds widespread use for direct and selective measurement of analyte concentrations, mainly in routine analyses, and for endpoint determinations of titrations. Direct potentiometric measurements provide a rapid and convenient method for determining the activity of a variety of cations and anions. The most frequently determined ion in water is the hydrogen ion (pH measurement). Ion chromatography combined with potentiometric detection techniques using ISEs allows the selective quantification of selected analytes, even in complex matrices. The sensitivity of the electrodes allows sub-ppm concentrations to be measured. 

The pyridinium ion (acid 2) as the analyte can be titrated with quaternary ammonium hydroxide (base 3) as it concerns the determination of H+ of the Brensted acid pyridinium, a potentiometric measurement of the pH titration curve and its inflection point is most obvious. In the aprotic, but protophilic, solvent pyridine no stronger acid can exist (see reactions 4.37 and 4.38) than the pyridinium ion itself hence there is a levelling effect but in theory only on the acid side. 

For the different values of pAHX and pA H+ see the summary Table 4.5 later of pKa data in various solvents of low e in comparison with pAa(H20). The mutual agreement of pffHX values obtained by spectrophotometry, DVP, potentiometry and titration was reasonably good the typical form of the curves for titration of the dinitrophenols with TMG can be explained by homoconjugation and more especially by its influence on the potentiometric measurements, calculated on the basis of simple dissociation hence the major discrepancies in the spectrophotometric and potentiometric pK values. In order to... 

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