Zero Current Potentiometry

If the input impedance of the measuring instrument is high ( 10 Q), then the negligible current passes through the cell, and, therefore, the measured AE value is close to the emf. 

If the diffusion potential is resulted by the concentration difference of one salt, then the value of fijig can be given by the following equation  

The chemical potential differences of all ions between the two solutions as well as the mobility differences take part in generating the For the multicomponent electrolyte, the Henderson form describes its value  

Based on this equation, we can see that, if a salt with % Uq is added in a high and equal concentration into both sides of the liquid junction, then only small will show up. 

The signal used in potentiometric analysis is generated at the boundary between the electrode and the sample solution. When no current passes through the cell then, the charge separation process is in dynamic equilibrium. For equilibrium, we can write 

For this technique it is simply required to have access to an indicator electrode whose potential is a function of the activity of the species to be titrated. A cell is then made by placing this indicator electrode with a suitable reference electrode in the solution to be titrated. The cell is connected in the circuit shown in Fig. 6.6 and its e.m.f. measured after each addition of titrant. Zero 

Basic circuit for classical potentiometry a = indicator electrode b = reference electrode. 

The equivalence point of the titration may be determined either from the inflexion point of the graph of indicator potential versus titrant volume (Fig. 6.7a) or from the derivative plot of d /dt versus V (Fig. 6.7b). 

The indicator and reference electrodes are now dipping into a solution containing mainly ferrous ions but a small amount of ferric ions. The potential of the platinum electrode will now adopt a characteristic value which is a function of the activity ratio of ferric to ferrous ions as expressed in the Nernst equation. 

As more ceric ions are added the ratio of ferric to ferrous ions increases. The potential adopted by the indicator electrode does not, however, vary greatly. Its value increases only slowly over the range a-b (Fig. 6.7a) in accordance with the Nernst equation. A point will be reached where all the ferrous ions have been removed by oxidation and a small excess of ceric ions have been added. The potential of the indicator electrode will now be a function of the ratio of activities of ceric and cerous ions according to 

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Three broad classifications of electrochemical methods are used in this chapter. Po-tentiometric methods include zero-current potentiometry and methods in which current of controlled magnitude is apphed to the working electrode, such as in potentiometric stripping analysis (PSA). Amperometric methods consider all techniques in which current is measured these include constant-potential amper-ometry and amperometric measurements made in response to a variety of applied potential waveforms in voltammetric methods. Impedimetric methods comprise a final classification in these methods, faradaic currents are generally absent, and impedance, conductance, or capacitance is the measured property. 

Nonselective, metallic indicator electrodes have been used for potentiometric measurements in complex biological media, for example, Pt electrodes have been used to monitor the redox potential of fermentation broths as cultures grow [17]. However, zero-current potentiometry more often involves ISEs based on solid membranes, composed of a sparingly soluble salt of the ion of interest or liquid membranes, in which an ion-selective reagent is dissolved, with the membrane separating reference... 

Enzymes, Antibodies, and Nucieic Acids with Zero-Current Potentiometry... 

Potentiometry (discussed in Chapter 5), which is of great practical importance, is a static (zero current) technique in which the information about the sample composition is obtained from measurement of the potential established across a membrane. Different types of membrane materials, possessing different ion-recognition processes, have been developed to impart high selectivity. The resulting potentiometric probes have thus been widely used for several decades for direct monitoring of ionic species such as protons or calcium, fluoride, and potassium ions in complex samples. 

Potentiometry is suitabie for the analysis of substances for which electrochemical equilibrium is established at a suitable indicator electrode at zero current. According to the Nemst equation (3.31), the potential of such an electrode depends on the activities of the potential-determining substances (i.e., this method determines activities rather than concentrations). 

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... 

The electrochemical detection of pH can be carried out by voltammetry (amper-ometry) or potentiometry. Voltammetry is the measurement of the current potential relationship in an electrochemical cell. In voltammetry, the potential is applied to the electrochemical cell to force electrochemical reactions at the electrode-electrolyte interface. In potentiometry, the potential is measured between a pH electrode and a reference electrode of an electrochemical cell in response to the activity of an electrolyte in a solution under the condition of zero current. Since no current passes through the cell while the potential is measured, potentiometry is an equilibrium method. 

Potentiometry is the most widely used electroanalytical technique. It involves the measurement of the potential of a galvanic cell, usually under conditions of zero current, for which purpose potentiometers are used. Measurements may be direct whereby the response of samples and standards are compared, or the change in cell potential during a titration can be monitored. 

Potentiometry deals with the electromotive force (EMF) generated in a galvanic cell where a spontaneous chemical reaction is taking place. In practice, potentiometry employs the EMF response of a galvanostatic cell that is based on the measurement of an electrochemical cell potential under zero-current conditions to determine the concentration of analytes in measuring samples. Because an electrode potential generated on the metal electrode surface,... 

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. 

In summary, then, the biggest difference between measurements of potential and current (potentiometry and amperometry, respectively) is that potentials are measured with a zero current wherever possible, implying that no compositional changes occur inside the cell during measurement, whereas compositional changes do occur during the measurement of current. 

Potentiometry The techniques and methodology of determining an activity as a function of potential (at zero current). Activity and concentration can often be interchanged at low ionic strength. 

The previous chapters dealt with ISE systems at zero current, i.e. at equilibrium or steady-state. The properties of the interface between two immiscible electrolyte solutions (ITIES), described in sections 2.4 and 2.5, will now be used to describe a dynamic method based on the passage of electrical current across ITIES. Voltammetry at ITIES (for a survey see [3, 8, 9, 10, 11, 12,18]) is an inverse analogue of potentiometry with liquid-membrane ISEs and thus forms a suitable conclusion to this book. 

Potentiometry is a method of obtaining chemical information by measuring the potential of an indicator electrode under zero current flow. It is based on the Nernst equation, which expresses the electrode potential as a function of the activity (or activities) of the chemical species in solution. The information obtained varies with indicator electrode, from the activity (concentration) of a chemical species to the redox potential in the solution. The potential of the indicator electrode is measured against a reference electrode using a high inptit-impedance mV/pH me-... 

Potentiometry is a method of electroanalytical measurement in which the equilibrium voltage of the cell consisting of an indicator electrode and a proper reference electrode is measured using a high-impedance voltmeter, i.e., effective at zero current. The potential of the indicator electrode is a function of particular species present in solutions and their concentration. By judicious choice of electrode material, the selectivity of the response to one of the species can be increased, and thus, interferences from other ions can be minimized. The method allows the determination of concentrations with detection limits of the order of 0.1 pmol per liter, although in some cases, as little as lOpmol differences in concentration can be measured. 

In contrast to potentiometry that operates under zero current conditions, other electroanalytical methods impose an external source of electricity to the sample solution, to induce an electrochemical reaction that would not otherwise spontaneously occur. It is thus possible to measure all sorts of ions or organic compounds that can either be reduced or oxidized electrochemically. 

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. 

Electrochemical transducers are commonly used in the sensor field. The main forms of electrochemistry used are potentiometry (zero-current cell voltage [potential difference measurements]), amperometry (current measurement at constant applied voltage at the working electrode), and ac conductivity of a cell. 

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