Potentiometric Measurement of pH

A wide variety of methods has been used in studies of oligomerization reactions. The most important quantitative method is potentiometric measurement of pH as a function of the total metal concentration and of the concentration of the analytical excess of acid or base. Other quantitative methods which are often used are potentiometric determination of metal ion concentration, calorimetry, spectrophotometry, and ion exchange. These, together with a number of other techniques, have recently been discussed thoroughly by Baes (22). 

Use of the potential of a galvanic cell to measure the concentration of an electroactive species developed later than a number of other electrochemical methods. In part this was because a rational relation between the electrode potential and the concentration of an electroactive species required the development of thermodynamics, and in particular its application to electrochemical phenomena. The work of J. Willard Gibbs1 in the 1870s provided the foundation for the Nemst equation.2 The latter provides a quantitative relationship between potential and the ratio of concentrations for a redox couple [ox l[red ), and is the basis for potentiometry and potentiometric titrations.3 The utility of potentiometric measurements for the characterization of ionic solutions was established with the invention of the glass electrode in 1909 for a selective potentiometric response to hydronium ion concentrations.4 Another milestone in the development of potentiometric measurements was the introduction of the hydrogen electrode for the measurement of hydronium ion concentrations 5 one of many important contributions by Professor Joel Hildebrand. Subsequent development of special glass formulations has made possible electrodes that are selective to different monovalent cations.6"8 The idea is so attractive that intense effort has led to the development of electrodes that are selective for many cations and anions, as well as several gas- and bioselective electrodes.9 The use of these electrodes and the potentiometric measurement of pH continue to be among the most important applications of electrochemistry. 

The usefulness of pH as a measure of the acidity and alkalinity of aqueous media, the wide availability of commercial glass electrodes, and the relatively recent proliferation of inexpensive solid-state pH meters have made the potentiometric measurement of pH perhaps the most common analytical technique in all of science. It is thus extremely impoitant that pH be defined in a manner that is easily duplicated at various times and in various laboratories throughout the world. To meet this requirement, it is necessary to define pH in operational terms—that is, by the way the measurement is made. Only then will the pH measured by one worker be the same as that measured by another. 

Figure 4.6. Flow-injection manifold employed for the enzymatic determination of urea by potentiometric measurement of pH. The sample S (30 jjlL) is injected into a carrier stream of 1.0 mM TRIS buffer in 0.14 M NaCl (pH 7.70), containing dissolved urease, and then passed to the reaction coil a, placed in a themostated water jacket, where the enzymatic degradation of the injected urea takes place. The sample zone is then led to a capillary glass flowthrough electrode (pH) and finally via the reservoir, housing the reference electrode (REF), to waste, W.

The direct potentiometric measurement of pH provides a measure of the equilibrium... 

Measurement of pH With the availability of inexpensive glass pH electrodes and pH meters, the determination of pH has become one of the most frequent quantitative analytical measurements. The potentiometric determination of pH, however, is not without complications, several of which are discussed in this section. 

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. 

Bacarella, A. L. Grunwald, E. Marshall, H. R Purlee, E. L., The potentiometric measurement of acid dissociation constants and pH in the system methanol-water. pKa values for carboxylic acids and anilinium ions, j. Org. Chem. 20, 747-762 (1955). 

The potentiometric measurement of physicochemical quantities such as dissociation constants, activity coefficients and thus also pH is accompanied by a basic problem, leading to complications that can be solved only if certain assumptions are accepted. Potentiometric measurements in cells without liquid junctions lead to mean activity or mean activity coefficient values (of an electrolyte), rather than the individual ionic values. 

Measurement of pH-dependent equilibria can also be used to identify coordination isomerization reactions in addition to stepwise dissociation, such as in the case of the iron(III) complex of exochelin MN (59). Here, a combination of spectrophotometric and potentiometric titration characterized multiple equilibria involving second-sphere protonation, coordination isomerization, and stepwise dechelation, and is illustrated in Fig. 8. 

It is possible to monitor the course of a titration using potentiometric measurements. The pH electrode, for example, is appropriate for monitoring an acid-base titration and determining an end point in lieu of an indicator, as in Experiment 10 in Chapter 5. The procedure has been called a potentiometric titration and the experimental setup is shown in Figure 14.11. The end point occurs when the measured pH undergoes a sharp change—when all the acid or base in the titration vessel is reacted. The same... 

The equilibrium solubility of an Fe oxide can be approached from two directions -precipitation and dissolution. The first method involves precipitating the oxide from a supersaturated solution of ions with stepwise or continuous addition of base und using potentiometric measurements to monitor pH and calculate Fej- in equilibrium with the solid phase until no further systematic change is detected. Alternatively the oxide is allowed to dissolve in an undersaturated solution, with simultaneous measurement of pH and Fejuntil equilibrium is reached. It is essential that neither a phase transformation nor recrystallization (formation of larger crystals) occurs during the experiment this may happen with ferrihydrite which transforms (at room temperature) to a more condensed, less soluble phase. A discussion of the details of these methods is given by Feitknecht and Schindler (1963) and by Schindler (1963). 

Measurement of pH is a potentiometric technique frequently used for measuring the degree of the deterioration of materials that are subjected to natural aging. The determination of pH levels is commonly carried out on ethnographic objects manufactured with parchment or leather, and it is especially relevant in altered paper due to the formation of acidic compounds from the decomposition of the woodpulps and other raw materials, which can induce the hydrolysis of the cellulose and then decrease the resistance and mechanical properties of the document [29]. 

Table 5), and several are now being used, or are potentially useful, for measuring key ocean elements. The most common use of direct potentiometry (as compared with potentiometric titrations) is for measurement of pH (Culberson, 1981). Most other cation electrodes are subject to some degree of interference from other major ions. Electrodes for sodium, potassium, calcium, and magnesium have been used successfully. Copper, cadmium, and lead electrodes in seawater have been tested, with variable success. Anion-selective electrodes for chloride, bromide, fluoride, sulfate, sulfide, and silver ions have also been tested but have not yet found wide application. 

The measurement of pH in cheese making is extremely important to control fermentation/acid production and hence the final quality. While there are no standard methods available for measuring cheese pH, there have been few methods reported in the literature. One method involves preparing a slurry of 10 g of grated cheese in water and measuring the pH potentiometrically (Fox et al., 2004a). However, this method may alter the balance between colloidal and soluble calcium phosphate and hence it is preferable to measure the pH of the cheese directly. The quinhydrone electrode method (Marshall, 1992) measures the pH directly. The potential (mV) created by a paste of cheese and quinhydrone in saturated KC1 is measured and used to determine the pH at a particular temperature. 

The potentiometric measurement of hydrogen sulfide via a Ag/Ag2 electrode is well known and such electrodes are commercially available. Boulggue (22) has used the electrode to measure changes in potential during a titration with HgC. This method can measure sulfide, polysulfide, thiols, sulfite and thiosulfate. Weaknesses of die method are the following there is no method of sample preservation the pH must be adjusted to pH 13 for the measurement of sulfide, thiol and polysulfide then adjusted to pH 7 for the measurement of thiosulfate and sulfite polysulfide is determined by difference after attack on the polysulfides with added sulfite (if elemental sulfur is present in the sample it will also participate in this reaction) the identity of the organic thiols is unknown and sulfite can not be measured in seawater samples because of competition with halides (21). 

Acetylcholineesterase Miniaturized multichannel transduc-tor with planar Au electrode which was first covered with a choline-selective liquid membrane made from 66% PVC-polyvinyl acetate (PVA), 33% 2-nitrophenyl octyl ether plasticizer and 1% ion-pair choline phosphotungstate. A second layer of 2% AChE in the PVA-polyethylene dispersion was spread on the top. The electrode was used as working electrode versus Ag/AgCl for potentiometric measurement of Ch and ACh in 0.1 M Tris buffer at 7.4. Optimum pH range for the sensor was 7-9. The calibration graph was linear from 0.02-10 mm ACh and detection limit was 5 pM. Response time was 3-5 min. Sensor was suitable for determination of ACh in biological fluids. [86]... 

C. The potentiometric determination of pH The most advanced and precise method of the measurement of pH is based on the measurement of the electromotive force (e.m.f.) of an electrochemical cell, which contains the solution of the unknown pH as electrolyte, and two electrodes. The electrodes are connected to the terminals of an electronic voltmeter, most often called simply a pH-meter. If properly calibrated with a suitable buffer of a known pH, the pH of the unknown solution can be read directly from the scale. 

How does information supplied by a direct poten-tiometric measurement of pH differ from that obtained from a potentiometric acid/base titration ... 

The direct potentiometric determination of pH provides a measure of the equilibrium concentration of hydronium ions in the sample. A potentiometric titration provides information on the amount of reactive protons, both ionized and nonionized in the sample. 

By the systematic work on the complexation equilibrium analyses of both weak acidic and weak basic polyelectrolytes, the Gibbs-Donnan approach is validated to provide deep insights into the complexation behaviors of linear polymer ligands. This concept does not need any adjustable parameter it only uses the logic of phase separation of polyelectrolyte aqueous solutions. The electrostatic nonideality (polyelectrolytic effect) observed in the acid dissociation equilibria of a polyion can directly be used to correct for the electrostatic nonideality for metal complexation. The potentiometric titration technique with concurrent measurements of pH and pM is most suit-... 

The quinhydrone electrode can be used for the potentiometric determination of pH. The solution to be measured is saturated with quinhydrone, an equimolar mixture of quinone (Q) and hydroquinone (HQ), and the potential of the solution is measured with a platinum electrode. The half-reaction and its standard potential are as follows ... 

The potentiometric technique involves the measurement of pH, which falls as acetic acid released by hydrolysis of the acetylated enzyme occurs however, this method lacks sensitivity (Michel 1949). The radioisotopic method uses tritiated acetylcholine as substrate. The method is highly sensitive but requires a scintillation counter (Winteringham and Disney 1964 Johnson and Russel 1975). The manometric method is based upon the release of carbon dioxide evolved from the action of the bicarbonate in the system with the acid formed by hydrolysis of the ester (Augustinsson 1948). 

In Fig. 4.68 is shown a microconduit incorporating two potentiometric pH electrodes and a common reference electrode, the introduction of sample solution being executed by means of an exteriorly placed injection port. The measurement of pH requires a system with limited dispersion coefficient, and no chemical reaction is needed in the flow channel. Consequently, a short residence time was chosen, and the two pH-sensitive PVC-based membrane electrodes, containing as electroactive material tri- -dodecylamine [778], were placed in a single-line system and very close to the injection position (cf. Fig. 4.3), the Ag/AgCl wire reference electrode being situated in a side channel and connected to the main channel downstream from the indicator electrodes. The manifold construction is such that the reference solution and thus the liquid junction are renewed... 

Binding constants in solution are usually determined from potentiometric titrations or plots of cyclic voltammetry peak potentials vs. CyD concentration after assessing the guest/host ratio in the complex [3, 4]. Potentiometric measurements are more frequently used, and ion-selective electrodes are employed, for the direct measurement of the guest activity in the solution. Measurements of pH allow the evaluation of concentrations of several reaction components. 

The pH was originally measured by judicious choice of acid-base indicator dyes and has in recent years given way to potentiometric methods due to the inherent limitations of color. A dye serves no useful purpose when the wastewater sample is light brown. Advances in both the instrument as well as the glass electrode have taken the measurement of pH very far since the early days when Arnold Beckman in 1935 was first asked to measure the pH of a lemon. 

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