Acidity constant electrochemical measurement

One important application of the Nernst equation is the measurement of pH (and, through pH, acidity constants). The pH of a solution can be measured electrochemically with a device called a pH meter. The technique makes use of a cell in which one electrode is sensitive to the H30+ concentration and the second electrode serves as a reference. An electrode sensitive to the concentration of a particular ion is called an ion-selective electrode. One combination is a hydrogen electrode connected through a salt bridge to a calomel electrode. The reduction half-reaction for the calomel electrode is... 

To determine the effect of acid catalysed decomposition of NADH on the electrochemical response in our experiments, the decrease in oxidation current for NADH was recorded as a function of time. The results of this experiment were compared with the decrease in NADH concentration as spectrophotometrically determined. The rates of decrease of the current and the concentration of NADH are both first-order and occur on similar timescales (Fig. 2.14). Analysis of the data for the two experiments provide first-order rate constants of 1.68 and 1.16 x 10-4 s-1 for the electrochemical and spectrophotometric measurements, respectively. The small difference between these two constants can be explained by the additional consumption of NADH by reaction at the electrode during the electrochemical measurement. This electrochemical process is also a first-order rate process, and the extent of the effect can be determined by using the treatment of Hitchman and Albery [50] for electrolysis using a rotating disc electrode. The results are consistent with the observed difference in the two rate constants. 

The acidity of these products cannot be determined by means of direct pH determination. However, the virtual acidity constant, AT l, is accessible from optical and electrochemical measurements. Comparison of the pX i values of selected Knoevenagel products and the pX values of some typical carboxylic acids in methanol establishes that they exhibit comparable acidity (Scheme 7). Using the o -values, the pX i values of benzylidene derivatives of Meldrum s acid are in good accordance with the Hammett equation. The Lewis acidity of Knoevenagel products is due to the formation of labile pseudobase adducts, the so-called anbadons (10), upon reaction with nucleophiles. Structures of the anbadons have... 

The combination of the high sensitivity of SEIRAS and a rapid-scan FT-IR spectrometer enables the spectral collection simultaneously with electrochemical measurements such as cyclic voltammetry and potential-step chronoamperome-try. The time-resolved measurement can give some information on electrode kinetics and dynamics, as has been shown in Fig. 8.24. Figures 8.25 and 8.26 represent another example of millisecond time-resolved ATR-SEIRAS study of current oscillations during potentiostatic formic acid oxidation on a Pt electrode [123]. At a constant applied potential F of 1.1 V, the current oscillates as shown in Fig. 8.25 a. Synchronizing with the current oscillations, the band intensities of linear CO and formate oscillate as shown in Fig. 8.26 (and also in Fig. 8.25 c). 

The measurement of formal potentials allows the determination of the Gibbs free energy of amalgamation (cf Eq. 1.2.27), acidity constants (pATa values) (cf. Eq. 1.2.32), stability constants of complexes (cf. Eq. 1.2.34), solubility constants, and all other equilibrium constants, provided that there is a definite relationship between the activity of the reactants and the activity of the electrochemical active species, and provided that the electrochemical system is reversible. Today, the most frequently applied technique is cyclic voltammetry. The equations derived for the half-wave potentials in dc polarography can also be used when the mid-peak potentials derived from cyclic voltammograms are used instead. Provided that the mechanism of the electrode system is clear and the same as used for the derivation of the equations in dc polarography, and provided that the electfode kinetics is not fully different in differential pulse or square-wave voltammetry, the latter methods can also be used to measure the formal potentials. However, extreme care is advisable to first establish these prerequisites, as otherwise erroneous results will be obtained. 

Electrochemical methods for NO determination offer several features that are not available with spectroscopic approaches. Perhaps the most important is the capability of microelectrodes to directly measure NO in single cells in situ, in close proximity to the source of NO generation. Figure 2 shows sensors that have been developed for the electrochemical measurement of NO. One is based on the electrochemical oxidation of NO on a platinum electrode (the classical Clark probe for detection of oxygen) and operates in the amperometric mode [17]. The other is based on the electrochemical oxidation of NO on conductive polymeric porphyrin (porphyrinic sensor) [24]. The Clark probe uses a platinum wire as a working electrode (anode) and a silver wire serves as the counterelectrode (cathode). The electrodes are mounted in a capillary tube filled with a sodium chlo-ride/hydrochloric acid solution separated from the analyte by a gas-permeable membrane. A constant potential of 0.9 V is applied, and direct current (analytical signal) is measured from the electrochemical oxidation of NO on the platinum anode. In the porphyrinic sensor, NO is catalytically oxidized on a polymeric metalloporphyrin... 

Another way to assemble films of Au MFC on an electrode surface is through carboxylate-metal ion linkages. For example, monolayers of mercaptoundecanoic acid (MUA) on Au electrodes can be formed by immersion in an ethanol solution of MUA. Au MFCs can be attached by immersing the MUA-functionalized electrodes first into ethanol solutions of Zn and then into solutions of mixed monolayer hexanethiolate/MUA-protected MFCs, subsequently rinsing away MFCs not bound to the electrode surface. This yields a monolayer of MUA-carboxylate-Zn+ —carboxylate-MUA Au MFCs on the electrode surface, as depicted in Figure 3.14. The ET rate constant was measured using cyclic voltammetry (100 s ), electrochemical impedance (90-160 s ), and potential step voltammetry (40-150 s ). 

An acidic pKi and a basic pK2 are associated with the equilibrium constants Ki and K2 of these two reactions. Electrochemical measurements allow a determination of the isoelectric point of the surface IEPS) which is the pH value of a solution in contact with the surface which yields an equal concentration of MOH J and M0 sites. The surface then bears a net zero charge and the proton concentration is equal to ... 

A simplification of the polarization resistance technique is the linear polarization technique in which it is assumed that the relationship between E and i is linear in a narrow range around E . Usually only two points ( , 0 are measured and B is assumed to have a constant value of about 20 mV. This approach is used in field tests and forms the basis of commercial corrosion rate monitors. Rp can also be determined as the dc limit of the electrochemical impedance. Mansfeld et al. used the linear polarization technique to determine Rp for mild steel sensors embedded in concrete exposed to a sewer environment for about 9 months. One sensor was periodically flushed with sewage in an attempt to remove the sulfuric acid produced by sulfur-oxidizing bacteria within a biofilm another sensor was used as a control. A data logging system collected Rp at 10-min intervals simultaneously for the two corrosion sensors and two pH electrodes placed at the concrete surface. Figure 2 shows the cumulative corrosion loss (Z INT) obtained by integration of the MRp time curves as ... 

As was already mentioned, the constitution of reagents 5 in ethereal solutions and of the complexes formed by them with a number of Lewis acids was investigated by electrochemical methods and the nature of existing ions and electroactive species was elucidated. It was also possible to evaluate the equilibrium constant for the Schlenk equilibrium for 5b, 5e and 5i in DME solutions from polarographic measurements . 

Like all the other acidity functions, W0(H) equals pH in dilute aqueous solution. In strong acids, this function should be a logarithmic measure of the proton activity as long as the normal potential of the redox system, ferrocene-ferricinium, is constant. This was, however, not the case in very strong acid solutions because ferrocene underwent protonation. Other electrochemical pH indicators have been proposed, such as quinine-hydroquinone or semiquinone-hydroquinone, the basicity of which can be modified by substitution on the aromatic ring. These electrochemical indicators have been used with success by Tremillon and co-workers48 for acidity measurements in anhydrous HF and HF containing superacids. 

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