Nonaqueous Solutions potential standards

Aromatic amines can be determined by measuring the difference of their UVV absorption spectra, taken at identical concentrations but different pH of the solution. Also, standard mixtures and samples of the amines isolated from coke processing products were tested LOD 0.1-1 ppm. The procedure is potentially useful for waste waters and industrial effluents, where techniques such as GC and nonaqueous titrations may prove difficult to apply333. A determination of certain metabolites symptomatic of pancreatitis... 

J. P. Hoare, in Standard Potentials in Nonaqueous Solutions (Eds. A. J. Bard, R. Parsons, J. Jordan), Marcel Dekker, New York, 1985. 

Standard Reduction Electrode Potentials for Inorganic Systems in Nonaqueous Solutions at 25°C Redox Potentials for Some Biological Half Reactions... 

STANDARD REDUCTION ELECTRODE POTENTIALS FOR INORGANIC SYSTEMS IN NONAQUEOUS SOLUTIONS AT 25°C... 

A reference electrode is needed to provide a potential scale for E° valnes as all voltages are relative. Any electrochemical reaction with a stable, well known potential can be nsed as a reference electrode. The NHE or standard hydrogen electrode (SHE) (Pt/H2,1.0 M H+) was the first well known reference electrode and is used as a reference in most tables of redox potentials. An NHE is difficult to construct and operate and therefore, is not typically used experimentally. Since the NHE is widely accepted, potentials are still often referenced to the NHE, converted from other reference electrodes. For aqueous solvents the SCE (Hg/Hg2Cl2 (KCl)) and the silver/silver chloride (Ag/AgCl) electrode are now commonly used as reference electrodes. To convert from the SCE to the NHE, E (vs. NHE) = E (vs. SCE) + 0.24 V. For nonaqueous solvents the silver/silver nitrate (Ag/AgNOs) reference electrode is often used. A pseudo-reference electrode can also serve as a reference point for aqueous or nonaqueous solutions. A silver or platinum wire can be used as a... 

Where a pH meter is standardized by use of an aqueous buffer and then used to measure the "pH" of a nonaqueous solution or suspension, the ionization constant of the acid or base, the dielectric constant of the medium, the liquid-junction potential (which may give rise to errors of approximately 1 pH unit), and the hydrogen ion response of the glass electrode are all changed. For these reasons, the values so obtained with solutions that are only partially aqueous in character can be regarded only as apparent pH values. 

Numerous electrochemical measurements have been carried out with the ruthenium diimin complexes [15], mainly with the aim of comparing electron-transfer processes in the ground and in the excited state, and for the determination of the character of the frontier orbitals. Much less data are known for the cyclometallated analogs. By far the most popular method for the electrochemical measurements is cyclic voltammetry (CV). The measurements are mostly done in nonaqueous solutions (acetonitrile, dimethylfor-mamide, etc.). A general difficulty in such measurements is the reference potential, and the use of an internal standard like, for example, Ru(bpy)2 + is therefore highly recommended. Table 1 contains a compilation of electrode potentials of cyclometallated complexes of the type considered in this review. For comparison, the values of Ru(bpy) + are included in the table. 

Measurement of pH in a nonaqueous solvent when the electrode is standardized with an aqueous solution has little significance in terms of possible hydrogen ion activity because of the unknown liquid-junction potential, which can be rather large, depending on the solvent. Measurements made in this way are usually referred to as apparent pH. pH scales and standards for nonaqueous solvents have been suggested using an approach similar to the one for aqueous solutions. These scales have no rigorous relation to the aqueous pH scale, however. You are referred to the book by Bates (Ref. 3) for a discussion of this topic. See also M. S. Frant, How to Measure pH in Mixed Nonaqueous Solutions, Today s Chemist at Work, American Chemical Society, June, 1995, p. 39. 

The most common reference electrode systems used in aqueous solutions are Ag/AgCl and the calomel electrode. If aqueous-based references are used in nonaqueous solution, however, large liquid junction is produced and often more serious, aqueous contamination of the nonaqueous cell occurs. Thus this combination is not recommended. The use of an Ag/Ag non-aqueous-based reference is suggested for nonaqueous electrochemistry. To avoid large junction potentials, the RE solvent should be as close in nature as possible to the cell solvent system. Often potentials are calibrated with a standard, such as ferrocene or cobaltocene. Suggested standards are listed in Table 2-2, along with reduction potentials and other properties. Construction of an Ag/Ag reference for nonaqueous use is shown in Figure 2-6. Reference electrodes can drift with time and must be carefully maintained. 

Most often an aqueous pH buffer solution is used to standardize the pH measuring system. If the measurement is to be made on a nonaqueous sample, the correlation between hydrogen ion activity in an aqueous standard and in a nonaqueous sample is not valid. Nonaqueous buffers which provide a more realistic pH value in nonaqueous samples through standardization under conditions of similar medium effect and liquid junction potential are described in Chapter 4. 

The most stable oxidation states for protactinium are Pa(V) and Pa(IV). The chemical behavior of Pa(V) closely mimics that of Nb(V) and Ta(V), and experimental data are consistent with a 5f(l) rather than a 6d(l) electron configuration for the Pa(IV) species [37]. The electrochemical literature for Pa is mainly focused on the characteristics of the Pa(V)/Pa(IV) couple and the electrodeposition of Pa metal films from aqueous and nonaqueous electrolyte solutions. In aqueous solutions, only Pa(V) and Pa(IV) ions are known to exist, and the standard potential for the Pa(V)/Pa(IV) redox couple is in the range of —0.1 to -0.32 V [38]. 

Although the entire discussion of electrochemistry thus far has been in terms of aqueous solutions, the same principles apply equaly well to nonaqueous solvents. As a result of differences in solvation energies, electrode potentials may vary considerably from those found in aqueous solution. In addition the oxidation and reduction potentials characteristic of the solvent vary with the chemical behavior of the solvent. as a result of these two effects, it is often possible to carry out reactions in a nonaqueous solvent that would be impossible in water. For example, both sodium and beryllium are too reactive to be electroplated from aqueous solution, but beryllium can be electroplated from liquid ammonia and sodium from solutions in pyridine. 0 Unfortunately, the thermodynamic data necessary to construct complete tables of standard potential values are lacking for most solvents other than water. Jolly 1 has compiled such a table for liquid ammonia. The hydrogen electrode is used as the reference point to establish the scale as in water ... 

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