Reference Electrodes Nonaqueous Solvents

One caveat to using aqueous REs in nonaqueous solvents is that the potential measurements carried out cannot be related (59) from solvent to solvent even though the same RE is used. This is due to the difference injunction potentials that exist between systans. The acceptability of various REs in different solvents is summarized in Table 4.9. 

As described in Section 4.3.S.2, the AglAg RE can be used with a variety of organic solvents. Using the same solvent in the RE filling solution as in the electrochemical cell will minimize the junction potential of the RE. The limitations of this electrode are that only solvents in which a silver salt is soluble and not oxidized by the Ag+ can be used. Similar to the difficulty in comparing potentials between different solvents using aqueous 

RE Acetonitrile Propylene carbonate Dimethyl formamide Dimethyl sulfoxide 

S satisfactory stability and reproducibility NR not recommended (unstable and/or not reproducible) the symbol ir denotes a salt bridge. 

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Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

In some nonaqueous solvents, it is necessary to use a standard reaction other than the oxidation of molecular hydrogen. At present, there is no general choice of a standard reaction (reference electrode). Although in some cases... 

The main properties of the double layer of solid lead electrodes have been already described in the Encyclopedia [1]. New achievements in this field have been the subject of reviews [for example [2-6]. Some of the new results relate to impedance of polycrystalline Pb electrodes in aqueous [7-9] and nonaqueous solvents (references in [3, 6[). Special attention has been paid to chemically and electrochemically polished polycrystalline electrodes, mainly in aqueous [10-12] and methanolic [13] fluoride solutions. 

More data regarding Au nonaqueous solvent interfaces, including surface preparation problems, can be found elsewhere [4] and in references cited therein. Table 2 summarizes selected values of the potential of zero charge for the pc-Au electrodes in contact with some solvent-electrolyte systems. 

Semiaqueous or Nonaqueous Solutions. Although the measurement of pH in mixed solvents (e.g., water/organic solvent) is not recommended, for a solution containing more than 5% water, the classical definition of a pH measurement may still apply. In nonaqueous solution, only relative pH values can be obtained. Measurements taken in nonaqueous or partly aqueous solutions require the electrode to be frequently rehydrated (i.e soaked in water or an acidic buffer). Between measurements and after use with a nonaqueous solvent (which is immiscible with water), the electrode should first be rinsed with a solvent, which is miscible with water as well as the analyte solvent, then rinsed with water. Another potential problem with this type of medium is the risk of precipitation of the KC1 electrolyte in the junction between the reference electrode and the measuring solution. To minimize this problem, the reference electrolyte and the sample solution should be matched for mobility and solubility. For example, LiCl in ethanol or LiCl in acetic acid are often used as the reference electrode electrolyte for nonaqueous measurements. 

Store a glass electrode in aqueous solution to prevent dehydration of the glass. Ideally, Do not leave a glass electrode out of water the solution should be similar to that inside the reference compartment of the electrode. If the (or in a nonaqueous solvent) any longer than... 

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 ... 

One advantage of the amperometric titration is its ease of automation. A titrator can be signaled to shut off when a specified current level is reached. The advantage of the two-working-electrode variation is the elimination of a reference electrode, which can be troublesome in nonaqueous solvents. 

Circuitry similar to that presented in Figure 8.13b has been used to analyze cells with impedances ranging from 102 to 1011 Q with 1% accuracy and resolution better than 1 part in 104 over a frequency range of 0.005 Hz to 10 kHz [14]. The technique has been especially useful for studies of the reaction kinetics of moderately fast chemical reactions. Kadish et al. [15] used phase-selective techniques to make ac impedance measurements to evaluate reference electrodes for use in nonaqueous solvents. Recent decreases in the cost of integrated function modules such as analog multipliers, oscillators, and phase-locked loops make this type of phase-selective instrumentation more accessible than ever. 

By far the biggest problems with the stability and the magnitude of the liquid junction potentials arise in applications where the osmotic or hydrostatic pressure, temperature, and/or solvents are different on either side of the junction. For this reason, the use of an aqueous reference electrode in nonaqueous samples should be avoided at all cost because the gradient of the chemical potential of the solvent has a very strong effect on the activity coefficient gradients of the ions. In order to circumvent these problems one should always use a junction containing the same solvent as the sample and the reference electrode compartment. 

A reference electrode based on (6.28) can be realized by immersing Ag wire in a solution of AgN03. In order to maintain its activity constant this solution is placed in the inner reference electrode compartment, which is then connected to the sample solution through the liquid junction. Because of its good solubility, silver nitrate has often been used in reference electrodes for nonaqueous solvents. 

For most potentiometric measurements either the saturated calomel reference electrode or the silver/silver chloride reference electrode are used. These electrodes can be made compact, are easily produced, and provide reference potentials that do not vary more than a few millivolts. The discussion in Chapter 5 outlines their characteristics, preparation, and temperature coefficients. The silver/silver chloride electrode also finds application in nonaqueous titrations, although some solvents cause the silver chloride film to become soluble. Some have utilized reference electrodes in nonaqueous solvents that are based on zinc or silver couples. From our own experience, aqueous reference electrodes are as convenient for nonaqueous systems as are any of the prototypes that have been developed to date. When there is a need to rigorously exclude water, double-salt bridges (aqueous/nonaqueous) are a convenient solution. This is true even though they involve a liquid junction between the aqueous electrolyte system and the nonaqueous solvent system of the sample solution. The use of conventional reference electrodes does cause some difficulties if the electrolyte of the reference electrode is insoluble in the sample solution. Hence the use of a calomel electrode saturated with potassium chloride in conjunction with a sample solution that contains perchlorate ion can cause erratic measurements due to the precipitation of potassium perchlorate at the junction. Such difficulties normally can be eliminated by using a double junction that inserts another inert electrolyte solution between the reference electrode and the sample solution (e.g., a sodium chloride solution). 

Potentiometric redox measurements are often performed in nonaqueous or mixed-solvent media. For such solvents various potentiometric sensors have been developed, which, under rigorously controlled conditions, give a Nemstian response over a wide ranges of activities, particularly in buffered solutions. There are some experimental limitations, such as with solvent purification and handling or use of a reference electrode without salt bridges, but there also ate important advantages. Solutes may be more soluble in such media, and redox... 

When voltammetry measurements are made in nonaqueous solvents, the problems of an adequate reference electrode are compounded. Until the 1960s the most common reference electrode was the mercury pool, because of its convenience rather than because of its reliability. With the advent of sophisticated electronic voltammetric instrumentation, more reliable reference electrodes have been possible, especially if a three-electrode system is used. Thus, variation of the potential of the counter electrode is not a problem if a second non-current-canying reference electrode is used to monitor the potential of the sensing electrode. If three-eleetrode instrumentation is used, any of the conventional reference electrodes common to potentiometry may be used satisfactorily. Our own preference is a silver chloride electrode connected to the sample solution by an appropriate noninterfering salt bridge. The one problem with this system is that it introduces a junction potential between the two solvent systems that may be quite large. However, such a reference system is reproducible and should ensure that two groups of workers can obtain the same results. 

Some Practical Considerations in the Use of Salt Bridges. Salt bridges are most commonly used to diminish or stabilize the junction potential between solutions of different composition and to minimize cross-contamination between solutions. For example, in working with nonaqueous solvents an aqueous reference electrode often is used that is isolated from the test solution by a salt bridge that contains the organic solvent. However, this practice cannot be recommended, except on the grounds of convenience, because there is no way at present to relate thermodynamically potentials in different solvents to the same aqueous reference-electrode potential furthermore, there is a risk of contamination of the nonaqueous solvent by water. 

Until recently, the most popular reference half-cell for potentiometric titrations, polarography, and even kinetic studies has been the saturated aqueous calomel electrode (SCE), connected by means of a nonaqueous salt bridge (e.g., Et4NC104) to the electrolyte under study. The choice of this particular bridge electrolyte in conjunction with the SCE is not a good one because potassium perchlorate and potassium chloride have a limited solubility in many aprotic solvents. The junction is readily clogged, which leads to erratic junction potentials. For these practical reasons, a calomel or silver-silver chloride reference electrode with an aqueous lithium chloride or quaternary ammonium chloride fill solution is preferable if an aqueous electrode is used. 

Figure 5.17 Design for simplified silver-silver ion reference electrode and salt bridge for the use with nonaqueous solvents.

Figure 6 presents a scheme of an electrolysis cell for the isolation of reduction and oxidation products of nonaqueous solutions [15]. The electrolyte of the W.E. solution must be an alkyl ammonium salt because the reduction products of most of the commonly used solvents in the presence of metal cations precipitate as insoluble metal salts. The counter- and reference electrode compartments are separated from the working electrode compartment by two frits each. The separating units have pipes which enable the sampling of their solutions in order... 

Active metals such as lithium and sodium can be used as stable reference electrodes in nonaqueous solutions in which they are apparently stable. To a limited extent this may be true for the Mg/Mg2+, Ca/Ca2+ and A1/A13+ couples as well (though they must be checked separately for each specific solution). It is important to note that in most of the commonly used nonaqueous systems, the above active metals are thermodynamically unstable and react readily with the solvent, the salt anions and the unavoidably present atmospheric contaminants. However, the active metals are apparently stable in many systems because the above reaction products, which are usually insoluble (metal salts), precipitate as protective passivating surface films. These films prevent further corrosion of the active metals in solutions [21], Hence, the active metal covered by the surface films may... 

The ferrocene bisCn-cyclopentadienyl)iron(II)/ferricenium couple has been found to have a very stable potential in many nonaqueous solvents. It can be used as a reference system for many applications in which a separate compartment for the reference electrode is possible because it is difficult to totally separate the ions from the measured electrolyte. The recommended use and potential of this system are described in Ref. 20. 

Table 9 Decomposition Potentials of 0.5 M LiAsF6 Solutions in Nonaqueous Solvents versus Aqueous Ag/AgCl/KCl Sat. Reference Electrode...

Empirical formulas exist to correct for the temperature dependence of the reference potentials in aqueous solution. When one must work in nonaqueous solvents, because of their conveniently large "window," one must add a 0.1 M to 1.0 M salt (see Fig. 11.67) to help conduct current, but there can be a problem with referencing the working electrode potential to a standard electrode. SCE can be used in many nonaqueous solvents, but in some cases such a direct experiment does not work one must use the Ag Ag+ ion... 

The silver-silver chloride reference electrode is one of the most reproducible and reliable reference electrodes for use in aqueous solutions, as well as one of the easiest to construct and use [ii]. For nonaque-ous solutions, the silver-silver ion reference electrode is widely used. It appears to be reversible in all apro-tic solvents except those oxidized by silver ion [ii]. The electrochemistry of silver is discussed in detail in [iii]. 

Mercury and solid s-p metal electrodes show stable electrochemical behavior in nonaqueous media, in particular in dipolar aprotic solvents. This knowledge was important for the advancement of electrochemical methodology, e.g., the special branch of polarography in nonaqueous systems has emerged. When performing electrochemical experiments in nonaqueous media, special attention should be paid to the reasonable choice of reversible reference electrodes. 

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