Nonaqueous solvents liquid-junction potential

The influence of interfaeial potentials (diffusion or liquid junction potentials) established at the boundary between two different electrolyte solutions (based on e.g. aqueous and nonaqueous solvents) has been investigated frequently, for a thorough overview see Jakuszewski and Woszezak [68Jak2]. Concerning the usage of absolute and international Volt see preceding chapter. 

Other difficulties of measuring pH in nonaqueous solvents are the complications that result from dehydration of the glass pH membrane, increased sample resistance, and large liquid-junction potentials. These effects are complex und highly dependent on the type of solvent or mixture used... 

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. 

For most solvents, standard buffers have not been established for standardizing the pH scale. Many measurements of pH in nonaqueous solvents have therefore been made by extension of the pH scale in water by use of aqueous buffers. Even for hydrogen-bonding solvents similar to water, extrapolation procedures are uncertain since the effect of both liquid-junction potential and transfer activity coefficient are... 

An increasing interest in nonaqueous media, which began in the seventies of the XXth century, also resulted in the extended studies on the properties of the relevant Hg[solvent interface. Most of the papers discussing the dependence of Epzc of Hg on the solvent used were published in the eighties [2]. The pzc values for selected solvents, including water for comparison, are collected in Table 1. They are expressed versus standard hydrogen electrode (SHE) and, in order to eliminate the unknown liquid junction potential, also versus the bis(biphenyl)chromium(l)/(0) standard potential, that was assumed to be solvent independent. 

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 mutual correlation of pH scales in different solvents creates more problems as a number of effects must be considered, such as electric permitivity and the solvation of ions. Potentiometric measurements of the hydrogen ion activity can be made using the hydrogen electrode in many solvents however, more difficulties are concerned with the reference electrode. When using an aqueous reference electrode the liquid junction potential on the boundary of two solvents is large. For a reference electrode in a nonaqueous solvent the interactions of various ions must be considered. Some good approximation can be reached when the reference electrode uses a... 

The glass electrode responds to hydrogen ion activity when immersed in a mixture of solvents of which water is at least a few percent. With nonaqueous measurements, however, significant error can occur due to the medium effect, discussed in Chapter 1, on the activity and on the liquid junction of the reference electrode giving rise to large liquid junction potentials. 

Added to the medium effect on activity is the hindering of the pH glass functioning by the solvent dehydrating the glass, by high sample resistance, and by large liquid junction potential developed at the reference electrode. These factors make nonaqueous pH difficult to measure and interpret. 

The low ionic strength and low conductivity of some nonaqueous solvents (see Table A.4) may result in severe noise pickup and large liquid junction potentials. These effects can be minimized by increasing the ionic strength of the solvent with a neutral electrolyte such as a quaternary ammonium salt. The addition of a neutral salt to the solvent increases its ionic strength, however, and consequently affects the hydrogen ion activity. Normally this effect is insignificant when compared with the potential error without the salt. 

When measuring the pH of a sample in mixed solvents or nonaqueous solvents, a large liquid Junction potential is developed. This may result in an unstable reading or require a long time for stabilization. The junction potential is developed because of the... 

Such an assumption was proposed, namely that a bridge consisting of a 0.1 mol dm tetraethylammonium picrate in acetonitrile suppresses the liquid junction potential between two different nonaqueous electrolytes [6]. The argument in favor of such a salt bridge for nonaqueous electrolytes is the similar electrical mobility of the tetraethylammonium cation and the picrate anion in acetonitrile. This assumption was later expanded to allow for other nonaqueous solvents [28]. Agreement for the electrochemical data was found if the nonaqueous solvents did not have acidic hydrogen atom(s) in the solvent molecule (aprotic solvents) [29], 0.1 mol dm solutions of either tetrabutylammonium picrate or pyridinium trifluorosulftMiate [30] were also used. 

There are two aspects to reference redox systems. One point is the possibility of compiling electrode potentials in a variety of solvents and solvent mixtures, which are not affected by unknown liquid junction potentials. Unfortunately very frequently aqueous reference electrodes are employed in electrochemical studies in nonaqueous electrolytes. Such data, however, include an unknown, irreproducible phase boundary potential. Electrode potentials of a redox couple measured in the same electrolyte together with the reference redox system constitute reproducible, thermodynamic data. In order to stop the proliferation of—in the view of the respective authors— better and better reference redox systems, the lUPAC recommended that either ferrocenium ion/ferrocene or bw(biphenyl)chromium(l)/te(biphenyl)chromium(0) be used as a reference redox system [5]. 

In this book, there are other chapters related to nonaqueous systems. Chapter 1 by Inzelt is on the electrode potentials and includes a section on the problem to relate the electrode potentials between different media. Chapter 2 by Gritzner is on the reference redox systems in nonaqueous systems and their relation to water. Chapter 3 by Tsirlina is on the liquid junction potential and somewhat deals with the problem between different solvents. Chapter 7 by Bhatt and Snook is on the reference electrodes for room temperature ionic liquids. See these chapters as well. 

It is obvious that aqueous reference electrodes are not well suited for use in nonaque-ous solvents, since unknown and irreproducible liquid junction potentials exist between the aqueous and nonaqueous phases. Furthermore, contamination of the sample by water and other ions associated with the reference electrode may occur. 

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). 

When studying nonaqueous systems by means of galvanic cells with aqueous or mixed reference electrodes, we cannot avoid liquid/liquid junctions and estimate the corresponding potential drop from any realistic model. In protic nonaqueous media (alcohols, dioxane, acetone, etc.), a hydrogen electrode can be used it is also suitable for some aqueous/aprotic mixtures. However, the io values for the hydrogen reaction are much lower as compared with purely aqueous solutions. When studies are carried out in nonaqueous media, in order to avoid liquid/liquid junction preference should be given to the reference electrodes in the same solvent as the electrode of interest. 

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. 

With the reference electrodes of this group, there is a liquid junction between different solvents, i.e., between the solvent of the solution under smdy and that of the solution of the reference electrode. Of course, the reference electrodes themselves should fulfill the requirements that the electrode potentials are stable and reproducible. However, in this case, the LJP between different solvents should also be stable and reproducible. The reference electrodes of this group can be divided into two subgroups the case using aqueous solutions and the case using nonaqueous solutions. 

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