Organic solvents reference electrodes

TABLE 8.21 Potentials of Reference Electrodes (in Volts) at 25°C for Water-Organic Solvent Mixtures Electrolyte solution of M HCl. 

The membrane phase m is a solution of hydrophobic anion Ax (ion-exchanger ion) and cation Bx+ in an organic solvent that is immiscible with water. Solution 1 (the test aqueous solution) contains the salt of cation Bx+ with the hydrophilic anion A2. The Gibbs transfer energy of anions Ax and A2 is such that transport of these anions into the second phase is negligible. Solution 2 (the internal solution of the ion-selective electrode) contains the salt of cation B with anion A2 (or some other similar hydrophilic anion). The reference electrodes are identical and the liquid junction potentials A0L(1) and A0L(2) will be neglected. 

The reduction is usually made in a multi-compartment electrochemical cell, where the reference electrode is isolated from the reaction solution. The solvent can be water, alcohol or their mixture. As organic solvent A,A-dimethyl form amide or acetonitrile is used. Mercury is often used as a cathode, but graphite or low hydrogen overpotential electrically conducting catalysts (e.g. Raney nickel, platinum and palladium black on carbon rod, and Devarda copper) are also applicable. 

Once the electrodes have been prepared for a given aqueous-organic solvent, the pan determinations can be made at each temperature, either graphically or by direct reading on commercial pH meters calibrated for poH measurements. In this procedure the pH meter is used as a milli-voltmeter. A solution A (10 M HCl in the aqueous-organic solvent considered) is selected as the standard reference solution, its pan being calculated for any temperature according to the Debye-Hilckel formula. After the electrodes have been immersed in this solution, the... 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

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. 

Tab. 6.4 Potentials of the Ag/Ag+ and Hg/Hg2+ reference electrodes and Fc/Fc+ reference system in various organic solvents (V vs BCr reference redox system in 0.1 M Bu4NCI04 unless otherwise stated in footnote at 25 °C)... 

The symbol for the electron in tables of values of E° in liquid ammonia is thus equivalent to NH3(1) + /iH2(g /= 1) NH4+(liq NH3 a = 1). As for aqueous solutions, several secondary reference electrodes have proved more convenient for the actual measurement of E° in liquid ammonia, e.g. silver/silver chloride. This procedure has been applied to other inorganic solvents and numerous organic solvents, and tables of values are readily available.32... 

Reference electrodes are divided Into two groups. One comprises the saturated calomel electrode, its variants (such as the "lithium S.C.E.", Hg/Hg2Cl2(s.), LiCl( s)> and others of the same ilk), and the normal hydrogen electrode. These are almost Invariably prepared with water, so that their use with a non-aqueous solution entails a liquid-junction potential between the non-aqueous solution of the compound being studied and the aqueous solution In the reference electrode. Some workers have sought to circumvent this by preparing similar electrodes In the same solvents or solvent mixtures that contain the compounds they study when this has been done, the symbol "(o)" (for "organic") follows the abbreviation that would denote the ordinary aqueous form of the reference electrode. 

Reference system complex metal cyanides, 20 ml. of a 0.03M aqueous solution and 200 ml. 15W S0rensen buffer containing organic solvents (for detailed description see Experimental), apparent pH 7.45. Electrodes combined platinum electrode with Ag/AgCl in saturated KC1 as reference electrode. Reaction temperature 20°C. Abbreviations Reference systems named as oxidants—Fe, potassium ferricyanide Mo, potassium molybdicyanide. Mv.—millivolts. 

TABLE 8.9 Potentials of Reference Electrodes (in volts) at 25°C for water-organic solvent mixtures Electrolyte solution oflMHCl... 

Choice of electrolyte for salt bridges and reference electrodes. Many of the difficulties encountered in potentiometric measurements can be attributed to erratic or drifting junction potentials caused by clogged junctions. Certain elementary rules should be observed in choosing the filling solution for a salt bridge or reference electrode, particularly when they will be used in organic solvents or solutions that are only partially aqueous. 

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. 

In the half-cell of Eq. (5.24), the concentration of AgClj" must be small compared to that of Cl-, or a liquid-junction potential will result because the mobilities of AgClJ and Cl- are not the same. Thus, for a reference electrode of the second kind to be elfective in cells without appreciable junction potentials, the equilibrium constant for the reaction of Eq. (5.25) must be smaller than unity (preferably <0.1). In water, methanol, formamide, and V-methyl-formamide, this criterion is met, but in most organic solvents the equilibrium constant for the reaction of Eq. (5.25) ranges from 30 to 100. The silver chloride electrode is not recommended for general use in organic solvents.27... 

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