Solvent voltage limits

Electron Transfer in Electrochemistry. In electrochemical cells electron transfer occurs within the electrode-solution interface, with electron removal (oxidation) at the anode, and with electron introduction (reduction) at the cathode. The current through the solution is carried by the ions of the electrolyte, and the voltage limits are those for electron removal from and electron insertion into the solvent-electrolyte [e.g., H20/(H30+)(C10j ) (Na )(-OH) ... 

Figure 5.19 Voltage limits for various electrode materials in several solvents (TEAP = tetroethylammonium perchlorate).

Figure 5.19 summarizes the positive and negative voltage limits for some commonly used electrode materials in several solvents. Wherever possible, the data for a particular solvent has been referred to a single reference electrode. Absolute values of the electrode potential for different solvent systems cannot be directly compared, however, because they are often referred to different reference electrodes and because of the uncertainty in our knowledge of junction potentials between different solvent systems. 

The Physicochemical Properties of Solvents and Their Relevance to Electrochemistry. The solvent properties of electrochemical importance include the following protic character (acid-base properties), anodic and cathodic voltage limits (related to redox properties and protic character), mutual solubility of the solute and solvent, and physicochemical properties of the solvent (dielectric constant and polarity, donor or solvating properties, liquid range, viscosity, and spectroscopic properties). Practical factors also enter into the choice and include the availability and cost of the solvent, ease of purification, toxicity, and general ease of handling. 

Only the (F20TPPT)Fem/ alkyl-metal porphyrin anion exhibits another reversible couple ( 1/2, —1.63 V) to yield a dianion [the four electron-withdrawing pentafluorophenyl groups on the porphyrin ring make possible the addition of an election to give the (F2oTPP2-)Fe dianion within the voltage limit of the solvent]. 

Another solvent that should be mentioned is methylene chloride (CH2C12). This solvent is important for electrochemical studies of nonpolar substances and for studies at low temperatures (accessible temperature range - 97° —> 40°C at ambient pressure). An advantage to this solvent lies in the stability of its radical ions. The cathodic and anodic voltage limits for this solvent with TBAP as the electrolyte and noble metal electrodes (Pt, Au) are 1.7—1.8 V versus SCE [68],... 

We have also seen (1.3.1) that the presence of hydronium ions in solution is a very important factor in determining the cathodic voltage limit, and any solvent containing water will contain hydronium ions. Finally the relatively high solubility of oxygen in protic solvents presents problems. 

It is very important for you to realise that the cathodic voltage limit is often set by the potential at which the supporting electrolyte cation is reduced. Typically for salts of the type KCl used in aqueous solutions, the limit is about —1.5 to —2 V (SCE). For the tetraalky-laramonium salts in non-aqueous solvents the limit is about —2.5 to -3.0 V (SCE). 

You should now be able to select a suitable solvent/supporting electrolyte system for your purpose. The limitations on choice are set by the solubility of the analyte and of the supporting electrolyte, the requirement for a low electrical resistance, and the necessity to have a voltage window available for the required analyte reaction. This latter limitation usually amounts to ensuring that an adequate cathodic voltage limit is available. 

Although you are now able to design a system and you have a wide choice of solvents and supporting electrolytes, take care not to over design. For example it is only necessary to provide a cathodic voltage limit just sufficiently negative to accommodate the analyte reaction. The system H2O/O.I mol dm KCl/Pt (anode, SE)/D1VIE (cathode, WE) is a well tried system for heavy metal cation analysis and rather like an old shoe this system would only be reluctantly discarded by analysts for this application. 

We can now turn to the limitations of the method and first of these is the voltage limit of about 0 to —2 V (SCE) in aqueous solution. This is not a serious limitation for metallic cation analysis but it is for organic analysis. The use of non-aqueous solvents and supporting electrolytes such as tetraalkylammonium salts (1.4) enable the negative limit to be extended to about —3 V (SCE). Little can be done about the anodic limit which is determined by the tendency of mercury to oxidise at potentials in the range 0-0.2 V (SCE). 

A key criterion for selection of a solvent for electrochemical studies is the electrochemical stability of the solvent [12]. This is most clearly manifested by the range of voltages over which the solvent is electrochemically inert. This useful electrochemical potential window depends on the oxidative and reductive stability of the solvent. In the case of ionic liquids, the potential window depends primarily on the resistance of the cation to reduction and the resistance of the anion to oxidation. (A notable exception to this is in the acidic chloroaluminate ionic liquids, where the reduction of the heptachloroaluminate species [Al2Cl7] is the limiting cathodic process). In addition, the presence of impurities can play an important role in limiting the potential windows of ionic liquids. 

In view of its position in the e.m.f. series ( °aj3+/ai = 166V (SHE)), aluminium would be expected to be rapidly attacked even by dilute solutions of relatively weak acids. In fact, the rate of chemical attack is slow, owing to the presence on the aluminium of a thin compact film of air-formed oxide. When a voltage is applied to an aluminium anode there is a sudden initial surge of current, as this film is ruptured, followed by a rapid fall to a lower, fairly steady value. It appears that this is due to the formation of a barrier-layer. Before the limiting thickness is reached, however, the solvent action of the electrolyte initiates a system of pores at weak points or discontinuities in the oxide barrier-layer. 

A point meriting attention is the voltage difference above. Doped polymers are rather electropositive (up to more than 4 V vs. a lithium electrode in the same solution), so much so that charging may have to be limited in order not to exceed the stability limits of the electrolyte (typically, propylene carbonate or acetonitrile as aprotic nonaqueous solvents). 

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