Conductance studies weak acids/bases

The self ionisation of water results in H30" "(aq) and OH (aq) always being present in any aqueous electrolyte solution. The measured resistance, or conductance, is therefore a composite quantity made up from the contribution made by the ions of the electrolyte and the H30 (aq) and OH (aq). The contribution from the HsO+Caq) and OH (aq) can be obtained by measuring the resistance, or conductance, of the H20(l) used in preparing the electrolyte solutions under study. The conductivity of the water can then be found from this measured conductance and the cell constant. This conductivity must then be subtracted from the conductivities found for each electrolyte solution. Values below 5 x 10 S cm are acceptable. This may seem to be a small contribution, but with accurate work it could represent a significant contribution especially if low concentration solutions are being studied, e.g. solubilities of sparingly soluble salts (see Section 11.15), or solutions such as very weak acids or bases (see Section 11.14). 

There are many weak acids, particularly phenols, which have pAaS >5, and some very weak bases, especially substituted anilines which have pAbS > 5 or whose protonated fonns have pAaS < 9. For these substances 100% ionisation or protonation cannot be achieved and the question can be asked how does this affect the possibility of determining a from conductance studies ... 

Some of the earliest chemical studies of nonhydrocarbons, which resulted in the isolation of numerous strong acids and bases, were conducted at the University of Texas under the leadership of Lochte and Littman (I). This work used the classical extraction of petroleum with mineral acids and caustic followed by chemical identification (boiling point, refractive index, derivatives, etc.). Since only strong Bronsted acids and bases can be isolated by aqueous solvents, their identifications were limited to low-molecular-weight species aqueous solvents also do not isolate weak acids or bases (low pK values). 

The two components of ammonium fluoride behave in very much the same way as do donor and acceptor impurities in electronic semiconductors, leading to more or less complete compensation. This behaviour is illustrated, as far as static conductivity is concerned, by the careful study of Levi et al. (1963) in which the ratio of hydrogen fluoride to ammonia concentrations was varied over a wide range. The experimental results are shown in fig. 9.9 and illustrate the expected behaviour. This can be calculated by the normal chemical methods used for a solution containing a weak acid and a weak base but, to obtain reasonable agreement with experi-... 

General Characteristics. In accordance with the high tendency for formation of Co(CO)4 (2 ) and the acidity of (3), the former componnd behaves in many respects as a psendohalide see Pseudohalide). Relative rates of electrophilic snbstitntion with Mel and MeOTs classify (2 ) as a hard and relatively weak nncleophile see Hard Soft Acids and Bases). Compounds of the type RCo(CO)4 are prepared by two different methods reaction of an electrophihc halide, snlfate, or tosylate (RX) with (2 ), or reaction of HCo(CO)4 with alkenes. Simple alkyl derivatives have low stability nevertheless, many studies have been conducted in which species have been identified as RCo(CO)4 intermediates. Apart from alkyl and aryl derivatives, for example MeCo(CO)4 and PhCo(CO)4, (2 ) forms derivatives with main group residues R3E and R2E and with the main group metals Zn, Cd, and Hg as well as with transition metals, as shown by examples in Scheme 4. Co E bonding in these compounds appears to be predominantly covalent. The bond distances can be long, and consequently, the coordination number of Co can be high. 

In formic acid (1 1) salts were found to be dissociated. HCl is associated Ka = 90 mol 1) due to the acidic nature of the solvent, and formate and H ions have excess mobility suggesting a proton-jump transport mechanism for these ions. Association of weak bases in acetic acid has been studied by conductance. ... 

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