Picric acid conductometric measurements

The failure of the Fuoss equation to reproduce experimental data appears particularly evident by conductometric measurements of hydrochloric acid (18) and by spectrophotometric measurements of picric acid (21) which we have carried out in TMS at 35° and 30°C, respectively. 

Pi(HPi( "] is the sum of anionic picrate species, and c is the analytical concentration of HPi). As may be seen, there are two linear portions with slopes of 0.46 for c < 0.08M and 1.56 for c > 0.08M. A very similar graph has been observed by Kolthoff and co-workers for HPi in acetonitrile (37) with slopes of 0.48 and 1.45 for concentrations below and above 0.1M, respectively. The analysis carried out by Caruso and co-workers (31) on these data confirms the hypothesis made by Kolthoff and co-workers that the predominating species in acetonitrile at higher concentrations is the ion Pi(HPi)2. Moreover, conductometric measurements on HPi in TMS have been interpreted by Eller and Caruso presuming that HPi polymers are formed in solutions (38). On these bases we have hypothesized that picric acid in TMS too forms the complex species Pi(HPi)2" according to the following reactions ... 

The conductance method is satisfactory only if the solvent can be rigorously purified. Through failure to appreciate this, the first values of pKa of picric acid in acetonitrile proved to be much too small, 5.6 and 8.9 as compared with 11.0 from electromotive force measurements on buffered solutions. D Aprano and Fuoss found that acetonitrile having a satisfactory specific conductance of about 10 cm still contained a trace of ammonia. This was converted to ammonium pic-rate when acid was added to the solvent giving a spurious contribution to the conductance of picric acid solutions. This discovery moved them to make the flat assertion that dissociation constants of weak acids cannot be determined in aprotic solvents conductimetrically . This may be an overly pessimistic view, conductance values of pKa for acids in di-methylsulphoxide and dimethylformamide agree well with those from spectrophotometric and electromotive force measurements. Approximate values of pKa and pKf can be obtained from conductometric titrations of a weak acid with a weak base. ... 

Conductance measurements on unbuffered solutions are unreliable, but useful information can be obtained from conductometric titra-tions. ii ii i Solubility - 2.Fii8 nd spectrophoto-metric measurements give more precise results. Kolthoff and his collaborators - have made a detailed spectrophotometric study of acid-base indicators in acetonitrile. Indicators with cationic acid forms can give trouble because of formation of ion pairs with anions. The most satisfactory indicators are ones like 2,6-dinitrophenol, 2,6-di-t-butyl-4-nitrophenol, and picric acid which have little tendency to form homo- or heteroconjugate ions because of steric hindrance. ... 

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