The rates of the allialine fading of bronlphenol blue, phenolphthalein, crystal violet, and malachite green have been studied from atmospheric pressure to 16,000 pounds per square inch. The rates for phenolphthalein and malachite green were also measured over a range of temperatures in order to determine the activation energies and entropies. The reactions went essentially to completion with the exception of the fading of phenolphthalein, for which the effects of temperature and pressure on the back reaction and equilibrium constant were also studied. No effect of pressure was found for the fading of crystal violet, but the reactions of the other three dyes were accelerated by pressure, so that there are negative volumes of activation. These are correlated with the entropies of activation and are interpreted in terms of the reaction mechanisms. INTRODUCTIONPrevious work (1, 2, 3, 4) has shown that for a number of reactions in aqueous solution the effects of pressure may be interpreted in terms of the electrostriction of the solvent around ions and dipoles. The present investigation consists of a study of reactions i~lvolving the attack by a hydroxide ion on large organic dye molecules. These reactions are of such a nature that electrostriction effects are not as powerful as in many of the previous reactions studied, so that structural effects are expected to play a part in determining the influence of pressure. Since the reactioils chosen are structurally similar to one another, but are quite different electrically, they provide a useful meansof separating the two effects. The dyes used were bromphenol blue, phenolphthalein, crystal violet, and malachite green, and of these the first two are negatively charged and the second two positively charged.The kinetics of the alkaline fading of bromphenol blue were first studied under atmospheric pressure by Panepinto and Kilpatrick ( 5 ) and by Amis and La Mer (6). The results of these workers led to the following proposed mechanism, which was suggested by Amis and La Mer:Quinoid form (blue) Carbinol form (colorless)The reaction went to completion.The alkaline fading of phenolphthalein was studied by Kober and Marshall (7), Biddle

Ionization constants of phenol, the three cresols, the six xylefiols and o-chlorophenol were measured at seven temperatures from 5 to 38"C, using a spectrophotometric methcd. The results were analyzed in terms of the Harncd and Robinson equation, and values of heat, free energy, entropy and heat capacity of ionization were determined. The free energies are additive, but the heats and entropies are not. A linear relationship between heat and entropy of ionization was found. The results are interpreted in terms of inductive and resonance effects, and of solventsolute interactions.In spite of a considerable amount of thermodynamical work on ionization processes in aqueous solution, there is still no satisfactory theoretical understanding of the problem. Valuable information is provided by studies of the effects of substituents on the thermodynamical quantities for ionizations, and the present work is concerned with phenol and certain substituted phenols. This work is related to a recent theoretical treatrnent,l and to an experimental study of phenol ionizations using a microcalorimeter 2 ; the present results are, however, at variance with the calorimetric ones and are considered to be more reliable.The work was done using a spectrophotometric method, essentially in the form developed by Robinson and Biggs,3 but measurements were made over a range of temperatures from 5 to 35°C. EXPERIMENTALThe apparatus used is shown in fig. 1. It consists of a glass cell A, 8 cm long and 1.2 cm in diam., with quartz windows sealed at each end using Hysol epoxy-adhesive cement (many adhesives are unsatisfactory as they give rise to a slight change in light absorption). The solution was continually pumped, using the glass pump D, between the container C (which was in a thermostatically-controlled water bath) and the cell A. All connections were ground-glass joints tightly clamped, and air was excluded using a mercury seal; the work was done under nitrogen. A stream of dry nitrogen was used to prevent condensation on the windows of the cell.The phenol solutions were approximately 0.000036 M, and also contained either 0.1 M NaOH, 0.01 M HCl, 0.01 M buffer or 0.025 M buffer. The spectra were taken, always in duplicate, with a Beckman DK-2 ratio-recording spectrophotometer.The phenols used were of the highest purity obtainable, and were further purified either