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Dissociation constant

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Description of key information

Two key studies are available showing a dissociation constant for 2,6-xylenol of 10.7 at 20 °C and 10.6 at 25 °C (Chen and Laidler, 1961; spectrophotometric analysis) and 10.6 at 25 °C (Fisher et al., 1966; potentiometric titration). 

Key value for chemical safety assessment

pKa at 20°C:
10.6

Additional information

Both key studies are well documented publications that were awarded a reliability score of 2 in accordance with the criteria for assessing data quality as set forth by Klimisch et al. (1997).

 

In Chen and Laidler, 1961, spectrophotometric analysis was conducted at different temperatures. The general procedure was to adjust the temperature of the water bath until the temperature was 25 °C at each end of the cell; the temperatures were measured using mercury thermometers which had been calibrated. After taking the spectrum the temperature was then changed to 5.0 °C, and the spectrum taken again. The procedure was repeated at 10, 15, 20 and 25 °C. The work was then done at 38 and 30 °C with a further check at 25 °C. Measurements were always made using buffer solution having a pKa as close as possible to that of the test solution.

The pKa value for 2,6-xylenol was 10.62 at 25 °C and 10.68 at 20 °C.

 

In Fisher et al., 1966, an aqueous solution was titrated with barium hydroxide solution and pH measured. The dissociation constant was determined by potentiometric titration of the test material solution with carbonate-free barium hydroxide. The initial phenol concentrations were of the order of 10^-3 M.

Before each titration, the glass electrode was tested for linearity with 0.05 M phthalate and 0.05 M borax. In alkaline solutions the linearity up to a pH of 12.5 was confirmed with a saturated calcium hydroxide buffer. The calomel and glass electrodes were than both wasted with water, dried with tissue, and lowered into the solution to be titrated. The titrant was added in ten or more equal portions and the pH (± 0.001) was recorded as soon as equilibrium was reached after each addition. Complete mixing between readings was accomplished by magnetic stirring. The stirring was discontinued while readings were being taken. When the titration was complete the electrodes were immediately washed and placed in a buffer solution of pH close to that of the pKa found. The results were discarded if the pH of the buffer was not reproduced to ± 0.01 pH without adjustment of the set.

The mean pKa value for 2,6-xylenol was 10.598 at 25 °C.

As it can be assumed that the variability of measurement results is higher than the temperature dependency, the pKa of 2,6-xylenol at 20 °C is calculated as the mean value of the available key information.

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