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EC number: 604-569-1
CAS number: 147126-62-3
Acid and base titrations were both conducted
because, although the structure of the test substance includes base
titratable groups such as alcohols, it was clear, based on other
experiments as reported herein that the material hydrolyses rapidly in
water giving unknown end products. The acid titration results showed
that there are no acid titratable groups present. The sodium hydroxide
titration indicated the presence of a weakly acidic group with a pKa of
7.36. The individual measurements, titration curves and details of the
calculations are included in Appendix 5. The results are summarized in
Table 27. The precision of the final calculated value falls within.the
expected precision of the method.
Table 27 - Summary of Dissociation Constant
Data for the Test Substance:
Method: Potentiometric titration
Test Temperature: 22 °C
Preliminary estimate of pKa from initial
titration: 7.5 based on single calculated value using 0.0189 N sodium hydroxide
Standard deviation: ±0.05 (n=14)
Expected precision: ±0.06 (See Albert &
Serjeant, pg 32)
Based on our measurements and observations
we concluded that the test substance in water contained a single weakly
acidic group with a pKa of7.36 ± 0.05 at 22.0°C. A pKa of this
magnitude rules out the presence of a carboxylic acid group in the
breakdown products present in water. Such a strongly acidic group would
be expected to display a pKa in the range of 4. This suggested that the
hydrolysis in water does not involve hydrolysis at the ester group but
rather opening of the oxathiolane ring. Such a ring opening would be
predicted to give rise to an enolizeable keto group. Such a group would
be expected to display a pKa in the range of 7 or 8. The enolizeable
keto groups in, for example, acetoacetone, display a pKa of 8.2, while
diacetyl acetone, displays a pKa of 7 .4. The keto groups themselves are
not acidic (Albert and Serjeant, 1962).
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