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Physical & Chemical properties

Dissociation constant

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In accordance with section 1, Annex XI of Regulation (EC) 1907/2006 (REACH), the study does not need to be conducted. This property is applicable only for ionisable organic substances. Calcium oxide is an inorganic salt, which reacts with water to give a solution of calcium dihydroxide. Calcium dihydroxide on the other hand will be completely dissociated into Ca2+ and OH- ions when dissolved in water. The concept of dissociable functional groups is therefore not applicable. Therefore, the determination of a dissociation constant is not considered to be required (ECHA Guidance on information requirements and chemical safety assessment Chapter R.7a: Endpoint specific guidance, section R.7.1.17).

For calcium carbonate, no experimental testing was considered possible according to OECD TG 112 for the following reasons:

1). The test material has been determined to be essentially insoluble in water. Therefore, determination of the dissociation constant(s) using a potentiometric titration, spectrophotometric and/or conductivity method would not be feasible.

2). The test material contains no significant chromophoric group(s) and thus, will absorb very little in the ultra-violet/ visible region of the spectrum. Therefore, determination of the dissociation constant(s) using a spectrophotometric method would not be practical.

Therefore, experimentally determined dissociation constants were taken from Albert and Serjeant, Ionisation Constants of Acids and Bases, A Laboratory Manual, 1971. The values presented below are those for carbonic acid, the free acid form of the test material:

pKa1 = 10.4 (functional group - carboxylic acid)

pKa2 = 6.4 (functional group - carboxylic acid)

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