Calcium dihydroxide

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

22 June 2015 - 21 August 2015

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: Study performed under GLP, but not to guideline

Qualifier:

no guideline followed

Principles of method if other than guideline:

The test item was shaken with purified water for 24 hours at 25±0.5 °C. A headspace of nitrogen was used to prevent further adsorption of atmospheric carbon dioxide. The pH of the solution after shaking was compared to a literature value of the pKa for the calcium (II) ion. A sample blank of purified water was also prepared and the pH compared to the literature value.

GLP compliance:

yes (incl. QA statement)

Dissociating properties:

not determined

The pH of the test item solution after shaking was 12.4. This compares to the literature value taken from the CRC Handbook of Chemistry and Physics 94th Edition (5 -93) of 12.6 at 25 °C for the dissociation constant of the calcium (II) ion.

The pH of the purified water sample blank was 5.5 which correlated with the literature value.

Conclusions:

The pH of a saturate solution of calcium dihydroxide was found to be 12.4. This is in good agreement with the literature value for the dissociation constant for the calcium (II) ion of 12.6.

Executive summary:

The dissociation constants in water of calcium dihydroxide could not be determined using OECD TG 112 as the test item was an inorganic and the test methods were not applicable. Alternatively, the pH of a saturated solution of test item was measured to be 12.4, which correlated with the value of 12.6 for the dissociation constant of the calcium (II) ion found in the literature.

Description of key information

For calcium dihydroxide and calcium carbonate, the pKa could not be determined according to OECD TG 112. For calcium dihydroxide the pH of a saturated solution was 12.4, which correlated to the pKa of 12.6 reported in the literature for the Calcium (II) ion. For calcium carbonate, pKa values of 6.4 and 10.4 for carbonic acid, the free acid form of the test material, are available in the literature.

Key value for chemical safety assessment

pKa at 20°C:

12.6

Additional information

The dissociation constants in water of calcium dihydroxide could not be determined using OECD TG 112 as the test item was an inorganic and the test methods were not applicable. Alternatively, the pH of a saturated solution of the test item was measured to be 12.4 which correlated with the value of 12.6 found in the literature for the pKa of calcium (II) ion.

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)