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Physical & Chemical properties

Dissociation constant

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Description of key information

A result of 9.19 pKa was determined for the read-across substance 5,5-dimethylhydantoin (DMH) from e.m.f measurements on the cell H2/Acid-base solution/saturated KCl/HgCl/Hg.
Due to the degradability of 5,5-dimethylhydantoin, sodium salt (NaDMH) it wasn't possible to conduct a Dissociation Constant study according to
OECD Method 112. Instead a Dissociation Constant prediction was carried out using specialist prediction software.
The structure of the submission material could not be inputted completely into the predictive software as the software is unable to process metallic
elements. The structure was therefore inputted with a hydrogen atom replacing the sodium atom. As the substance degrades rapidly to its parent
material in the presence of water, this would be the structure of the test item when in contact with water.
Predicted pKa's were as follows:
Functional Group 1: 8.93 ± 0.10
Functional Group 2: 16.33 ± 0.40
Functional Group 2: -1.13 ± 0.40

Key value for chemical safety assessment

pKa at 20°C:
9.19

Additional information

Both reports have been assigned a reliability rating of 2 however the decision was taken to assign the read-across study as the key study for the following reasons: It is recognized that in water, 5,5 -dimethylhydantoin, sodium salt (NaDMH) dissociates rapidly and becomes converted to the substance dimethylhydantoin (DMH).This process of protonation of NaDMH by water to form DMH occurs relatively easily in the presence of water or any other similar proton-donating substance.The half-life for this transformation is considered to be relatively short. It was not possible to conduct either a full study or a dissociation constant prediction on the registerable substance due to the degrgadability of the test material and limitations with the prediction software. As a result the prediction was based on the structure of the test item when in contact with water, this being the parent material 5,5 -dimethylhydantoin (DMH). Furthermore as the justification for using read-across material is that 5,5 -dimethylhydantoin, sodium salt dissociates to 5,5 -dimethylhydantoin and that an experimental result was able to be obtained on the read-across material, it was considered that the read-across study should be the key study of interest.