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Physical & Chemical properties

Dissociation constant

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Description of key information

Phosphonic acid has dissociation constants of 1.80 and 6.20.

Key value for chemical safety assessment

pKa at 20°C:
6.2

Additional information

The dissociation constants of hydrochloric, orthophosphoric, phosphorous, methylphosphonic, dimethylphosphonic, ethylphosphonic,

phenylphosphonic, diphenylphosphonic, phenylphosphourous acids in methyl, ethyl, n-propyl, n-butyl, and isopropyl alcohols have been determined from potentiometric titration curves. For Phosphonic acid this has resulted in dissociation constants in methyl- (5,86), ethyl- (7,13), n-propyl- 7,49), n-butyl- 7,75), and isopropyl- (7,44) alcohols. Dissociation constants for phosphonic acid in water have been reported in literature 1,80 and 6,20.

As the dielectric constant and the basicity of the solvent decrease, the acid strength falls but differentiating power of the solvent with the respect to the acid becomes enhanced, which makes possible differential titration of mixtures of acids which cannot be titrated differentially in aqueous solutions.