Dicopper hydroxide phosphate

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

REPORTING FORMAT FOR THE ANALOGUE APPROACH
For further information please refer to read across justification in IUCLID section 13.

Reason / purpose for cross-reference:

read-across source

Dissociating properties:

yes

No.:

#1

pKa:

2.16

Temp.:

25 °C

Remarks on result:

other: 1st proton

No.:

#2

pKa:

7.21

Temp.:

25 °C

Remarks on result:

other: 2nd proton

No.:

#3

pKa:

12.32

Temp.:

25 °C

Remarks on result:

other: 3rd proton

Description of key information

No experimental determination of the dissociation constants in water was performed for the test materials as it was anticipated that on performance of the test procedures, as detailed in Method 112 of the OECD Guidelines for Testing of Chemicals, 12 May 1981, that the resulting dissociation constants determined would be that of the parent anions only, for which literature values are available. For example, it is anticipated that different types of orthophosphate will demonstrate significantly different pH values in water due to increasing numbers of protons being substituted with the particular cation on titration with acid or base as appropriate, the actual dissociation constants determined for each compound would be common, i.e. that of the triprotic acid anion. Titrations would also be expected to be similar irrespective of the counter ion. Read-across is justified on the basis that phosphoric acid is the parent acid for all inorganic phosphates. 

Additional information

The dissociation of the read-across substance phosphoric acid occurs in three steps. The first dissociation results in the dihydrogen phosphate anion. The second dissociation leads to the hydrogen phosphate anion.Phosphate or orthophosphate anion is the resulting product of the third and last dissociation.For each of the dissociation reaction steps a dissociation constant can be determined. For the first dissociation step the pKa = 2.12, for the second dissociation reaction step the pKa = 7.21 and the third dissociation has a pKa = 12.67. All dissociation constants were determined at a temperature of 25 °C. The dissociation constants show an increase from dissociation step to the next. This effect is due to the former loss of a H proton, resulting in a negatively charged molecule and a higher energy demand for further dissociation steps. 

No.	pKa	Temperature (°C)
#1	2.16	25
#2	7.21	25
#3	12.32	25