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Dissociation constant

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Description of key information

Dissociation constant (DTPMP-xNH4): DTPMP acid has ten ionisable groups

Key value for chemical safety assessment

Additional information

In aqueous solution, DTPMP-xNH4 will ionise to form DTPMP acid and ammonium ion.

Dissociation constant are obtained for the ammonium salt by valid read-across from the parent acid. Ten dissociation constant values as listed below are reported in secondary literature which is considered reliable. The result (Tomson 1994) is considered to be reliable and selected for key study.

1.03 
2.08 
3.11
4.15 
5.19 
6.23 
7.23 
8.30 
1.18 
12.58

Eight of the ten pKa values were determined in a second reliable study (Martell 1968) and are listed below. They are considered as supporting data.

pKa3 = 2.8
pKa4 = 4.45
pKa5 = 5.5
pKa6 = 6.38
pKa7 = 7.17
pKa8 = 8.15
pKa9 = 10.1
pKa10 = 12.04

The speciation of ammonia/ammonium in solution is understood in relation to the pKa of ammonium, which has a value of 9.25 (published in Lide, 1999 and Budavari, 1996 and reported in the Ammonia Category SIAR).