Questions regarding the Phreeqc SIT database (and the NEA TDB)

[GCanciani]

Hello everyone, 'im modelling the speciation of actinides in HNO3 with respect to increasing concentrations of H2SO4 and have been extrapolating SIT coefficients and modifying my database as appropriate.

However, i noticed a curious inconsistency. As i don't know if it's a mistake from my part (i'm missing something) or a mistake in the databases i use, i thought i'd ask here.

The problem i have is that the logk value for some reactions ( for example the formation of Pu(SO4)2 from Pu4+ 2SO42-) differs significantly from that i find in the reference.

In the case of the databases (either SIT or NEA TDB, they are both the same) the logk for the complexation above is given as 11.14, however from the "Chemical thermodynamics of neptunium and Plutonium" book (the reference quoted in the SIT database provided with phreeqc) the log(beta) is given as 7.18

Am i just missing a conversion between the two values which occurs when a database is converted to phreeqc or is this a type-o in the databases? or do i have the wrong reference?

any help on the matter will be greatly appreciated.

[dlparkhurst]

First, make sure the log Ks refer to the same chemical reactions.

Otherwise, you are on your own; I try to stay away from thermodynamic data. The sit.dat database that we release is as Thermodynamic database ANDRA/RWM - THERMOCHIMIE-TDB from  www.thermochimie-tdb.com.

[GCanciani]

Thank you for your reply, in my case i need to start using logk data as there are some Pu-HNO3 (and H2SO4) species not taken into account in the NEA database which i'd be keen to simulate. I will certainly double and triple check that the reactions are all the same!

i'll look at the thermochimie database and see if that helps further.

Giacomo