Saturation Index in output file not log(IAP/Ksp) ?

[GTG]

Hi everyone,

I am new in using PhreeqC and have a question regarding the Saturation Index given in the output file.
The SI is calculated by log(IAP/Ksp). Using the values given in the output file in this equation I get very different values than those given as SI. Am I doing something wrong?

Thanks for the help!

[dlparkhurst]

I don't know of any problem with the saturation index calculations; they have been part of PHREEQC for 30  years.

My guess is that there is an inconsistency between the reaction you are using to calculate IAP and the log K that you are using. The reaction and log K in the "Saturation index" section of the output should be consistent with the definition of the phase in PHASES in the database (or input file).

[Agabamud]

@dlparkhurst I have found that with all the reactions that I run, there is no link between the SI i calculate with the IAP and Ksp from the output file and the SI value generated by PHREEQC.

Secondly, for the gas-phase phases that have a positive SI value, can we assume that they have left the system or that they are still in the reaction mixture?

[dlparkhurst]

Apparently, you are not calculating SI correctly. Here is a script that calculates SI(calcite). You can find the values printed in the 'User print' section in the output sections 'Distribution of species' and 'Saturation indices'. See how the calculations are made in the USER_PRINT section of the input file.

Code: [Select]

PHASES
Calcite
CaCO3 = CO3-2 + Ca+2
-log_k -8.48
SOLUTION
Ca 1
Alkalinity 2
USER_PRINT
10 log_iap = LA("CO3-2") + LA("Ca+2")
20 log_k = LK_PHASE("Calcite")
30 si_cal = log_iap - log_k
40 PRINT 'LA("CO3-2"): ', STR_F$(LA("CO3-2"), 10, 2)
50 PRINT 'LA("CO3-2"): ', STR_F$(LA("Ca+2"), 10, 2)
60 PRINT "Log IAP:     ", STR_F$(log_iap, 10, 2)
70 PRINT "LK:          ", STR_F$(log_k, 10, 2)
80 PRINT "SI:          ", STR_F$(si_cal, 10, 2)
90 PRINT "Compare to values below in 'Distribution of species' and 'Saturation indices'."
END
Saturation indices alone have no affect on the system. No moles of elements are added to or removed from solution when a saturation index is calculated.

To calculate equilibrium by mole transfers of elements into another phase, you must use one of the reactant keyword data blocks. If you include CO2(g) in EQUILIBRIUM_PHASES, then CO2 will enter or leave the solution to achieve the specified log partial pressure of CO2(g). There is also a GAS_PHASE data block that can be used to model gas-water equilibria.