Activity calculations

[robertaasa]

Hello, how are you? First, I?d like to thank you for your help and for answering my questions.

I have three questions regarding the method PHREEQC uses to calculate activities. I know there are several models available, but I?m unsure which one is used in each case.

Question 1: I?m using the WATEQ database, but I have modified it by adding my own species. When I add the reactions for these species, the only parameter I include is the log_k of the reaction. In this case, what method does PHREEQC use to calculate the activity coefficients of these species?

Question 2: Suppose I use the PITZER database but modify it by adding my own species, only specifying the log_k values. Would PHREEQC calculate the activity coefficients of the original species using the Pitzer model and use a different method for the new species I added? Or does it always calculate all activity coefficients using the same method for the entire system?

Question 3: I assume that in the first and second cases, PHREEQC would use the Davies equation, since it doesn?t require additional parameters. Is this method reliable for very concentrated solutions?

[dlparkhurst]

Q1: Look at the manual for SOLUTION_SPECIES, -gamma option. Here is the text, but equations are included in the manual for phreeqc.dat. Llnl activity coefficients are similar, but allow only one b_dot value for all species, and an optional Sechenow parameter for uncharged species; the same parameter for each species selected.

-gamma --Indicates activity-coefficient parameters are to be entered. If -gamma is entered, then the equation from WATEQ (Truesdell and Jones, 1974) is used, . In this equations, is the activity coefficient, is ionic strength, and A and B are constants at a given temperature. If -gamma is not input for a species, then for a charged species the Davies equation is used to calculate the activity coefficient:

; for an uncharged species the following equation is used .

Q2: You should not add new species to pitzer.dat unless you know what you are doing. Pitzer interaction coefficients are used for all activity coefficient calculations. If you include no interaction coefficients for a species, your results will be unjustifiable.

Q3: Phreeqc.dat, wateq4f.dat, minteq.dat, and other do revert to the Davies equation if -gamma is not defined. I'm not sure what llnl-type databases do. Pitzer databases do not revert to Davies. The Davies equation gives a kind of averaged response, but individual ions (like SO4-2, for example) show quite different ionic strength dependence of activity coefficients.