pH Fix and Precipitation

[NicolevJ]

Good evening Sir Parkhurst,

According to the tips on pH Fix written before I have tried to implement the pH fix and Halite to fixate the pH.
However I come to a warning stating:
WARNING: Fix_pH, Pure phase with add formula has not converged.
    SI may be a local minimum.   Residual: 3.544615e-01

Do you know how I can solve this warning?

Aside that, I encounter another problem of which I am not sure if it is related to the above mentioned warning or due to the way phreeqc executes its keywords. I have let phosphates precipitate and created a graphical output where the phosphate removal percentage is calculated. One graph (G7) is based upon increasing temperature with fixed pH, the other graph (G13) an increasing pH by NaOH addition with at fixed temperatures. The problem I encounter is that if I were to compare the P removal efficiencies at at, for example T 25degC and pH 7.2 they do not align. Is there a reason why this is so? 

In the attachment the two graphs and a simplified script.

I hope you can help me,

Kind regards,
Nicole

[dlparkhurst]

Fix_pH may not arrive at the correct pH when multiple minerals are defined in equilibrium phases. Use -force to require that the correct pH is attained.

Code: [Select]

EQUILIBRIUM_PHASES 1 #Fix pH 6.8
ATCP 0 0
DCPD 0 0
DCP            0 0
Struvite 0 0
Fix_pH      -6.8 HCl #10
-force
Halite  -20  10
END
You have been inconsistent in the two calculation approaches. In one case, you simply specify a pH of 7.2; whereas in the other, you add NaOH to attain a pH of 7.2. The second approach has two effects (1) the ionic strength of the solution is about 0.6, which has a significant effect on activity coefficients and in turn the aqueous speciation of P, and (2) you have much more Na in solution so that the aqueous species NaHPO4-  is much more significant.

I think you should define only one solution, rather than 5. I am not sure what the pH or composition should be. All of your solutions have a rather large charge negative imbalance of 14 percent or more. Once you decide on a composition, then allow the Fix_pH to add NaOH or HCl, which should work with your definitions. When you do the second set of calculations, again start with the same solution and add NaOH. If necessary, you may have to use HCl explicitly to obtain lower pH values.

(For debugging, I think you should have simplified more to look at two solutions that were inconsistent.)

[marcomaro90]

Hi,

Could you please reupload the file To solve ph temp graph equals.phr or write the whole code?

Thanks,

Maro