PhreeqcUsers Discussion Forum

Beginners => PHREEQC basics => Topic started by: tianshiyu on October 14, 2019, 02:23:06 PM

Title: Environment
Post by: tianshiyu on October 14, 2019, 02:23:06 PM
Thank you for reading my post. I'm now  using phreeqc to simulate precipitation in urine. It's basically adding calcium to an ammonia buffer and to forming Calcium hydroxyphosphate(Ca5(PO4)3OH). The initial phosphorus concentration is 0.3g/L and the initial ammonium is 7g/L. But when I add 3g/L Ca(OH)2 in the solution and the pH up to about 8.5, the concentration of PO4(3-) is still high(0.14g/L), which in my actual experiment the phosphorus concentration will reach to a very low level.
I checked the difference, found that the logk of Ca5(PO4)3OH is -3.42 and the literature reported that the log k of  Ca5(PO4)3OH is -57.536. So  this may be the cause of the error between the model and the reality.
Now I'm confused. How can I solve this problem.
Thank you for answer me!
Title: Re: Environment
Post by: dlparkhurst on October 14, 2019, 03:15:07 PM
First, I am sure the chemical equations are different for the two log Ks; for example, one may use PO4-3 and the other H2PO4-. You will need to add other equations and log Ks from the database to convert one equation to have the same form as the other (like PO4-3 + 2H+ = H2PO4- with associated log K).

After you convert to the same equation, you will be able to make a better comparison of the log Ks.  At that point, it is up to you to decide which log K is more appropriate. Note that there very well may be differences in the literature (and other PHREEQC databases) for the reactions that you are using to convert the equations, and the model used to derive the log Ks may differ from the PHREEQC databases. So, there may be many issues of uncertainty and data consistency.