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Log K and more help please!




>Temperature dependence of log K is defined either by (1) the delta_H and
>the Van't Hoff equation, or (2) by an analytical expression. It does not
>sound like you have the information except at 30 degrees, so you probably
>can't fit the coefficients for an analytical expression. The only
>alternative would be to use or guess the enthalpy of reaction. The log_k
>value for phreeqc is the log K at 25C, so you should correct the K from 30
>to 25 degrees to be consistent. The K is also the zero ionic strength K, so
>if you have the information, it should logically be corrected back to zero
>ionic strength. In actuality, there are probably more uncertainties in the
>K than you would find in these corrections. I'd probably just use the 30C
>log K. You may want to do some sensitivity runs to determine the effects of
>reasonable uncertainties.

Dear David(if it is ok to call you David),

Thank you for explaining the logK concept to me, however, I am still a little confused. In theory, I should convert the K into the zero ionic strength K before I enter it into the input file to be consistent with the Ks listed in the Phreeqc database. In reality, all my experiments are carried out in 0.1 M sodium nitrate medium. Does PHREEQC take the K at zero ionic strength condition, convert it to the value that suits the ionic strength at which my experiments are conducted and hence does the output file list the species distribution at 0.1 M, i.e. the ionic strength I used?

If I enter the K at 0.1 M into the input file, does Phreeqc assume this is the K at zero ionic strength and hence go through the normal conversion process, i.e. convert this K that is already at 0.1 M to a 'wrong' value at 0.1 M before it predicts the species distribution?

I apologize if I bother you too much and thank you for your help.

Regards,
Zhi Ling





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