Question of the Gaines-Thomas convention in PHREEQC

[Jeonghwan Hwang]

Hello, this is Jeonghwan Hwang.
I have a question for the Gaines-Thomas convention in PHREEQC.

In case of K-Ca exchange, the exchange reaction can be written in PHREEQC as below;
2KX + Ca2+ = CaX2 + 2K+  ; log_K   1.0 #example

In PHREEQC, what is the selectivity coefficient (log_K  1.0) refers to?

Ksel = [CaX2]^0.5[K+]/[KX][Ca2+]^0.5  ---- (1)
or
Ksel = [CaX2][K+]^2/[KX]^2[Ca2+]        ----  (2)

At first, I understood that (2) was correct because I think that (1) has to be expressed by"KX + 0.5 Ca2+ = 0.5 CaX2 + K+  ; log_K   1.0". But I showed the (1) in the "Calculating cation exchange with PHREEQC (Version 2)" by Appelo.

I have a little confusion to understand.
Can I get your comments?

Sincerely,
Jeonghwan Hwang

[dlparkhurst]

Just calculate it. You will need to define EXCHANGE_SPECIES. Here are a couple ways to do it, but the first is preferred.

phreeqc.dat uses Na+ + X- = NaX; log_k 0 as the reference half reaction, so usually you would defined log_k of KX and CaX2 relative to NaX. Here, I simply made KX with log_k 0.

Code: [Select]

EXCHANGE_SPECIES
K+ + X- = KX
-log_k 0.0

Ca+2 + 2X- = CaX2
      -log_k      1.0
END

SOLUTION 1
Ca 1
Cl 2
END
USE solution 1
EXCHANGE 1
KX 1e-3
USER_PRINT
10 PRINT LOG10[ACT("CaX2")*ACT("K+")^2/ACT("KX")^2/ACT("Ca+2")]
20 PRINT LOG10[ACT("CaX2")^0.5*ACT("K+")/ACT("KX")/ACT("Ca+2")^0.5]
END
EXCHANGE_SPECIES
2KX + Ca+2 = CaX2 + 2K+
 log_K   1.0
END
USE solution 1
USE exchange 1
END