Simple cation exchange model for example

[Jeonghwan Hwang]

Hello, this is Jeonghwan Hwang
While reading a textbook, I found a cation exchange model example.
To model this, I wrote the following code.
But when verified with the results,
It was confirmed that the selectivity coefficient does not appear close to 2.85.
I haven't found any grounds for the error to appear in the code.
Is there anything I've missed?
If you let me know, it will help you study.

Thank you

Sincerely,

Jeonghwan Hwang

====================================================
 EXCHANGE_MASTER_SPECIES
    Q   Q-
 
 EXCHANGE_SPECIES
    Q- = Q-
    log_k 0.0
   Na+ + Q- = NaQ
    log_k 0.0
    Ca+2 + 2NaQ = CaQ2 + 2Na+ 
    log_k 0.455

 end
 
 EXCHANGE 1
      NaQ  0.002
 end
 
 solution 1
    ph 7
    temp    23.0
    units mg/l
    Ca 40
    Na 20
 end
 
 USE EXCHANGE 1
      use solution 1
      EQUILIBRIUM_PHASES 1;
 end

[dlparkhurst]

Code: [Select]

EXCHANGE_MASTER_SPECIES
    Q             Q-           
 
EXCHANGE_SPECIES
Q- = Q-
    log_k     0
Na+ + Q- = NaQ
    log_k     0
Ca+2 + 2NaQ = CaQ2 + 2Na+
    log_k     0.455
END
EXCHANGE 1
      NaQ  0.002
END
SOLUTION 1
    ph 7
    temp    23.0
    units mg/l
    Ca 40
    Na 20
END
USER_PRINT
10 eqf_Ca = MOL("CaQ2")*2/TOT("Q")
20 eqf_Na = MOL("NaQ")/TOT("Q")
30 q = (eqf_Ca/eqf_Na^2)*ACT("Na+")^2/ACT("Ca+2")
40 PRINT "IAP, CaQ2: ", q
USE exchange 1
USE solution 1
END