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Checking exchanger reactions
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Topic: Checking exchanger reactions (Read 424 times)
peterwadeuk
Top Contributor
Posts: 29
Checking exchanger reactions
«
on:
July 02, 2017, 10:51:05 AM »
Greetings good people,
A colleague wished to solve an exchange reaction with Na, K, Ca, Mg, H analytically.
I tried to persuade him to use PHREEQC.
So as to simplify the initial induction I stripped down the WATEQ4F database, as attached.
We ran the program, output attached, and received the following output which he performed calculations on to verify.
For the divalent ions:
Reaction Ca + MgX2 = Mg + CaX2
Species Activities
Ca 9.980E-04
Mg 6.698E-04
CaX2 4.977E-04
MgX2 2.107E-04
Rxn No. Reaction LogK
(1 Ca + 2X = CaX2 0.8
(2 Mg + 2X = MgX2 0.6
(1 – (2 Ca + MgX2 = Mg + CaX2 0.2
Calculated LogK (Mg * CaX2) / (Ca * MgX2) 0.20
The calculated LogK is in agreement with the LogK calculated from the component exchange reactions.
For combination monovalent and divalent ions:
2Na + MgX2 = Mg + 2NaX
Species Activities
Mg 6.698E-04
MgX2 2.107E-04
Na 9.011E-04
NaX 1.388E-05
Rxn No. Reaction LogK
(3 2Na + 2X = 2NaX 0
(4 Mg + 2X = MgX2 0.6
(3 – (4 2Na + MgX2 = Mg + 2NaX -0.6
Calculated LogK (Mg * NaX^2) / (Na^2 * MgX2) -3.12
!! The calculated LogK is not in agreement with the LogK calculated from the component exchange reactions.
We are doing something wrong here which I believe has something to do with the definition of the exchange equilibrium constants.
Can someone advise where the problem may lie?
Thank you,
Peter
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dlparkhurst
Top Contributor
Posts: 1270
Re: Checking exchanger reactions
«
Reply #1 on:
July 02, 2017, 06:30:17 PM »
I calculate the following:
Species log(activity)
Na -3.045
Mg -3.174
NaX -2.034
MgX2 -0.551
Log(IAP) = la(Mg) + 2*la(NaX) - 2*la(Na) - la(MgX2) = -0.6
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peterwadeuk
Top Contributor
Posts: 29
Re: Checking exchanger reactions
«
Reply #2 on:
July 03, 2017, 11:01:03 AM »
Thank you David.
I realise now that I should get the activities by explicitly requesting them e.g. SELECTED_OUTPUT 1 as opposed to trying to find them in the phreeqc output file.
All the best, Peter
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Checking exchanger reactions