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Ionic strength limits

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This is a helpful Q and A from

IONIC STRENGTH LIMIT: I've been perusing the user's guide to PHREEQC and note that it works well at low ionic strength. What's the upper ionic strength limit you recommend?

You also say it MAY be reliable at higher ionic strength solutions for sodium chloride-dominated systems. How high do you mean by higher? (upper limit). Does seawater qualify as sodium chloride-dominated, or do you mean virtual absence of ions other than sodium and chloride?

Hedging, I say between .1 and 1 molal (seawater is ~0.7). A non-sodium- chloride solution would be toward the lower end and a sodium-chloride (or at least chloride-) dominated solution would be toward the higher end.

For the major cations, the activity coefficients as a function of ionic strength are derived from chloride salts. In a pure sodium-chloride solution, the activity coefficients of sodium and chloride are fit to perhaps 6 molal. For other strong electrolyte cations that are fit from chloride salts, the calculations are probably not too bad as long as chloride is the dominant anion. As anions other than chloride are introduced, things get progressively worse. In addition, most complexes have only the non-ion-specific Davies equation for the activity coefficient, so large amounts of complexing at high ionic strength is much less reliable. In general I would be very cautious (skeptical) with calculations at ionic strengths over 2 molal. Between .5 and 2 molal, I would consider the calculations as qualitiative and check to the extent possible with the specific interaction approach.

You can use other programs (EQ3/6, PHRQPITZ, Geochemists Workbench) that implement the specific interaction approach of Pitzer. This approach fits activity coefficients, frequently to 2-6 molal, in mixed electrolyte solutions. This is definitely the better approach for high ionic strengths, but the drawback is the data are not available for redox reactions or aluminosilicate reactions.


You can check the ionic strength of a solution with:

-file  example.xls
-ionic_strength  true

Note that PHREEQC now implements the Pitzer model. It is a direct reimplementation of PHRQPITZ, so that PHRQPITZ is now obsolete.

Thanks David. 

For everyone: Pitzer.dat can be selected from the database options.

In the book "Groundwater Geochemistry" of Merkel and Planer-Friedrich, they recommend the use of the following equations regarding the ionic strength:

DEBYE-HÜCKEL equation (Debye & Hückel 1923) --> I < 0.005 mol/kg
extended DEBYE-HÜCKEL equation --> I < 0.1 mol/kg
Güntelberg equation (Güntelberg 1926) --> I<0.1 mol/kg
Davies equation (Davies 1962, 1938) --> I<0.5 mol/kg
"Wateq" DEBYE-HÜCKEL equation (Truesdell & Jones 1974) --> I < 1 mol/kg

For higher ionic-strength go for Pitzer or SIT.


What are the actual limits to ionic strength and Pitzer.dat?


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