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Topic: Calculating total ionic strength?  (Read 15775 times)

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Doubleoblivious

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Calculating total ionic strength?
« on: September 18, 2008, 09:17:06 AM »
Hello all,
how do i calculate the total ionic strength of the following solution?
0.3M Boric acid, 0.05M Citric acid in the same solution (water).

B(OH)3 + H2 B(OH)4− + H+
C6H8O7 + H2 C6H5O 3- + 3H+

http://en.wikipedia.org/wiki/Ionic_strength
is this correct?

I = 1/2(3 * ((+1)2 * 0.05) + (+1)2 * 0.3 + (-3)2 * 0.05 + (-1)2 * 0.3) = 0.6M

Borek

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Re: Calculating total ionic strength?
« Reply #1 on: September 18, 2008, 10:37:42 AM »
Boric acid is so weak, you can safely ignore it - it is almost not dissociated. Ionic strength is solely due to the citric acid presence - but it is a weak acid as well, so it is not completely dissociated. To be precise you should calculate citric acid dissociation, calculate ionic strength, recalculate dissociation using activity coefficients, calculate ionic strength again and so on. This is iterative process. Luckily solution is diluted so ionic strength is not large. Much smaller than you calculated (you tried OK, you just wrongly assumed everything is 100% dissociated).
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

AWK

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Re: Calculating total ionic strength?
« Reply #2 on: September 19, 2008, 02:01:21 AM »
Calculate concentration H3O+ = H2citrate from K1 of citric acid (quadratic equation is preferable) and treat this solution as 1-1 electrolyte (then concentration of H3O+ will be equal to the ionic strength with an error for ionic strentgh about 0,0001 M)
AWK

Doubleoblivious

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Re: Calculating total ionic strength?
« Reply #3 on: September 19, 2008, 07:01:00 AM »
Thanks a million for the advice guys, its much clearer now.