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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Doubleoblivious on September 18, 2008, 09:17:06 AM

Title: Calculating total ionic strength?
Post by: Doubleoblivious 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 + H₂O  :rarrow: B(OH)⁴⁻ + H₊
C₆H₈O₇ + H₂O  :rarrow: C₆H₅O ^3- + 3H⁺

http://en.wikipedia.org/wiki/Ionic_strength (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

Title: Re: Calculating total ionic strength?
Post by: Borek 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).

Title: Re: Calculating total ionic strength?
Post by: AWK 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)

Title: Re: Calculating total ionic strength?
Post by: Doubleoblivious on September 19, 2008, 07:01:00 AM

Thanks a million for the advice guys, its much clearer now.