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Ionic strength of Mcilvaine's Buffer

  1. Nov 26, 2016 #1
    Table 1. Preparation of Constant Ionic Strength McIlvaine Buffered Solutions
    Reference: P.J. Elving, J.M. Markowitz, I. Rosenthal, Analyt. Chem. 28(7): 1179-1180 (1956)
    pH Desired Composition, G/Liter of Solution Composition, Moles/Liter of Solution Buffer System G/Liter of KCl added per liter
    at 25 C H3C6H5O7.H2O Na2HPO4.12H2O H3C6H5O7.H2O Na2HPO4.12H2O Ionic Strength of Solution to produce IS of
    (MW 210.141) (MW 358.142) 210.141 358.142 (IS), M 1.0 M 0.5 M
    pH H3C6H5O7.H2O Na2HPO4.12H2O H3C6H5O7.H2O Na2HPO4.12H2O Ionic strength ( gm/L) (gm/L) (moles/L) (moles/L) (IS)
    4.8 10.7 35.3 0.0509 0.0986 0.2320
    5 10.2 36.9 0.0485 0.1030 0.2560
    6.8 4.79 55.4 0.0228 0.1547 0.3920

    The above table is taken from the reference cited by Elving et al. This table shows the concentrations (in grams/Liter) for Citric Acid monohydrate (MW 210.141) and Na2HPO4.12H2O (MW 358.142). I converted these to moles/Liter for the 3 pH values of interest (4.8, 5.0, and 6.8). The 3 ionic strength values are those provided by Elving et al. Unfortunately they do not show their calculations of IS in their publication and I have been unable to figure out how they arrived at these 3 values. I would greatly appreciate any help you can provide to show me how these values of IS were calculated.
    Thank you.
  2. jcsd
  3. Nov 27, 2016 #2


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  4. Nov 27, 2016 #3
  5. Nov 27, 2016 #4


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    Staff: Mentor

    Basically you should find all equilibrium concentrations of ions involved (rather simple conceptually, but producing some cumbersome math) and plug them into the ionic strength definition.

    Trick is, things don't end here - after that you should calculate activity coefficients, recalculate the equilibrium concentrations and plug them into the definition again. Repeat until the result converges. These are iterations you were told about elsewhere.

    Calculating these things manually is a waste of time. It is better to use devoted software, like my Buffer Maker.
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