Ionic strength of Mcilvaine's Buffer

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Discussion Overview

The discussion centers on the calculation of ionic strength (IS) for McIlvaine's buffer solutions, specifically referencing a table from a publication by Elving et al. Participants are seeking clarification on the methods used to derive the ionic strength values presented in the table, which includes specific compositions and pH levels.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a table detailing the preparation of McIlvaine buffered solutions, including concentrations and ionic strength values, and expresses difficulty in understanding how these ionic strength values were calculated.
  • Another participant provides a link to a Wikipedia page on ionic strength, but this is deemed unhelpful by the original poster who seeks specific methods used by Elving et al.
  • A later reply suggests that calculating ionic strength involves finding all equilibrium concentrations of ions and using the ionic strength definition, noting that this may require iterative calculations to account for activity coefficients.
  • This same participant recommends using dedicated software for these calculations, implying that manual calculations could be inefficient.

Areas of Agreement / Disagreement

Participants express differing views on the complexity of calculating ionic strength, with some suggesting that it is straightforward while others indicate it involves cumbersome mathematics and iterative processes. There is no consensus on the specific methods used by Elving et al.

Contextual Notes

The discussion highlights the lack of detailed methodology in the original reference, which contributes to the uncertainty in replicating the ionic strength calculations. The iterative nature of the calculations and the need for software assistance are also noted as potential limitations in manual approaches.

Jim B
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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.
 
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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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