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1.16.3: Ionic Strength: Ional Concentration

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    The ionic strength of a salt solution I containing \(\mathrm{i}\)-ionic substances is defined by equation (a); \(\mathrm{m}_{j}\) is the molality of ionic substance-\(j\), charge number \(\mathrm{z}_{j}\) [1].

    \[\mathrm{I}=(1 / 2) \, \sum_{\mathrm{j}=\mathrm{i}}^{\mathrm{j}=\mathrm{i}} \mathrm{m}_{\mathrm{j}} \, \mathrm{z}_{\mathrm{j}}^{2} \nonumber \]

    The sum is taken over all \(\mathrm{i}\)-ionic substances in the solution. The situation is slightly complicated by the fact some authors use the term ‘ionic strength’ where the concentration \(\mathrm{c}_{j}\) (expressed using the unit, \(\mathrm{mol dm}^{-3}\)) replaces \(\mathrm{m}_{j}\). The substitution is reasonably satisfactory for dilute salt solutions at ambient \(\mathrm{T}\) and \(\mathrm{p}\) where the mass of water, volume \(1 \mathrm{~dm}^{3}\), is approx. \(1 \mathrm{~kg}\).

    The ional concentration of a salt solution \(\Gamma\) is defined by equation (b) where \(\mathrm{c}_{j}\) is expressed using the unit, \(\mathrm{mol dm}^{-3}\) [2].

    \[\Gamma=\sum_{j=i}^{j=i} c_{j} \, z_{j}^{2} \nonumber \]

    Footnotes

    [1] An aqueous solution contains \(\mathrm{K}_{2}\mathrm{SO}_{4}\) (\(0.1 \mathrm{~mol}\)) in \(1 \mathrm{~kg}\) of water(\(\ell\)).

    \(\mathrm{m}\left(\mathrm{K}_{2} \mathrm{SO}_{4}\right)=0.1 \mathrm{~mol} \mathrm{~kg}^{-1} ; \mathrm{m}\left(\mathrm{K}^{+}\right)=0.2 \mathrm{~mol} \mathrm{~kg}^{-1}\)

    and \(\mathrm{m}\left(\mathrm{SO}_{4}{ }^{2-}\right)=0.1 \mathrm{~mol} \mathrm{~kg}^{-1}\)

    Hence \(\mathrm{I}=(1 / 2) \,\left[(0.2)+\left(2^{2} \mathrm{X} 0.1\right)\right]=0.3 \mathrm{~mol} \mathrm{~kg}^{-1}\)

    [2] H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolytic Solutions, , Reinhold, New York, 2nd. revised edn.1950, p.33.

    This page titled 1.16.3: Ionic Strength: Ional Concentration was last modified on Mon, 20 May 2024 07:02:53 GMT and is shared under a Public Domain license and was authored, remixed, and/or curated by Michael J Blandamer and Joao Carlos R Reis via source content that was edited to the style and standards of the LibreTexts platform.