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This answer to an earlier question regarding the electrical conductivity of sulfuric acid provides a graph showing the conductivity of sulfuric acid/water mixtures ranging from 0% to 100% sulfuric acid:

enter image description here

(Image by Horace E. Darling in "Conductivity of sulfuric acid solutions" [Journal of Chemical & Engineering Data 9.3 (1964): 421-426.], via M. Farooq here at ChemSE.)

As can be seen, the conductivity of the solution rises smoothly from 0% to a peak at approximately 30% sulfuric acid, and declines thereafter. However, at approximately 85% sulfuric acid, conductivity reaches a local minimum, after which it actually rises slightly with increasing sulfuric-acid concentration until reaching a local maximum at approximately 92% sulfuric acid, before again dropping off, more steeply, as the concentration of sulfuric acid in the solution continues to increase to 100%.

Why does the trend of decreasing conductivity with increasing sulfuric-acid concentration temporarily reverse in the ~85%-~92% range?

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  • $\begingroup$ interestingly the plot in this question doesn't even go out that far in concentration $\endgroup$
    – uhoh
    Commented Jun 19, 2021 at 1:12
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    $\begingroup$ Presumably the concentration in the graph is percent by weight. If so, then it's interesting to note that an equimolar mixture of $\ce{H2O}$ and $\ce{H2SO4}$ represents a 84.5 wt% solution, whereas a 1:2 molar mixture represents a 91.6 wt% solution. I'll be surprised if this is purely coincidental. $\endgroup$
    – Nicolau Saker Neto
    Commented Jun 19, 2021 at 1:34
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    $\begingroup$ Ať 85%, the ion pairs stop being solvated by water and start being solvated by H2SO4. Solvation means ion separation and conductivity increase. $\endgroup$
    – Poutnik
    Commented Jun 19, 2021 at 9:08

1 Answer 1

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The comment by Vikki made me dig even older papers. Since conductance (not conducitivity note that Darling is using an incorrect terminology from today's standards) is inversely related to viscosity, I thought there must a sharp change in the viscosity of sulfuric acid solution as a function of concentration. This guess is not that bad. This is from 1923 paper. Rhodes, F. H., and C. B. Barbour. "The viscosities of mixtures of sulfuric acid and water." Industrial & Engineering Chemistry 15.8 (1923): 850-852.

There is a sharp increase in viscosity at 85%, which indicates there is a major structural change in sulfuric acid solution in the range 85-92%. Sulfuric acid forms a hydrate in this range. When the viscosity is high, the conductance goes down, there is a depression in the curve. This viscosity jump is causing the double hump. Once we are past the high viscosity range, conductance goes up again.

It is amazing how simple molecules do not stop from surprising us!

Image

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