# Why does the graph of the electrical conductivity of sulfuric acid/water solutions have this knee in the ~85%-~92% range?

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:

(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?

• interestingly the plot in this question doesn't even go out that far in concentration
– uhoh
Jun 19 '21 at 1:12
• 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. Jun 19 '21 at 1:34
• Ať 85%, the ion pairs stop being solvated by water and start being solvated by H2SO4. Solvation means ion separation and conductivity increase. Jun 19 '21 at 9:08