While researching chromate conversion coating for edits to this answer in Space Exploration SE, I came upon the following passage in Corrosion Resistance of Stainless Steel to Sulfuric Acid

Sulfuric acid is quite corrosive in water although it makes a poor electrolyte due to the fact that very little of it will dissociate into ions, according to Chemical Land 21’s description of sulfuric acid. The concentration of the acid is what determines its corrosive effectiveness, as British Stainless Steel Association (BSSA) explains. Most types of stainless steel can resist low or high concentrations, but it will attack the metal at intermediate temperatures. The concentration is affected by temperature.

Wikipedia's Sulfuric_acid; Polarity and conductivity says:

In spite of the viscosity of the acid, the effective conductivities of the $\ce{H3SO4+}$ and $\ce{HSO4-}$ ions are high due to an intramolecular proton-switch mechanism (analogous to the Grotthuss mechanism in water), making sulfuric acid a good conductor of electricity. It is also an excellent solvent for many reactions.

Question: So one source explains that "Sulfuric acid is... a poor electrolyte due to the fact that very little of it will dissociate into ions", and the other seems to suggest that the "effective electrical conductivities" of the resulting ions are high "making sulfuric acid a good conductor of electricity". Is the fact that these appear to disagree to me due to my failure to understand what each means, or is one incorrect or at least incomplete?

  • 4
    $\begingroup$ What has the last image to do with any of the question? Given that the image source (please cite the actual source with words, because links can just die) has no obvious policy on reuse of their images, I'll simply remove it. cc @MelanieShebel $\endgroup$ Jun 17 at 21:18
  • 2
    $\begingroup$ Does this answer your question? Conductivity as a function of acid concentration $\endgroup$
    – Mithoron
    Jun 17 at 21:34
  • $\begingroup$ @Martin-マーチン thanks for your edit; yes I shouldn't have included the image, and in the past I've also addressed "[Source]" myself: Are meaningless link titles like “Source” sub-optimal, leaving the answer unrepairable if a link rots/breaks? $\endgroup$
    – uhoh
    Jun 18 at 6:09
  • $\begingroup$ Besides the relative aspect treated by comments and answer.... The two sentences aren't in contrast. The second refers to a mechanism of conduction that isn't available in, just to give an example, Na+ and Cl- solution. Namely, you may think to freeze in place the ions, still current can flow between locations due to multiple proton exchange reactions. $\endgroup$
    – Alchimista
    Jun 21 at 9:58

The best known conducting aqueous solutions are that of strong acids in water because the hydronium ion (=protonated water) has the highest electrical conductivity known today. The infinite dilution conductivity of of hydronium ion are compared below from the Book Chapter "Proton Transfer Reactions and Kinetics in Water" by Stillingerenter. The units are cm$^2$/ohm equivalents. As you can see, the best conducting ions in water are the hydronium ion and the hydroxide ion. The "rest" of the ions are nowhere close.

$$ \begin{array}{rllll} \hline t\left({ }^{\circ} \mathrm{C}\right) & \lambda_{\mathrm{H}^{+}}^{\circ} & \lambda_{\mathrm{OH}^{-}} & \lambda_{\mathrm{Na}^{+}}^{\circ} & \lambda_{\mathrm{Cl}^{-}}^{\circ} \\ \hline 0 & 225 & 105 & 26.5 & 41.0 \\ 5 & 250.1 & & 30.3 & 47.5 \\ 15 & 300.6 & 165.9 & 39.7 & 61.4 \\ 18 & 315 & 175.8 & 42.8 & 66.0 \\ 25 & 349.8 & 199.1 & 50.10 & 76.35 \\ 35 & 397.0 & 233.0 & 61.5 & 92.2 \\ 45 & 441.4 & 267.2 & 73.7 & 108.9 \\ 55 & 483.1 & 301.4 & 86.8 & 126.4 \\ 100 & 630 & 450 & 145 & 212 \\ \hline \end{array} $$

So, when people say good or poor aqueous electrolytes, these are all relative terms. You can choose these above values as your measuring bar. Dilute sulfuric acid in water therefore a very good conductor with respect to other salt solutions because it furnishes "protons" in water.

When it comes to the conductivity pure sulfuric acid, it will be an apple to orange comparison with its aqueous solution. There is no water (or very little), small amount of hydronium ions. The current carrying ions are now different. Look at the conductance (reciprocal of ohms, the old paper labels the y axis as conductivity) as a function of sulfuric acid concentration, it follows a very non-linear behavior and its resistance is relative high as compared to aqueous solution.

Thus the Wikipedia claim "sulfuric acid a good conductor of electricity." is a very relative comparision. Good conductor of electricity as compared to (??). The writer is silent after that! From this graph one can easily deduce that conc. sulfuric is not a very good conductor when compared with its aqueous solutions.

(Ref: Darling, Horace E. "Conductivity of sulfuric acid solutions." Journal of Chemical & Engineering Data 9.3 (1964): 421-426.)

enter image description here

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    $\begingroup$ Interesting - why the knee in the graph at 85-92% H2SO4? $\endgroup$
    – Vikki
    Jun 18 at 23:05
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    $\begingroup$ No idea...these types of questions have kept chemistry or physics alive. Otherwise these sciences would have been dead by now. $\endgroup$
    – M. Farooq
    Jun 18 at 23:46
  • $\begingroup$ @Vikki-formerlySean that's quite a compelling (and different and more specific and unique) question! Would you consider asking that as a separate question? I think it's worth asking if it's not answered in answers here or in any at the proposed dupe. $\endgroup$
    – uhoh
    Jun 19 at 0:32
  • $\begingroup$ @uhoh: Aaaaand asked. $\endgroup$
    – Vikki
    Jun 19 at 1:01

The conductivity might be viewed relative to other electrolytes and by concentration.

  • By concentration: Up to about 25% $\ce{H2SO4:H2O}$, conductivity increases fairly linearly with concentration, reaches a maximum at ~35%, and then drops precipitately (no pun intended)! And sulfuric acid has such "affinity" for water that oleum, or fuming sulfuric acid, has more $\ce{SO3}$ than $\ce{H2O}$. Oleum is much less strongly ionized, and is a comparatively poor conductor.
  • In comparison to hydrochloric acid: At concentrations less than 25%,, $\ce{HCl}$ is the better conductor, but above that, $\ce{H2SO4}$ is better, up to ~50%. Above 50%, as mentioned, $\ce{H2SO4}$ conductivity decreases.

However, when you start a car, consider that a battery weighing perhaps 20 kg, with about seven liters of 35% $\ce{H2SO4}$, provides between 100 and 200 amps to turn over a four or six cylinder Otto-cycle engine... so $\ce{H2SO4}$ must be at least a fairly good conductor.

  • $\begingroup$ if you didn't mean the pun "precipitously" (like a precipice ) $\endgroup$
    – Jasen
    Jun 19 at 6:04

The apparent contradiction is due to the fact that none are absolute statements and to a misunderstanding of the term electrolyte.

The dissociation of sulphuric acid is concentration dependent, so more ions are in solutions at lower concentration.

Beside this, the second statement refers to a mechanism of conduction that is not general for ions in solution, namely proton exchange between anions. This intermolecular process, absent in, e.g. a NaCl solution, is an additional channel for the current flow. You can hypothetically think of not moving anions, still current can flow by the above protons exchange.

This nevertheless requires enough ions and a relative low distance between them.

As a result, the conductivity peaks at medium-low concentration, as in the graph you have attached.

In fact, a substance can be a relatively poor electrolyte in the sense that its dissociation is far then complete. However, as in the current case, its conductivity can be relatively high, at least higher than expected.


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