Is "Sulfuric acid... makes a poor electrolyte... very little of it will dissociate into ions" really true? What does that actually mean?

Question

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?

Answer 1

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.)

Answer 2

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.

Answer 3

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.