8
$\begingroup$

What is the historical perspective for the choice of 33% concentration that is often found in commercial sales of Hydrochloric acid?

Is this the same reason we find sulfuric acid sold at 37% and Nitric at 68%?

I am guessing these are some limit at which further concentration, synthesis or purification reaches a chemical or economic limit but am curious to find out about the specific numbers selected. Obviously, all of these are also sold in higher purities/concentrations as analytical reagents but that obviously takes more effort to produce and will cost more.

Improve this question
$\endgroup$
1

2 Answers 2

Reset to default
9
$\begingroup$

The user ssavec prompted me to research further on my own and post an answer.

It seems the limits are practical ones.

  • For hydrochloric acid — $\ce{HCl}$ a practical limit is 38% with absolute limit around $40~\%$ and commercial concentrations ranging from 30 to 35% for convenience in transportation infrastructure.
  • For nitric acid — $\ce{HNO3}$ the $68~\%$ limit is the maximum that can be reached by direct distillation, higher concentration up to $98~\%$ (fuming nitric acid) needs alternative dehydration or synthesis techniques.
  • For sulfuric acid — $\ce{H2SO4}$, which can be distilled to $98.3~\%$ (and can be used to concentrate nitric acid) it is sold at concentrations that are directly suited for the intended use ranging from $20~\%$ to $80~\%$. With concentrations around $30~\%$ common for lead-acid battery electrolyte preparation.

So it seems the limits are all practical but for different reasons, one was close to the absolute limit for efficient transport. The second was at the direct distillation limits and the last one was to suit the most popular consumer applications.

The spread in the numbers had caused me to wonder on their relation but it seems there was no direct relation to the concentrations we see for consumer sale.

Improve this answer
$\endgroup$
1
  • 1
    $\begingroup$ Simply put, HCl is only soluble to 40% in water. HNO3 is only stable up to 68%, higher concentrations (slowly, depending on the excess) loose nitrous oxides until they're down to 68%. This also makes transportation and storage somewhat inconvenient. $\endgroup$
    – Karl
    Jun 28, 2016 at 14:27
5
$\begingroup$

The accepted answer doesn't actually give a reason why HCl is not sold above 40% and the comments on it are not quite right. Concentrations above 40% w/w are possible, but the rate of evaporation off such a solution at room temperature / atmospheric pressure is so high that the concentration will quickly drop to below 40%. Where a concentration higher than 40% is required, special measures (typically a pressurised container) must be used to maintain it.

Improve this answer
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.