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topic 53343

What does "20% by volume hydrochloric acid" mean?

October 14, 2009

What I need is to calibrate the corrosive conditions of CASS test chamber. I have difficulty in following the procedures described in ASTM B368-97 (2003) "Standard Test Method for Copper-Accelerated Acetic Acid-Salt Spray (fog) Testing (CASS Test). When talking about cleaning exposed nickel panels, B368-97 (2003) states in senction 8.7.3: "Immerse each panel in 20% by volume hydrochloric acid prepared with distilled water for 2 min at 20 to 25C ...". But I don't understand by volume of what concentration of HCl. Is there a general convention about concentration by volume of hydrochloric acid that I don't know? Hydrochloric acid is produced in solutions up to 38% HCl (concentrated grade). I know a concentration of 6 Normal of HCl is 20% by mass HCl. I like to know how others make 20% by volume hydrochloric acid.


Peter Zhang
product validation engineer - Portland, OR

October 16, 2009

Perhaps you are being too analytical in trying to apply the instruction. It's true that hydrochloric acid does not exist as an individual substance, as do nitric and sulfuric acids. It can exist only as a solution in water. So trying to interpret from a chemistry point of view, an instruction for a routine mechanical operation, can lead to confusion. It would be better if the writers of standards and procedures used a few more words to make their intention known unambiguously, but they usually don't.

I believe that you should simply accept the term "hydrochloric acid" to mean concentrated hydrochloric acid, and ignore the underlying chemistry detail of whether it is a solution or an individual substance.

So "20% by volume hydrochloric acid" means take 20 mL of hydrochloric acid (that is, of concentrated solution which we are now calling simply hydrochloric acid), and dilute it with water to give 100 mL.

Bill Reynolds
Bill Reynolds
   consultant metallurgist
Ballarat, Victoria, Australia

We sadly relate the news that Bill passed away on Jan. 29, 2010.

First of two simultaneous responses -- October 19, 2009

In addition to what Bill said, concentrated HCl is 37.3% .
Reagent HCl can be 36% in many places.
IF I remember correctly, commercial grade HCl is 34%. Now pool acid is HCl, but a lot weaker! For your use, 20% refers to 20% commercial strength acid and 80% water. Note also that their are different qualities of acid as well as strengths.

Now the fun starts, most acids are sold by Baume which is a strange scale for specific gravity. It can be converted to % with the proper chart.

James Watts
- Navarre, Florida

Second of two simultaneous responses -- October 19, 2009

Thank you very much for answering my question. I thought taking "hydrochloric acid" as concentrated hydrochloric acid was the only way I could make "20% by volume hydrochloric acid". But I was uncertain if it was a generally accepted convention in industries to simply call concentrated hydrochloric acid as "hydrochloric acid". Now at least you interpret the term that way.

Again Thanks,


Peter Zhang
- Portland, Oregon

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