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What does "66 degree" sulfuric acid mean (66 °Baumé)

(to help readers better understand the Q&A's)

"Baumé" is one of many ways to describe the concentration of an acid, so it is possible to convert between it and other descriptions of concentration. But many knowledgable people believe that you should do your best to avoid describing things in 'Baumé' if possible because not only is it archaic, but it can be ambiguous.

Current question and answers:

Q. When I make specific gravity 1.4 in sulfuric acid (50% water and 50% H2SO4) mix, is this combination right or wrong?

vinoth selvaraj
- Chennai tamilnadu India
April 3, 2021

A. Hi Vinoth. If you're asking what is the specific gravity of a mixture of one volume of water at specific gravity 1.00 plus one volume of acid at specific gravity 1.84, the answer is you'll have two volumes of the mixture, weighing a total of 2.84 units, so the specific gravity will be 2.84/2 or 1.42.

But I'm confident you didn't come here to ask someone to check trivial arithmetic, but rather because you're unsure about some principal. So please introduce yourself and what you're trying to do so readers have the opportunity to help you by spotting possible errors in your understanding :-)

Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
April 2021

⇩ Related postings, oldest first ⇩

Q. I am a retired Firefighter doing research into early Twentieth Century Firefighting. I am currently examining the Chemical Engines of that era, which were oversized soda acid extinguishers on wheels. In the equipment catalogues, there are advertisements for the sulfuric acid used in these engines as being "66 degree sulfuric acid." I cannot figure out if the "66 degree" refer to the purity, specific gravity of the acid or something else. I am hoping you can tell me.

Yours truly,

Mark Hannon
historical research - Baltimore, Maryland, U.S.A.

A. It's 66 degrees Baumé, which is a hydrometer scale for density, and which equates to specific gravity of 1.84 times as heavy as water. What it's actually saying is that they contained sulfuric acid of full commercial concentration (about 98 percent pure). If the acid were more dilute, i.e., of lower concentration, its density would be lower, which means the degrees baumé would be lower.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

A. I consulted my CRC Handbook of Chemistry & Physics [adv: on Amazon & AbeBooks & eBay (adv.)] and 66°Bé has a specific gravity of 1.8354, is 93.19% H2SO4, and is as pure as you can get. This would be a reagent grade acid in most cases.

James Watts
- Navarre, Florida

April 6, 2010

thumbs up sign Hi Mark,
Thanks for raising this issue. I was recently visiting my son in Revelstoke, Canada and in the Fire Museum they had glass container that held sulfuric Acid and I was wondering why? The answer is now evident. Good luck with the rest of your research.

Ray Wildman
- Nelson Bay, NSW, Australia

A. As stated, Sulfuric acid that is 66 Baumé is right at 93% in concentration. However, we have purchased it in a tech grade at 99% from Univar. The problem with this concentration is that it has a tendency to freeze in cold climates.

Harvey Wade
- Daggett, California, USA
July 19, 2011

Q. Okay all you chemists out there. Every once in a while I get brain lock up and I can't break it. Here is my question.

How much 66 Be Sulfuric Acid (93.2%) do I need to add to a 1 gal solution to raise the concentration by 1 oz/gal?

The way I see it is 1 / 0.932 = 1.073 oz.

My laboratory SPC program says to add .403 gal to raise 100 gal by 1 oz/gal. I see that formula as impossible. You would have to add at least 107 oz to raise it by 1 oz/gal. Unless there is something I'm missing. Hence the brain lock up.


Tim Hamlett
Tim Hamlett, CEF
- West Palm Beach, Florida, USA
January 29, 2015

January 2015

A. Hi Tim. Your method has perhaps a loose end and a tiny error. The loose end is that liquids are usually measured by volume rather than weight, and your acid addition has a specific gravity of 1.835. So you'd probably want to add 1.073/1.835 = .585 fluid oz. of the 66 ° Bé stuff. The tiny error is that if you originally had a gallon of solution (i.e., 128 fluid oz.), and you've added .585 fluid oz. to it, the new volume will be 128.585 fluid oz., whereas you only added enough to raise the concentration of 128 fluid oz, not 128.585 fluid oz. by one ounce per gallon :-)
The error is small for small adds, but grows more for larger adds.

Your SPC program says add .403 gallons, i.e., 51.6 fluid oz for 100 gallon, which is not horribly far from your .585 fluid oz. for 1 gallon.

 I'd probably try the calculation backwards: one fluid oz of the 66 ° Bé stuff would weigh 1.835 oz and would contain 1.71 oz of pure H2SO4. If you added this to a gallon of solution the volume would now be 129 fluid ounces and would contain 1.71 more ounces of H2SO4 than before the add or 1.71 x 128/129 = 1.7 more ounce per gallon than before the add. So to get just 1 more oz per gallon instead of 1.7 more, add 1/1.7 = .588 fluid oz. 

I could be wrong, but I think that the general answer is that to raise the concentration of one gal of solution by "X" avoir. oz. per gallon,
you must add .585 * X * (X + 128) / 128 fluid ounces of 66 ° Bé

So if you want to increase the concentration by 1 avoir. oz. per gallon of pure H2SO4, you need to add .588 fluid oz of 66 ° Bé. I think your answer was plenty close for a small add.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

thumbs up signThanks Ted. That makes a lot more sense. The only other thing I could think of was that the lab software was making the addition by weight rather than volume.

Tim Hamlett
Tim Hamlett, CEF
- West Palm Beach, Florida, USA
January 30, 2015

A. In almost all plating and cleaning operations 1 oz per gal means 1 fluid oz.
Normally, if it means an avoir weight it will refer to it as sulphate, not sulfuric acid.
When in doubt, go back to the driving specifications, somewhere in those publications it will define it or refer to another publication.

James Watts
- Navarre, Florida
January 30, 2015

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