I work as an amino acid lab technician in a production facility, and we've been tasked with figuring out how to determine the amount of sulfuric acid remaining in a solution made in our plant. Ideally, the amount would be zero, or almost zero. We already have a sulfate test we use for a fertilizer product we produce, and someone suggested using that test. It's a Hach spectrophotometer test kit.

My question is, if we use that test kit to determine total sulfates in the sample, is there a conversion factor to determine sulfuric acid, or is it more of a 1:1 result? I realize total sulfates won't necessarily all be from sulfuric acid. Without divulging too much info, the solution is a reaction with soybean meal and sulfuric. Ideally, the sulfuric is completely used up, but production is wanting to have a test available to confirm this.

I hope I've given enough info. Thanks in advance for any and all help.


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    $\begingroup$ I would either use pH measurement either titration by NaOH. $\endgroup$
    – Poutnik
    Aug 31, 2023 at 18:03
  • $\begingroup$ I suggested titration with Barium hydroxide also. If we use a known concentration (50%) of NaOH, what would be the calculation to determine sulfuric acid concentration? Sorry if this is a dumb question. I'm really good at analyzing pig feed, and I can understand some chemistry, but it's not my background. I'm still learning. $\endgroup$
    – Joe
    Aug 31, 2023 at 19:19
  • $\begingroup$ Just to be clear, your sample is a supernatant solution with residual sulfate/sulfuric acid after the product has been isolated? $\endgroup$
    – Buck Thorn
    Sep 1, 2023 at 6:11
  • $\begingroup$ You give NO info. If the acid reacts acid-base the amount of sulfate remains constant analysis of SO4= is not useful, a pH should work. A reaction such as H3PO4 from calcium phosphate uses sulfate, a sulfate test will work. Specific info is needed to recommend a test. $\endgroup$
    – jimchmst
    Oct 2, 2023 at 8:28
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    $\begingroup$ The question is, if there is really the need to determine sulfuric acid content, or if there is enough to determine the remaining acidity, expressed in the equivalent amount of sulfuric acid. As if a major part of the acid is already neutralized, sulfate content is irrelevant to the acid content. $\endgroup$
    – Poutnik
    Jan 29 at 8:40

1 Answer 1


In a solution that contains various sources of hydrogen ions, it will be impossible to calculate the amount of free sulfuric acid by any means. You have a natural product broth (perhaps soya sauce?), so that is even more complicated because sulfate can also come naturally from food products. Remember that all classical analytical chemistry (titration or gravimetry) is very accurate and precise but only when the sample consists of one or two components.

The only accurate solution is to determine free sulfate by ion chromatography on an anion exchange column or quite crudely by a spectrophotometry. This will require extensive sample clean up. One can only quote total sulfate but not free sulfuric acid. It will be hard to make a correlation.


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