I have a mystery acid which isn't citric, boric, or oxalic acid. When mixed with water, it smells pungent, like hydrochloric acid--though I haven't compared them side by side.

Which other white powders form a strong acid upon being combined with water? Is there any powder that decomposes into HCl when mixed with water?

(Note: the mystery acid was sold as part of a set, along with sodium chlorite, for generating chlorine dioxide for use in aquariums.)

  • 1
    $\begingroup$ Does it dissolve in water completely? $\endgroup$ – Ivan Neretin Apr 6 '17 at 7:26
  • $\begingroup$ @IvanNeretin It dissolves completely except for some chalky looking impurities. (The other powder, the sodium chlorite, also leaves impurities visible in the water when dissolved.) I'll measure the solubility when I get home. $\endgroup$ – piojo Apr 6 '17 at 9:38
  • 1
    $\begingroup$ Is it sodium acetate? Because it is very soluble in water to form acetic acid. It has pungent smell and has ph of 2.4. $\endgroup$ – Nilay Ghosh Apr 6 '17 at 11:36
  • 2
    $\begingroup$ @NilayGhosh Don't confuse people. Sodium acetate solution does not have pungent smell; more importantly, it has alkaline pH rather than acidic. True, it does contain some minuscule amounts of acetic acid resulting from hydrolysis, but that doesn't make it an acid. $\endgroup$ – Ivan Neretin Apr 7 '17 at 6:21
  • 1
    $\begingroup$ Did I say "salts"? I meant "chlorides" (including covalent chlorides). $\ce{PCl5}$ would behave like that, but it is most likely too aggressive to be actually used. $\endgroup$ – Ivan Neretin Apr 7 '17 at 8:49

From the pH and solubility information alone, I would propose sodium bisulfate: cheap, readily available, safe in the powerfully oxidizing solution. (Possibly also potassium bisulfate.)

One way of manufacturing $\ce{HCl}$ produces sodium bisulfate as a byproduct: $$\ce{NaCl + H2SO4 -> HCl + NaHSO4}$$ The odor of $\ce{ HCl}$ could be a minor impurity in this mystery acid. The other impurities also suggest it is not a reagent grade product.

  • $\begingroup$ Nice suggestion! It looks the same as the bisulfate photo on wikipedia, and based on its packaging in a plastic bag within a bottle, I suspect the mystery acid is also hygroscopic. The one wrinkle in this theory (and it's minor because I don't trust my perception) is that I thought I also smelled chlorine (like a pool smell). Still, you're likely right. I'll try to think of a way to confirm. $\endgroup$ – piojo Mar 7 '18 at 1:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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