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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.)

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    $\begingroup$ 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
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    $\begingroup$ 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
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    $\begingroup$ @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
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    $\begingroup$ 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
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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.

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  • $\begingroup$ 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

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