So, I wasn't trying to do a chemistry experiment this evening; it was entirely unintentional!

I was mixing up a batch of fertilizer for my plants (no nitrates involved, so I wasn't worried about potentially dangerous reactions). The main goal was to add some sulfur (which over time lowers the pH) and potassium (in the form of potassium sulfate); to balance it out there also were lesser amounts of calcium superphosphate and magnesium sulfate, a little iron sulfate, and tiny amounts of trace elements (boric acid and some simple compounds of manganese, copper, zinc, and molybdenum)

The potassium sulfate tends to come in big chunks because it's hygroscopic, and I know that hot water helps dissolve it (and that the sulfur doesn't dissolve in water, even hot water), so I heated the mixture (in water) in the microwave, then stirred with a plastic utensil. I noticed a surprising amount of offensive-smelling offgassing coming off of it. So I put in my analog pH meter, and it pegged instantly, hard (its limit is 3,5, so it must have been much lower). Even after adding some calcium oxide (which instantly fizzed and started gelling), the pH was still pegging hard. And after 1 dilution (maybe 7:1?), it was still pegging, and only after the second dilution did it get stop pegging (but still showed strongly acidic). There was heavy brown discoloring in the bottom of the plastic container, and it looked like the sulfur might have been reacting as well.

I assume I created sulfuric acid (and/or possibly phosphoric?) somehow, but I'm not sure what the reaction was! I wasn't expecting this at all. Normally in horticulture, sulfur takes ages to acidify soil, as it's decomposed by bacteria. I know that combusted sulfur yields sulfur dioxide, which - oxidized over a vanadium pentoxide (or other) catalyst - yields sulfur trioxide, and thus (with water) sulfuric acid... but that's clearly not what happened here. Any clue what reaction we're looking at?

  • $\begingroup$ Perhaps anhydrous calcium chloride could work? Sigma Aldrich sells it with a 99% purity, with the remaining impurities being traces of metals basis. $\endgroup$ Mar 26, 2018 at 12:38
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    $\begingroup$ Dissolve some of the potassium sulfate in distilled water and measure its pH. If the resulting solution is highly acidic it is possible that you have potassium hydrogensulfate instead of potassium sulfate. $\endgroup$
    – aventurin
    Apr 15, 2018 at 19:06
  • 3
    $\begingroup$ Potassium sulfate isn't hygroscopic (contrary to the description in the question); however, potassium hydrogen sulfate is. $\endgroup$
    – Loong
    Apr 10, 2023 at 11:38

1 Answer 1


Probably you are looking at two different phenomena:


If the potassium sulfate is hygroscopic, then as seen inthe comments it may have been potassium hydrogen sulfate, $\ce{KHSO4}$ instead of the normal sulfate $\ce{K2SO4}$. The $\ce{HSO4^-}$ ion is a fairly strong acid with $\pu{pK_a}\approx2$, so reasonably solutions of salts with this ion and no normal sulfate would be expected to have $\pu{pH}<2$ even at concentraions as low as 0.1 molar. So they require much dilution to get the $\pu{pH}$ within your reading range. Thus your accidental acid may be potassium hydrogen sulfate instead of normal potassium sulfate.


There is no indication that sulfur reacts with sulfate or bisulfate salts to give sulfur dioxide. But it can react with air, especially when heated (probably your microwaving destroyed microbes). It's supposed to go to sulfuric acid, but sulfur dioxide can form as an intermediate product and potentially gas out. A neutral or basic solution might convert the intermediate to sulfite or bisulfite ions, but an acid solution (see above) would promote formation of the gas.

  • $\begingroup$ SO2 is not easily oxidized to SO3 or sulfuric acid. $\endgroup$
    – jimchmst
    Apr 12, 2023 at 20:13
  • $\begingroup$ But it could be formed. Neutral or basic solutions containing sulfite ions would be easier to oxidize to sulfate. $\endgroup$ Apr 12, 2023 at 20:15

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