Looking for suggestions on how to identify an unknown white solid we received as a donation. It was originally labeled as $\text{MgSO}_4$, but clearly is not. We suspect it might be $\text{CaSO}_4$. So far, I can say that it is practically insoluble in water. We are a DIYbio lab and so were aren't equipped for complex chemical analyses. We also don't have a fume hood so we can't utilize any reactions that might produce noxious gases. Any ideas would be greatly appreciated if only so I don't have to see the bag sitting on my desk anymore.

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    $\begingroup$ Either $\ce{CaSO4}$ (~55 USD per 1 kg) or $\ce{MgSO4}$ ((~85 USD per 500 g) is not seriously expensive in fairly pure form. Your chemical is in very crude form judging by the color (probably in tech grade). So why waste money to analyze? $\endgroup$ Commented Aug 10, 2022 at 21:01
  • $\begingroup$ You can take advantage of the difference in pH required to precipitate calcium and magnesium hydroxides for this one. I would acidify a solution of it, then slowly add in a 0.1 molar solution of KOH while recording pH, until you see a precipitate of the hydroxide salt. Off-hand, I cannot recall which precipitates first as pH increases but this will give you an idea if you are dealing with Ca or Mg sulfate. $\endgroup$ Commented Aug 10, 2022 at 21:23
  • $\begingroup$ @Todd, CaSO4 is not soluble in acids, it is very hard to dissolve. However, insolubility does not prove that it is CaSO4. It is best to buy the material from a reliable source. $\endgroup$
    – ACR
    Commented Aug 10, 2022 at 22:06
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    $\begingroup$ A good reason not to accept “donations” of unidentified materials… $\endgroup$
    – Jon Custer
    Commented Aug 10, 2022 at 22:30
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    $\begingroup$ @AChem - I disagree. See this for example. $\endgroup$ Commented Aug 10, 2022 at 22:32

1 Answer 1


Since you don't know what it might be contaminated with, it might need to be disposed as hazardous waste. It could be contaminated with biological material, or even with radioactive wastes, which would not show on any but the most sensitive chemical assay.

As the old riddle goes:

What do you call 1,000 liters of sewage mixed with 1 ml of Chateau Mouton-Rothschild 1945? Sewage.

What do you call 1,000 liters of Chateau Lafite 1869 mixed with 1 ml of sewage? It's still sewage.

  • $\begingroup$ This isn't really an answer, just moralism. However, I think you are correct that it's easier just to handle it as potentially hazardous waste since it is definitely not the compound of interest. $\endgroup$ Commented Aug 15, 2022 at 21:54

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