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I dissolved aluminum sulfate in DI water and expected to see just a clear water solution with aluminum sulfate dissolved as ions. However, a precipitate was visible. I addd only 0.6g of Aluminum sulfate so there is no way that the precipitated is saturated aluminum sulfate. Furthermore, it does not seen to be aluminum hydroxide, so then what is the precipitate? Aluminum sulfate has the property of filtering out impurities in water, however, I am using DI water so then what is the precipitate?

Also, the precipitate is not aluminum hydroxide because aluminum hydroxide is dissolvable in sulfuric acid; however, when I added sulfuric acid to the precipitate solution, the precipitate did not disappear.

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    $\begingroup$ I'll say your sulfate is not what you think it is. $\endgroup$ – Karl Mar 25 '16 at 19:28
  • $\begingroup$ what does that mean? $\endgroup$ – user510 Mar 25 '16 at 19:34
  • $\begingroup$ It's something else. I think you proved that quite convincingly. $\endgroup$ – Karl Mar 25 '16 at 19:40
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    $\begingroup$ Alternatively your water is not what you think it is. $\endgroup$ – Rudi_Birnbaum Mar 25 '16 at 19:54
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    $\begingroup$ Deionized water contains not much ions, but the ionexchange columns or filters used tend nevertheless to slowly get contaminated with bacteria or other germs. So they should be regular exchanged. Now Al$_2$(SO$_4$)$_3$ has the property to denaturate proteins, this is why it is used also for leather tanning occasionally. So my guess is that its some protein precipitate oringinating from biologically impure water. $\endgroup$ – Rudi_Birnbaum Mar 25 '16 at 20:27
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According to Wikipedia, "When dissolved in a large amount of neutral or slightly alkaline water, aluminium sulfate produces a gelatinous precipitate of aluminium hydroxide, Al(OH)3."

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Try dissolving the precipitate properly! Did you separate and dry the ppt? Or like try removing significant amount of supernatant, it should be Al(OH)3 ....why don't you perform a lake test to confirm the hydroxide?

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