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I bought a box of A&H Washing Soda, the packaging of which implied that it was pure washing soda (sodium carbonate).

On dissolving it in osmotically-purified water, however, I found that the water took on a brown tinge, which would seem to indicate a contamination or adulteration of some type. Also, the washing soda tended to form hard chips or clumps, which I have not seen in previous experiments doing the same thing.

I suspect there may be some kind of organic compound being used to adulterate the washing soda.

How can I determine if there is an organic adulterant and remove it?

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  • $\begingroup$ In addition to the answer given by Curt: It might be interesting to see whether the same effect can be observed using destilled water, rain water or any (tab) water that didn't go through the RO purifier. $\endgroup$ – Klaus-Dieter Warzecha Mar 2 '15 at 11:10
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If there is an organic contaminant in the $\ce{Na2CO3}$, you could try to rinse the sodium carbonate with an organic solvent in which $\ce{Na2CO3}$ is insoluble. Based on this paper, acetone and methanol would qualify. Since those solvents, especially methanol, are toxic, I'm not recommending either one of those. Based on the data for methanol and acetone, I'm guessing absolute ethanol would also be incapable of dissolving sodium carbonate, so it would probably the best (relatively) non-toxic solvent for extraction. If you can't get absolute ethanol due to tax or beverage laws, you could probably used denatured ethyl alcohol instead. It usually has small percentages of methanol, acetone, and/or isopropyl alcohol added to make it non-potable.

After you wash the sodium carbonate in this solvent, you should air dry it for several days, and then dry it in an explosion-proof drying oven at 100 - 200 °C. Be very careful when combining flammable solvents with heated environments!

The problem could also be with the water you used, although you did say you used osmotically purified water. The reason I think the water may be the culprit is that if the washing soda contained a brown contaminant, I would think it render the powder visibly brown. If no brownness is evident until adding the soda to the water, it makes me think the high pH from the washing soda is precipitating minerals from the water, in particular iron.

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    $\begingroup$ It's also possible that the brownness is from an inorganic contaminant in the washing soda. This would be difficult to remove with organic solvents and would require a totally different purification procedure. Recrystallization maybe? $\endgroup$ – Curt F. Mar 2 '15 at 4:26
  • $\begingroup$ Life is short. I would recommend getting sodium carbonate from an alternate source. It probably comes in different grades - avoid technical or practical grade and favor reagent grade if available. If the problem persists, the contamination would appear to be from the water. $\endgroup$ – iad22agp Mar 2 '15 at 19:00

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