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Author: Subject: amber discoloration after heating 75% phosphoric acid

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[*] posted on 11-3-2018 at 12:32
amber discoloration after heating 75% phosphoric acid

Hi I'm trying to concentrate 75% H3PO4 to around 85%.

I heated the liquid in a beaker with stirring never allowing the temperature to exceed 90C. Left it overnight because it was taking a while. Anyway came back in the morning and I lost give or take the amount of water I was looking to remove, but the liquid has turned a dark amber color. Is this the result of impurities?
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[*] posted on 12-3-2018 at 03:55

Yes, this is due to impurities, almost certainly of organic nature. The heating, combined with the more concentrated acid, cause charring of organic matter.

Pure reagent grade phosphoric acid is colorless and it remains colorless on heating.

You could try adding some H2O2 to the phosphoric acid (e.g. 10 ml of appr. 10% H2O2 added to 100 ml of acid) and then heating again until the volume is down to 100 ml again. The organic material may be destroyed oxidatively and the heating decomposes any excess H2O2.

[Edited on 12-3-18 by woelen]

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[*] posted on 12-3-2018 at 05:25

Also refluxing with activated carbon may work which can then be filtered off. Phosphoric acid shouldn’t react with it, AFAIK.

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