Which type of citric acid crystalline powder do they sell in grocery stores?
Is the concentration of citric acid in the crystals 100%?Is it anhydrous or a monohydrate?
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The box in my kitchen is labelled "reine Zitronensäure" (= pure citric acid) and "lebensmittelsauber".
According to the manufacturer, the latter means that citric acid in this quality can be used to descale (decalcify) kitchen items, but it is not intended to be used as a food supplement and should not be digested!
This may have real technical reasons, i.e. the product may in fact contain impurities resulting from production, processing and packaging. The manufacturer however does not specify the kind or amount of impurities.
On the other hand, this kind of labelling may just reflect different legal treatment (certification, monitoring, etc.) of products intended as food supplement and those only used to clean kitchen equipment.
Concerning waterfree vs monohydrate, neither the package in my kitchen, nor the website of the manufacturer provide any information.
If you know a chemist, this question is easily solved: Let him measure the melting point of a sample. Waterfree citric acid melts at 153 °C, the monohydrate at 100 °C.