[pdc]

In reviewing Principles of Pharmacy by W.B. Saunders (1917), p. 679-680, I came across a recipe for citrated caffeine.

"Dissolve 50 g citric acid in 100 ml hot distilled water, add 50 g caffeine and evaporate to dryness on a water bath. Caffeine citrate contains, when dried to constant weight at 80° C, not less than 48% of anhydrous caffeine. Forms a white powder; slightly bitter, acid taste; forms syrupy solution with small amount of water, precipitates caffeine on addition of more water, but makes a clear solution when more water is added. For details see U.S.P., p. 86. ... Remarks: Citrated caffeine, as given above, is made by blending caffeine with citric acid. The union is not a definite one, like the alkaloidal salts, hence the product is called 'citrated' and not 'citrate of'. It is chiefly used for making the effervescent salt."

Not very much anhydrous caffeine dissolves in room temperature water. I had assumed a caffeine salt would dissolve in copious amounts in room temperature water and thought it may exist as caffeine sulphate or caffeine hydrochloride. Research indicates caffeine citrate is preferred.

I would like to know why caffeine citrate is preferred as it appears from the above quote that the citrated caffeine is not a "definite union" as the mere addition of some water precipitates caffeine. I would think this has something to do with the composition of caffeine citrate, in that the above quote states caffeine citrate is composed of some 48% caffeine.

Does either caffeine sulphate, caffeine hydrochloride or another form of caffeine exist as a stable solid in a definite union so that copious amounts readily dissolve in room temperature water?

I would be most grateful for any input or comments regarding my query. Thank you very much for your time and consideration.