When my students prepare the caffeine standard in a range of 5mg/L to 100mg/L they all turn yellow with Bromocresol reagent. Our blank is distilled water, but also turns yellow with Bromocresol. What could be the problem?
Dear Faith,
In aqueous solution (water), Bromocresol green will ionize to give the monoanionic form (yellow). This because the pKa of water is around 14-15.
Similarly, the pKa of caffeine is 14, thus it will produce the same color with bromocresol.
Hoping this will be helpful,
Rafik
Thanks MP and RK, I have used the method from the same paper you have provided. I think I should change the water I use and see. But MP the sentence you like in the paper (Caffeine standard solution was made by dissolving 1mg pure atropine (Sigma Chemical, Bangalore) in 10 ml distilled water) always confuses, what is the truth? because I have failed to explain this simple chemical change Caffeine to atropine!.
@MP that is true! I will monitor the water pH, any experience with a proper pH for this or just 7/
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