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How to measure the unknown quantity of phosphoric acid of soft drinks

I wish to measure the unknown volume of phosphoric acid in carbonated beverages. However if I titrate a sample of 'coca cola' with standardised NaOH, would the presence of carbonic acid affect my results? What procedure would allow me to eliminate the presence of carbonic acid in coca cola?

Lillian Lam
- Brisbane, QLD, Australia
2003

Nothing is that simple. There is probably a small amount of citric acid and very probably some ascorbic acid also. If you chose to ignore these, you can reduce the carbonic acid to a tolerable amount by boiling it for a couple of minutes and then bringing it back up to original volume after it has cooled so you get an honest amount. Cover with a watch glass while cooling. You might want to look at a chemistry book and see if you want to try a gravimetric analysis for phosphates. A pain! I would titrate with a 0.1 or even better with a 0.05 N NaOH. Use a pH meter if you can and titrate slowly. Graph your results and see if you can find the 3 inflection points from the phosphoric acid. If you find a tiny 4th or 5th one, you will know that there were other acids. Go slow and record pH vs mL of NaOH.

James Watts
- Navarre, Florida
2003

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To reduce the amount of carbonic acid to tolerable levels you could simply degas the soft drink sample by mixing the sample in a milk shake shaker/stirrer container for approx. 9 minutes.

Regards

Clifford Name
- Australia
2005