Antioxidant Activity III

The second week of the antioxidant lab students analyzed the antioxidant power and polyphenol content of four commercial bottled teas. We used TPTZ (2,4,6-tripyridyl-s-triazine) FRAP assay to measure antioxidant power of tea and compare it with a gallic acid standard. We used the same serial dilution technique that we had performed the previous week. The biggest problem with measuring the antioxidant power with a colorimetric method is to make sure that the absorbance is within a range where the spectrophotometer is working well. The bottled teas often gave an undiluted absorbance of over 2A which is not acceptable. However, with dilution the absorbances fell in the acceptable range between 2 and 0.1 A. The gallic acid standard is run in parallel with the teas because the antioxidant power of the tea is calculated in units of mM gallic acid. The tea values have to be corrected for dilution. The calculations are a little involved because each tea absorbance (between 2 and 0.1 A) had to be converted to gallic acid equivalents and then averaged. The averaged gallic acid equivalent values for the four teas from Monday’s lab section were 0.77, 1.26, 1.55, & 2.23 mM gallic acid. The averaged Folin-Ciocalteau equivalent values for the four teas from Monday’s lab section were 3.58, 3.68, 3.52, & 4.91 mM gallic acid (in the same order as the FRAP teas). The correlation between antioxidant power and polyphenol content was not as strong as expected. It does show, however, that different teas do vary somewhat in both antioxidant power and polyphenol content. Student handout available on OChemOnline Wiki.


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