Colorimetric Nitrate Determination of Tea Extract with Activated Carbon Pretreatment

Compounds, such as polyphenols, pigments，and ascorbic acid, in tea leaves darken the color of the extract that interferes the nitrate determination by salicylic acid colorimetry. This study applied two kinds of activated carbon, one made from wood (AC1) and another from coconut shell (AC2), for the decolorization pretreatment. The color removal was effectively achieved by addition of AC1 at 1% or AC2 at 1‰ to the filtered tea leaf extract. The nitrate recovery rates of the pretreatments for the colorimetric determination were compared by using a series of NO₃^--N standard solutions in the concentrations of 0 , 2.5, 5, 10, 15, 20, 30, 40, 50，and 60 mg·L^-1 with the addition of 1% of AC1 or 1‰ of AC2. From the resulting color measurements, nitrate recovery rate of each pretreatment was obtained for a regression analysis. A higher linear correlation coefficient of R²=0.9998 was found under 1% AC1 treatment that rendered 96.61% ～ 115.04% recovery of NO₃^--N. On the other hand，the 1‰ AC2 treatment showed R²=0.9396 with a nitrate recovery varied with the nitrateconcentration—i.e. , 0% in the range of 0 ～ 5 mg·L^-1 and from 2.24% increased to 29.24% with increasing NO₃^--N from 10 mg·L^-1 to 60 mg·L^-1. Thus, for the determination of nitrate in tea leaf extract by salicylic acid colorimetric method, a pretreatment by addition of 1% of AC1 for decolorization was recommended.