Colorimetric Nitrate Determination of Tea Extract with Activated Carbon Pretreatment
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摘要: 茶叶中含有大量的多酚类化合物、色素和抗坏血酸等物质，在采用水杨酸比色法测定茶叶硝态氮时发现，茶叶前处理滤液的颜色较深，存在色素干扰。本研究通过加入粉末活性炭对样品滤液进行脱色，比较木质（AC1）和椰壳（AC2）两种材质活性炭对滤液色素的去除效果和硝态氮回收率的影响。结果表明，样品滤液脱色所需活性炭AC1和AC2的量分别为1%和1‰。对系列硝态氮浓度（0、2.5、5、10、15、20、30、40、50和60 mg·L-1）的标准液分别进行1% AC1和1‰ AC2处理，发现1% AC1处理条件下，测试范围内线性较佳（R2=0.9998） ，硝态氮回收率为96.61%～ 115.04%；而1‰ AC2处理下的线性较差（R2=0.9396） ，回收率随硝态氮的浓度而改变，在硝态氮浓度为0～5 mg·L-1时，回收率为0%，10～60 mg·L-1范围时，回收率随硝态氮浓度的增加由2.24%增加到29.24%。因此，1% AC1脱色可用于比色法测定茶叶硝态氮。Abstract: 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.
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Keywords:
- Tea leaf /
- NO3--N- /
- activated carbon /
- decolorization /
- recovery rate
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