Determination Method on Total Nitrogen and Total Phosphorus in Sea-buckthorn Stem Flow Liquid with Combined Digestion

National Standards HJ 636-2012 and GB 11893-89 illustrated that total nitrogen (TN) is digested with alkaline potassium persulfate and then determined using ultraviolet spectrophotometry and that total phosphorus (TP) is digested with neutral potassium persulfate and then determined using spectrophotometry. However, the analysis efficiency is low due to individual sampling and digestion for TN and TP. Based on national standard methods, a modified and effective method to respectively determine the contents of TN and total TP in one sample from trunk stem flow liquid, which would be digested once using potassium persulfate as the oxidant combined with spectrophotometry is focused on, and reported here. The results indicated that nitrogenous compounds from the sample in the alkaline potassium persulfate solution would be oxidized and decomposed into the form of NO₃^--N, whose absorbance is proportional to the concentration of TN under high temperature. Phosphorous compounds from the sample in the acidic potassium persulfate solution were oxidized and decomposed into the form of PO₄^3--P, whose absorbance was proportional to the concentration of TP under high temperature. The precision (RSD, n=5) of this method for determination of the actual samples is TN < 2% and TP < 4%. The standard addition recovery rates of TN and TP are 98.0%-104.2% and 94.0%-107.0%, respectively. Therefore, two standard series and high pressure digestion twice, which were needed in the national standard methods, were modified to only one standard series and high pressure digestion once saving 50% of the sample amount and enhancing the analytical efficiency. This modified determination was proved to be effective in the analysis of samples with low contents of organic matter from stem flow liquid, due to the difficulty of digestion and interference of determination of TN and TP in those samples with high contents of organic matters under the before mentioned conditions.