The blank absorbency is always too high to obtain the exact result when we analyze the total nitrogen using alkaline potassium persulfate digestion-UV spectrophotometric method.We studied the phenomenon and the results were as follows.The purity of water and potassium persulfate was the most essential factor in the analysis.Water with high purity and imported K2S2O8 with the purity of more than 99.0% could insure the blank absorbency less than 0.030.Instead of using imported K2S2O8,we also obtained perfect results using domestic K2S2O8(AR) as long as we made the reagent secondarily crystallized.If the purity of water or the reagent of K2S2O8 was not high enough,we could improve the experimental operation to reduce the blank absorbency by adding HCl(1+9)solution into alkaline potassium persulfate solution,extending the digestion time to 50 minutes or cooling rapidly after digestion.But these methods could only reduce the blank absorbency to some degree but couldn't ensure an absorbency less than 0.030.Different instruments for digestion or absorbency analysis showed no significant effect on the results.