Determination of Total Nitrogen in Tobacco by Coulometric Titration

The pretreated tabacoo sample (0.20 g) was digested in 0.2 g of copper sulfate, 0.3 g of potassium sulphate and 6 mL of sulfuric acid at 170 ℃ for 1 h, and then the mixture was heated to 370 ℃, which was continuously digested for 2 h. After cooling to room temperature, 8 mol · L⁻¹ KOH solution was added drop-wise to adjust pH of digestion solution to 4.0－5.0, and the mixture was made its volume up to 50 mL by water. In the condition of platinum electrode - platinum wire electrode as working electrode pairs, platinum electrodes as indicating electrode pairs, sodium tetraborate - potassium bromide solution (KBr concentration of 1.2 mol · L⁻¹, pH 8.6－8.9) as electrolyte solution, with appropriate electrolysis current, the titrant BrO⁻ quantificationally titrated NH₄⁺ in sample solution after pretreatment. Total nitrogen was determined by electrolysis time that reached to titration end-point according to Faraday's law. As shown by the results, test for recovery was made by the standard addition method, giving results in the range of 95.1%－109%, with RSDs of the determined values (n=5) less than 2.0%. This method was applied to the analysis of actual samples, and there was no significant difference between this method and continuous flow analysis method in YC/T 161－2002. The results obtained by this method were similar to those of the Dumas combustion method. However, this method did not require a large amount of reagents to be prepared in advance, and it was easy to operate with short time consumption.