Determination of Sulfur Dioxide Residues in Dried Fruit Products by Kjeldahl Distillation-Automatic Photometric Titration

A method for the determination of sulfur dioxide residues in dried fruit products was established by Kjeldahl distillation-automatic photometric titration, and test conditions, including the ratio of water volume (mL) to sample mass (g) (liquid-solid ratio), concentration of hydrochloric acid solution, ultrasonic extracting time, steam output, were optimized by single factor test and response surface Box-Behnken design test. 5 g of Crushed sample were weighted and extracted with 270 mL of water and 10 mL of 7 mol·L^-1 hydrochloric acid solution for 15 min by ultrasound, and distilled for 9 min with the steam output of 90%. The condensed liquid was collected by 25 mL of absorbent (20 g·L^-1 lead acetate solution), 10 mL of hydrochloric acid and 1 mL of 10 g·L^-1 starch indicator solution were added into the absorption solution, and the mixed solution were titrated by automatic photometric titrator. The signal drift value was set as 15 mV·min^-1, and the detection wavelength was set as 640 nm, and the titration endpoint was obtained from the first derivative of the titration curve. The results showed the determined value (0.367 g·kg^-1) was in conformity with the predicted value (0.365 g·kg^-1) under the optimal test conditions. Tests for recovery and precision on actual samples were made, and values of recovery were found in the range of 94.3%-99.7%, with RSDs (n=7) of the detemined values not more than 0.90%. The results obtained by the proposed method were compared with those obtained by full glass distillation-artificial titration method given by GB 5009.34-2016, and no significant difference was found between the two methods at the 95% confidence level.