ASCORBIC ACID IN TOMATOES 13 



Dye Indicator Strength 



It has been suggested by some investigators that a 0.1 percent 2, 6-dichloro- 

 phenolindophenol dye solution, instead of the usual 0.05 percent solution, will 

 result in closer titration values for ascorbic acid. To determine the accuracy of 

 this suggestion, the sera extracted from ten centrifuged SO cc. samples of tomato 

 juice were diluted to 100 cc. each. Three aliquot portions were titrated for 

 ascorbic acid with 0.05 and 0.10 percent dye solutions. The results show that in 

 five samples of tomato juice an average maximum variation of 2.87 percent was 

 found with the 0.10 percent concentration of dye and 6.04 percent with the 0.05 

 percent dye concentration. That is, much greater accuracy is obtained by the 

 use of the stronger dye. 



Probable Error 



In any chemical titration there is an element of error in determining the end 

 point. To establish this value in both the modified iodine and the 2, 6-dichloro- 

 phenolindophenol methods the following equation was used: 



probable error = ±0.6745 



where Sd^ is the summation of the squares of the deviations from the arith- 

 metical mean, and n^ is the square of the number of determinations. 



The acid-extracted supernatenl liquid from centrifuged tomato juice was used 

 to determine the probable error in reading the end point. The serum from50cc. 

 of juice was made up to 100 cc. and titrated in aliquot portions of 25 cc. Ten 

 samples of juice titrated in triplicate, a total of 30 titrations, were used for the 

 0.10 percent dye method and 30 titrations for the iodine method. The results 

 show a probable error of ± .05 cc. on the three successive titrations of each sample. 

 A variation of +.05 cc. in each titration of a tomato sample which averages 

 3 cc. of dye, gives a probable error of ± 2 percent on each sample. Under like 

 conditions the probable error in determining the titration end point in the iodine 

 method was only +.02 cc. These data show that uniform results can be obtained 

 by using chemical titration methods for determining ascorbic acid with only 

 slight error. 



Standardization of the Dye 



Early literature on the estimation of ascorbic acid by the chemical titration 

 method is reported in terms of quantity of the dye reduced. When crystalline 

 ascorbic acid was finally synthesized, it became possible to standardize the dye 

 solution in terms of the pure acid. This eliminated the necessity of preparing 

 fresh solutions of the dye daily. However, for accurate quantitative work, it is 

 considered inadvisable to use dye solutions made up for periods longer than five 

 days. 



The method of standardizing the 2, 6-dichlorophenolindophenol dye against 

 pure ascorbic acid is not accurate at best. Besides being expensive, the small 

 quantities of the pure acid used may lead to inaccuracies in the standardization. 

 Moreover, aqueous solutions of pure ascorbic acid are rapidly oxidized in the at- 

 mosphere; this necessitates rapid titration of the dye. 



Birch, Harris, and Ray (1933) suggested that the ascorbic acid be standardized 

 with iodine to eliminate the possible error caused by the presence of impurities 

 in the ascorbic acid. One molecule of ascorbic acid reduces two atoms of iodine. 



