ASCORBIC ACID IN TOMATOES 15 



react with the oxidizing agents used for the determination of ascorbic acid. 

 Samples of juice expressed from raw tomatoes without contacting any metal were 

 used to determine the effect of metallic ions. Results show that 1 or 100 parts 

 per million of either stannous or ferrous ions or 200 parts per million of both, in 

 tomato juice do not interfere with the chemical determination of ascorbic acid. 

 However, if the end point is not reached and read promptly during the titrations 

 the metallic ions obscure the end point and make it difficult to obtain a true 

 reading. Normal solutions of either or both ions do not react with the 2, 6-di- 

 chlorophenolindophenol dye used in the titration. Thus the presence of iron and 

 tin salts in foods canned in tin containers does not interfere with the titration 

 of ascorbic acid. 



DESCRIPTION OF PERFECTED CHEMICAL TITRATION METHOD FOR 

 DETERMINATION OF ASCORBIC ACID 



Preparation of indicator solution (0.10 percent). 



1. Dissolve approximately 0.10 gram of dry 2, 6-dichlorophenolindophenoI 

 indicator with successive portions of warm water, filter, and add water 

 to make up to a volume of 100 cc. 



2. Do not store for periods longer than five days. 



3. Standardize indicator solution daily. 

 Preparation of iodine solution (0.01 N). 



1. Dissolve approximately 1.3 grams of iodine and 25 grams of potassium 

 iodide in distilled water and make up to one liter. 



2. Store iodine solution in a dark or paper wrapped bottle at ordinary re- 

 frigerator temperature. 



Preparation of sodium thiosulfate solution (O.OIN). 



1. Dissolve 25 grams of sodium thiosulfate in distilled water and make up 

 to a liter. This makes a O.ION stock solution. 



2. Dilute ten times to prepare a O.OIN solution. 

 Preparation of starch solution (0.5 percent). 



1. Dissolve 0.5 gram of soluble starch in 100 cc. of cold water and store at 

 a cold temperature. 

 Preparation of acid mixture (3 percent trichloroacetic acid and 2 percent meta- 

 phosphoric acid). 

 1. Dissolve 30 grams of trichloroacetic acid and 20 grams of metaphosphoric 

 acid in one liter of water. 

 Standardization of iodine solution against pure ascorbic acid.^ 



1. Accurately weigh about 10 mg. of pure ascorbic acid. 



2. Dissolve in acid mixture and make up to 100 cc. volume with acid mixture. 



3. Dilute three 10 cc. aliquot portions of ascorbic acid solution with 25 cc. 

 of acid mixture. 



4. Add 5 to 10 drops of starch solution to each portion. 



5. Add iodine solution from a burette until the blue starch-iodine color 

 appears, indicating the end point. 



6. Repeat the above procedure until the amount of iodine reduced by three 

 samples checks closely. 



7. Standardize the iodine solution bi-monthly. 



^While this method of standardization was followed in all of the determinations reported in this 

 bulletin, the authors now recommend the use of standard iodine and thiosulfate solutions as des- 

 cribed by Buck and Ritchie (1938) and also by Menaker and Guerrant (1938). The data reported 

 in this bulletin were largely obtained before the publication of this technic. 



