16 MASS. EXPERIMENT STATION BULLETIN 354 



Example of Calculation 

 Weight of ascorbic acid — 12.5 mgs. 

 Weight of ascorbic acid in 10 cc. aliquot — 1.25 mgs. 

 0.77 cc. iodine = 1.25 mgs. ascorbic acid. 

 1 cc. iodine = 1.62 mgs. ascorbic acid. 

 Standardization of 2, 6-dichlorophenolindophenol dye. 



1. Dilute 2 cc. of lemon juice with 20 cc. of water. 



2. Add dye solution from burette until a pink end point is continuous for 

 one minute. 



3. Repeat until results check. 



Example of Calculation 

 2.64 cc. indicator = 2 cc. lemon juice. 

 1.32 cc. indicator = 1 cc. lemon juice. 



4. Dilute 5 cc. of lemon juice with 20 cc. of water and add 5 to 10 drops of 

 starch solution. 



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



6. Repeat until results check. 



Example of Calculation 

 1.81 cc. iodine = 5 cc. lemon juice. 



1 cc. iodine = 2.76 cc. lemon juice = 1.620 mgs. ascorbic acid (from 

 standardization of iodine). 



2.76 cc. lemon juice = 3.64 cc. indicator = 1.620 mgs. ascorbic acid. 

 1 cc. indicator =0.445 mg. ascorbic acid. 

 Summary: Indicator = lemon juice = iodine =ascorbic acid. 

 Preparation of sample from fresh tomatoes. 



1. Accurately weigh a cross section or quarter section of approximately 

 25 grams. 



2. Thoroughly triturate sample in a mortar with about 5 grams of acid-washed 

 white sand and 50 cc. of the acid mixture. 



3. Pour into centrifuge tubes. 



4. Rinse out mortar with more of the acid mixture and pour into the same 

 tubes. 



5. Centrifuge at 1800 revolutions per minute for 15 minutes. 



6. Decant serum (colorless liquid) from centrifuge tubes into a 200 cc. volumet- 

 ric flask. 



7. Cover residue in centrifuge tubes with acid mixture and extract a second 

 time by centrifuging for same' period of time and speed. 



8. Decant serum into previous extraction. Make up to 200 cc. volume with 

 acid mixture. 



9. Titrate 25 cc. aliquot portions of this 200 cc. volume with the 2, 6-di- 

 chlorophenolindophenol dye. The pink end point should persist for one 

 minute. 



10. Make three separate titrations. 



Example of Calculation 

 Weight of sample — 36.7 gms. 

 Weight of sample in 25 cc. aliquot — 4.59 gms. 

 3.19 cc. indicator =4.59 gms. tomato. 

 1 cc. indicator = 1.48 gms. tomato. 

 1.48 gms. tomato = 0.445 mg. ascorbic acid (from standardization of 



the indicator). 

 1 gm. tomato = 0.301 mg. ascorbic acid. 



