There are two kinds of natural ascorbic acid: reduced ascorbic acid and dehydroascorbic acid. Reduced ascorbic acid contains enol (COH=COH) in its molecular structure, so it is a very sensitive reducing agent. It can lose two hydrogen atoms and be oxidized to dehydroascorbic acid. Dye 2.6- Dichloroindophenol Sodium (C 12H6O2NCl2Na) can oxidize ascorbic acid, and its body is reduced to colorless derivatives.
2.6- Dichloroindophenol sodium salt is easily soluble in water, and its alkaline or neutral aqueous solution is blue and pink in acidic solution. This change is used to identify the titration end point.
Because ascorbic acid is easy to change under the influence of many factors, the operation time should be reduced as much as possible to avoid contact with metals such as copper and iron and prevent oxidation.
The colored sample solution can be decolorized with neutral clay, and the clarified filtrate can be absorbed for determination.
Second, drugs and equipment.
Tomatoes (cyan and red), peppers, cabbages, onions, oranges, candied dates, fresh dates, persimmons, apples, etc.
Ascorbic acid (pure), 2.6- dichloroindophenol sodium salt, 2% oxalic acid, clay.
Micro burette, 100ml volumetric flask, 10ml pipette, beaker, mortar (or pulverizer), aluminum box, funnel, analytical balance and centrifuge.
Three. Operations and steps
1. reagent preparation
(1) Ascorbic acid standard solution: accurately weigh 50mg (0.1+0mg) of Ascorbic acid, dissolve it with 2% oxalic acid, carefully transfer it to a 250ml volumetric flask, add oxalic acid to dilute it to scale, and calculate the milligrams of Ascorbic acid in each ml of solution.
(2) Calibration of 2.6-dichloroindophenol solution. Weigh 50 mg of 2.6- dichloroindophenol sodium salt, dissolve it in 50 ml of hot water, cool it, dilute it to 250 ml with water, filter it, put it in a brown medicine bottle, refrigerate it, and calibrate it with standard ascorbic acid just prepared.
Absorb 2ml of ascorbic acid standard solution, add 5ml of 2% oxalic acid, titrate with 2.6- dichloroindophenol dye solution until the pink color lasts 15s, and calculate the mg of ascorbic acid per 1ml dye solution according to the known ascorbic acid standard and dye dosage.
2. Preparation and determination of sample solution
Weigh 20 grams of chopped fruit and vegetable samples (or 5 grams of candied dates), put them into a mortar and add a small amount of 2% oxalic acid solution for grinding (or weigh100g of samples and100g of 2% oxalic acid and pour them into a pulverizer for pulping, then weigh 40g), pour them into a 200ml volumetric flask, add 2% oxalic acid solution for dilution to scale, and then weigh them. If the filtrate is colored, it is difficult to identify the end point during titration, and white mud can be used for decoloration, filtration or centrifugal precipitation for later use.
Suck 10 ml filtrate into a beaker, titrate with a calibrated 2.6- dichloroindophenol sodium salt solution until the pink color does not fade within15s, and record the dye amount.
Absorb 10ml of 2% oxalic acid solution, use dye as blank titration, and record the dosage.
Calculation formula:
(V-V 1) X A b
w =――――X—X 100
bachelor's degree
W = 100 The sample contains mg of ascorbic acid.
V = number of milliliters of dye used to titrate the sample.
V 1 = number of milliliters of dye used for blank titration.
A = 1 ml dye solution equivalent to mg of ascorbic acid.
B = number of milliliters of sample solution absorbed during titration.
B = total ml of diluted sample solution.
A = grams of sample.
Note: For the samples treated with fuming sulfuric acid or sulfurous acid and its salts, 5 ml of formaldehyde (pure) should be added when preparing the sample solution to eliminate the influence of sulfur dioxide, and then the volume will be fixed later.
IV. Results and Calculation
1. Fill in the measurement data in the table below.
Calibration of (1) dyes
Average concentration of ascorbic acid standard solution (mg/ml) for the first time, the second time and the third time per 1 ml dye solution.
(2) Calculation of ascorbic acid content in the sample
Sample Name SampleNo. (g) Total volume of sample solution (ml) Amount of sample solution for titration (ml) Amount of dye for titration Sample (ml) Amount of dye for blank titration (ml) Vitamin C content (mg/ 100g) 123 average value 1234.
2. List formulas and calculation results.
I can't give you the data, haha