Agriculture and Food 7–12 · Years 11–12

Vitamin C in juice and its loss on heating (microscale iodine titration)

Nutrition: diets for optimum nutrition

Practical, model not builtLow risk

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The idea

Ascorbic acid (vitamin C) reacts one to one with iodine, so a titration measures how much a juice contains and shows how heating and storage destroy it.

Safety card

Low riskAn adult supervises

Hazards

1 mol/L sulfuric acid irritates skin and eyes
hot juice when boiling

Controls

eye protection throughout
acid in a dropper bottle
no tasting in the laboratory

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

What you need

microscale titration apparatus: a 2 mL graduated pipette used as the burette, filled and emptied with a syringe and read to 0.01 mL (the RSC microscale arrangement); 10 mL beaker, 1 mL and 2 mL glass pipettes, 25 mL measuring cylinder
sodium thiosulfate, 0.010 mol/L; potassium iodate, 0.001 mol/L; potassium iodide, 0.005 mol/L
sulfuric acid, 1 mol/L, in a dropper bottle; freshly made starch solution
fresh orange juice; the same juice boiled for 10 minutes in a covered pan, cooled and made back up to its starting mass with deionised water; the same juice stored open in the refrigerator for 3 days
eye protection

How to do it

Weigh the juice to be boiled, boil it for 10 minutes with the lid on, cool it and weigh it again, then add deionised water until it is back to its starting mass. Record both masses and the mass of water added: boiling an open vessel for 10 minutes drives off a large and uncontrolled part of the water, which concentrates whatever ascorbic acid survives and works in the opposite direction to the loss this practical is built to show, so without this step a result in mg per 100 mL is not comparable with the fresh juice and can cancel or reverse the real loss.
Fill the titration apparatus with sodium thiosulfate solution.
Pipette 2.0 mL of potassium iodate into the beaker, add 3 mL of potassium iodide and three drops of sulfuric acid (iodine forms) and a few drops of starch.
Do a blank titration with no juice: titrate the iodine with thiosulfate until the blue-black colour disappears.
Repeat with 1.0 mL of juice added before titrating, and do a duplicate for each juice until two titres agree.
Calculate vitamin C in mg per 100 mL for the fresh, boiled and stored juices and compare with any vitamin C figure on the label.

What you should see

The blank titre is 1.20 mL (6.00 x 10^-6 mol of iodine). A juice with 40 mg of vitamin C per 100 mL gives a titre of 0.75 mL, and 50 mg per 100 mL gives 0.63 mL: the more vitamin C, the smaller the titre. The boiled juice gives a larger titre than the fresh juice because heating destroys vitamin C, and this comparison holds only because the boiled juice was made back up to its starting mass: a juice boiled in an open vessel and titrated as it stands has lost water as well as vitamin C, and the concentrating effect can hide or reverse the loss. Report the mass of water added, because it sets how much the boiled sample was diluted back. The juice stored open for 3 days may show a smaller loss, which can be hard to separate from titration error. The learner knows it worked when duplicate titres agree to within one drop (about 0.02 mL, the RSC figure for the apparatus with a fine tip), the blank is within a drop of 1.20 mL, and the boiled juice went back to its starting mass before it was titrated.

What changes

What you change: juice treatment (fresh, boiled 10 minutes, stored open 3 days)
What you measure: titre (mL) and vitamin C (mg per 100 mL)
What you keep the same: juice source
juice mass restored to its starting value after boiling, with the mass of water added recorded
1.0 mL sample
reagent volumes and concentrations
temperature

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Cooking does not change the vitamins in food.
All orange juices contain the same amount of vitamin C.
A bigger titre means more vitamin C.
Boiling a juice for ten minutes changes only the vitamins in it.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Food Technology Stage 6 Syllabus (2013)P2.1 P4.3
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities