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Vit C Back-Titration help / guidance

Hey all,

Deadline for my A2 chemistry (Salters) investigation plan is approaching. The coursework aim is to investigate methods of finding an unknown concnetration of Vit C (e.g. orange juice)

I have chosen to look at 3 methods:

Back Titration - add excess I3- ions to vit C solution & titrate with Sodium Thiosulphate. using formulas, excess can be found which allows amount of Vit C to be found.

Titration with NBS - NBs reacts with ascorbic acid, and add starch & potassium iodide will show end-point (turn a blue colour)

Colorimetry - calibrate colorimter with iodine solutions of known concentration (make up a standard solution of iodine in aqueous potassium iodide) to get a calibration curve. Add sample into stock iodine solution & read result of curve to find concentration via some calculations.

Basically, I am lost about what concentrations of solutions to use.

Should I just make up random concnetrations which are easy to dilute & calculate with?

Reply 1

jrrowleyws

Do some moles calc's. Your going to need to have a rough idea on what to use. Guess work in coursework is never good and you would generally be expected to state how you worked out the concentrations.

Its not terribly hard, you just need a few equations and a rough guess of how much ascorbic acid is in the OJ -look on packets.

Otherwise you might be titrating very small amounts or litres of a low concentrated solution.

If you need a 4th method, look into titrating with DCPIP.