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the number of moles of ascorbic acid

The endpoint is shown by an indicator, in this case a solution of starch. Iodine, I2, and starch

form a blue-black product when they react. When there is still Vitamin C in the reaction

vessel, it will react with the iodine, I2, and convert this to the product I- which does not react

with the starch indicator. However, once the Vitamin C has all been removed by reaction,

the excess iodine, I2, will react with the starch indicator and cause the solution to change


Although you will be doing a redox titration rather than an acid-base titration, the method

and technique are exactly the same. You will use the pipetting and titration skills learnt in

the last prac.

In Part One you will perform a titration using a solution of iodine and a standard solution of

Vitamin C.

Chemistry connections…

Recall from Experiment 0 that a standard solution is one of known concentration. The

concentration of standard solutions is usually known because the chemist has made the

solution themselves using volumetric glassware. For example, in your last practical you used

a standard solution of sodium hydroxide (exact concentration provided on the dispenser)

and whilst you didn’t originally know the sulphuric acid concentration, that solution too had

been prepared volumetrically (you checked your titration skills at the end by discovering the

actual concentration at the servery).

In this experiment you must first prepare the Vitamin C standard solution very accurately so

you can use it to confirm the concentration of the iodine solution by titration in Part One.

This procedure is referred to as standardisation – you are said to standardise the iodine

solution. The accurate Vitamin C standard solution is prepared by dissolving pure Vitamin C

(ascorbic acid, a white powder) in water using a volumetric flask.