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iodine solution standardisation

Chemistry connections…

In the last titrations prac the solution of known concentration was placed in the burette and the “unknown” was placed in the conical flask beneath it. Does it matter which solution goes where?

10 Clean and rinse a 250mL conical flask with deionised water and, using a volumetric pipette, transfer to it a 25.00mL aliquot of the Vitamin C standard solution from the beaker (note: you will always pipette from this beaker and NOT from the volumetric flask).

Chemistry connections…

A chemist will never pipette directly from the volumetric vessel that contains a standard solution. The point of a standard solution is that it has a precisely-known concentration that you can use to your advantage (in this case performing a titration to calculate an unknown concentration). In these experiments the results are therefore only as reliable as the standard solution. Why is it not good practice to pipette directly from the volumetric flask?

11 Add 10 drops of starch indicator to the solution in the conical flask.

12 Titrate the solution in your flask. Like in the last experiment, it is generally worthwhile to do the first titration quickly at the cost of accuracy to determine the approximate end-point for the next more accurate runs. This is referred to as the “Rough” run. Record all your titre values in Table 2.1. Note: the end-point is reached at the first sign of blue colour that remains after 20 seconds of swirling the conical flask.

13 Repeat the titration until you have a minimum of two concordant titres (readings that differ by less than 0.1mL). Top up the iodine in the burette as required.

Question 1 – iodine solution standardisation

a) Calculate the mean (average) volume of the titre values you have chosen. Justify any

exclusions you have made.

b) What is the number of moles of Vitamin C present in the 25.00mL you pipette into

each conical flask?

c) Using the balanced Equation 2.1, how many moles of iodine, I2, must have been

present in the amount of iodine solution you titrated?

d) Given this number of moles and the average titre value, what is the concentration of

your iodine solution?