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VITAMIN C DETERMINATION

2 Weigh out this calculated mass (you only need to weigh the amount approximately but record the weight exactly) then deliver it to a 250mL volumetric flask using a funnel (be sure to get all of the ascorbic acid into the flask, otherwise the concentration will not be correct – rinsing the funnel into the flask with some deionised water may help).

3 Add approximately 100mL of de-ionised water to the volumetric flask. Swirl the flask until all of the Vitamin C is dissolved.

4 Fill the remaining volume to the aliquot line with de-ionised water (do not do this if there is un-dissolved solid present still).

5 Replace the stopper and invert the flask a number of times to ensure that the solution is homogenous and its concentration is uniform.

Chemistry connections…

If the solid dissolves in the water at the very bottom of the volumetric flask and you do not invert it, what affect might this have on the titrations you do? Would the results from these titrations be reliable?

6 Clean two 250 mL beakers carefully and rinse them at least twice with de-ionised water. Shake them to remove most of the water. The remaining water must be removed by drying with paper towel.

Chemistry connections…

Using your knowledge from the last titrations prac can you explain why the beakers must be dry and have no water present in them before use?

7 In one of the clean and dry 250 mL beakers place approximately 150 mL of iodine solution. (It is the concentration of this solution that you are trying to determine. Ie you are about to “standardise” this iodine solution.) In the other beaker pour a similar quantity of Vitamin C solution you prepared in Step 1. (It helps to label your solutions!)

8 Rinse a 50 mL burette well with de-ionised water.

9 Rinse the burette at least twice with the iodine solution, fill the burette (to near the top), wipe the outside with paper towel and mount it on a burette stand at your bench. Make sure the space in the burette beneath the tap also has solution in it because the volume graduations on the side account for this.