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health effects or irritation of the respiratory tract

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.

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F


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?