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The iodine solution.

vITAMIN C (ASCORBIC ACID) STANDARD SOLUTION PREPARATION AND IODINE SOLUTION STANDARDISATION

PROCEDURE

1 The standard solution of Vitamin C (ascorbic acid) has a desired concentration of 0.005mol/L. Using the molar mass, M, of ascorbic acid and the two equations – n=cxV and n=m/M – calculate the mass of ascorbic acid required to make up 0.25L (250mL) of solution. (Hint: refer to Experiment 0 Introductory Experiment, Question 3 if you need a refresher on how to do this type of calculation. Be sure to check your units.) Check your calculated mass with your demonstrator before proceeding.

Chemistry connections…

Notice that there are no longer questions worth marks in your Report Book about solution preparation calculations (unlike experiments 0 and 1F). Calculations and then solution preparation are standard skills that all chemists are assumed to have. They are therefore never discussed in scientific articles (beyond stating a standard solution’s concentration) and consequently will not appear in your Report Books anymore. Remember that the molar mass and two equations described above can be used to determine the mass of ANY compound in a solution of ANY concentration and volume. In a regular scientific article the experimental procedure would simply state “A 1L solution of 0.005mol/L ascorbic acid was prepared”, so you need to be becoming familiar with the method/calculations you’d require to prepare this yourself.

Foundations of Chemistry Laboratory Manual VITAMIN C DETERMINATION 3F

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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…