Analysis Of Commercial Vitamin C Tablets

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Experiment 5

Date: 18-10-2005

Analysis Of Commercial Vitamin C Tablets

Objective

To determine the mass of Vitamin C in 1 pill of Vitamin C tablet.

Introduction

        In this experiment, the concentration of sodium thiosulphate solution is not given. So, we need to standardize it through titration. Sodium thiosulphate reacts with iodine in the following reaction:

                        I2 + 2S2O32- ---------------→2I- + S4O62-

After an amount of S2O32- is added, the solution of I2 turns pale yellow. When starch solution is added and more S2O32- is added, the solution reaches its end point, which is colorless. In this way, the molarity of the thiosulphate solution is determined.

        Vitamin C could be oxidized by iodine in the presence of acid in the following equation:

        Due to the low solubility of iodine, direct titration of iodine solution and Vitamin C is unsuitable. Then how could the experiment be done? The experiment could be done by adding acidified Vitamin C solution into potassium iodide solution. Then, we add potassium iodate to allow the following reaction to take place:

                                IO3- + 5I- + 6H+ --------------→ 3I2 + 3H2O

        I2 formed in this reaction could react with the Vitamin C in as mentioned in the above equation. The I2 not yet reacted would then be titrated against thiosulphate solution, like the first equation, to determine its amount.

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        This method is a kind of back titration. It is the excess amount of substance which is being titrated, but not the object being studied. This method is used when the direct titration of the object being studied is not suitable, which is why back titration is used in this method.

Procedure

  1. 0.682g of potassium iodate solid was weighed out. It was then made up to 250cm3 using a volumetric flask.
  2. Sodium thiosulphate solution was added to 25cm3 of the iodate solution until the solution turns to pale yellow. Iodide and sulphuric acid was added beforehand. ...

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Spelling, grammar and punctuation were all fine, with the correct technical phrases being used when necessary. All the reaction equations have been shown with experimental data and working out set out in a clear and easy to follow manner.

The writer has demonstrated all the necessary skills expected and it is clear that they understand the area well. The discussion section also adds some further depth to the study and shows that they are able to make evaluative comments based on both current knowledge and experience gained from doing the experiment. The calculations are all correct and lead to the correct conclusion of the amount of vitamin C in one pill. The student could have further discussed possible errors, what their implications are and how they worked to reduce them.

This experimental write-up was writtenin a very logical order, with sub-titles marking all the sections to make it easy to access the information. The objective is clearly expressed at the beginning, and then the student, methodically, answers the question by showing all the phrases of the experiment, with precise explanations of why things happen. One small gripe would be that the final section with the question answers could have had a copy of the questions so it would be easier for the examiner to know whether they answered the question correctly or not.