A guide to our Lab Work

Preparation of Standard solutions

  • Standard solutions were prepared in 50cm3 volumetric flasks labelled from A to F. All solutions contained 5cm3 of 5% Ethanoic Acid, 5cm3 of 2% KI and varying amounts of Diluted Milton Reagent as outlined in Table

  • Samples of these solutions were placed in cuvettes and placed in the Colorimeter; the absorbance values were read and a calibration curve produced.

  • At this stage we proceeded to extract the Iodide from the food samples.

    • Analysis of Iodide in Food Samples.

    1. Removal of interfering Pigments.

  • From this list it is obvious that some of the samples (Broccoli, Carrots, Kelp, Cauliflower) contained pigments that would interfere with the identification of the Iodide pigment, as a result we removed these interfering pigments by extracting them in Methylated spirits, using the same procedure used to extract Chlorophyll from a green leaf. We felt that this procedure would not affect our overall analysis, since the iodide ion I- is polar and it would not be extracted in the non-polar solvent Methylated spirits. (Likes dissolve likes).

  • 2. Extraction of Iodide from food Samples.

  • Using the principle of Likes dissolves Likes as used above we extracted the Iodide from the food samples using water. Solid food samples were liquefied using a food processor and water was added (in known quantities) to improve the texture of the sample to make it suitable for reading using the colorimeter.

  • Volumetric Analysis to determine the concentration of Diluted Milton Solution.

  • In order to determine the precise concentration of the hypo chlorite ion (diluted Milton Reagent) we performed a volumetric analysis using a 0.12M standard solution of sodium thiosulphate. The reaction occurs according to the following equations:

    • ClO- + 2I- + 2H+ goes to Cl- +I2 + H20

    2 S203- + I2 goes to S406 - + 2I-

  • The end point was determined using a starch indicator, which was added to the conical flask when the solution turned a straw yellow colour; on addition of the starch the contents of the flask became a blue/black colour. The titration was continued until the blue/black colour disappeared.

    •