Practical investigations
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I'm investigating methods of determining Vitamin C in juice drinks. I've researched titrations using DCPIP and NBS but I need another method for comparison. I think I could use a colorimeter (like industry) but can't find any references. Can you point me in the right direction? Thank you.
Igloo writes ..
The conventional way of using colorimetry to determine vitamin C concentration is to make use of its reaction with iodine. The colour of iodine varies from dark brown to pale yellow in aqueous solution (the colour depends on the concentration of iodine), so a colorimeter can be used to measure its concentration quite accurately.
First, though, the colorimeter has to be calibrated with iodine solutions of known concentration. To do this you need to make up a standard solution of iodine in aqueous potassium iodide (in which it is fairly soluble). Having done this, the stock solution can be carefully diluted using a pipette and volumetric flask to provide you with several other more dilute solutions for which you can calculate the concentrations of iodine, [I2], Successful use of a colorimeter depends on an appropriate choice of filter, so you will use various filters in turn, measure the transmittance with the stock solution of iodine and that with the lowest concentration, to see which filter gives the greatest range of values.
Next, having measured the transmittance of each of the iodine solutions using the chosen filter, a plot of transmittance against the concentration of iodine, [I 2], needs to be drawn. This is the calibration curve.
Next a portion of the juice drink should be added to a known volume of the original stock solution of iodine (which must be in excess), and then the colorimeter can be used to measure the transmittance of the mixture (which of course now contains the iodine at a lower concentration). The calibration curve can be used to read off [I2], and a calculation can be carried out to determine the quantity of iodine destroyed by the vitamin C, hence the concentration of vitamin C in the juice. Finally, knowing the original volume of extract, the quantity of vitamin C present in the sample of juice used can be calculated.
If the juice is in any way coloured, you might like to try stirring the extract with a spatula measure or two of activated charcoal. After stirring, this mixture can be filtered, and hopefully the filtrate will be less coloured than it was before. Do not heat the mixture, otherwise you risk accelerating the breakdown of vitamin C. After this decolourisation procedure you can carry out the procedure with the colorimeter as mentioned above.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 29 January 2008
