Practical investigations
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I am doing my A-level chemistry project on the chemistry of wine. I understand the majority of the theory, but I am however confused on some aspects.... whilst I was researching for the theory behind how to determine the sulphur dioxide content of wine (by adding iodine, and starch indicator etc) I was confused to find that you must first standardise the iodine solution. What is this and how do you do it? Thanks.
Igloo writes ...
Iodine is not a "primary standard".
In other words, it is not easily obtained in a 100% state of purity. When a solution containing 0.10 g dm-3 is made up by weighing out the appropriate mass of solid iodine and making this up into 1 dm3 of solution, the actual concentration will undoubtedly be lower than predicted. As a consequence it is necessary to check its true concentration via a titration with a primary standard.
Fortunately, sodium thiosulphate, with which iodine reacts, can be obtained in a high state of purity, so that solutions of known concentration can be made up.
The procedure is therefore as follows:
Make up your solution of iodine to the approximate concentration required.
Make up a solution of sodium thiosulphate to a known concentration.
Carry out a sodium thiosulphate-iodine titration.
Use your results to calculate the true concentration of the iodine solution.
Use this (standardised) solution with your wine samples to determine the sulphur dioxide concentrations.
Always carry out a risk assessment and check with your teacher before starting any practical work.
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.
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updated: 17 January 2006
