Practical investigations
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We're doing the investigation of copper concentration in aqueous solution by iodometric titration. I was wondering why do we have to titrate the Cu(II) sol'n immediately after the addition of KI? Also, why do we have to delay the addition of starch until the endpoint is nearly approached?
Ulex replies
I think the only significance of the ‘immediately’ component of the instructions is to indicate that you don’t have to do anything to the mixture before titrating it.
The addition of starch is delayed because if the starch suspension is added to a high concentration of iodine some of the starch precipitates and the individual starch grains adsorb iodine giving black specks suspended in an almost colourless solution. This is very difficult to titrate accurately to an end-point. I encourage students to try (once!) adding the starch before titration just to see what does happen!
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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: 09 January 2007
