Practical investigations
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For my chemistry coursework I want to investigate the solubilities of metal halides, probably using potassium. I know that I need to start by making a saturated solution, but will these substances dissolve in water? I am then thinking of titrating the solution with silver nitrate. What would be a suitable indicator? Also, how much of each substance should I use? I have to plan my experiment but I am unsure about the quantities! Many thanks.
Igloo writes ...
You are correct in thinking that you need to use saturated solutions, and yes, all the potassium halides are soluble in water. Titrations with silver nitrate are appropriate, but since you are dealing with a highly expensive reagent, the standard solution you make up should be dilute, e.g. of concentration 0.05 mol dm-3. As a result you will need to dilute your saturated solutions by a known factor in order to be able to obtain sensible titres. Why? Potassium chromate(VI) is the usual indicator, and if you’re unsure of the theory behind these titrations I suggest you use www.google.co.uk, with “silver nitrate titrations” as your search phrase and you should find several relevant and helpful references.
As for recommending “suitable quantities” this is all a matter of trial and error. Typical solubility values for group I halides are in the region of about 2-5 mol dm-3 at room temperature, so, having made some of your filtered saturated solution, you will need to dilute this about 50-100 times (using a pipette and volumetric flask) before the titration with silver nitrate solution. Finally, having calculated the concentrations of these diluted solutions, the values will need to be scaled up by the dilution factor in order to calculate the solubility of the original saturated solution, hence the solubility.
Do remember that it is vital that the silver nitrate solution is made up in de-ionised water. I hope you know why!
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updated: 13 December 2006
