Monday, June 28, 2010

The titration method

The titration sequence of events:
1. Rinse a conical flask with water.
2. Rinse the pipette with the HCl solution and then pipette, a 25 mL sample of the hydrochloric acid and place it in a conical flask, (an aliquot). Repeat so that you have 3 conical flasks set up and ready to go.
A pipette is a piece of apparatus which accurately delivers a given volume -before filling it is always rinsed with a small amount of the solution.
3. A few drops of an appropriate acid-base indicator, in this case phenolphthalein, is added to the flask. The solution will remain colourless.
4. A burette is then rinsed with a sample of sodium hydroxide (the solution it is to be filled with) and then filled to just below the 0.00 mL mark
A burette is another piece of glassware that accurately measures out a volume of liquid.
5. The initial volume of aqueous NaOH in the burette is carefully read (to 2 decimal places - giving an accuracy of ± 0.02 mL).
6. Carefully add the NaOH to the aqueous HCl, finishing the titration as soon as the first permanent pink colour is observed.
The colour change is referred to as the end-point, and assuming the correct indicator has been used, is the point when the acid and base have reacted in the molar ratio given by the balanced neutralisation equation.
7. By taking the difference between the initial and final volumes in the burette, the total volume of NaOH(aq) added can be carefully determined.
8. The titration is repeated at least 3 times, or until you have obtained 3 concordant results i.e. burette volumes (titres) that, ideally, agree to within ± 0.2 mL.

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