A Level Titration Lab Report
A titration is a method of analysis that will allow you to determine the precise endpoint of a reaction and the precise quantity of reactant in the titration flask. A burette is used to deliver the second reactant to the flask and an indicator is used to detect the endpoint of the reaction. So a chemical reaction is set up between a known volume of a solution of unknown concentration and a known volume of a solution with a known concentration
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Method: You will need: * Solution of ‘acid rain’ (dilute sulphuric acid) * Sodium hydroxide solution, 0. 1mol dm-3 * 250cm3 conical flask * 10cm3 pipette and filler * Burette * Phenolphthalein indicator. * Clamp * Funnel * White tile (to help reading endpoint) Firstly prepare your burette, your burette should be filled with 0. 1mol dom-3 of the sodium hydroxide solution. When filling a burette, close the stopcock at the bottom and use a funnel. Check for air bubbles and leaks, before proceeding.
To remove any air bubbles tap the side of the burette while the solution is flowing slowly. This is because if an air bubble is present during a titration the volume readings may be wrong. Take a volume reading and record the volume of solution in a table (see fig 1). Read the bottom of the meniscus. You need to make sure your eye is at the level of meniscus, not above or below because reading from an angle, rather than straight on will result in a parallax error. Use a pipette to transfer 10cm3 of the ‘acid rain’ sample to a 250cm3 conical flask.
Add 5 drops of Phenolphthalein indicator. Use the burette to deliver small amounts of the solution to the titration flask by turning the stopcock. The indicator will change colour when the required amount for neutralisation mixes with the solution in the flask, it will become a permanent pale pink colour. This is the endpoint and should be approached slowly; it needs to be a permanent pale pink for 20 seconds. Carry out several more titrations approaching the endpoint more slowly and watch the colour of your flask carefully.
Rinse the flask with distilled water each time until you reach two volumes that agree to within 0. 1cm3. Record your readings in your table (fig 1) and calculate the volume of sodium hydroxide you have used. The first attempt will be a ‘rough’ titration; it will have gone beyond the end-point and added more sodium hydroxide than is needed to react with the acid in the flask. However this will give you a general idea of what the end-point is.
Results and Calculations: Figure 1. The table shows the results from the titration: | | Accurate| | | Rough| 1| 2| 3| Final burette reading (cm3)| 15. 20| 14. 70| 29. 35| | Starting burette reading (cm3)| 0. 00| 0. 00| 14. 70| | Volume used from burette (cm3)| 15. 20| 14. 70| 14. 65| | Titre: 14. 70cm3 Figure 2. The table shows the calculation of the concentration in the acid rain sample: | SodiumHydroxide| SulphuricAcid| SodiumSulphate| Water| Molar Ratio| 2| 1| 1| 2| Concentration| 0. 10 mol dm-3| 0. 0735 mol dm-3| | | Volume| 0. 0147 dm-3| 0. 010 dm-3| | | Mol| 0. 00147 mol| 0. 000735 mol| | |
The titre is 14. 70cm3 so I was able to calculate in the above table that the acid concentration is 0. 0735 mol dm-3. Evaluation: Sources of procedural errors: 1. *The first drop of titrant that causes colour change of the indicator is hard to judge. Even in the more careful titration (after determining the rough endpoint), the precise endpoint might be missed by a drop or two due to missing the indicator colour change. In our example this might give higher acid concentration than the real concentration. . Mistake in reading the volume in the burette due to misreading the meniscus. (parallax, reading the volume at an angle), this can cause over or underestimation of the analytic concentration.
Air bubbles in the burette tap, might lead to accidently adding more NaOH as the titrant shoots up (this leads to overestimation of the acid concentration). 8. Unclean glassware contaminated from previous experiments. 9. Not transferring all acid solution (shaky hands leading to loosing part of the solution while transferring and thus underestimating acid amount). 10. Losing solution from titration flask before the end point due to too vigorous swirling.
Leaking burette – giving at the endpoint higher reading than what was really used (overestimation of the acid). I have identified that 1 and 3 of the procedural errors would have the most impact on my overall results because firstly I rinse the flask with distilled water not H2SO4 and secondly it was first my titration and do not have the experience to judge the exact required shade of pink.