The purpose of this experiment is to determine the concentration of a solution of sodium hydroxide by titration against a standard solution of potassium hydrogenphthalate.
In a previous experiment you made a standard solution of potassium hydrogenphthalate (Making a standard solution). The substance has the formula C8H5K04, but because it behaves as a monoprotic acid in producing one mole of hydrogen ions per mole of compound, we can simplify the formula to HA. This simple formula is often used to represent an acid with a complicated structure.
Sodium hydroxide reacts with potassium hydrogenphthalate according to the equation:
To show you when the reaction is complete – the stoichiometric point or equivalence point – you use an indicator called phenolphthalein, which is colourless in acid and pink in alkaline solution. The point at which the addition of one drop (or even less) of alkali changes the solution from colourless to just faintly pink is called the end-point and, in this case, shows that the reaction is just complete.
- safety spectacles
- filter funnel, small
- burette, 50 cm3
- 2 beakers, 100 cm3
- sodium hydroxide solution, approx. 0.1 mol dm3 NaOH (CORROSIVE)
- pipette, 25 cm3
- pipette filler
- standard potassium hydrogenphthalate solution, 0.1 mol dm3 (IRRITANT)
- conical flask, 250 cm3
- phenolphthalein indicator solution
- white tile
- wash-bottle of distilled water
- Using the funnel, rinse the burette with the sodium hydroxide solution and fill it with the same solution. Do not forget to rinse and fill the tip. Record the initial burette reading in the ‘rough’ column of the Results Table.
- Using a pipette filler, rinse the pipette with some of the potassium hydrogen phthalate solution and carefully transfer 25.0 cm3 of the solution to a clean 250 cm3 conical flask.
- Add 2-3 drops of the phenolphthalein indicator solution.
- Run sodium hydroxide solution from the burette swirling, until the solution just turns pink.
- Refill the burette with the sodium hydroxide solution, and again record the initial burette reading to the nearest 0.05 cm3 (one drop).
- Using the pipette, transfer 25.0 cm3 of the potassium hydrogenphthalate solution to another clean conical flask. Add 2-3 drops of the phenolphthalein indicator solution.
- Carefully titrate this solution to the end-point, adding the alkali drop by drop when you think the colour is about to change.
- Repeat steps 5, 6 and 7 at least twice more.
- Empty the burette and wash it carefully immediately after the titration, especially if it has a ground glass tap.
You should record burette readings to the nearest 0.05 cm3 (approximately one drop). Consecutive titrations should agree to within 0.10 cm3 and, strictly, you should repeat the titration until this is achieved. However, you may have neither the time nor the materials to do this. With practice, your technique will improve so that it is not necessary to do more than four titrations.
Calculate the mean of the two (or preferably three) closest consecutive readings and quote this also to the nearest 0.05 cm3
Note that this does not introduce a fourth significant figure; it merely makes the third figure more reliable.
- What effect would each of the errors described below have on the calculated value of the concentration of sodium hydroxide?
(a) The burette is not rinsed with the sodium hydroxide solution.
(c) The tip of the burette is not filled before titration begins.
(d) The conical flask contains some distilled water before the addition of potassium hydrogen phthalate.
- In using phenolphthalein as an indicator, we prefer to titrate from a colourless to pink solution rather than from pink to colourless. Suggest a reason for this.
- Why is it advisable to remove sodium hydroxide from the burette as soon as possible after the titration.