Apparatus:
Burette Sodium Hydroxide (0.1 M)
Pipette Various Indicators
Beakers Various Vinegars
Conical Flask Deionised water
White Paper Volumetric Flask
Glass Stirring Rod
Funnel
Preliminary Experiment
Preliminary Experiment (choosing indicators):
We have been given two Indicators to try, they are Screened Methyl Orange which changes colour at pH 3.7 and Phenolphthalein which changes colour at pH 8.4. I have to plan an experiment to find out which is the best indicator to use in the titration experiment.
An indicator is used to identify the end point of a titration. The end point is when the correct number of moles have been added for complete reaction. N.B. In Acid/Alkali reactions the pH may not be 7. An indicator changes colour as the pH changes. Different indicators change colour at different pH. Thus different indicators are needed.
Strong Acid : Strong Alkali - Any Indicator
Weak Acid : Strong Alkali - Phenolphthalein
Strong Acid : Weak Alkali - Methyl Orange
Weak Acid : Weak Alkali - Very difficult
Preliminary Exp. prediction:
I predict that the best indicator to use will be the Phenolphthalein indicator. I Predict this because if we look at the table above a weak acid reacting with a strong alkali is easiest to see with Phenolphthalein. We are dealing with very dilute amounts of acid and strong alkali in this experiment, so following this table it should be Phenolphthalein that works best.
Method:
1) Take 4 beakers and label them 1,2,3 and 4.
2) We know that there is roughly 1M acetic acid (CH3 СOOH) in the 2 vinegars. This means to make the experiment fair we have to dilute it by a factor of 10 as the NaOH solution is 0.1M. Take a sample of both the colourless and brown vinegar. Add 10cm3 of each vinegars into separate measuring cylinders. Once this has been done, add 90cm3 of deionised water to each of the measuring cylinders.
3) Now the colourless and brown vinegars have been diluted, add 10cm3 of the diluted colourless vinegar to beakers 1 and 2 and 10cm3 of the diluted brown vinegar to beakers 3 and 4.
4) Add a few drops of Methyl Orange Indicator to beaker 1 and 3, and add a few drops of Phenolphthalein Indicator to beakers 2 and 4.
5) As we will not have performed the titration experiment yet, we will not know how much NaOH is needed to change the colour of the indicator. Pour some of the NaOH solution into each of the four beakers, containing the diluted vinegars.
6) I cannot swirl the solution as I will be able to in the main experiment because I am not using a conical flask. Instead I will have to stir the mixture with a glass rod. When a colour change has been observed, stop adding the NaOH and record the colour change.
7) When the colour change has been observed and recorded, dismantle and wash out the apparatus carefully.
Preliminary Results:
Conclusion:
As I predicted, it was much easier to determine the end point with the Phenolphthalein Indicator than with the Methyl Orange. It was a much sharper change with the Phenolphthalein indicator and it took longer to determine the end point with Screened Methyl Orange Indicator. This means that in the titration experiment it will be much clearer when I have reached the end point using Phenolphthalein, giving me more accurate results. It also changed at pH 8.4 which means this is definitely the right indicator to choose.
Prediction:
I already know there is roughly 5% acetic acid in commercial vinegar so hopefully after the experiment has been completed and all calculations carried out, I should have roughly 5% concentrate of acid. If I get nothing like 5% I know I have anomalous results and either the experiment will have been carried out incorrectly or the solutions are contaminated.
As I have already mentioned, the aim of this experiment is to find the acid concentration of different vinegars in ( mol/dm3, g/dm3 and % ). I need to plan a simple, safe procedure before I can begin.
Method:
Safety Goggles must be worn!
1) Collect all the apparatus that I will be using in this experiment, beakers, pipette, conical flask, burette, sodium hydroxide (0.1 M), phenolphthalein indicator, clear & brown vinegar, deionised water, volumetric flask, white paper, funnel, clamp.
2) Measure out 10 cm3 of either the clear or brown vinegar using a pipette, and empty it into a 100 cm3 volumetric flask and fill up the rest of the flask to the line with deionised water. We know that there is roughly 1M acetic acid (CH3 СOOH) in the 2 vinegars. This means to make the experiment fair we have to dilute it by a factor of 10. as I did in the preliminary experiment, as the NaOH solution is 0.1M.
3) Rinse the pipette through with the new solution of water and vinegar and extract 10 cm3 from the volumetric flask and place it into the conical flask, then add a few drops of phenolphthalein indicator.
4) Clamp the burette vertically with the tap closed, place a funnel in the top of the burette and fill with (0.1 M) sodium hydroxide. When full, let a bit of the solution out into a beaker until the bottom of the meniscus is level with the 0.0 mark on the burette.
5) Place the conical flask with the vinegar and water solution in, under the burette with a piece of white paper under the flask to see the colour change more clearly.
6) Slowly let the NaOH solution into the flask, swirling the flask around as this is done. When enough NaOH has been added the solution should start to change purple. When the solution changes purple and does not change back again, turn the burette tap off and record the amount used. It must be taken into account that all the readings must be taken from the bottom of the meniscus to produce a fair experiment.
7) Record the results in a table and repeat the experiment until consistent results are achieved, using the same solution to make it fair.
Diagram:
Results:
Brown Vinegar:
Rough
If we look at the results collected, they are fairly consistent which is a good thing. The first one we do not really need to count as it is a rough result and may not be very accurate. There are 2 results of 8.4 cm3 so this looks like a good result to use in the calculations.
NaOH - 0.1M : 8.6cm3
CH3СOOH - ?M : 1.0cm3
CH3COOH (aq) + NaOH (aq) —> NaC2H3O2 (aq)+ H2O
N.o mole of NaOH = MV
1000
= 0.1 * 8.4
1000
= 0.00084 mol
N.o mole of CH3СOOH = MV
1000
M = N.o mole of CH3СOOH * 1000
V
= 0.00084 * 1000
1
= 0.84 mol dm-3
Concentration of CH3COOH = ( Mass of 1 mole of CH3COOH ) * N.o of mole of CH3COOH
= ( 12 + 3 + 12 + 16 + 16 + 1 ) * 0.84
= 60 * 0.84
= 50.4 g/dm-3
CH3СOOH percentage in brown vinegar = 50.4 * 0.10
= 5.04 %
Clear Vinegar:
Rough
NaOH - 0.1M : 7.6cm3
CH3СOOH - ?M : 1.0cm3
N.o mole of NaOH = MV
1000
= 0.1 * 7.6
1000
= 0.00076 mol
N.o mole of CH3СOOH = MV
1000
M = N.o mole of CH3СOOH * 1000
V
= 0.00076 * 1000
1
= 0.76 mol dm-3
Concentration of CH3COOH = ( Mass of 1 mole of CH3COOH ) * N.o of mole of CH3COOH
= ( 12 + 3 + 12 + 16 + 16 + 1 ) * 0.76
= 60 * 0.76
= 45.6 g/dm-3
CH3СOOH percentage in clear vinegar = 45.6 * 0.10
= 4.56 %
Key:
* = multiply
Evaluation:
This experiment seems to have been quite accurate, so the equipment used was a good choice. No anomalous results seem to have been achieved which means it is safe to say that the calculations should be quite accurate. If I were to repeat this experiment I would take more readings to get a wider range of results to take an average over. This would give me much more reliable results.
The one thing that was the hardest to measure was the end point. It is very difficult to accurately determine the exact end point. One way of combating this could have been by using a pH probe to see when the pH changes. The amount of vinegar in commercial vinegar varies form brand to brand, so it is not possible to say, on my findings, that every brown vinegar contains 5.04 % acetic acid and all clear vinegar contains 4.56 % acetic acid.
Conclusion:
As I predicted, in both cases, clear and brown vinegar, I achieved a result that was fairly close to 5% concentration of acetic acid. The results that I achieved were very small amounts which is one of the reasons why titration had to be used. It was also very important to be very accurate because the slightest degree in accuracy could have created anomalous results. This experiment seems to have been fairly successful, with consistent results achieved, and finding around 5% acetic acid in the vinegar as predicted earlier. I don’t appear to have got any anomalous results which means they are good enough to make a reasonable conclusion on.
