Investigation to find out if vinegar from chip shops is watered down

To find out if samples of vinegar have been watered down I’m going to use the titration method. First I’ll test vinegar that hasn’t been watered down. I’ll do this by adding (triturating) a solution of known concentration to the vinegar solution to determine its concentration. The solution I’ll be adding from a burette into the vinegar will be an alkaline, as seen as the vinegar is an acid. An indicator (phenolphthalein) will show when the solutions reach neutralisation because it will turn them magenta in colour.

The reaction that I’m using is [H + OH H O] neutralisation

By doing this you will be able to see how much alkaline it needed to neutralise the solution, therefore how much hydrogen ions was present in the vinegar. Then I will do the same test on watered down vinegars and then I will be able to compare these results to it.

For the alkaline I will use sodium hydroxide with the concentration of 0.5m. I have decided to use sodium hydroxide with the concentration of 0.5m by doing some preliminary work and I have picked this one because the concentration at 0.1 took too long to neutralise the vinegar and I wouldn’t have time to use this and the concentration at 1.0 neutralised the vinegar after a few drop therefore the results would have been too inaccurate. So I went with the one in the middle at 0.5m which neutralised the vinegar in a reasonably amount of time.

The titration technique for carrying out this neutralisation reaction is very accurate; this is because the apparatus used is calibrated to give a high degree of accuracy in the measurements made. I will do a rough titration for all 4 vinegar types before hand, this will give me a rough idea of the volume of acid needed to neutralise the acid. So I could quickly run alkaline in, mixing all the time until I get close to the rough volume then I’ll add the alkaline drop wise, mixing after each drop until I reach the end point. This will make it more accurate. Also I will measure the volume to the smallest scale possible (to 0.1mm�).

The results will be reliable because I’m doing them 3 times so if there were any anomalies they could be spotted and not used when averaging the results for each.

I will be using colourless vinegar so I can see easily when the solutions neutralises because the phenolphthalein indicator will turn the solution magenta in colour when it does.

I predict the more watered down the vinegars are the less alkaline (sodium hydroxide) would be needed to neutralise it. This is because the more water added, the less concentrated it would be and therefore there would be less hydrogen ions in the same volume. So less hydroxides ions will be needed to neutralise them.

Method

First clean the equipment that I am going to use to ensure that no other chemicals are left on them that could affect the experiment. I will do this by swilling water round them then swilling the solution that’s going to go into the equipment. I will use a burette to add the sodium hydroxide (alkaline) into the vinegar (acid).

To fill the burette:

1. Make sure the tap is closed!

2. Place an empty beaker underneath.

3. Use a funnel when filling the burette.

4. Pour in a little sodium hydroxide solution at a time; don’t fill the funnel up.

5. Fill to just above the 0.00ml line.

6. Remove the funnel.

7. Open the tap and run the liquid out very slowly until on 0.00ml

With the 25cm� pipette measure out 25cm� of each 4 types of vinegar including the vinegar that isn’t watered down, and put them in 4 separate conical flasks and label.

To use the pipette;

1. Put the pipette filler on the end of it

2. Squeeze the button marked 1 on the pipette filler to push all the air out

3. Submerge the tip of it into the vinegar solutions,

4. Squeeze button marked 2 to suck the solution up to the brown line

5. Squeeze the button marked 3 to expel the vinegar into a conical flask

Into the vinegars put 2-3 drops of phenolphthalein indicator. Put a conical flask with a vinegar solution in under the tip of the burette on the stand. I’ll also put a white tile under the conical flask, so that the colour changes can be seen more easily. The conical flask can be held by the neck and swirled easily to mix the two solutions; the sloping sides of the flask prevent the liquid from splashing. The flask will be washed out thoroughly in between titrations. Adjust the tap of the burette with your left hand and mix the flask with your right hand. Keep adding the sodium hydroxide from the burette until the volume gets close to the rough titration then add in the sodium hydroxide slower until the solution turns magenta in colour (the end point). Take the reading from the burette to give the volume of alkaline added and put in on the table. I will do the same for all the vinegar solutions.

To ensure that I’m carrying the experiment safely I will be careful with all the equipment, as they are all delicate and expensive to replace. I have done preliminary work so I have already used the equipment so I know how to use them properly. I have chosen the correct equipment for the job they need to do so everything should work and go as planned.

Apparatus & chemicals

Burette, Sodium hydroxide,

4 conical flasks, 3 types of colourless vinegars

Beaker, from different shops,

Pipette, Original colourless vinegar that hasn’t

Pipette filler, been watered down,

Burette stand, Phenolphthalein indicator,

White tile, Funnel,

Fair Test

1. Use same volume of vinegar.

2. Use same concentration of sodium hydroxide of 0.5m for the alkaline to neutralise the vinegars.

3. Use clean equipment as not to affect the chemicals that I’m using.

4. Use the same equipment.

5. Use the same method.

6. The only variable will be the vinegar type, everything else stays the same.

7. Compare all the results to the original vinegar type.

Obtaining evidence

During titration the colour change can be rapid to counter this I have used a weak concentration of sodium hydroxide, which is why I used a concentration of 0.1 mol rather than 0.5 mol. This is another limitation in the procedure

I repeated each experiment 3 times to show of any anomalies and to get an average this made my investigation reliable. My results are accurate to 1 decimal place. I carried my experiment out exactly as explained in the plan, keeping it the same for each experiment as to be fair and also give accurate results.

I ensured I carried out a safe experiment, by firstly doing preliminary work. Doing this I knew how everything would work and that they would work according to my plan. Also I learnt how to use the apparatus properly and safely. To use them like stated in the plan and not force them to do anything as they are made out of glass and are delicate. The equipment was suitable to help me carry out my experiment and get the results I wanted.

Analysing and concluding

There is no trend in the results because different vinegars were used from random chip shops. You can see by my results that I found out that all the vinegars from the 3 different shops have been watered down and by different amounts, by comparing them to the original vinegar. I’ve found out that the shops cheat you by watering down their vinegar to make it last longer. Shop C has added the most water, 76.2% of their vinegar is in fact water. Then Shop A with 59.2% of their vinegar being water and Shop B added the least water to their vinegar of 12.2%.

I predicted that the more watered down the vinegar, the less alkali (sodium hydroxide) would be needed to neutralise it. This is backed up by my experiment, because vinegar C has got 76.2% added water and this only took 4.5cm� of alkaline to neutralise it comparing to the original vinegar that took 18.9cm� of alkaline to neutralise it. This occurs because the more water added, the less concentrated the vinegar would be and therefore there would be less hydrogen ions in the same volume. So the less hydroxides ions will be needed to neutralise them. Meaning that the vinegar watered down the most, with the least hydrogen ions in it, would need the least sodium hydroxide, for the neutralisation reaction to occur because less hydroxides ions would be needed to neutralise the hydrogen ions.

The results shown in this investigation are true and are believed to be as accurate as can possibly be attained, you can tell this because they are all give very close results from the others, only up to 0.4cm� away from each other at the most.

Evaluating

I feel that my experiments went well and was suitable for the task given. This is proven by the fact that I achieved consistency and proves that this experiment can be both reliable and accurate. I got mostly identical titres or ones that was only 0.1cm� to 0.4cm� off. Also by choosing relatively accurate instruments I minimise error through instrument limitations; this is because the pipette, and burette used is calibrated to give a high degree of accuracy in the measurements made So any error through equipment is minimal, yet it still has to be taken into consideration.

I only feel I have 1 anomaly and that on the shop A the first accurate triturate. I think this is a anomaly because it is more than the rough triturate and its meant to be less because its rough therefore I would of probably added more alkali than needed. Also the other results from that experiment was 7.6cm� and the anomaly is 0.7cm� higher than this, so I just scrapped that result as not to affect the accuracy of the average result.

I feel that there was little limitation in the procedure and any limitation was minimal. An example of this is the minute splashes that occur in the conical flask during the titration, which some of it gets on the walls of the conical flask.

Some errors are likely to come from measurements. A source of error would be parallax error, which would involve the meniscus. The parallax error would be present in the burette and pipette. This error must also be taken into consideration. Another form of human error would be the judgement in colour change. In titration it is impossible for someone to judge the solution during the end point of the indicator to be the exact same colour every time with just the naked eye. This is another limitation in the procedure. During titration the colour change can be rapid to counter this I have used a weak concentration of sodium hydroxide, which is why I have used a concentration of 0.1 mol rather than 0.5 mol.

The experiment can be improved by using a computer, camera or keep another neutralised solution to compare the colours with. If the computer could hold the colour of the first titration then one could do the second titration while looking at the computer screen, so all the titrations would be stopped at exactly the same colour. This would totally reduce the human error when it comes to the colour change of the indicator, making the results more fair, accurate. Another way I could have improved the experiment was if I was to use a larger quantity of vinegar for the titration and, as a whole would reduce the percentage error.

I feel that I collected enough evidence to make me confidence about my conclusion, but next time I could test more vinegar from different shops to give me a bigger range and variety.