Apparatus
- pH meter
- Density bottle of known volume
- Balance accurate to 3 decimal places
- Distillation apparatus
- Heating Mantle
- Round bottomed flask
- Dropper
- Anti bumping granules
- Burette
- Measuring pipette
- Measuring cylinder
- Beaker
- Volumetric Flask (50ml)
- Glass Funnel
Calibrating the pH meter
In order to calibrate the pH meter, a pH 7 buffer solution had to be made up. This was done by first measuring 100cm3 of deionised water using a measuring cylinder. Then a pH 7 buffer table was placed in this water and allowed to fully dissolve. This creates a pH 7 buffer solution, a solution of pH 7 which is resistant to any changes to pH which might normally occur in a solution.
The instructions for the pH meter were then followed closely in order to calibrate the meter, in order that the pH values it gives are accurate and reliable.
To obtain the % Ethanol against Density Calibration Graph
In order to make a 1% solution of ethanol, 1cm3 of ethanol was measured out using a measuring pipette. A dropper was originally going to be used, but increased inaccuracies in the volumes measured out meant that the more accurate measuring pipette was used instead.
This was made up to a 100cm3 solution by adding 99cm3 of water, measured out using a burette to keep the accuracy of the measurement high. The result is a solution of 1% ethanol.
Some of this solution was then transferred to a density bottle which was then weighed on a balance correct to 3 decimal places, to ensure a high standard of accuracy. From this, the mass of ethanol could be calculated by subtracting the mass of the density bottle while empty from the mass of the density bottle with the ethanol solution.
With the mass of ethanol and volume of the density bottle known, the density of the solution could be calculated using the formula:
Where the mass is the mass of ethanol and volume is the volume of the density bottle.
The above process was repeated for solutions of ethanol up to 20% by adding 1cm3 more of ethanol while reducing the volume of water by 1cm3 at the same time, keeping the overall volumes of the solutions constant but adding 1cm3 more ethanol each time, to create solutions of 1%, 2%, 3% up to 20%. A solution of 0% ethanol was also made up by using 100cm3 of water (ie no ethanol) and calculating it’s density, in order to make the calibration graph more reliable and accurate.
From this, a graph of density against % ethanol solution was created using Microsoft excel, which resembled a straight line.
This experiment was done in duplicate, which meant that each solution of ethanol was made up twice and weighed, in order to gain more accurate results.
A solution of known ethanol concentration was made up and its density calculated as if it were a solution of unknown ethanol concentration, in order to check the calibration graph created is accurate. The density of this “unknown” solution is then compared to the calibration graph and if the graph gives a close enough result to the actual concentration then it can be deemed sufficiently accurate.
To calculate the Ethanol Content of the Wine
50cm3 of wine was measured out using a burette to keep a high degree of accuracy in the volume of the wine.
The wine was made alkaline by adding 0.1M Sodium Hydroxide until the solution reached a pH of 7.5. This solution of Sodium Hydroxide was not needed to be standardised because the actual concentration of the alkali used does not matter, it is just the alkaline effects of the alkali that were needed. Each wine was brought to the same pH before distillation so that it would not affect any results.
The liquid was then transferred to a round bottomed flask and this was connected to the distillation apparatus as shown above. Anti-bumping granules were also added to the liquid before distillation, to ensure the liquid did not distil over, meaning the experiment would have to be restarted.
This liquid was then heated until it reached around 100°C, then heating was stopped and the distillate was transferred to a 50cm3 volumetric flask. This was then made up to the mark with deionised water, in order that the distillate is the same volume as the wine before distillation. This should give a solution of ethanol which has the same concentration as the wine.
Some of this solution was transferred to the density bottle, and its density calculated in the same way as the solutions of ethanol in the first part of the experiment.
This process was then repeated for each wine.
The experiment was also carried out in duplicate, so two 50cm3 samples of each wine were measured out and distilled and their density calculated in order to gain more reliable results as an average of the two could then be taken.
Results
To obtain the % Ethanol against Density Calibration Graph
Volume of Density bottle: 26.650cm3
Mass of Density bottle: 19.702g
Attempt 1
Attempt 2
Taking an Average
Calibration Graph
To Check the Accuracy of the Graph
A solution of 12.5% ethanol was made up and compared to the graph to ensure the graph is sufficiently accurate for reading off ethanol content. This solution was treated as an unknown and its ethanol concentration taken from the graph and compared to the true value.
Mass of density bottle + ethanol = 45.804g
Mass of ethanol = 26.102g
The density of this solution can be found:
This value can now be put into the graph, to find out the ethanol concentration of the solution.
The equation of the line is used, in order to gain the most accurate result.
Where y = Density (g/cm3 ) and x = Ethanol concentration (%)
This is very close to the actual value of 12.5% so the graph can be deemed sufficiently accurate.
Determining the Ethanol Concentration of the Wine
The same process as above was used to calculate the ethanol concentrations of the three wines.
Attempt 1
Attempt 2
Taking an Average
Discussion
Conclusion
This experiment has determined that the Italian and Australian wine has a higher ethanol content than quoted on the label while the South African wine has a slightly lower ethanol content than quoted on the label.
Evaluation
Overall, this experiment can be concluded as being successful, as the duplicate attempts of each part of the experiment are very close to one another, suggesting a high degree of accuracy each time it was repeated.
To ensure this accuracy, the most accurate equipment available was used when measuring out volumes of liquids, such as using burettes instead of measuring cylinders. Burettes were used instead of measuring cylinders when measuring out volumes of liquids because their scale divisions are much smaller, meaning there is a much smaller uncertainty in the volume of the liquid when compared to a measuring cylinder. The smallest scale division which can be read from a burette is 1cm3 , resulting in the uncertainty in the value of the liquid being measured out as
±0.05cm3 , an almost negligible amount when measuring out the quantities of liquid needed for this experiment.
When making up the solutions of ethanol, it was first suggested that the volume of ethanol start at 0.5cm3 and distilled water 49.5cm3 , with the quantity of ethanol increasing by 0.5cm3 each time, while the volume of distilled water decrease at the same rate to alter the ethanol concentration. However, this would have created a much larger uncertainty, as the uncertainty in the ethanol would be 10% it’s total value. To overcome this, all values were doubled, so the smallest volume of ethanol was 1cm3 , which halved the uncertainty to 5% of it’s total volume.
When using the density bottle to weigh the solutions of ethanol and the samples from the wine, it had to be ensured that the density bottle was completely full, so that the solutions took up the total volume of the bottle without any air bubbles. This was to ensure that the mass of the solutions were recorded correctly to keep the experiment accurate. Also, the density bottle had to be completely dried on the outside, and this was done using a paper towel, to ensure that no solution on the outside of the bottle was weighed, which would affect results. The density bottle was also thoroughly rinsed out with the solution about to be weighed, to ensure that the previous solution did not contaminate the solution about to be weighed.
The calibration graph was created using concentrations of ethanol ranging from 0% to 20% in 1% increments. This could have been made more accurate by increasing in smaller increments of ethanol concentration or going to a higher concentration of ethanol, but time constraints meant that this was not a possibility. However, because the points in the density against ethanol concentration calibration graph resembled a straight line, the calibration graph can be seen as mostly accurate.
Overall, because the differences between densities and masses in each attempts of the experiment were quite small, the experiment can be seen as mainly accurate providing meaningful results.
