To Make up 0.1M Sodium Hydroxide solution
Because the formula mass of sodium hydroxide is 40g, 4g of NaOH was weighed out, and dissolved in 100cm3 of deionised water. This was then made up to one litre, in a volumetric flask, and the resulting solution should have a concentration of 0.1mol l-1
Standardising the Sodium Hydroxide solution
In order to standardise the NaOH solution, 20cm3 of Oxalic acid was pipetted into a volumetric flask and a few drops of phenolphthalein was added.
Sodium Hydroxide was then put into a burette, and the a titration was carried out until the phenolphthalein changed colour from colourless to pink, as this indicates the end point of the titration. A rough titre was carried out, and a further two titrations were carried out, in order to have concordance in the volumes ie two volumes within 0.1cm3 of each other.
To determine Total Acidity
1 – pH meter 2 – Burette 3 – Magnetic stirrer 4 – Stands 5 – Beaker with wine
100cm3 of wine was measured out using a burette and placed into a beaker, along with a magnetic stirrer. This beaker was placed onto a magnetic stirring plate and the plate was switched on, causing the magnetic stirrer to spin.
A burette was filled with NaOH and the pH of the wine was recorded before and had been added.
At the start of the titration, the pH was read after every 3cm3 of NaOH was added, but as this came closer to the equivalence point of pH 8.2 this was read more often.
A titration curve of volume against pH could then be plotted and the volume for the wine to reach a pH of 8.2 was determined. Using this and Tartaric Acid as the representative acid, the total acidity of the wine could be calculated, in mol l-1 , with regards to the Tartaric acid.
To determine Volatile Acidity
1 – Steam Evaporator 2 – Evaporating basin with wine
25cm3 of wine was measured out using a burette then placed in an evaporating basin. This evaporating basin was then placed on the steam evaporator and the wine was evaporated until very little remained.
This was topped back up to 25cm3 with deionised water, and this process was repeated twice, to ensure that all the volatile acids had evaporated away.
After the final evaporation, the remaining liquid was again made up to 25cm3 with deionised water, ensuring that the evaporating cylinder was thoroughly washed out, to ensure that all of the liquid was transferred.
This liquid was then transferred to a beaker, and a magnetic stirrer added, and titrated as above, using the pH meter to record pH against volume.
From this, a pH against volume titration curve could be plotted and from this the fixed acidity of the wine could be calculated. The volatile acidity of the wine could then be calculated by subtracting the concentration of the fixed acids from the concentration of the total acids.
Results
Standardising the Sodium Hydroxide
Rough Titre = 20.8cm3
1st Titre = 20.9cm3
2nd Titre = 20.8cm3
Average Titre = 20.85cm3
The number of moles of oxalic acid now needs to be calculated:
Because Oxalic acid is diprotic, one mole of oxalic acid neutralises two moles of sodium hydroxide, so 1x10-3 moles of acid neutralises 2x10-3 moles of sodium hydroxide.
This means that there is 2x10-3 moles of NaOH on 20.85cm3 , so the concentration of the solution can now be calculated:
This is the actual concentration of the NaOH solution and this value shall be used in future calculations.
Total Acidity
Australian
Italian
South African
Calculations
Calculating the Total Acidity of Each Wine
Australian
As can be seen from the results, it took 89cm3 to bring the wine to the equivalence point of pH 8.2. The number of moles of NaOH needed for this is given by:
Tartaric acid is now used as our representative acid in the wine.
Since 2 moles of NaOH reacts with 1 mole of Tartaric Acid, 8.535x10-3 moles of NaOH will react with 4.268x10-3 moles of Tartaric Acid.
Therefore there are 4.268x10-3 moles of Tartaric Acid in 100cm3 of wine, so the concentration of acid in the wine can be calculated.
Italian
It took 88cm3 of NaOH to reach the equivalence point of pH 8.2 so the number of moles of NaOH needen for this is given by:
Therefore 8.439x10-3 moles of NaOH react with 4.220x10-3 of Tartaric Acid in 100cm3 of wine.
South African
It took 79cm3 of NaOH to reach the equivalence point of pH 8.2so the number of moles of NaOH needed for this is given by:
Therefore 7.576x10-3 moles of NaOH react with 3.788x10-3 moles of Tartaric Acid in 100cm3 of wine.
Volatile Acidity
Australian
Italian
South African
Calculations
Calculating the Volatile Acidity of Each Wine
Australian
It took 19.7cm3 of NaOH to reach the equivalence point of pH 8.2. The number of moles of NaOH needed for this is given by:
Since 2 moles of NaOH reach with 1 mole of Tartaric Acid, 1.889x10-3 moles of NaOH react with 9.445x10-4 moles of Tartaric Acid in the 25cm3 solution of wine after evaporation, the fixed acidity concentration of the wine is given by:
This is the fixed acidity of the wine, and the volatile acidity is given by subtraction:
Volatile Acidity = Total Acidity – Fixed acidity = 4.286×10-2 – 3.778×10-2
=5.08x10-3 mol l-1
Italian
It took 18.5cm3 to reach the equivalence point of pH 8.2, so the number of moles NaOH needed for this is given by:
Since 1.774x10-3 moles of NaOH reacts with 8.87x10-4 moles of Tartaric Acid in the 25cm3 solution of wine after evaporation, the fixed acidity concentration of the wine is given by:
The Volatile Acidity of the wine is now given by:
Volatile Acidity = Total Acidity – Fixed Acidity
= – 3.548×10-2
= 6.72x10-3 mol l-1
South African
It took 17.1cm3 of NaOH to reach the equivalence point of pH 8.2. so the number of moles of NaOH needed for this is given by:
Since 1.640x10-3 moles of NaOH reacts with 8.20x10-4 moles of Tartaric Acid in the 25cm3 solution of wine after evaporation, the fixed acidity concentration of the wine is given by:
The Volatile Acidity of the wine is now given by:
Volatile Acidity = Total Acidity – Fixed Acidity
= 3.788×10-2 – 3.280×10-2
= 5.08x10-3 mol l-1
Summary of Results
Fixed Acidity Volatile Acidity
Australian 4.286×10-2 mol l-1 5.08x10-3 mol l-1
Italian 4.220×10-2 mol l-1 6.72x10-3 mol l-1
South African 3.788×10-2 mol l-1 5.08x10-3 mol l-1
Discussion
Conclusion
This experiment has determined that the Australian has the highest overall acidity compared to the Italian and South African wines, and the Italian wine has the highest volatile acidity compared to the Australian and South African wines. The South African wine had the lowest total acidity of the three, while both the Australian and South African had the same lower volatile acidity compared to the Italian wine.
Evaluation
In order to top up the wine in the evaporating basin in the second part of the experiment back up to 25cm3 , the only way that was found viable, due to the small quantities of wine left after evaporation, was to mark a line at 25cm3 on the basin, and add deionised water until the solution hit that line again. This might not be very accurate, but it shouldn’t matter very much, because the deionised water, along with the volatile acids, will evaporate away again, and the same number of moles of fixed acids will remain in the basin. In order to top the volume up to 25cm3 for the titration, a 25cm3 volumetric flask was used, as it was decided that this was needed to be as accurate as possible, to ensure the concentration value obtained is correct.
During each titration, the pH was read less frequently at the start of the titration, and more frequently when the pH approached the equivalence point of 8.2. this was done to ensure that the volume at pH 8.2 could be read as accurately as possible, and this was achieved by adding sodium hydroxide in 0.5cm3 around pH 8.2, and in 0.1cm3 , the smallest volume a burette can read, when the pH was around 8.
The standardisation of the sodium hydroxide also helped to improve accuracy, because after making the solution of NaOH up, it could just have been taken to be a 0.1M solution of NaOH. Since there are always likely to be errors in the making up of the NaOH, it was decided best to standardise it, and this was done with Oxalic Acid, a primary standard, in order to ensure that the exact concentration of the NaOH was known.
