The purpose of this experiment is to determine the total and volatile acidities of each of the wines and compare them.
Acidity is a major contributor to the taste of wines. This is especially important in white wines, because there are very little tannins found in it, so acidity can affect the taste of the wine much more than in red wines.
In this experiment, 0.1M Sodium Hydroxide solution is needed for titrations. Because this is not a standard solution, it is first standardised using oxalic acid.
In order to calculate the total acidity of the wine, a titration with 0.1M sodium hydroxide is carried out with a pH meter. The pH of the wine and sodium hydroxide solution is measured when a certain volume of NaOH is added each time, and a titration curve of volume against pH is plotted. The volume for the solution to reach a pH of 8.2 is recorded. This is because NaOH is a strong alkali and wine is a weak acid, so the pH lies more to the side of the alkali. A pH of 8.2 as the equivalence point is a value agreed on by winemakers.
In order to calculate the total acidity of the wine, a representative acid must be used. This must be chosen because wine contains multiple different acids, which require different moles of NaOH to neutralise them. Tartaric acid was chosen as the representative acid for the wine, because it is thought to be the most abundant. Tartaric acid requires two moles of NaOH for it to be neutralised.
In order to calculate the volatile acidity of the wine, a sample of wine is evaporated using a steam evaporator, then made up to the original volume with deionised water and this process was repeated. The volatile acids evaporate away, while the remaining acids constitute what is known as the fixed acidity of the wine. This is made up to the original volume with deionised water and titrated with NaOH as before to give the acidity of this solution, which is known as the fixed acidity of the wine.The representative acid used here will again be Tartaric acid. In order to calculate the volatile acidity of the wine, this value is subtracted from the value calculated for the total acidity in part one of the experiment.
* Oxalic Acid
* Sodium Hydroxide
* 3 white wines from different regions
* Deionised Water
* Phenolphthalein Indicator
* Steam Evaporator
* Evaporating Basin
* pH Meter
* One Litre Volumetric Flask
* 25cm3 Volumetric Flask
In order to standardise the sodium hydroxide solution, solutions of both oxalic acid and sodium hydroxide had to be made up.
To Make up Oxalic Acid solution
In order to make up the oxalic acid solution, 1.575g of oxalic acid was weighed out and dissolved in 100cm3 of deionised water. This was then made up to 250cm3 , so the resulting solution had a concentration of 0.05mol l-1 .
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.