One of the reasons, people consume beers, wines and spirits is to take advantage of their anxiolytic properties to use the opportunity to relax and “let loose”. However, while moderate consumption reduces stress, an excessive amount leads to a plethora of side effects including high blood pressure – so drink responsibly! Let’s get into what’s in wine and how we analyse it.
Ethanol
All alcoholic drinks contain some percentage of ethanol which has the following structure:
Ethanol has the property of being a versatile solvent that is miscible with water and many organic solvents. Why is that? The presence of the hydroxyl group gives the molecule polar properties allowing dipole-dipole and/or hydrogen bonds to form between ethanol and polar solvents. Conversely, the short hydrocarbon chain provides non-polar properties allowing it to form dispersion forces with other non-polar solvents. You may have heard the term “like dissolves like” which means ethanol can dissolve in both polar and non-polar solvents (Module 7, Alcohols) and it’s all because of intermolecular forces.
Intermolecular forces are forces between molecules such as dispersion, dipole-dipole and hydrogen bonds. Breaking these bonds result in a change in state such as going from a liquid to gas (Module 7, Alcohols). |
We will utilise the chemical properties of ethanol to perform a titration to determine its concentration in wine.
This stepwise oxidation occurs because ethanol is a primary alcohol – meaning the carbon bonded to the hydroxyl is bonded to one other carbon.
It is this property that we can use to determine the concentration of ethanol in samples of wine (Module 7, Alcohols).
Back Titration of Wine
In this case, we can’t perform a simple titration because the wine is already coloured so it would be difficult to determine the endpoint. Therefore, we have to take a sample of wine, react the ethanol with an oxidising agent, and then react the oxidising agent with something else in a very simple titration process.
The method is split into two parts: the oxidation reaction and back titration (Module 6, Quantitively Analysis). The general procedure is that we oxidise ethanol using K2Cr2O7 and take the unreacted K2Cr2O7 and react it with HCl.
Will the titration be accurate? Well, not quite. To sum up, we now know that ethanol is a very versatile solvent which is actually a property that allows it to be miscible in wine. Interestingly, it is also able to undergo oxidation reactions and it is this chemical property that allows us to perform a redox titration to determine the concentration of ethanol. Just by knowing a bit of chemistry, the central science, we’re able to usefully apply our knowledge to an industrial application!
Part 1) Oxidation
Method:
Pipette 20.0 mL of wine into a 100 mL volumetric flask and dilute to volume with distilled water.
Pipette 5 mL of the aliquot into a conical flask and stand in a conical flask of ice and water.
Using a volumetric pipette, add 25 mL of potassium dichromate dropwise. The reaction will generate heat, so it is important that the mixture remains cold to prevent the loss of ethanal vapour.
Place the conical flask in a water bath at 60-65°C and leave sample to react for 45 minutes.
Now we have oxidised all our ethanol with excess potassium dichromate. We take this excess and reacted with another chemical species, as characteristic of back titration.
Part 2) Back titration
The excess dichromate ions are reacted with KI to produce brown I2, which is subsequently reacted with Na2S2O3 in titration. The volume of Na2S2O3 is recorded.
An excess of KI needs to be added to ensure all chromate ions have reacted.
Method:
Add 2 g of KI to the conical flask. The colour of the solution will look brown due to the presence of I2.
Add Na2S2O3 into the burette and titrate the contents of the conical flask. The S2O3^{2-} ions act as a reducing agent, reducing brown I2 into colourless I^-.
Record titre.
Repeat titration until concordant titres are obtained.
Now that we have collected our titres, we need to discard our rough titration and average our concordant titres. At this point, you should know how to do back titration calculations, and the general method with this particular calculation would be:
Calculate the moles of Na2S2O3, then the number of moles of I2 reacted (second equation).
Using stoichiometry, calculate the moles of K2Cr2O7 using the number of mols of I2 (first equation). This is how much Cr2O7^{2-} was in excess.
Calculate the amount of Cr2O72- that reacted with ethanol using:
Then we can work out how many mols of ethanol was in solution (nreacted).
4. Calculate the moles of ethanol pre-dilution.
5. Calculate your concentration using the volume of ethanol in the conical flask.
Discussion
If you perform the calculation and compare your alcohol percentage with that on the wine label it may be different. If the error is large, other compounds within wine may react with potassium dichromate giving you an inaccurate value. We know that other sources of error in titration include overshooting the endpoint, misreading volumes or general faulty techniques – so it’s up to you to familiarise yourself with the process of titration to minimise these human errors.
To sum up, it is possible to determine the concentration of ethanol in wine using its oxidising properties, followed by back titration. It can be a long and convoluted process but if you are able to understand the general principles that goes into this process then congratulations, you now know how to calculate the percentage alcohol in wine!
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