Practical investigations
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1. Why does wine have to be distilled in order to determine the alcohol content, instead of simply measuring the density of the wine?
2. How many steps would you expect for a base titration curve of tartaric acid? How many for potassium hydrogen tartrate to the same final pH?
3. Which increases as a wine goes bad - total acidity, fixed acidity, volatile acidity?
4. Will tartaric acid exist in wine largely as the undissociated acid, H2T, or as the hydrogen tartrate ion HT-?
5. In an experiment, why is it assumes that the distillation is over when the temperature of the boiling liquid has reached the boiling point of water? (when the boiling liquid is not even water).
6. When measuring ethanol content in wine my text book says I must add 2M NaOH to 25ml of the wine to adjust the pH to just alkaline. But I don't understand why NaOH was added before the distillation and why it had to be 2M as opposed to 0.1M?
Igloo writes ...
1. This is to separate the ethanol from all the other components in the wine. When density tables are used to convert density measurements into ethanol concentration, the assumption is that ethanol and water are the only components of the mixture.
2. Tartaric acid is diprotic so there are two steps in the neutralisation. Potassium hydrogen tartrate is monoprotic so there is only one step.
3. Levels of all three acids generally rise, but that of the volatile acids can decrease slightly with time. An extremely complex set of reactions take place during spoilage and much depends on the type of wine too.
4. Tartaric acid is a fairly weak acid so the species predominantly present in wine, where the pH is appreciably less than 7, will be the undissociated molecule H2T.
5. You are presumably referring to the removal of the ethanol by distillation. Most of the ethanol distils over in the early stages when the boiling temperature of the liquid is in the region of 80-85°C. The eventual boiling point (if the process is continued) is indeed likely to be over 100°C, but by the time this temperature is reached it can safely be assumed that virtually all the ethanol has distilled over.
6. The wine needs to be neutralised so that all the volatile acids (which would otherwise distil over with the alcohol) are neutralised and converted to non volatile salts. Use of 2M NaOH means that only a relatively small volume needs to be used to complete this neutralisation process. To ensure complete neutralisation with 0.1M NaOH, 20 times more NaOH would need to be added making the mixture too bulky for subsequent distillation. You would be correct in thinking, however, that 0.1M NaOH is far safer to handle than 2M NaOH.
Do rely on what is said here. Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 08 November 2007
