How To Measure Titratable Acidity
Posted Nov 26th 2012 | By:
Why measure titratable acidity (TA)?
Grapes naturally contain acid, and this acid contributes to the taste of wine and also helps topreserve it. A wine made from grapes that are too low in acid will have a flat, flabby taste and will be more likely to grow spoilage bacteria. A wine made from grapes that are too high in acid will be too sour. Wineries that grow their own grapes can control the acidity of their wines by controlling the time of harvest. The more ripe the grapes are, the less acidic they will be. Home winemakers don't have that option, and even wineries might want to adjust acid after harvest. You can use a simple kit available from winemaking suppliers to test a sample of juice or wine to determine whether the acid levels should be adjusted.
How do titratable acidity kits work?
Titration is, in general, a method to determine the concentration of an unknown substance. It depends on the addition of a known amount of a substance that reacts with the unknown amount of another substance in a predictable way. In our case, the unknown is the amount of acid in wine or juice. The reaction is the neutralization of the acid by a base. A base, or alkali, combines with an acid to neutralize it. You may have mixed vinegar (an acid) with baking soda (a base) to see a reaction between an acid and a base. Kits to test titratable acidity typically use sodium hydroxide as the base. Because the concentration of the base is known, you can measure the amount of the base you need to add to neutralize the acid, and that will tell you how much acid was there.
Because acids and bases in the wine or juice are not visible, you also need an indicator dye so that you know when you've added enough base to use up the acid. Indicator dyes are special dyes that change color when you change their pH. The indicator dye most often used in titratable acidity kits is phenolphthalein (pronounced FEE-nol-THAY-leen). This dye is clear in an acid, but in a base it is bright pink. Because of this, you know that you've neutralized all the acid when the solution turns pink.
How To Measure Titratable Acidity
The materials you will need to measure titratable acidity of a sample of wine or juice are: a test tube or other small container made of clear glass, sodium hydroxide (the base) at a known concentration (typically 0.1 Normal), phenolphthalein, and a syringe with volume markings to measure the amount of base added. All of these things may be purchased together as a kit.
The first step is to use the syringe to transfer a sample of the wine or juice to the test tube. It isimportant to use an exact volume. My kit calls for 3 mL. Because different kits may have different concentrations of the base, use the amount your kit calls for. Because red wine makes the pink color difficult to see, it should be diluted with a bit of distilled water so that it is lighter in color. The exact amount of water is not important because it does not change the amount of acid in the wine.
The second step is to add a small amount of the indicator dye phenolphthalein. The exact amount is not crucial. Because the wine or juice is at least a little acidic, the indicator dye will be clear and will not change the color of the wine.
The third step is to add a known amount of the base. This can be done with a syringe or a dropper with volume markings on it. Add the base one drop at a time and mix after each drop. Stop when it turns pink and stays pink. Before that point, it will look pink when you first add a drop but the pink will go away when you mix it. After it is completely pink, make a note of how much base you added.
The final step is to calculate the titratable acid. The calculation will vary depending on the concentration of the base and the amount of wine or juice you started with. If you started with 3 milliliters (mL) of sample and the base was 0.1 Normal concentration, you multiply the volume of base added in mL by 0.25 to find percent titratable acid.
To make a titratable acid kit last longer, you can use only 1 mL of wine or juice and 1 drop ofphenolphthalein. In this case, you would multiply the volume of 0.1 Normal base used by 0.75 to find percent titratable acid. This is because when you start with a third of the acid, you will only need a third of the base to neutralize it, so you need to multiply the volume of the base by three to figure out how much you would have used if you did the full test.
Typical titratable acidity for wine is .60%. If you need more acid, one teaspoon of acid per gallon will raise the titratable acidity about 0.15%. If you need less acid, 3 teaspoons of calcium carbonate per gallon will lower titratable acidity about 0.15%. Acid is more commonly added to wine or juice than is calcium carbonate.
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