How to Test the Amount of Acidity in a Vinegar Fermentation

The acid content of homemade vinegar is easily determined with baking soda.

To determine the acidity of a solution such as vinegar, chemists normally use a method called "titration," in which a base is added, one drop at a time, to acid until the acid has been neutralized. A similar procedure can be carried out using simple kitchen utensils and a pH indicator prepared from red cabbage leaves. In this case, the "base" is baking soda (also known as sodium bicarbonate).

Things You'll Need

• 16 oz. baking soda
• 8 oz. store-bought white vinegar
• 1 eyedropper
• 8-oz. measuring cup
• 4 clear drinking glasses
• 1 tablespoon
• 1 coffee filter
• 1 small strainer
• 1 gal. distilled water

Instructions

1. Prepare the cabbage indicator by chopping the head of red cabbage into small pieces. In a large sauce pan, combine the chopped cabbage with 16 oz. of distilled water and bring to a boil until the water turns deep purple. Pour the mixture through the strainer into a container and refrigerate until needed.

2. Prepare a saturated solution of baking soda by placing about 4 oz. of warm distilled water in a drinking glass and adding baking soda. Add the baking soda one tablespoon at a time and stir after each tablespoon has been added. When no more baking soda will dissolve in the water, the solution is "saturated." Expect a small amount of undissolved baking soda to remain in the glass.

3. Place the coffee filter into the flour sifter or strainer. Place the strainer on top of another drinking glass and pour the saturated solution of baking soda through the filter paper. This will remove any undissolved baking soda.

4. Add 2 tablespoons of the cabbage juice prepared in Step 1 to an 8-oz. measuring cup and fill to 4 oz. with distilled water. Add exactly 20 drops of store-bought vinegar using an eyedropper. Transfer this mixture to a clear drinking glass.

5. Rinse the eyedropper with distilled water. Add the saturated baking soda solution to the vinegar/cabbage juice mixture, one drop at a time, with the eyedropper until the solution turns from purple to blue. Periodically stop to stir the solution with a tablespoon to ensure the components are sufficiently mixed. Count the number of drops and record this number as Ds (drops for store-bought vinegar). Set this solution aside for comparison in Step 6.

6. Rinse the measuring cup with distilled water and repeat Steps 4 and 5 using the homemade vinegar solution. Try to achieve the same shade of blue attained for the store-bought vinegar. Record the number of drops as Dh (drops for homemade vinegar).

7. Calculate the acid concentration of the homemade vinegar, Ah (percent acid in homemade vinegar). Note the percent acetic acid in the store-bought vinegar and record this value as As (percent acid in store-bought vinegar); this information will be printed on the label. Using the notations in Steps 5-7,

   Ah = (Dh x As) / Ds

   As an example, if store-bought vinegar containing 6% acetic acid required 15 drops of baking soda solution and the homemade vinegar required 25 drops of baking soda solution to achieve the same shade of blue, then

   Ah = (25 x 6) / 15 = 10% acetic acid

Tips & Warnings

• Do not be concerned about adding too much baking soda solution to the store-bought vinegar. As long as the same shade of blue is achieved with the homemade vinegar, the results will be valid.

References

• Centre College: Determination of Acetic Acid in Vinegar
• Stanford University: Red Cabbage Lab