Determination of Acidity (Titrable Acidity) of a given sample (Milk)

TITRATION: a technique used to measure the volume of a solution of known concentration that is required to react with a measured amount (mass or volume) of an unknown substance in solution. Titrable acidity of a solution is an approximation of the solution's total acidity.

Material Required:

1. Milk sample.
2. N/10 NaOH solution.
3. Phenolphthalein indicator. (C₂₀H₁₄O₄)- Phenolphthalein is an indicator that turns from clear to pink in a basic solution, in a strongly acidic solution, the indicator is orange, but in the pH range of 0 to 8.2 it is clear.

Apparatus:

1. 10 ml capacity pipette.

2. 100 ml conical flask or porcelain dish.

3. 50 ml capacity burette with stand.

4. Porcelain tile.

5. Glass rod.

Procedure:
1. Fill the burette with N/10 NaOH solution.
2. Mix the milk sample thoroughly by avoiding incorporation of air. But in case of fruit juice obtain at least 50 mls of clear juice by one of the following methods: 1. Cut fruit, press with a hand press, and filter through cheesecloth, or2. Cut fruit into a blender, homogenize, centrifuge slurry, and pour off clear liquid for analysis. After preparation of sample take 6 grams of juice into a 100 ml beaker and add 50 mls of water.

3. Transfer 10 ml milk with the pipette in porcelain dish/conical flask

4. Add equal quantity of glass distilled water.
5. Add 3-4 drops of phenolphthalein indicator and stir with glass rod.
6. Take the initially reading of the alkali in the burette at the lowest point of meniscus.
7. Rapidly titrate the contents with N/10 NaOH solution continue to add alkali drop by the drop and stirring the content with glass rod till first definite change to pink colour which remains constant for 10 to 15 seconds.
8. Complete the titration within 20 seconds.
9. Note down the final burette reading.

Calculation: