Question

You need to perform a titration of lactic acid. What indicator would you use to best align the end point with the equivalence point? You will need to support your decision with calculations, including the pH at the equivalence point.

Answer 1

A sample calculation is provided as the lactic acid concentration varies from sample to sample

Example : Lactic acid in milk (CH3-CHOH-COOH)

Titrate against : NaOH solution (0.4 N)

Indicator : phenolphthalein

End point: pink to constant color variation

equivalent point: 30 sec after end point

Experiment procedure:

1. Rinse and fill burette with titrant (NaOH = 0.1 N).

2. take 25 ml of milk in a beaker and add the indicator (phenophthalein).

3. titrate till sample is colorless

4. proceed untill the color variation is constant for more than 30 seconds.

5. We write down the volume of titrant used to reach our equivalent point.

Calculation:

Equivalent point (NaOH used) = 4.3 mL

Titrated vol = 25 ml

moles of lactic acid = 4.3 X 10^-3 ) X0.01 = 4.3 X 10^-4 mol/L

gr of lactic acid = moles X mol mass = 4.3 X 10^-4 X 90.08 = 3.88 X 10^-2 g