Experiment 18

Objective

To determine the strength of sodium hydroxide (NaOH) using a standard solution of oxalic acid.

Introduction

Oxalic acid, a dibasic weak acid, reacts with sodium hydroxide, a strong base, in a neutralization reaction. Phenolphthalein is used as an indicator due to its color change from pink in basic solutions to colorless at the endpoint of the reaction.

Reaction:

(COOH-CHOH-CHOH-COOH) + 2NaOH →

(COONa-CHOH-CHOH-COONa) + H2​O

Using the equation V1 M1 = V2M2, where:

●       V1=10 mL (volume of NaOH),

●       M​1 is the strength of NaOH (unknown),

●       V2 =10mL (volume of oxalic acid),

●       M2 = 0.1M (strength of oxalic acid).

Calculation:  M1 = V2 x M2 / V1 = 0.1 M NaoH

Materials

Chemicals

●      Sodium hydroxide (NaOH)

●      Distilled water

●      Oxalic acid (0.1 M)

●      Phenolphthalein indicator

Apparatus

●      250 mL flask

●      Pipette

●      Burette

Procedure

1.    Wash and dry the 250 mL flask.

2.    Add 10 mL of the 0.1 M oxalic acid solution to the flask.

3.    Add 1–2 drops of phenolphthalein to the oxalic acid, turning the solution pink.

4.    Using a burette, add NaOH solution dropwise to the flask.

5.    Observe the color change to colorless, indicating the endpoint.

6.    Record the volume of NaOH required.

7.    Repeat the titration if necessary for accuracy.

Observation

As NaOH is added, the pink color fades to colorless upon neutralization, indicating the strength of NaOH as approximately 0.1 M.

Questions

1.    What is an acid-base reaction?

2.    What is an indicator, and why is phenolphthalein used in this titration?

3.    What color change is observed with phenolphthalein in acidic vs. basic solutions?