Freezing Point of Acetic Acid:
The strength of acetic acid may be easily and accurately tested by finding the freezing point of acetic acid. Since acetic acid may easily be adulterated with some strong acid to give the required acid strength (if tested by usual acidimetric titration), it always better test any one its physical property. The freezing point of Glacial Acetic acid (98 to 100%) is 16.7°C. If the test sample freezes below 16.7°C, then we could fix that the acid is adulterated with water and the strength will be lower than Glacial acetic acid. On the other hand if the freezing point is higher than 16.7°C, some other acid admixture would have changed it and in this case also it is not pure. So while testing the acid strength, we could allow a maximum deviation limit of +/- 5% from the freezing point for some instrumental errors.
1. Attach a test tube clamp to the ring stand so the tube is about 25cms above the base. Place the test tube in the clamp near the top of the tube and tighten the clamp securely.
2. Place a thermometer in the slotted cork so that the temperature scale is visible through the slot. Position the cork so that the thermometer is about 1 cm from the bottom of the test tube when the cork is place as shown in the figure.
3. Measure 10ml of pure acetic acid using a graduated cylinder. Pour the acid into the test tube and replace the thermometer.
4. Adjust the temperature of acetic acid to about 25°C by warming or cooling the test tube in beaker of water (with or without ice cubes).
5. Empty the beaker and fill it with ¾ with crushed ice. Add cold water until the ice is almost covered. Place the beaker of ice water below the test tube assembly.
6. Carefully read the thermometer and record the temperature as the 0 minute reading in a data table.
7. Lower the test tube assembly in to the ice water beaker. Be sure to lower the test tube until all of the acetic acid is under water. Refasten the clamp to the ring stand.
8. Loosen the cork slightly and agitate the acetic acid slightly using the thermometer. Be careful not to bang the thermometer bulb on the glass and keep the thermometer bulb completely immersed in the acid. Take temperature reading every 30 seconds as the acid cools. Begin with the 0.5 minute reading.
9. Stop stirring the acid as soon as you are sure the crystals are forming. Keep taking readings until a total of 15 minutes as passed. Stir the ice water occasionally to help it at a constant temperature.
10. After completing the temperature readings remove the test tube assembly from the water. Replace the ice water in the beaker with warm water. Immerse the test tube assembly in the warm water and begin taking temperature readings every thirty seconds as the acetic acid melts. Once the acid has melted a little and moves easily in the test tube agitate the solid mixture with the thermometer. Continue taking readings until all the acetic acid is melted.
1. Draw a smooth curve using the readings. Time vs temperature.
2. From this data we can find out the freezing point and melting point of acetic acid. Infer that if it is close to 16.7°C, then it is a pure acid.