How to Teach Acid Base Balance

A pH titration is a basic chemical and analytical procedure. A titration of one percent acetic acid with a five percent solution of sodium bicarbonate is a useful demonstration and model of titration. The reaction is of weak acetic acid with a weak, buffer base of NaHCO3. The bicarbonate is added in measured amounts and allowed to react to the endpoint of the reaction which is a neutral pH. A calibrated burette (a long tube, marked with volume amounts and a stopcock at the terminus) is used to add measured titration amounts. The addition is halted when an equivalence or specific defined endpoint is reached.

Things You'll Need

• Beaker, 250 ml
• Acetic acid, 50 ml of a 1% solution
• NaHCO3 (baking soda) made up at 5% concentration
• Burette
• Stand and clamps to hold burette
• Magnetic bar and stirrer
• Measuring cylinder
• pH color indicator, litmus
• Notebook for recording of readings

Instructions

1. Place the empty beaker on the magnetic stirrer (set to the off position). Place the magnetic stir bar gently into the bottom of the empty beaker.

2. Clamp the burette to the holding stand. Check and make sure that the stopcock is in the closed position. Add five percent NaHCO3 to the topmost calibrated burette fill mark.

3. Add a 50 ml of the one percent acetic acid to the empty beaker. Turn on the magnet stirrer to a slow spin that mixes the solution thoroughly without splashing or bubbling.

4. Add 10 to 20 drops of the litmus color indicator for pH. A distinct red color should be evident.

5. Record the starting points on the calibrated burette for the NaHCO3 fall into the acetic acid. Count the drops, if possible, and watch for a color change from red to blue, which indicates that the titration of the acid is complete and finished. When the color changes, shut off the stopcock to stop NaHCO3 flow into the beaker.

6. Record the final reading on the burette and determine the volume of sodium bicarbonate needed to neutralize the acetic acid. The titration is complete when the pH has changed to alkaline. The NaHCO3 has neutralized the acetic acid and during the titration CO2 was released and sodium acetate was formed.

Tips & Warnings

• Adding the drops of the NaHCO3 slowly to the acetic acid permits an accurate result.

• Counting the drops added during titration is useful for calculation of the volume per drop. This can calculated from the total volume used (as measured on the burette) divided by the total drops used. Example" if 100 drops expended 20 ml of NaHCO3, then each drop would be equivalent to approximately 0.2 ml.

• Avoid contact with acid or base to eyes, mouth and skin. Accidental exposure to acid requires thorough and copious washing with water.

References

• Titration; 15 th ed.; Micropaedia; The New Encyclopedia Brittanica, 1990.
• Burette, Stopcock and Titration Solution Added to Flask