Water Treatment pH Lab
Treated drinking water resists pH changes because of dissolved bicarbonate (alkalinity). Add acid or base to one liter of water and see how buffering protects the pH, and what happens when it runs out.
Controls
Mg/L as calcium carbonate. 0 is like distilled water; 80–120 is typical treated water.
Left of center adds base (like lime or caustic soda). Right of center adds acid (like acid rain). Units: millimoles per liter of strong acid or base.
Model: closed carbonate system with pK₁ = 6.35, pK₂ = 10.33, starting at pH 7.6 when alkalinity is above zero (pH 7.0 for pure water). Real water chemistry is more complex, but the buffering behavior is realistic.
pH
Buffer used
of the bicarbonate
Pipe corrosion risk
Try this
- With alkalinity at 100, add acid until the pH leaves the guideline range. How much does it take?
- Set alkalinity to 0 and add just a little acid. Compare the pH swing with the buffered case.
- Find the “cliff” where the buffer runs out and the pH drops suddenly.
What to notice
- Low alkalinity means a small acid input causes a big pH change.
- The steep part of each curve is where the buffer is exhausted.
- Treatment plants adjust pH and alkalinity so water is less corrosive in pipes.
Connect to the lab
- Use with Testing Our Water and the PhET pH Scale simulation.
- See the teacher key for sample data and the Flint discussion.