Investigation 15: Buffering Activity
This lab tests whether a household product like shampoo is a buffer by adding measured amounts of acid or base and tracking pH, then comparing that curve to plain water's.
A buffer is a weak acid paired with its conjugate base (or a weak base with its conjugate acid), both present in solution at the same time. Add H+ and the conjugate base reacts with it; add OH- and the weak acid neutralizes it. Either way the equilibrium shifts to soak up the stress, so pH barely moves. Plain water has no such pair sitting around, so even a small amount of added acid changes its pH sharply.
The lab design is a comparison, not a single measurement. You measure the starting pH of the product, then add a strong acid (or base) in small, equal increments, stirring and recording pH after each addition. You run the identical procedure on a water control using the same volumes and the same increments. Plotting pH versus volume of acid added for both on one graph is the payoff: water's curve drops steeply right away, while a true buffer's curve stays nearly flat over a stretch before it eventually drops.
That flat stretch is the buffering region, and its length depends on how much weak acid and conjugate base are actually present. A short flat region before the pH finally falls means the buffer's supply of acid or base got used up fast, meaning lower concentrations; a long flat region means more reserve capacity. This connects to the error-analysis and experimental-design practices running through the unit: a flat-looking pH reading only counts as evidence of buffering when it is compared against an unbuffered control and tested with a large enough addition to actually challenge the system.
Key terms
- Buffer
- A solution containing a weak acid and its conjugate base (or weak base and conjugate acid) that resists pH change when acid or base is added.
- Conjugate base
- The species left behind after a weak acid donates a proton, capable of accepting a proton back.
- Buffering region
- The flat stretch of a pH-versus-titrant graph where pH changes very little despite continued additions.
- Unbuffered control
- A plain solution, usually water, treated identically to the test product so its sharp pH change shows what 'no buffering' looks like.
Watch it explained
- How it worksHow do you test a household product for buffering activity in a fair, repeatable way?
- Worked exampleGraph pH against added acid for water and for a buffered product on the same axes.
- Common mistakeWhy isn't a product with a stable pH necessarily a buffer?
- Going furtherWhat does the length of the flat region tell you about the concentrations in the product?
Check yourself
End of topic.