By the end of this lesson
- Plan an investigation of pH and enzyme activity using buffers.
- Process endpoint times into rates and identify an optimum range.
- Evaluate control variables, safety, repeatability and endpoint resolution.
BIG QUESTION
How can an investigation change pH without accidentally changing temperature, concentration or the way the endpoint is judged?
Buffers isolate pH as the independent variable; standardised sampling turns starch disappearance into comparable rate evidence.
Use a buffer to define the independent variable
Prepare buffer solutions spanning a suitable pH range. Mix the same volume and concentration of amylase with one buffer for each trial. A bufferA solution that resists changes in pH, used to keep the pH of a reaction mixture constant. resists pH change when starch and enzyme solutions are combined.
Measure or verify pH where equipment allows. Label clearly and avoid carrying one buffer into another with shared pipettes.
Standardise the reaction and endpoint
Place starch and buffered amylase in a constant-temperature water bath to equilibrate. Add iodine drops to a spotting tile. Mix equal volumes of starch and enzyme, start the timer immediately, and sample at fixed intervals.
Record the first time a sample remains orange-brown. Calculate rate as 1/time. If starch remains after the maximum observation period, record this transparently rather than inventing an endpoint.
Keep variables other than pH constant
Keep temperature, enzyme and substrate concentrations, volumes, total volume, mixing, sampling volume and sampling interval constant. Use at least three repeats per pH.
The volume of buffer should also remain constant because changing it changes dilution as well as pH. This is a common hidden confounding variable.
Safety and practical judgement
Wear eye protection. Iodine is an irritant and stains; buffer solutions at extreme pH may be irritant or corrosive depending on composition and concentration. Use small volumes and follow the laboratory risk assessment.
Use pipettes rather than mouth pipetting, clean spills promptly and handle glassware carefully. Moderate-temperature water baths avoid unnecessary burn risk.
Analyse without overstating precision
Plot mean rate against pH. The highest tested mean identifies the best-performing tested pH, while the true optimum may lie between values. Narrower pH steps around the peak can refine the estimate.
Endpoint colour is subjective and discrete sampling creates interval uncertainty. A colorimeter or data logger may improve objectivity, but only if the measurement is calibrated to starch disappearance and does not introduce a different confounder.
Graph skills: SLAPUK and choosing a scaleRates with awkward values: choose an even scale, not the raw data.
Variables in the pH investigation
A valid plan states not just what is controlled, but how.
| Variable class | Variable | How it is handled |
|---|---|---|
| Independent | pH. | Use labelled buffers at chosen pH values and equal buffer volumes. |
| Dependent | Amylase activity. | Measure starch-disappearance time; calculate rate = 1/time. |
| Controlled | Temperature. | Use one thermostatically controlled water bath and verify it. |
| Controlled | Concentrations and volumes. | Use the same stocks and measured volumes each trial. |
| Controlled | Sampling. | Use equal drops and a fixed interval with the same endpoint rule. |
| Reliability | Random variation. | Repeat each pH at least three times and compare spread. |
Language that earns precision
- buffer
- A solution that resists pH change when small amounts of acid or alkali are added.
- confounding variable
- An uncontrolled factor that changes with the independent variable and offers an alternative explanation.
- repeatability
- Agreement when the same person repeats a method with the same equipment.
- resolution
- The smallest interval or difference that a method can distinguish.
- anomaly
- A result inconsistent with the pattern and repeats, investigated rather than automatically deleted.
Common misconceptions
Claim: Iodine measures pH.
Correction: The buffer defines pH; iodine reports whether starch remains.
Claim: Adding more buffer only changes pH.
Correction: It also changes total volume and concentrations, so buffer volume must be controlled.
Claim: The fastest tested pH is the exact optimum.
Correction: It is the best tested value; the true optimum may lie between tested points.
Claim: An anomalous result should always be removed.
Correction: It should be checked against repeats and excluded only with a justified reason.
Match the answer to the command word
Give a repeatable sequence plus variables, repeats, safety and data processing.
State how each control variable prevents an alternative explanation.
Connect a specific limitation to its effect and a feasible improvement.
Recall, apply, then justify
Recall, apply, then justify (continued)
Explain why a buffer is used in this investigation.
Hint
Connect stable pH to validity.
Suggested marking points
- It resists pH change during the reaction.
- This keeps the intended pH stable so differences in rate can be attributed to pH.
A reaction finishes in 125 s. Calculate the rate.
Hint
Use 1/time and give s⁻¹.
Suggested marking points
- 1 ÷ 125.
- 0.0080 s⁻¹.
Explain why equal buffer volume is a control variable.
Hint
Think beyond pH to dilution.
Suggested marking points
- Different buffer volumes change total reaction volume.
- This changes enzyme and substrate concentrations by dilution.
- Equal volumes ensure pH is not confounded with concentration.
Suggest two improvements to a method that tests only pH 3, 7 and 11 once each.
Hint
Address reliability and resolution.
Suggested marking points
- Repeat each pH and calculate a mean.
- This reduces random error and shows spread.
- Test more pH values with narrower intervals near the apparent peak.
- This estimates the optimum more precisely.
Design a decision rule for handling a trial in which iodine never remains orange-brown during the ten-minute observation period.
Planning hint
Avoid assigning a false exact time; preserve the information for analysis.
What a strong design includes
- Record the result as endpoint greater than 600 s or rate less than 1/600 s⁻¹.
- Do not record 600 s as though the endpoint occurred exactly then.
- Check a control experiment and reagent function before interpreting low activity.
- Repeat the trial to test reliability.
- Extend observation only if the same rule is applied consistently and substrate depletion remains measurable.
- Use censored-data notation or plot transparently rather than fabricating a zero rate.