Chemistry IA · Reactivity 3 · Mechanisms of chemical change · topic 23 of 25

The pKa of an indicator · IB Chemistry IA idea: guide, sample graph and step-by-step plan

advanced but very neat. good choice

An indicator is just a weak acid whose two forms have different colours. Measure how much of each is present across a range of pH and the pKa drops out of the data.

Difficulty●●● Stretch
KitAdvanced: spectrophotometer, or colorimeter with a suitable filter
TimeAbout 12 hours
FitsHL (pKa and indicators are AHL)
  • ✓ you like acid–base equilibria with a colour change
  • ✓ you can prepare solutions carefully
  • ✓ you enjoy a graph that gives a constant

The big idea

← your hook: one constant from one curve

Bromothymol blue is a weak acid, HIn, with a yellow acid form and a blue base form. The Henderson–Hasselbalch relationship says the two forms are present in equal amounts when the pH equals the pKa. A spectrophotometer measures the base form's absorbance at its peak, so you can follow its fraction across buffers of different pH. Plot absorbance against pH and look for the midpoint, or linearise the data and find the intercept. The chemistry is clean: the hard part is keeping every other thing equal across a set of buffers.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Linearise the data

Plot pH against the logarithm of a ratio built from your absorbances, and use the intercept.

→ Your analysis changes from reading a midpoint to fitting a line. The gradient is a test of the model: what should it be?
Use the whole spectrum

Record spectra, not one wavelength, and find where the curves cross (an isosbestic point).

→ A strong check that only two coloured forms are present. The crossing point also tells you whether the indicator concentration was constant.
Change the indicator

Repeat with another indicator, such as methyl red or phenol red, in a different pH range.

→ A new pKa and new buffers, with the same method. You compare the two results with literature values.
Test ionic strength

Make one set of buffers at different ionic strengths by adding a neutral salt.

→ A second study on whether the apparent pKa changes. A good source of discussion about what constant really means.
Test temperature

Repeat the series at two or three temperatures with a water bath.

→ Asks whether the pKa shifts with temperature. The pH of buffers also changes, so you must measure it at each temperature.
Change the context

Link to pH indicators in swimming pools, aquariums or soil testing kits.

→ Gives your introduction a purpose; the method and graph stay the same.

Your Chemistry IA research question: run the check

write it FIRST, then check it

Write your own question first. Then make sure it ticks every box:

  • Names the independent variable (the pH of the buffers, with a range and how many) and how you will measure each pH.
  • Names the dependent variable (the absorbance at a stated wavelength) and why that wavelength.
  • Says exactly what stays the same in each cuvette (indicator concentration, temperature, path length).
  • Says how a pKa will come from the data, and which method you use to get it.
  • Is narrow enough for about 10 to 12 hours, with at least six buffers and repeats.

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The set-up

One way to set it up. Draw your own with your real wavelength and buffers for the Method section.

pH 5.5pH 6.5pH 7.0pH 7.5pH 8.5 Asame volume ofindicator in eachread each real pH on a calibrated meterfilter near the blue form's peakblank: buffer, no indicator

How your Chemistry IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question; why indicators matter (titrations, test kits); your prediction for the pKa and where it comes from.
Background chemistry~450
The indicator equilibrium; the Henderson–Hasselbalch relationship; the colour of each form; how absorbance relates to concentration; what your graph should show.
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or keep it constant, and why it matters.
Method~450
How you made the buffers, how you checked their pH, how you added the indicator, how you chose the wavelength, how you blanked the instrument; repeats. Repeatable by someone else.
Safety, ethics, environment~60
Bromothymol blue stock is usually in ethanol (flammable: no flames); the dilute buffers are low hazard, but the acid and alkali you adjust them with are irritant at 0.1 to 1 mol dm⁻³. Goggles; glass cuvettes. Dilute waste as your school directs.
Raw datatable
Measured pH of each buffer and each absorbance reading, with units and uncertainties in the headers.
Processing~250
How absorbance becomes a fraction in the base form; how you found the limits (fully acid, fully base); one worked example with the uncertainty.
Graphs and analysis~350
Absorbance against pH, or the linearised version, with error bars; how you read the pKa; the uncertainty in it.
Conclusion~400
Answer with numbers: your pKa with its uncertainty, compared with a literature value.
Evaluation~650, the biggest part!
Strengths, weaknesses ranked by their effect on the pKa, and improvements linked to each.
Referencesnot counted
Your literature pKa and spectra, and the buffer recipes.

What your Chemistry IA graph could look like

mark the half-way point clearly

Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only

5.55.86.06.26.56.87.07.27.57.88.08.28.50.00.10.20.30.40.50.60.70.80.9pH of buffer (± 0.05)Absorbance at the base-form peak (± 0.005)sigmoid for a weak acid (made up)half of the maximum absorbanceyour data, with error barswhere the curve crosses the dashed line,half is in each form: read the pH thereILLUSTRATIVE · MADE-UP DATA
  • Axis labels name the quantity and its unit. The pH axis has an uncertainty, because it comes from a meter.
  • Error bars on every point. The horizontal ones matter too: pH is measured, not set.
  • The dashed line marks half the maximum absorbance, so you must have a reliable maximum (fully base) and zero (fully acid).
  • Check that the curve flattens at both ends. If it does not, your pH range is too narrow.
  • Another graph to consider: pH against the logarithm of a ratio of absorbances. If the model holds, it is a straight line, and its intercept gives the pKa.

Which uncertainty matters most?

Rank by the effect on the pKa you report. Here the biggest effects come from the chemistry (ionic strength) and the pH meter, not from the spectrophotometer. Estimate your own.

Ionic strength varies (~0.10 pH)1.4%pH meter calibration (± 0.05)0.71%Maximum absorbance wrong by 3%0.36%Indicator amount differs by 2%0.24%Absorbance reading (± 0.005)0.15%biggest effect: discuss this first!% uncertainty in the pKa (made-up scenario)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

the trial run decides your wavelength

Tick steps off as you go. We'll remember on this device.

0 of 11 done

  1. Read and predictWrite the indicator equilibrium and the colour of each form. Predict how absorbance at the base-form peak should change with pH and where the midpoint should be.
  2. Find the spectraRecord or find the spectra of the acid and base forms and choose a wavelength. Decide what a blank should contain.
  3. Trial run (1 lesson)Add the same amount of indicator to one acidic and one basic buffer. Are both absorbances stable? Is the cuvette clean? Do the readings repeat?
  4. Decide the buffersChoose at least six pH values spanning the transition, how you will make them and how you will measure each pH. Decide how you will keep the ionic strength similar.
  5. Fix the indicator amountDecide how you will add identical amounts of indicator to each buffer, and check that the absorbance at one fixed point confirms it.
  6. Collect the dataZero with the blank, then read each buffer in order of pH. Measure the real pH of the solution you read, not the label of the buffer.
  7. ProcessFind the absorbance for the fully acidic and fully basic cases and convert each reading into a fraction in the base form. Carry the uncertainty through.
  8. Graph and fitPlot absorbance against pH and, if you linearise, add error bars and a line of best fit. Decide what the intercept and gradient should be.
  9. EvaluateWhich uncertainty or systematic error moved the pKa most? Rank them and make every improvement specific.
  10. Write, then cutWrite in the order of the layout above. Keep within 3,000 words: tables, graphs, equations and references are not counted, so let them carry detail.
  11. Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.

Chemistry you'll need

Traps to avoid in this Chemistry IA

  • Trusting the label on a buffer: measure the real pH of the solution in the cuvette.
  • Different amounts of indicator in each buffer, so absorbance changes for a reason that is not pH.
  • No reliable fully-acid and fully-base absorbances, so the midpoint is a guess.
  • A pH range that does not reach the flat ends of the curve.
  • Using a filter that does not match the base form's peak, so the signal is weak and mixed.
  • Buffers of very different ionic strength, which shifts the apparent pKa without being mentioned.

More Chemistry IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

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