Chemistry IA · Reactivity 3 · Mechanisms of chemical change · topic 22 of 25

How hard can a buffer be pushed? · IB Chemistry IA idea: guide, sample graph and step-by-step plan

great for HL: real chemistry, real maths

Add alkali to a buffer and the pH hardly moves, until it does. How much it takes depends on how concentrated the buffer is, and you can predict it before you start.

Difficulty●●● Stretch
KitStandard: pH probe, burette, stirrer
TimeAbout 10 to 12 hours
FitsHL (buffers); SL with extra background
  • ✓ you like acid–base chemistry
  • ✓ you can handle a pH meter patiently
  • ✓ you enjoy checking a model with a calculation first

The big idea

← predict BEFORE you measure

A buffer made from ethanoic acid and its salt resists changes in pH because the acid reacts with added alkali and the ethanoate reacts with added acid. The Henderson–Hasselbalch relationship links the pH to the ratio of the two, so you can calculate how much sodium hydroxide moves the pH by one unit. Then you test it. Maths first, then the lab: the calculation may tell you your burette is too small. The stronger the buffer, the more it takes. Your job is to find out how closely that holds and where real solutions depart from the ideal.

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 →

Vary the ratio instead of the concentration

Keep the total concentration fixed and change the ratio of acid to salt, then measure the alkali needed for a 1.0 pH change.

→ Your graph becomes volume against ratio. The model predicts the response is not symmetrical, which makes a stronger test.
Add acid instead of alkali

Titrate each buffer with hydrochloric acid, not sodium hydroxide.

→ Compares the two directions. If the buffer is 1:1 the model predicts the same volume in both directions, which is a test of the model.
Use a different buffer system

Compare ethanoic acid with another weak acid and its salt, such as a phosphate or a methanoic buffer.

→ Tests whether the pKa matters for the resistance to change. You must explain why the best buffer range differs.
Change the pH window

Measure alkali needed to move the pH from just above its start by 0.5 or 2.0 units.

→ Tests whether the response is steady across the pH range or not. The model predicts a curve.
Draw the whole titration curve

Add alkali in small steps from the start to well past the buffer region for one concentration.

→ A fuller picture of where the buffer works and where it breaks down. Needs a fine step size and a clear plan.
Change the context

Buffers in blood, in swimming pools, or in a food or shampoo you can test safely.

→ 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 and its range (which total concentrations, and how many).
  • Names the dependent variable as a volume and states exactly what you measured it to (the pH value or change).
  • Says how you make each buffer and how you check the ratio really is what you intended.
  • Says what stays the same (buffer volume, temperature, probe, stirring, concentration of alkali).
  • Is narrow enough for about 10 hours, and you have checked that the largest volume of alkali fits your burette.

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The set-up

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

pH0.10 M NaOH: small stepscalibrate with 2 buffersprobe clear of the stir barsteady stirring

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; where buffers matter (blood, water treatment, food); what you expect and why.
Background chemistry~450
The buffer equilibrium; what happens when alkali is added; the Henderson–Hasselbalch relationship; your pre-lab calculation of the volume of alkali you expect.
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or keep it constant.
Method~450
How you made each buffer and how you checked its starting pH; how you calibrated the probe; how you added alkali and when you read the pH; repeats. Repeatable by someone else.
Safety, ethics, environment~60
Ethanoic acid at your stock concentration (dilute, under 1 mol dm⁻³, is an irritant; glacial acid is corrosive and stays with your teacher); 0.10 mol dm⁻³ sodium hydroxide irritates eyes, so goggles; the probe is glass. Neutralised waste down the sink with plenty of water if your school allows.
Raw datatable
For each buffer: starting pH, volume of alkali at each pH reading, units and uncertainties in the headers.
Processing~250
How you found the volume for the 1.0 pH change (read off, or interpolated), with its uncertainty; one worked example.
Graphs and analysis~350
Volume of alkali against buffer concentration with error bars, the model line, and how your data differs from it.
Conclusion~400
Answer with numbers: how the volume changed with concentration and how it compares with the calculated one.
Evaluation~650, the biggest part!
Strengths, weaknesses ranked by their effect on the volume, and improvements linked to each.
Referencesnot counted
Your source for the pKa value and the buffer equation.

What your Chemistry IA graph could look like

draw the model line too

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

0.000.050.100.150.200.250.300.350.400.4505101520253035404550Total buffer concentration / mol dm⁻³ (± 0.002)NaOH (0.10 mol dm⁻³) for ΔpH = 1.0 / cm³ (± 0.5)ideal buffer model, 25.0 cm³ (made up)your data, with error barsvolume should scale with concentration.Where does your line differ, and why?ILLUSTRATIVE · MADE-UP DATA
  • Axis labels name the quantity and its unit, with the uncertainty in the volume and in the concentration.
  • Error bars on every point. The vertical ones come from reading the pH and the step size, not from the burette alone.
  • The model line is drawn from your pre-lab calculation, so the reader sees the data against a prediction.
  • A line that goes through the origin is a physical expectation: no buffer, no resistance. Say so, then check. Above about 0.45 mol dm⁻³ the volume no longer fits a 50 cm³ burette.
  • Another graph to consider: pH against volume of alkali for one concentration, to show the flat buffer region itself.

Which uncertainty matters most?

Rank by the effect on the volume for the 1.0 pH change in a 0.10 mol dm⁻³ buffer (about 10 cm³). Notice that how you take the readings can beat the instruments.

Reading step of 1.0 cm³ (interpolation)4.9%Probe calibration (± 0.05 pH)2.3%NaOH not standardised (± 2%)2%pH drift while waiting (± 0.02)0.9%25.0 cm³ pipette (± 0.06 cm³)0.24%biggest effect: discuss this first!% uncertainty in volume, 0.10 M buffer (made-up)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

step 2 saves you a wasted lesson

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

0 of 11 done

  1. Read and predictWrite the buffer equilibrium and the reaction with alkali. Use the Henderson–Hasselbalch relationship to predict how the volume should depend on concentration, before any apparatus.
  2. Do the arithmetic for your whole rangeCalculate the volume of alkali you expect at every concentration. Check the biggest volume against your burette, and decide how to deal with it.
  3. Trial run (1 lesson)Make one buffer, calibrate the probe, add alkali in steps and watch the pH. How long does a reading take to settle? Does stirring disturb the probe?
  4. Decide range, step size and repeatsChoose at least five concentrations, how big each step of alkali is, and how many repeats. Smaller steps near the target pH. Write the reason for each choice.
  5. Standardise or justifyDecide whether you will check the alkali's concentration or accept the label, and say why.
  6. Collect the dataCalibrate with two buffers before each session. Record every reading straight into your table, with the time you waited.
  7. ProcessFind the volume at which the pH has risen by 1.0 and its uncertainty. Decide how you will handle the volume added by the alkali itself.
  8. Graph and comparePlot volume against concentration with error bars and compare your line with the pre-lab model. Decide what a difference would mean.
  9. EvaluateWhich uncertainty or assumption moved the volume 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

  • A probe calibrated with one buffer, or not at all, so every pH is shifted.
  • Adding alkali in big steps, then finding the 1.0 pH change by guessing between two readings.
  • Not checking that the largest volume fits the burette until you are in the lab.
  • Mixing a buffer that is not really 1:1, so the starting pH is not where the model says.
  • Reading the pH before it settles, or with the probe touching the stir bar.
  • Forgetting that the buffer equation is an approximation and giving no reason for a difference between the model and your data.

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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