Controlling variables and improving quality
What "controlling variables" means here
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This is not a fair-test investigation with an independent variable. Nothing is being deliberately varied.
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Here, controlling variables means keeping everything the same across all your titrations, so that any difference between titres reflects only the measurement, not a change in conditions.
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Achieved asks you to describe how you controlled them. Merit asks you to explain how that control improved the quality of the investigation. Excellence asks you to justify it in terms of validity and accuracy.
The significant variables
| Variable | How it is controlled | Why it matters |
|---|---|---|
| Aliquot volume | Same pipette, rinsed with sample, drained the same way each time | A different aliquot volume changes the moles being titrated, so the titre changes for a reason unrelated to concentration |
| Concentration of the titrant | One batch of standardised solution used throughout | A change of batch mid-investigation makes titres incomparable |
| Concentration of the sample | One batch of diluted sample, made in one volumetric flask | Two separate dilutions can differ slightly |
| Amount of indicator | Two or three drops every time | Indicators are weak acids/bases and consume titrant |
| Endpoint judgement | Same person, same white tile, same "first permanent colour" rule | A drifting endpoint definition shifts every subsequent titre |
| Temperature | Room temperature throughout | Volumes of solutions change slightly with temperature; also matters for reactions warmed on purpose |
| Time before titrating | Titrate promptly after preparing | Some analytes (vitamin C, hypochlorite) decompose in air or light |
- The two that most often appear in reports and are worth naming explicitly are the aliquot volume and the amount of indicator, because both are entirely under your control and both have a direct effect on the titre.
Random and systematic error
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A random error varies in size and direction between repeats — it shows up as scattered titres.
- Sources: reading the burette, judging the endpoint, small splashes.
- Reduced by repeating and averaging concordant titres.
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A systematic error shifts every result in the same direction by about the same amount — it does not show up in the scatter.
- Sources: unstandardised titrant, wrongly rinsed conical flask, misread pipette calibration, consistent parallax.
- Not reduced by repeating. Only removed by fixing the cause.
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This distinction is worth learning properly, because it is the source of the single most quoted line in this standard: concordant titres prove precision, not accuracy.
Accuracy, precision and validity
| Term | Meaning | How you improve it |
|---|---|---|
| Accuracy | How close the result is to the true value | Standardise the titrant, use volumetric glassware, remove systematic errors |
| Precision | How closely repeats agree with each other | Repeat until three concordant titres, careful dropwise addition near the endpoint |
| Validity | Whether you measured what you set out to measure | A visible, sharp endpoint; a complete reaction; no interfering substances |
- A result can be precise but not accurate (three identical wrong answers), or accurate but not precise (a scattered set whose mean is right by luck). You want both.
Techniques that increase quality
- Repeat until at least three concordant titres are obtained, and average only those.
- Add dropwise near the endpoint, with a half-drop delivered by touching the burette tip to the flask wall and rinsing it in.
- Swirl continuously so no local excess builds up.
- Use a white tile and view against it consistently.
- Standardise the titrant on the same day if it is not a primary standard.
- Read at eye level to eliminate parallax.
Worked ExampleExplaining control of variables at Merit level
In a determination of the ethanoic acid concentration in vinegar, describe how three significant variables were controlled and explain how each improved the quality of the investigation.
Variable 1 — the volume of the aliquot
Control: the same 20.0 mL volumetric pipette was used for every aliquot, rinsed with the diluted vinegar before each use, and allowed to drain in the same way with the tip touched to the flask wall.
Why it improved quality: the calculation assumes every conical flask contained exactly 20.0 mL of diluted vinegar. Using one pipette, used identically, means each flask contained the same number of moles of acid, so any variation between titres came from the burette reading and the endpoint judgement alone — not from a varying amount of acid. This improves precision, and it makes the concordance test meaningful.
Variable 2 — the amount of indicator
Control: exactly three drops of phenolphthalein were added to every flask, from the same dropper bottle.
Why it improved quality: phenolphthalein is itself a weak acid, so it consumes a small amount of the sodium hydroxide titrant. That consumption is constant if the amount of indicator is constant, but if one flask received ten drops it would need measurably more titrant to reach the endpoint. Keeping it constant removes a source of variation between titrations and prevents a systematic overestimate.
Variable 3 — the concentration of the titrant
Control: a single batch of sodium hydroxide was standardised against a sodium carbonate primary standard at the start, and that same batch was used for all titrations, kept stoppered between uses.
Why it improved quality: the titrant's concentration is a direct multiplier in the calculation, so an error in it produces the same percentage error in every result. Standardising it establishes the true value rather than trusting the label, which improves accuracy; keeping the bottle stoppered prevents absorption of carbon dioxide, which would otherwise weaken the solution progressively during the investigation and make later titres inconsistent with earlier ones.
Answer: controlling the aliquot volume and the indicator quantity improved precision by removing sources of variation between titrations, while standardising and sealing the titrant improved accuracy by fixing and preserving the one quantity that multiplies through every calculation.