Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 8.22 Uncertainty in volumetric analysis-Study Notes - New Syllabus

8.22 Minimising Uncertainty in Volumetric Analysis & Estimating Overall Uncertainty

Volumetric analysis (titration) involves measurement uncertainties from apparatus and technique. These must be minimised and quantified to assess reliability of results.

(A) Sources of Uncertainty 

• Burette readings (±0.05 cm\(^3\))
• Pipette volume (e.g. ±0.06 cm\(^3\))
• End-point judgement (indicator colour change)
• Parallax error when reading meniscus

(B) Minimising Uncertainty

1. Technique Improvements

• Read meniscus at eye level
• Use white tile to see colour change clearly
• Add solution dropwise near endpoint
• Swirl continuously for complete mixing

2. Repeating Measurements

• Perform multiple titrations
• Obtain concordant titres (within ±0.10 cm\(^3\))
• Take average of concordant values

3. Apparatus Handling

• Rinse burette with solution used
• Remove air bubbles from burette tip
• Use volumetric pipette for accuracy

(C) Calculating Percentage Uncertainty

For a single measurement:

\( \mathrm{\%\ uncertainty = \frac{absolute\ uncertainty}{measured\ value} \times 100} \)

Burette Example

• Two readings (initial + final) → total uncertainty = ±0.10 cm\(^3\)

\( \mathrm{\%\ uncertainty = \frac{0.10}{titre} \times 100} \)

(D) Total Uncertainty in Result

• Add percentage uncertainties from:
  • Burette
  • Pipette

Rule:

Total % uncertainty ≈ sum of individual % uncertainties

(E) Key Insight

• Larger titre → smaller % uncertainty
• Small volumes → larger % error

Summary

• Minimise errors using careful technique and repetition.
• Calculate % uncertainty for each measurement.
• Add uncertainties to estimate total error.

Therefore, reliable titration results depend on both precision and proper uncertainty analysis.