Titration Skills and Error Sources

Detailed Explanations

A. Workflow: setting up and running a titration

1. Rinse the burette with distilled water, then with the titrant solution.
2. Fill the burette, run some solution through the jet to remove any air bubble, and remove the funnel.
3. Pipette a fixed volume into a conical flask (use a pipette filler), add indicator.
4. Record the initial burette reading (read the lower meniscus at eye level).
5. Titrate with swirling; near the end-point, add titrant dropwise.
6. Rinse flask walls with distilled water during titration (does not change moles; it just washes splashes down).
7. Record the final burette reading and calculate the titre.

Mini example (recording):

• initial = 0.10 cm³, final = 25.30 cm³ → titre = 25.20 cm³

B. Workflow: concordant titres and the mean

1. Do one rough titration to find the approximate end-point.
2. Do repeat titrations until two (or more) titres are concordant (use the exam’s tolerance).
3. Mean only the concordant titres.

Mini example:

• titres: 24.95, 25.05, 25.00 (within 0.10 cm³) → mean = 25.00 cm³

C. Writing “error → effect → improvement” (ACE marks)

Good evaluation answers are specific and directional.

Example: air bubble in burette tip Because some of the burette reading is used to fill the tip before any solution is delivered, therefore the recorded volume is larger than the true delivered volume (titre is too high).

Then the improvement targets that weakness:

• “Ensure the jet is filled and free of bubbles before starting.”

D. Preparing a standard solution

1. Accurately weigh a suitable primary standard. If the method uses weighing by difference, record both balance readings and subtract them.
2. Dissolve the solid completely in a beaker using distilled water.
3. Transfer the solution quantitatively into a volumetric flask. Rinse the beaker, stirring rod and funnel into the flask so that no measured solute is left behind.
4. Add distilled water to just below the mark, then use a dropping pipette until the bottom of the meniscus is on the mark at eye level.
5. Stopper and invert the flask several times so that the concentration is uniform.

Calculate the concentration only after converting the flask volume to dm³:

c = n/V

E. Redox titrations

The glassware technique is similar, but the chemistry at the endpoint is an electron-transfer reaction rather than neutralisation.

1. Write the balanced overall redox equation, using half-equations if needed.
2. Use the measured titre to find the amount of titrant.
3. Apply the stoichiometric ratio from the balanced equation. Do not assume a 1:1 ratio.
4. Use the endpoint method stated for that system. Potassium manganate(VII) can be self-indicating; other systems may need a suitable indicator.
5. If acid is required, use the acid specified. Another acid may take part in a side reaction.

F. Iodimetric and other indirect titrations

In an indirect titration, the substance being measured first produces a second substance, which is then titrated. In an iodometric titration, the analyte often liberates iodine from iodide ions. The iodine is then titrated with thiosulfate. This differs from direct iodimetry, in which iodine itself is the titrant.

I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻

Work backwards through both equations:

n(S₂O₃²⁻) → n(I₂) → n(analyte)

Add starch only when the iodine solution has become pale yellow. Adding it too early produces an intense iodine–starch colour that can make the endpoint slow and difficult to judge. Near the endpoint, add thiosulfate dropwise until the blue-black colour just disappears.