LC CHEMISTRY - EXPERIMENT: Sodium Thiosulphate and standardise it by titration against a solution of Iodine

REBECCA'S LC CHEMISTRY - EXPERIMENT: Sodium Thiosulphate and standardise it by titration against a solution of Iodine KNOWT

Explain how iodine, a non-polar substance of very low water solubility, is brought into aqueous solution

reaction with potassium iodide

The iodine solution was made up in a x cm³ volumetric flask. Describe the procedure for measuring 25.0 cm³ of this solution into a conical flask

• pour some iodine solution into a clean dry beaker

• use pipette

• rinsed with deionised water
  rise with iodine solution

Name a suitable indicator for this titration.

starch

At what stage is the indicator added?

straw yellow

State the colour change at the end point in the presence of the indicator

blue black to colourless

Explain why the use of distilled water instead of deionised water throughout this experiment would be likely to ensure a more accurate result

deionised water only ions removed

primary standard

• pure

• stable

• high molecular mass

• of known concentration

Describe how the crystalline thiosulfate was dissolved, and how the solution was transferred to the volumetric flask and made up to exactly 500 cm³

• rinse into beaker

• stir

• dissolve pour through funnel

• add risings

• add water until bottom of meniscus

• read at eye level

Pure iodine is almost completely insoluble in water. What must be added to bring iodine into aqueous solution?

potassium iodide KI

A few drops of freshly prepared starch solution were added near the end point as the indicator for this titration.
What sequence of colours was observed in the conical flask from the start of the titration until the end point was reached?

• red

• orange

• straw yellow

• blue-black

• colourless

what properties are NOT primary standard

• unpure

• unstable

• low molecular mass

ratio

1 I₂ : 2 S₂O₃²⁻

oxidisng agent

I₂

reducing agent

S₂O₃²⁻

what happens when u add starch too early

strong complex formed with starch which won’t decolourise