Chemical Analysis Bench · AnalytLab

The burette and the pipette

On a burette the 0 mark is at the top and readings increase downwards; the smallest division is 0.1 mL, so readings are estimated to 0.01 mL.

Before use: check the stopcock for leaks → wash with distilled water → rinse 2–3 times with the standard solution → fill above the 0 mark → run out quickly to flush the bubble from the tip → set the level at 0 or just below.

When reading, keep your eye level with the bottom of the meniscus. Drag “Eye level” in the reading panel to see what you get when looking up or down.

Titration: finding the concentration of hydrochloric acid

Titrate 25.00 mL of hydrochloric acid of unknown concentration with 0.1000 mol/L standard NaOH, using phenolphthalein as the indicator.

Left hand on the stopcock, right hand swirling the flask, eyes on the colour of the solution. Go faster at first; when the pink where the drops land fades more and more slowly, switch to drops and then half drops. When the last half drop turns the solution from colourless to pale pink and it does not fade within half a minute, you have reached the endpoint.

Do one rough titration, then accurate ones until two agree within 0.10 mL. c(HCl) = c(NaOH)·V(NaOH) / V(HCl).

Indicators and the titration curve

The beaker stands on a magnetic stirrer with a pH electrode dipped in, recording the pH as NaOH is added.

Near the equivalence point, half a drop (about 0.02 mL) can make the pH jump from about 4 to about 10 — the pH jump. As long as the indicator’s colour-change range lies within the jump, the endpoint error is tiny: phenolphthalein (8.2–10.0) and methyl orange (3.1–4.4) both work; litmus changes over a wide range and not clearly, so it is not used for titrations.

Switch to 0.0100 mol/L and do it again: the jump becomes narrower.

Error analysis

Every error comes down to one quantity: whether the V(NaOH) you read is too large or too small. If V is too large, the result is too high; if V is too small, the result is too low.

Tick one or more mistakes on the right, then “Run one titration automatically” and look at the result. Think: why does distilled water in the conical flask make no difference?

Testing for cations

Each of the 8 test tubes in the rack holds 2 mL of solution. Click a tube, then a dropper bottle (or pick a reagent on the right), and you can add drops to it.

First add NaOH solution drop by drop and note the colour of the precipitate; then keep adding in excess and see whether it dissolves. Repeat with aqueous ammonia and compare.

The Fe²⁺ precipitate changes colour on standing; for NH₄⁺, warm after adding NaOH and test the gas with damp red litmus paper. Fe³⁺ can also be tested with KSCN solution.

Testing for anions

Halide ions: acidify with dilute nitric acid, then add AgNO₃ solution — Cl⁻ gives a white, Br⁻ a cream and I⁻ a yellow precipitate, none of which dissolve in dilute nitric acid.

SO₄²⁻: acidify with dilute hydrochloric acid, then add BaCl₂ solution — a white precipitate. CO₃²⁻: dilute acid causes effervescence and the gas turns limewater milky. NO₃⁻: add NaOH solution and aluminium foil and warm — the ammonia given off turns damp red litmus paper blue.

Try adding AgNO₃ or BaCl₂ straight to Na₂CO₃ solution without acidifying first — why must you acidify?

Flame tests

Some metal elements give the flame a characteristic colour when heated.

Steps: dip the platinum wire in concentrated hydrochloric acid and heat it in the outer flame until the flame is its original colour → dip it in the sample → hold it in the outer flame and note the colour → clean it with acid again. The wire still carries sodium left by the previous student — burn it off first.

To see potassium’s flame colour, look through blue cobalt glass, which filters out sodium’s yellow light. Try the NaCl + KCl mixture.

Identify an unknown salt

You are given an unknown salt: two test tubes hold its solution and its solid sits on the flame-test bench. Using as few reagents as possible, work out which cation and which anion it contains and answer in the notebook on the right.

Plan the order before you start: look at the colour first, then choose reagents that can tell several possibilities apart at once.

In-class testing

Review titration and ion tests, then answer the multiple-choice questions below.