Neutralisation and titration.

Step 1 of 5

Neutralisation

When an acid meets a base, the hydronium and hydroxide ions join into water:

$\mathrm{H_3O^+ + OH^- \rightarrow 2H_2O}$⁠

That reaction is called neutralisation. On the bench in the Scene a titration is set up: the conical flask holds 25 mL of hydrochloric acid with a few drops of phenolphthalein, an indicator that is colourless in acid and pink in base. The burette above it holds sodium hydroxide. The tap at the bottom of the burette lets it run, and a magnet turning under the flask stirs it. Volume added says how much base has run out of the burette, and the Drop by drop chart draws the pH.

Step 2 of 5

The curve

Here Volume added and Look inside are fixed. Predict, then run it.

Step 3 of 5

The equivalence point

The point at which the acid and the base added hold exactly the same number of moles is called the equivalence point. There:

$c_a V_a = c_b V_b$⁠

Now the flask holds 20 mL of acid, and the base in the burette is more dilute: 0.08 mol/L. The readings beside the Scene show both solutions.

Step 4 of 5

Running it in

In a real titration you open the tap quickly at first, and slowly, drop by drop, near the equivalence point. Volume added moves in steps of 0.1 mL, about one drop.

Step 5 of 5

An acid of unknown concentration

That is titration’s main use: measuring an unknown concentration. You fill the burette with a solution of known concentration, titrate until the colour flips, and read off how much ran out. Here Look inside is open at Whole curve, and the readings do not show the acid’s concentration.