Basic acid/base titrations

Types of acid base titration There are four different types of acid/base titrations; 1) Strong acid with strong base. 2) Strong acid with weak base, 3) Weak acid with strong base, 4) Weak acid with weak base.

Titration of 25ml 0.1M HCl and 0.1M NaOH Strong acidStrong base

Choosing an indicator For strong acid/strong base pH at equivalence = 7 An indicator’s end point must correspond to the equivalence point. In simple terms it must be on the “steep” part of the curve.

Strong acid – Strong base with phenolphthalein pH changes rapidly from 4 – 10 Phenolphthalein has an effective pH range of c8.2 – 10 So phenolphthalein is a suitable indicator.

Strong acid – Strong base with methyl orange Methyl orange, has an effective range of 3.6 – 4.4 pH changes rapidly from 4 – 10 So methyl orange is a suitable indicator.

Titration of 25ml 0.1M HCl and 0.1M NH 3 Strong acidWeak base

Strong acid – Weak base pH at equivalence < 7 Rapid pH change occurs c4-8

Strong acid – Weak base Rapid pH change occurs c 4-8 Phenolphthalein has an effective pH range 8.2 – 10. Equivalence < effective pH range so phenolphthalein is not suitable.

Strong acid – Weak base Methyl orange, effective range 3.6 – 4.4 pH changes rapidly from 4 – 8. So methyl orange is suitable. Bromothymol blue, pH 6 – 7.6, would also be suitable.

Titration of 25ml 0.1M CH 3 CO 2 H and 0.1M NaOH Weak acidStrong base

Weak acid – Strong base pH at equivalence > 7 Rapid pH change c8 – 11

Weak acid – Strong base Rapid pH change occurs c 8 – 11 Phenolphthalein, pH range of 8.2 – 10 So phenolphthalein is suitable.

Weak acid – Strong base Methyl orange, effective range of 3.6 – 4.4 pH changes rapidly from So methyl orange is not suitable. Neither could Bromothymol blue, pH 6 – 7.6.

Titration of 25ml 0.1M CH 3 CO 2 H and 0.1M NH 3 Weak acidWeak base

Weak acid – Weak base pH at equivalence depends on relative strength of acid and base No rapid pH change.

Weak acid – Weak base Phenolphthalein is not really suitable.

Weak acid – Weak base But neither is methyl orange For a titration of weak acid and weak base there is no ideal indicator.

Summary Strong acid vs strong base rapid change 4 -1o Strong acid vs weak base rapid change 4-8 Weak acid vs strong base rapid change 8 – 11. Weak acid vs weak base no rapid change.

Summary for phenolphthalein Suitable for strong acid vs strong base and weak acid and strong base. But not for strong acid and weak base and weak acid and weak base.

Summary for methyl orange Suitable for strong acid vs strong base and strong acid and weak base. But not for weak acid vs strong base or weak acid and weak base.

Sketching titration graphs. 1) Start at an appropriate pH. Eg 4–5 for a weak acid and 1 for a strong acid. 2) Show neutralisation taking place at a suitable volume of base, using the stochiometry. 3) Show a suitable rapid pH change. Eg 4-10 for strong acids and bases, 6-10 for strong base and weak acid. 4) Finish at an appropriate pH, eg 14 for strong bases.

Graphical determination of pKa Titration graphs can be used to find pKa. K a. = [ H + ] [A - ] / [HA] At the mid point, half way from the start of the titration to its end point; [HA] = [A - ] K a = [ H + ] pKa = pH

Read off the volume at the end point. Halve it to get the mid point. 25 ml 12.5 ml Eg; For ethanoic acid and sodium hydroxide

Draw a line up to the graph at 12.5ml. Then draw a line to the y axis. Read off the pH, which = pKa