Q4(a). Indicators for 0.1 M ethylenediamine with HCl
Suggest indicators in the following titration: 0.1 M ethylenediamine and HCl. Given K_(b₁)=8.5×10⁻⁵, K_(b₂)=2.7×10⁻⁸ at 25°C. (Assume the quantitative formation of mono- and di-protonated species.)
Answer in brief
First equivalence point (enH⁺) pH =8.18: phenol red or cresol red. Second equivalence point (enH₂²⁺) pH =3.95: methyl orange or bromocresol green.
Key points
- Indicators with a range around 8: phenol red (6.8–8.4, yellow → red), cresol red (7.2–8.8) or thymol blue (second range 8.0–9.6). The break is only modest because pKₐ₂-pKₐ₁=3.5<4: use a mixed indicator or compare with a colour…
- Indicators with a range around 4: methyl orange (3.1–4.4, yellow → red) or bromocresol green (3.8–5.4, blue → yellow).
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
Let en = H₂NCH₂CH₂NH₂. Titration: en →enH⁺ →enH₂²⁺.
pK_(b₁)=4.07, pK_(b₂)=7.57 ⇒ pKₐ(enH₂²⁺)=14-7.57=6.43, pKₐ(enH⁺)=14-4.07=9.93
First equivalence point: enH⁺, amphiprotic
pH=12(pKₐ₁+pKₐ₂)=12(6.43+9.93)=8.18
Indicators with a range around 8: phenol red (6.8–8.4, yellow → red), cresol red (7.2–8.8) or thymol blue (second range 8.0–9.6). The break is only modest because pKₐ₂-pKₐ₁=3.5<4: use a mixed indicator or compare with a colour standard.
Second equivalence point: enH₂²⁺, a weak acid
Volume doubles: C=0.1/3V=0.0333 M; Kₐ=K_w/K_(b₂)=3.7×10⁻⁷.
[H⁺]=√(KₐC)=(3.7×10⁻⁷)(0.0333)=1.11×10⁻⁴ M pH=3.95
Indicators with a range around 4: methyl orange (3.1–4.4, yellow → red) or bromocresol green (3.8–5.4, blue → yellow).
| Stage | pH | Indicator |
| start (0.1 M en) | 11.46 | – |
| half-way I ([en]=[enH⁺]) | 9.93 | buffer, pKₐ |
| I equivalence, enH⁺ | 8.18 | phenol red / cresol red |
| half-way II | 6.43 | buffer |
| II equivalence, enH₂²⁺ | 3.95 | methyl orange / bromocresol green |
use methyl orange for the total end point when the first break is too shallow; ethylenediamine standardised against HCl is the primary standard for ion-exchange regeneration.
Coversdiprotic baseIndicators for 0.1 M ethylenediamine with HCl
All parts of Question 4
- 4(a) Indicators for 0.1 M ethylenediamine with HCl10 marks
- 4(b) Homogeneous precipitation10 marks
- 4(c) Working principle of redox indicators10 marks