In Kjeldahl's method for estimation of nitrogen, CuSO4 acts as:

Step 1: Understand Kjeldahl's Method

Kjeldahl's method estimates nitrogen in organic compounds. The compound is heated with conc. $\ce{H2SO4}$: $\ce{Organic-N + H2SO4 ->[Delta, \text{catalyst}] (NH_4)_2SO_4}$

The digestion requires a catalyst to speed up the conversion. Three reagents are commonly used:

1. $\ce{CuSO4}$ — catalyst
2. $\ce{K2SO4}$ — raises the boiling point of $\ce{H2SO4}$ (speeds up digestion)
3. $\ce{Hg}$ or $\ce{Se}$ — catalyst (but toxic)

Step 2: The Role of $\ce{CuSO4}$ Specifically

$\ce{CuSO4}$ acts as a catalyst in Kjeldahl's digestion. It accelerates the oxidative breakdown of the organic compound by conc. $\ce{H2SO4}$, speeding up the conversion of organic nitrogen to ammonium sulphate.

Step 3: Evaluate Each Option

| Option | Role | Correct? | |---|---|---| | (a) Reducing agent | $\ce{CuSO4}$ is oxidising in nature (Cu²⁺) | ❌ | | (b) Catalytic agent | $\ce{CuSO4}$ accelerates the digestion reaction | ✅ | | (c) Hydrolysis agent | Hydrolysis is not the mechanism in Kjeldahl's | ❌ | | (d) Oxidising agent | While $\ce{Cu^{2+}}$ can oxidise, it is used in catalytic quantity, not stoichiometric | ❌ |

Answer: (b)

Key Points to Remember:

• Kjeldahl's method: compound + conc. $\ce{H2SO4}$ + $\ce{CuSO4}$ + $\ce{K2SO4}$ → $\ce{(NH4)2SO4}$
• $\ce{CuSO4}$: catalyst for the digestion
• $\ce{K2SO4}$: raises bp of $\ce{H2SO4}$ → faster digestion
• After digestion: add NaOH → $\ce{NH3}$ gas evolved → absorbed in standard acid → titrate
• Kjeldahl formula: $\%N = \frac{1.4 \times N \times V}{W}$ where N=normality, V=volume of acid (mL), W=mass (g)