In Duma's method, nitrogen is estimated as `NH_3`.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Dumas' Method**: Dumas' method is a technique used to determine the nitrogen content in organic compounds. It involves the combustion of the organic sample in the presence of copper oxide (CuO) in a controlled atmosphere.

2. **Chemical Reaction**: When an organic compound (which can be represented as \( C_xH_yN_z \)) is heated with CuO, it undergoes combustion. The general reaction can be represented as:
\[ C_xH_yN_z + CuO \rightarrow CO_2 + H_2O + N_2 \]
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