How is `H_2S` used as an analytical reagent?

Explanation

**Step-by-Step Solution:**

1. **Understanding H₂S**: Hydrogen sulfide (H₂S) is a colorless gas with a characteristic odor of rotten eggs. It is important in analytical chemistry as a reagent.

2. **Role of H₂S in Analysis**: H₂S is primarily used to identify and separate certain metal ions in qualitative analysis. It acts as a precipitating agent for various metal sulfides.

3. **Target Group**: H₂S is specifically used to determine the basic radicals of Group 2 in qualitative analysis. These radicals include lead (Pb), copper (Cu), mercury (Hg), cadmium (Cd), and bismuth (Bi).

...