What is Nessler's reagent? How is it prepared ?

Explanation

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

1. **Definition of Nessler's Reagent:**
Nessler's reagent is an alkaline solution of potassium mercury(II) iodide, represented chemically as K2HgI4. It is used primarily for the detection of ammonia in water.

2. **Preparation of Nessler's Reagent:**
To prepare Nessler's reagent, you need to follow these steps:
- Start with a solution of mercury(II) chloride (HgCl2).
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