Product Short Description: NESSLERS REAGENT For ammonia & ammonium salt Product Description: Medical Definition of Nessler's reagent an alkaline solution of potassium mercuric iodide used in chemical analysis especially in a test for ammonia in aqueous solution (as when obtained from water, blood, or urine) with which it forms a yellowish brown color or precipitate called also Nessler's solution. : an alkaline solution of potassium mercuric iodide used in chemical analysis especially in a test for ammonia in aqueous solution (as when obtained from water, blood, or urine) with which it forms a yellowish brown color or precipitate. — called also Nessler's solution.Medical Definition of Nessler's reagent : an alkaline solution of potassium mercuric iodide used in chemical analysis especially in a test for ammonia in aqueous solution (as when obtained from water, blood, or urine) with which it forms a yellowish brown color or precipitate. — called also Nessler's solution.