Quantitative Range of Nessler's Reaction with Ammonia.

There is considerable variation in ways of preparing Nessler’s reagent as well as in the manner of forming the colored ammonobasic mercuric iodide. Two critical values are the ratio of KI to Hg19, i.e., x in HgL.xKI, and the final concentration of mercury. The value x determines the stability of solutions and to some extent the relative sensitivity to NH3. Vanselow (2) concluded that x should be 2.10, which is a 5% excess of KI over the theoretical 2 moles. Table 1 shows values of x and the concentration of mercury in several reagent preparations. Table 2 shows a range of x between 2 and 3. The mercury concentration in the final color solution ranges from 0.002 to 0.02 moles per liter. In view of a 10-fold range, practical consideration would indicate a need for knowledge of the lowest workable concentration.