Preparation of Nessler’s Reagent .L P. VANSELOW, University of California Citrus Experiment Station, Riverside, Calif.
D
ISCONCERTING experiences with Sessler’s reagent
the red precipitate was noted. These results are given in Table 11. Having thus determined t h a t increasing concentrations of potassium hydroxide decrease the magnitude of the excess of potassium iodide required t o prevent the precipitation of niercuric iodide, it remains t o see m-hat effect the potassium hydroxide and the excess potassium iodide have on the color formation n-ith ammonia. Reasoning from the equation usually as-igned t o the Sessler reaction (4)
prepared according to formulas found in textbooks and the literature include slowness in the development of color, formation of a red precipitate, and the development of off colors, especially with low concentration of ammonia. The studies herein presented were undertaken to find t.he reasons for these disturbing factors and t o formulate a Sessler’s reagent satisfactory in every respect. A consideration of the phase-rule diagram (Figure 1) of Dunningham (1) for the three-component system potassium iodide-mercuric iodide-water sheds light on t’he formation of the red precipitate which forms when some Sessler‘s reagents are diluted.
NH3
+ 2IC3HgIa + 3KOH
= 7KI
+ 2H10 + HgOH.XH.HgI
it follorv~that illcreasing the concentration of potassiurnlhydroxide should increase the rate of color formation, and conversely increasing amounts of excess potassium iodide should slow down the color formation. The influence of potassium iodide and potassium hydroxide concentrations on the rate of color formation with ammonia waq extensively investigated.
Point B on the diagram has approximately the composition: = 53 per cent, and HzO = 9 per cent: this corresponds rather closely to the hypothetical substance KzHg14. The two areas ABCDE and DEG are the only ones of great interest to us; the former is t’he only area of solutions with no solid phases present, and the latter represents solutions with solid mercuric iodide present. Dottedline X E represents the process of dilution of a solution having the composition represented by point X. Thus a concent,rated solution of pot,assium mercuric iodide represented by point X ivill, when diluted to the concentration ordinarily used in making the Sessler test, precipitate red mercuric iodide Tvhen the boundary of area EDG is reached. One cannot easily ascertain from Dunningham’s diagram how much potassium iodide must be present to avoid entering this area of mercuric iodide precipitation on dilution. Hon-ever, this can be determined approximately by simple dilution experiments. A stock 0.4 N solution of potassiuni mercuric iodide \\-as prepared; 5-ml. portions were diluted to 100 ml. with increasing percentages of excess potassium iodide present, and the time in minutes for the formation of tmhered precipitate of mercuric iodide was noted. These result’s are given in Table I.
KI = 38 per cent, HgIz
F-
The addition of a fourth coinp~nent,potassium hyclmxide, t o the three-component diagram of Dunningham is of little interest to us except as it influences t’he position of line D E . This, too, can be determined approximately by simple dilution experiments on the potassium mercuric iodide solution with varying amounts of potassium hydroxide atlded. Varying amounts of both pot’assium iodide antl potassium hydroxide were used in these dilution tests in which the final concentration of the mercuric ion was 0.01 S in every case. The time elapsing between the dilut.ion of the potassium mercuric iodide solution from 0.4 to 0.01 -I‘antl the first appearance of
-
E HaO
KI
G-Hjlt
FIGURE1. DCXXIXGH.AN’S DIAGR.AX FOR THE THREECOMPOXENT SYSTEM:I
