Louis Gordon and Eugene D. Salesin
Precipitation from Homogeneous Solution
Cose Institute of Technology Cleveland, Ohio
A lecture demonstration
The precipitation of nickel with dimethylglyoxime generated in situ from biacetyl and hydroxylaminel affords an ideal lecture demonstration of the advantages of precipitation from homog~neous solution (PFHS).2 I t is a simple demonstration and one which has worked successfully for the authors. When this demonstration is made in conjunction with the conventioual precipitation of uickel with alcoholic dimethylglyoxime, the difference in the physical characteristics of the two precipitates becomes strikingly evident. Although the exact mechanism3 involved in the precipitation of nickel dimethylglyoximate from homogeneous solution is not known, the following equations probably describe the process:
2
CH3-C=NOH
I
CHs-C=NOH
+ Ni+.
-[
Soluble Yellow Complex OH,.'OR
]
-
,
When the appropriate reagent solutions are first mixed, the resulting solution is virtually colorless. The solution begins to turn yellow apparently as the result of complex formation between nickel and
T h e cover
dimethylglyoxime (reaction 3). The solution becomes increasingly more yellow and subsequently long red needles of the nickel chelate begin to appear in about five minutes. I n the couventional procedure, precipitation occurs too rapidly to permit visual observation of the yellow complex except under certain condition^.^ If, from the start, a light beam is passed through the homogeneous solution, both the development of the yellow color and the formation of the precipitate are readily observed. The nickel dimethylglyoximate crystals precipitated from homogeneous solution exhibit a slightly different reddish hue than those obtained by conventional precipitation. The Experiment Solution A: Five g of amrnonit~mchloride, 3.0 g of hydraxylamino hydrochloride, and 80 mg of Nit? are dissolved in enough distilled water to hring the volume to about 200 ml. The pH of the solution is adjusted to i . 5 with ammonium hydroxide and the final volume is brought to 400 ml with distilled water. Solution B: Exactly 0.60 g purified biacetpl (Burdick and Jackson Laboratories, Muskegon, Michigan) is disnolved in enough distilled water to hring the total volume to 400 ml. Solution C: Five g of nmmonium chloride, 5 ml of eoncentrated ammonium hydroxide, and 80 mg of Nit' are dissolved in distilled water to a 400 ml volume. Solution D: Forty ml. of a one per cent solution of dimethylglyaxime in ethyl slcohol are nddod to sufficient1:1 ethyl nlcoholdistilled water solution to bring the total volume to 400 ml. Conuentional Precipitation: Pour Solution C into a one liter beaker. Add Solution D to Solution C with stirring to produce the voluminous nickel dimethylglyoximate. Pwn'pitation from Homogeneom Solution: Pour Solution A into a one liter beaker. Add Solution R with stirring. In a felr minutes crystals of nickel din~eth,vIglyoximstehegin to precipitate which will slowly grow several millimeters in length. It is possible to adjust the pH of Solution A so that the precipitation from homogeneous solution will begin in almost any desired amount of time. An approximate guide is: ~p
vH
Time of Start of Precioitat~on.Min
The difference in the physical characteristics of the two precipitates will become even more apparent upon filtration.
T h e couttmting a p p u w m r i of nii:lir,l ~ l i m c t l ~ y l ~ l y ~ , r ipremate h . sample cipitates is shown hy n pair of p l i r ~ t o r n i c r o ~ m l ~ Tho pictured a t t h e left was prcripitatrtl from hamogrncoos solution as described in t h e text. T h e one shown a t t h e right was obtained by conventional methods.
16
/
Journal o f Chemical Education
The authors wish to acknowledge the partial assistance of the Atomic Energy Commission under Contract AT(l1-1)-582. L., Talanta, 5, 81 (1960). SALESIN,E. D., AND GORDON, M. L.. AND WILLARD.H. H.. "PreGORDON. , I,.. - , SALUTSKY. eipitntion from Homogeneom Solution," John Wiley and Sons, Inc., New York, 1959. "ALEESIN. E. D., A N D GORDON, L., research in progress. ' D I E E L , H., "The Applications of the Diaximes to Analytical Chemistry," The G. Frederick Smith Chemical Co., Columbus, Ohio, 1940, p. 13.
