The Preparation of Potassium
Paul 5. Porkozim
Northeastern Ill~no~s State College Chicago, 62625
Hexacyanocobaltate(lll)
Experimentation on the undergraduate level in the area of inorganic coordination chemistry has made significant strides since the statement of Iiirschner (1) in 1956 concerning the almost complete exclusion of such topics from general chemistry courses. Recently, the aqueous cobalt cyanide system' has provided a most effective teaching expcriment for a freshman honors laboratory section. The elaboration of experimental Section I1 coupled with appropriate spectral investigations could extend the applicability of the experiment t o advanced undergraduate classes. When performed in an aqueous solution open t o the air, the reaction scheme is apparently as follows (2) Scheme A 2 CoCI.
-+ + -
+ 4 KCN
2 Co(CN)..2..5 H.0
2 C O ( C N ) ~H.0 . ~ . ~ 6 KCN 2 K,[Co(CN)J
Notes:
On
+ 4 KC1
2 Ka[Co(CN)d
Ks [(CN)sCoOOCo(CN)rI
(1)
(2) (3)
(a) Steps (2-4) occur quite quickly and are essentially over within the first 5 min after addition of reac-
tants. (h) Elemental analysis best places the hydrated cobalt(11) cyanide here in the intermediate 2.5 water form. Color of this compound varies from shades of gray to reds or red-browns but appears to be ineansequential for the most part.
If reactions ( 2 4 ) are run under an inert atmosphere of nitrogen, an entirely different mechanistic course is followed a t the important oxidation step, the product, however, eventually being potassium hexacyanocobaltate(II1) as above. Scheme B 2 CoCI?
+ 4 KCN
-r
2 Co(CN)*.2.5H?O
+
-+
2 Co(CN)a.2.5 H20
+ 6 KCN
+
2 HnO 2 KJ[CO(CN)~] 2 KCN 2 K,[Co(CN)al
+ 4 KC1
2 Ka[Co(CN)J Hdg)
+ 2 KOH
(1)
(2) (5)
Reaction (5) imparts an addit.ional factor. If the evolved hydrogen gas is collected and measured, ideal gas law calculations can be employed t o evaluate overall reaction stoichiometry by checking yield of product I
