A Simple Homogeneous Precipitation Preparation of%hromium(ll) Acetate John C. Reeve Building 207, The Technical University of Denmark, DK-2800 Lyngby, Denmark I have for a number of years in an introductory preparative inorganic chemistry course used the preparation of Cr" acetate as an example of an exercise requiring the use of typical oxygen-free atmosphere techniques. The apparatus used was a simplified version of the elaborate one in a well-known hook on preparative inorganic chemistry (1) and involved two vessels with sintered glass fiters and a supply of pure nitrogen. Several other rather elahorate procedures have appeared in Inorganic Syntheses (2). Most of these procedures involve adding a Cr" salt solution to concentrated sodium acetate solution giving a reactive, rather light pinkish rust-colored powder. I t seemed that the reverse addition would avoid undesirable local high pH values during the addition and give a slower precipitation and larger, possibly less air-reactive crystals. The product of reverse addition was indeed darker and reacted noticeably more slowly with the air. Homogeneous precipitation by slow pH rise due to urea hydrolysis resulted in large (1-3-mm), apparently almost black. elistenine crvstals. which could he filtered and washed quickl; in air. ;i'hiprod;ct could be kept in a normal wellstoppered reagent hottle with no sign of significant change after several months. If left in the laboratory air, crystals started to react and exfoliate. Under the microscooe. the crystals, which exhibit well-defined facets, are seen to he red-orange. A crystal that had been crushed to a powder resembled the usually prepared material. Procedure In a 300-ml conical flask with a B29 ground neck are placed 15 g C P chloride hexahydrate, 20 ml concentrated hydrochloric acid, 20 ml H20,9 ml of glacial acetic acid, and 6 g of urea. A powder funnel is used for all additionsof solids to keep the sides and neck of the flask clean. The flask is gently swirled until all solids are dissolved. Very coarse zinc powder is added in one 7.5-g portion, and the flask is gently swirled again. After a few seconds of vigorous reaction the flask is closed using a double Bunsen valve or a graund-glass nonreturn valve closed by a Bunsen valve (seefigure).When the reaction subsides,the flask is placed on a steam bath, and after some 20 min a clear, deep blue acid solution of Cr" salt is obtained. After 20-24 h on the steam bath the flask contains large, almost black crystals of Cr" acetate in a pale green solution. The flask is cooled, and the crystals are filtered quickly on a sintered glass suction filter, washed several times with water, alcohol, and ether, and sucked dry. Finally the crystals are spread on a filter paper and weighed. The yield is 70%to 90%. Analysls Only one of the preparative descriptions cited (2(d)) gives analytical results. Two other publications, one dealing with structure (3) and one dealing with magnetic measurements ( 4 ) , give analytical results. The results apparently depend on the method of washing and drvine. The method of oreoaration used here is most +da;to ihe method employed i n the structure determination work (3). Accordingly, the analysis is compared with theory and the results of ref. (3) (see table). The CrlI analysis was performed by dissolving the material in a cold sulfuric acid solution of excess persulfate containing a drop of silver nitrate, adding excess of potassium iodide and titrating the liberated iodine with thiosulfate after about 1h. Discussion The Crm is reduced by the zinc in the strongly acid solution, 444
Journal of Chemical Education
Analyllcal Results Atomic ratios
Analysis of prcducts (%) Calculateda Found Ref. (3
Cr
C
H
N
CI"ICr
C/Cr
H/C
27.6 25Bb 26.0
25.5 24.eb 25.2
4.29 4.15" 4.48
0 0"
1.00 0.97 0.97
4.00 4.19 4.19
2.00 1.99 2.12
...
'Based an C I ( C W ~ ) ~H20. . 'Analysis carried M by Microsnslytlcal Depamnont. Copenhagen Uniuenity, Uni-
verohetspanen 5. Copenhagen.
Sealing devices fw the conical reaction flask: B. Bunsen valve: T, rubber tube; C, cut; P, glass plug: V, ground glass nonreturn valve. and vigorous hydrogen evolution sweeps the flask free of air. 2 C P + Zn
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2Cr2++ Zn2+
The resultant solution is deeo blue due to the aauo-Cr" ion. The urea hydrolyzes graduaily in the acid solu&n, and pH slowly rises with the production of NH4+.
The COz-helps . to keep air excluded, and as the pH rises, the concentration of acetate ions rises slowly producing an acetic acid-acetate buffer system. This leads to a slow precipitation of Cr" acetate, a process which tends t o slow pH rise by consumption of acetate ions. If too much urea were used, there metal hvdroxides. Homoeecould he a risk of orecioitatine . . neous precipitation is a technique more commonly encountered in analytical chemistry. Higher homologs are known (6). The dark crystals dissolve readily in dilute hydrochloric acid free from oxygen to give a blue solution. The storeahle dark crystals could he useful as a Cr" shelf-reagent. The Cr" acetate produced is a dimer, Cr2(CH3C00)4,2Hz0, with two chromium atoms held together by four acetate groups so that there is electron pairing between the two chromium atoms (3). In solution the dimer is in equilibrium with the monomer, and it is the latter which reacts (7,s).No reference t o suitable liquids for recrystallization has heen found, although aqueous media have been used for very small quantities (3). The oxalate, which is stated to he even more stahle once
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Volume 62
Number 5
May 1985
445
