John C. Sheppard' and Reilly C. Jensen
Diego State College Sun Diego, California
San
A radiochemistry experiment
Extraction of Cobalt(l1) Complexes
The extract,ion of ch~oro-complexesof cobalt(I1) with long chain tertiary amines illustrates to students the use of tracer techniques as applied to distribution measurements2 and provides insight into the behavior of the cbloro-complexes of cobalt(I1). The experiment logically involves a study of the dependence of the distribution ratio upon hydrochloric acid concentration followed by a study of the effect of amine concentration. The former establishes the optimum hydrochloric acid concentration for the extraction of cobalt(I1). The latter provides information concerning tion of the cobalt(I1) species found in the organic phase. About 6 hrs. are required for a student to perform t,he necessary laboratory work. The experiment begins with the preparation of a 10% by volume solution of trilaurylamine3 in xylene. This amine solution is equilibrated with 6 M hydrochloric arid so t?at no net hydrochloric acid transfer will take place during the extraction operations. Small volumes of the resulting amine hydrochloride solution are subsequently equilibrated with equal volumes of 1 to 12 11l HC1 solutions each containing about 10' r/m of cobalt-60 tracer. After separation of the two 'Present address, Hmford Lahorstories, General Electric Co., Richlsnd, Washington. See far example: MAHLMAN, H. A,, L E D U I C OG~. , W., AND MOORE, F. L., Anal. Chem., 26, 1939 (1954). STrilrturylmine may be ohtained from Archer Daniels Midland, 700 Investors Building, Minneapolis, Minnesota.
Figure 1. A plot of the distribution rotio lc/m/ml of cobalt-60 in the amine-xylene phase divided b y the c/m/ml of cobalt40 in the hydrochloric acid phase1 0 3 a function of hydrochloric ocid mncentratior The eitrocting rolvent was 10% v / v hilourylamine in xylene, and the hydrochloric ocid concentrations are expressed in moler/liter.
phases by centrifugation a sample of each phase is transferred to counting tubes by means of transfer pipets and the relative activity measured in a well-type scintillation counter. The success of the experiment is in jeopardy at this point unless the student takes special rare not to contaminate the aqueous phase with the tracer-rich organic pkase. The distrihution ratio is obtained by dividing the c/m/ml of the organic phase by the c/m/ml of the aqueous phase. Typical student data for this experiment is illustrated in Figure 1. There is a maximum in the distribution ratio at about 8 M HCI. For the determination of the distribution ratio dependence on the amine concentration, the 10% trilaurylamine solution is diluted with xylene to make several solutions having trilaurylamine concentrations ranging from 0.1 to 5%. These solutions are tl-en equilibrated u-ith cobalt-GO, hydrochloric acid solutions and the distrihution ratio determined as described above. The hydrochloric acid concentration is 6 M in order to reduce the problem of contamination of the aqueous phase by the tracer rich organic phase while still keeping ronveniently measurable distribution ratios a t low amine concentrations. Under conditions of lorn amine concentrations the distribution ratio, D, skould obey a limit,ing expression, D
=
Q' (Amine)"
with deviations expected at higher amine concentrat,ions due to the formation of new complexes. Q' is a
Figure 2. A plot of the di%tribuIion ratio against the square of lhe omine concentration, rhowing the deviation from the second power d e pendence at higher omine mncentrotionr. The hydrochloric m i d concentration war 6M.
Volume 40, Number I, lonuory 1963
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formal equilibrium constant for the extraction of cobalt(I1) from a particular concentration of hydrochloric acid. Plots of the distribution ratio versus various powers of the amine concentration may be used to determine the value of n. Alternately, n may he determined from the slope of the line when the log of the distribution ratio is plotted versus the log of the amine concentration. Figure 2 is a typical student plot of the distribution ratio against the square of the amine concentration and shows the deviation from this relationship a t higher amine concentrations. Once the amine concentration dependence, n, is found, Q' may be calculated. The student should explain the significance of the maximum value observed in the plot of the distribution
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ratio versus the hydrochloric acid concentration. The student should also determine the composition of the cobalt(I1) species present in the organic phase. If, for example, the plot shows that there is a second power amine dependence, as our students have found, it then suggests that the cobalt(I1) species in the organic phase may be C O C L ( H ~ ~ Recent )~. work of Good and Bryan4have confirmed this assumption. Suggested variations to the above procedure are the extraction of cobalt(I1) from solutions of thiocyanate, oxalate, and other complexing anions. Other amines such as tri-n-octylamine, triisooctylamine, and tri-ndecvlamine can also be used.
' GOOD,M. L., AND BRYAN,S. E., J. In079. and Nuclear Chem., 20,140(1961).
