E
60
I
-6-
25-
20-
$ z
15-
S
io-
I
,
5c 0
-200
I
I
-400
-600
Miiliwolls v 5 S C E
l
I
l
-800
-200
I
-400 -600 M i l l ~ v o l l svs S.C.E.
I
[ Cd”]
-800.
Figure 2. 4 p F Cd(I1) 0.1F Na2S04 A . Average faradaic current over 4 seconds as a function of E.
B. Specific charge (pc/cm2)on the DME as obtained in same solu-
tion Specific charge on DME obtained by integration of ac determined differential double layer capacity in 0.1F Na2S04 was then integrated (by counting squares and by weight) from -0.200 V us. SCE to each potential (the constant of integration was chosen as the charge determined by the present technique at - 200 mV). The closed circles in Figure 2 show these data thus obtained from the polarographic integration method. The agreement is quite good and would be better if centered about the point of zero charge. A series of solutions varying the Cd(I1) concentration from lpM to 10pM was run. The “polarographic” curve as well as the qo us. E curve was obtained in the potential region -0.200 to -0.850 V 6s. SCE for each solution. Three runs
(pM)
Figure 3. Wave height of faradaic current as a function of concentration of Cd(I1) were made at each potential in each solution and the mean value was used. The wave height as function of concentration of Cd(I1) is shown in Figure 3. The method described appears to have considerable merit as a tool to determine both the specific charge and the point of zero charge us. the DME. It is somewhat useful for trace analysis. Although quite tedious to perform manually, the method is simple and straightforward with automatic equipment. The use of an on line computer (7) simplifies and enhances the utility and accuracy of the method. RECEIVED for review July 24, 1967. Accepted October 2, 1967.
(7) G. Lauer and R. A. Osteryoung, Division of Analytical Chemistry, 154th National Meeting, ACS, Chicago, Ill., September 1967.
Separation of Gaseous Organic Fluoronitrogens by Liquid Column Chromatography R. L. Rebertus, K. R. Fiedler, and G . W. Kottong Central Research Laboratories, Minnesota Mining and Manufacturing Co., S t . Paul, Minn. 55101 ORGANICFLUORONITROGENS are frequently synthesized as complex mixtures. For example, the direct fluorination of guanylurea (1) produces unsaturated compounds containing the N,N,N’-trifluoroamidino moiety together with saturated perfluoroamines containing up to three difluoroamino groups bonded to the same carbon atom. Gas chromatography has been used most extensively as a method for separating these compounds (1, 2). The main limitation of this technique is the risk of explosion involved in trapping relatively large quantities of the neat fluoronitrogens. In this paper we describe the liquid column chromatographic separation of some N,N,N’-trifluoroamidinesand saturated perfluoroamines. The explosion hazard is reduced through the use of inert solvents, and separations have been carried out on both micro and macro scales. Qualitative tests which (1) R. J. Koshar, 4th International Symposium on Fluorine Chem-
istry, Estes Park, Colo., July 1967. (2) R. J. Koshar, D. R. Husted, and R. A. Meiklejohn, J. Org. Chem. 31,4232 (1966).
distinguish between the saturated perfluoroamines and the N,N,N’-trifluoroamidino compounds in the column effluent are also described. EXPERIMENTAL
Materials. Bis(difluoroamino)difluoromethane, tris(difluoroamino)fluoromethane, tetrafluoroformamidine, and pentafluoroguanidine were prepared by the methods described previously (1, 2). Solutions of the fluoronitrogens were prepared by condensing measured volumes of the gases into 3M brand inert fluorochemical liquid FC-75 (b. 216” C, d. 1.76) or heptane. Silica gel (100-200 mesh) was dried at 110” C and exposed to the atmosphere for a minimal period of time during the packing step. Procedure. Depending upon the sample size, safety equipment ranged from small l/a-inch laboratory shields to 14-inch concrete barricades with remote control facilities, and the columns ranged in size from 0.8 X 8 cm to 10 X 100 cm. The sample reservoir, chromatographic column, and receiver were cooled to about 0” C to avoid excessive loss of the gaseous fluoronitrogens. A 2 to 4% (w/w) solution of the VOL. 39, NO. 14, DECEMBER 1967
0
1867
80
-E 60 \
rn
E
i
.-0
40
+ L
c 8) 0
s 0 20
I 0
2
4 6 8 IO Volume of Effluent, ml
I 12
Figure 1. Separation of tris(difluoroamino)fluoromethane and pentafluoroguanidine by liquid column chromatography Column: 0.8 X 8 em containing silica gel (100-200 mesh) in 3M brand inert fluorochemical liquid FC-75 Sample: 3 ml of fluorochemical containing 0.5% (w/w)tris(difluoramino)fluoromethaneand 2 % (w/w)pentafluoroguanidine fluoronitrogen mixture in the inert fluorochemical was allowed to percolate over a bed of silica gel at a flow rate of 1 cm min-’ until breakthrough of the saturated perfluoroamines was indicated by a positive test of a few drops of the effluent with a 1 M solution of potassium iodide in 90% (v/v) acetonitrile-water (3, 4). Fractions were collected and elution with the solvent was continued until breakthrough of the N,N,N’-trifluoroamidines was indicated by a positive test with 1Maqueous potassium iodide (4). At this point, elution with 65 % (v/v) trifluoroacetic acid in the inert fluorochemical was initiated, and a volume sufficient to saturate the bed (ca. 3.5 meq/ml) was introduced. Elution with the trifluoroacetic acid solution was discontinued just prior to its breakthrough, which was indicated by a pH test paper located inside the column near the bottom of the bed. The fractions from the fluorochemical elution and acid displacement were analyzed by gas chromatography. RESULTS AND DISCUSSION
Silica gel is a satisfactory adsorbent for organic Auoronitrogens. Exposure of the material to moist air for a few minutes, however, leads to the hydrolytic decomposition of the N,N,N‘trifluoroamidines (5). Alumina and molecular sieves also tend to decompose the unsaturated fluoronitrogens. The separation of tris(difluoroamino)fluoromethane and pentafluoroguanidine is illustrated in Figure 1 . Either the inert fluorochemical or a hydrocarbon such as heptane can be used as solvent and eluent; however, the former is preferred because of its much greater stability toward oxidation. Tris(3) R. L. Rebertus, J. J. McBrady, and J. G. Gagnon, J. Org. Ciiem., 32, 1944 (1967). (4) R. L. Rebertus and P. E. Toren, [bid.,32, in press (1967). (5) R. L. Rebertus and B. W. Nippoldt, Ibid.,32, in press (1967).
1868 * ANALYTICAL CHEMISTRY
(difluoroamino)fluoromethane is readily eluted with either of these solvents, but the pentafluoroguanidine is removed completely only if numerous bed volumes are used. On the other hand, displacement is readily accomplished with trifluoroacetic acid, which is completely miscible with the inert fluorochemical. Surprisingly, little heating of the column occurs even when it is contacted with a 65% (v/v) solution of the acid. In addition, the solvolysis of pentafluoroguanidine (5) in the displacing medium is too slow to be detrimental. The separation of bis(difluoroamino)difluoromethanetetrafluoroformamidine mixtures was also effected. The relative affinities of the fluoronitrogens for silica gel increase in the order: (F2N),CF < (F2N)J2F2
