V O L U M E 26, NO. 3, M A R C H 1 9 5 4
523
Table 111. Analyses by Chloranilic Acid Procedure after Removal of Excess Mineral Acid by IRA-410 R u n Solution 1 3X-HC1, l a ml. 2 3NHCI, 15 ml. 3 3NHC1, 15 ml. 4 3NHC1, 15 ml. 5 3 N HNOa, 5 ml. 6 3NHKO1, 5 nil. 7 3 N HClOa, 5 ml. 8 3”fHC101, 5 ml. 9 3NHC1, 2 ml. 10 3 N H C 1 , 2 ml. 0
I
Mg. of Sr in Successive 5-MI. Fractions of Eluent I1 111 IV J’
VI
0.59
0.59
0.65 0.44
0.05
0.11
0.23
0.57
0.62
0.61
0.23
. . , O
0.12
0.60
0.55
0.46
0.16
0.35
0.40
0.19
0.31
0 35 0 . 0 9
0.07
0.25
0.07
0.09
0.45
0.73
0.14
0.23
0.41
0.11
0.37
0.Oi
...a
...a
0.29
0.07
0.03
...a
...a
...a
0.07
...= ...= 0.11
. . . O
...= ... . . . 5
...
...... ... ...... ...... . . . 5
T o t a l Sr Found Bctual 2.43 2.50 2.26
2.50
1.73
1.67
1.21
1.17
0.82
0.83
0.86
0.83
0.87
0.83
0.82
0.83
0.44
0.33
0.39
0.33
Too small t o be detected.
I t was found possible to concentrate most of the strontium in the first 5-mI. fraction of the eluent by using dry resin. Runs 1 through 6 and 8 were carried out with a moist column-Le., the column had been regenerated, washed, and the excess water removed by gentle aspirator suction. In these runs, the strontium is distributed among the first three or four fractions. In runs 7, 9, and 10, the resin was thoroughly dried between filter papers after washing, then was replaced in the column. With this technique, the bulk of the strontium is found in the first fraction of eluent.
The calibration curve given in Figure 1 can, in certain cases, be used for the analysis of ions other than strontium(I1). For example, three 5.00-ml. samples of calcium chloride solution containing 0.20 mg. of calcium ion in each sample gave the following analytical results: 0.19,0.20, and 0.20mg. of calcium, respectively. Whereas the calcium chloranilate precipitates more rapidly than the strontium salt, the precipitation of barium from barium chloride by chloranilic acid is much slower than that of strontium, and low results were consistently obtained in analyses for barium. The analyses reported in this paper have been carried out on solutions which contain no cation (except hydrogen) other than the one of interest. I n view of the widespread interference that would result from the presence of other ions, (3) this method is recommended only for solutions of the type described above. LITERATURE CITED
(1) Barreto, A., BoZ. soc. b r a d agron. (Rio de J a n e i r o ) , 8 , 351 (1945). (2) Feigl, F., Mikrochemie, 2, 187 (1924). (3) Frost-Jones, R. E. U., and Yardley, J. T., Analyst, 77, 468 (1952). (4) Gammon, N., and Forbes, R. B., ANAL.CHEM.,21, 1391 (1949). (5) Jackson, C. L., and MacLaurin, R. D., Am. C h e m J., 37, 87 (1907). (6) Koroleff, F., Finska Kemistsamfundets Medd., 60, 56 (1951). (7) Le Peintre, M., Compt. rend., 231, 968 (1950). (8) Tyner, E. H., ANAL. CHEM.,20, 76 (1948). (9) Yardley, J. T., personal communication. RECEIVEDfor review August 24, 1953. Accepted December 1, 1953 Presented before t h e Division of Analytical Chemistry a t t h e 124th Meeting of the AMERICAN CHEMICAL SOCIETY, Chicago, Ill. Research supported in part b y the Atomic Energy Commission under contract No. AT (30-1)1256.
Detection, Estimation, and Removal of Impurities in Fluorocarbon liquids DANIEL GRAFSTEIN Westinghouse Research Laboratories, East Pittsburgh, Pa.
This work was initiated in order to minimize the corrosion problems associated with the use of perfluorinated liquids in electrical apparatus. Methods were sought by which impurities in commercial preparations of fluorocarbons could be quantitatively determined and removed. Ultraviolet absorption spectrophotometry proved to be a surprisingly sensitive tool for the estimation of trace olefinic impurities and a procedure was developed in which both olefinic and hydrogen-containing impurities are eliminated by degradation with potassium hydroxide pellets at elevated temperatures. The results should prove useful in applications requiring inert fluorocarbon media of known purity and in the preparationof perfluorocarbons where completeness of fluorination can now be appraised.
I
N COSXECTIOX with certain studies in this laboratory, a
need developed for fluorocarbon liquids free of reactive impurities. Polyamides, polyesters, and copper underwent visible deterioration upon continuous contact with commercially available perfluorinated compounds at elevated temperatures. Fractional distillation and repeated mashing of the liquids with aqueous alkali and water failed to alter these effects.
-4n apparent reaction was observed by ?*IcCulloch (3) of this laboratory between perfluorodimethylcyclohexane and alkali metal hydroxide pellets at room temperatures. The solid became coated with a brown substance which was shown by analysis to be composed primarily of alkali fluorides. ilfter repeated treatments, using fresh quantities of alkali, the liquid no longer exhibited this reaction. The treatment was accompanied at first by a rapid increase and then by a slow decrease in the amount of unsaturation in the liquid, as shown by titration with potassium permanganate in acetone, until finally the liquid was no longer reactive to permanganate. I n a further study, it was found that the reaction was accelerated a t elevated temperature and was given by other impure fluorocarbon liquids. Throughout the alkali treatments, less than 5% of the perfluorinated compounds treated were lost, mainly through handling. -4marked decrease in corrosion was observed with the treated fluorocarbons. Ultraviolet absorption spectra display a remarkable sensitivity towards purity. Figures 1 through 4 show the spectra of several commercial preparations of perfluorinated liquids, both before and after the alkali treatment. Throughout this paper, liquids exhaustively treated with potassium hydroxide are referred t o as purified. The recent availability of perfluorinated olefins permits a calibration of the ultraviolet spectra and allows a quantitative esti-
ANALYTICAL CHEMISTRY
524 mation of reactive olefinic impurities in perfluorinated liquids. Thus, after purification, data obtained indicate that perfluoroheptane contained a maximum of 0.023% by weight of olefinic impurities.
06 05 m
C-UNPURIFIEO C
04
EXPERIMENTAL
Y
A Beckman DU spectrophotometer with hydrogen source and 1-em. silica cells were used for the spectral measurements. The \\.ave-length scale was calibrated against the mercury spectrum. The instrument gave absorbance readings directly. Samples for measurement were prepared by diluting a stock solution containing 1.8127,. by weight (0.090i3 mole liter-l) of perfluoro-l-heptene in purified perfluorohpptane. IIolar extinction coefficients, e, are rrported as the avei age of those calculated dirertly from the optical density and the concentration in moles liter-' for dl points a t a given ware length. In these calculations, the value of the density ( $ ) of perfluorohcptane was taken as (E26.1 = l.il-14. The equation usrd \vas E
=
CURVE
$
16
A S C 0 E
I 2
F
(MOLES PE
00907 00451 00225
00111 00070 PURIFIED C,
4
220
225
230
02
9
01
-0 I 220
230
240
250
260 270 zao WAVE LENGTH ( m u )
290
300
310
Figure 2. Curves for Solutions of Perfluorodimeth)1cyclohexane (CEFM)and (1: clizo Ether (CkFlsO) in Iso-octane at 25 C.
over Drierite, and frartionally tiidilled (boiling point, 101' C.). The spectrum of a middle, constant h i l i n g fraction is shown in Figure 2. Purified Perfluorodimethylcyclohexane. This material was prepared in the same manner as was perfluorohept'ane, except that from five t,o six alkali treatments were required a t 138" C. Its spectrum is shon-n in Figure 2. Other Fluorocarbons. Samples of perfluorononane and perfluorotripropylamine \yere obtained from the Minnesota Llining and llanufacturing Co. The spectra of these samples are shown in Figures 3 and 4, respectively. Tn-o alkali treatments at 185" C. were sufficient to remove the impurities. Spcctra for the purified samples are included in Figures 3 and -1. Perfluoro CS Cyclic Ether. i\n ether having the molecular formula of CBF1~O and belleved to be a mixture containing both five- and six-membered rings !?-as obtained from the Minnesota Rlining and RIanufacturing Co. Its spectrum is shown in Figure 2; curve C. Purification as before, involving two alkali treatments, followed by fractional distillation (boiling point, 101.5' C.) gave a product whose spectrum is shown in Figure 2, curve D .
8
0
g
Concentration 108 prams per liter
20
2 8
03
D/Cd
where D = log (101I) IO = intensity of incident light I = intensity of transmitted light C = conrentration, moles liter-] tl = cell length, centimeters
eZ
y 2
235 240 245 250 WAVE LENGTH ( m u )
255
260
265
Figure 1. Curves for Solutions of Perfluoroheptene in Purified Perfluoroheptane at 26.3" C.
0 20 CURVE
a-
PIJRIFIEO
0 I5 I
Perfluoro-1-heptene. Perfluoro-1-heptene was prepared by the pyrolysis of the dry sodium salt of perfluorocaprylic acid ( 2 ) (boiling point, 80" to 80.5' C.). All boiling points in this paper are uncorrected. Perfluoroheptane. Perfluoroheptane (1044 grams), obtained from the D u Pont Co , was fractionally distilled a t a 60 to 1 reflux ratio through a 4-foot Podbielniak Heligrid column rated a t 120 theoretical plates and equip ed u i t h a Brown temperature recorder. All fractional distilLtions reported in this paper were made in this column. A constant boiling middle fraction (about 680 grams) was taken for this work (boiling point, 82" C.). Its absorption spectrum against a water blank is given in Figure 1. Purified Perfluoroheptane. Samples of distilled commercial perfluoroheptane (about 35 ml.) were sealed with about i grams of potassium hydroxide pellets (c.P., 85.1%) in borosilicate glass ampoules. The ampoules were heated a t 135" C. for 48 hours. After this time, the tubes were opened and the liquid was decanted from the dark brown residue. The liquid was resealed with fresh potassium hydroxide pellets and heated again a t 135' C. for 48 hours. After this second treatment, only a slight reaction was evident. The process was repeated a third time with no visible reaction. The fluorocarbon was washed several times with distilled water, dried over Drierite, and distilled from a Claisen flask. I t s spectrum is shown in Figure 1. Perfluorodimethylcyclohexane. Perfluorodimethylcyclohexane, obtained from the D u Pont Co., was washed with water, dried
Figure 3.
Curves for Perfluorononane (Undiluted Sample) at 25' C.
Decomposition of Perfluoro-1-heptene. Perfluoro-1-hpptene (1.06 grams) and 9.7 grams of potassium hydroxide pellets (c.P., 85.1%) were sealed in a 50-ml. borosilicate glass ampoule. The ampoule was heated for 68 hours a t 135" C., cooled, and opened. The contents TVere dissolved in 200 ml. of water and the fluoride ion was estimated quantitatively by a modified lead chlorofluoride method. Fluoride ion found, 0.82 gram; calculated, 0.81 gram. AYALYSIS OF SPECTRA
Figure 1 is a plot of absorbance versus wave length for various concentrations of perfluoro-1-heptene in purified perfluorohep-
525 .04
CURVE A CURVE B
.03
.02
x\
-- UNPURlFlED PURIFIED __
creasing absorption by the olefin. Thereby, a concentration of 0.0011 mole liter-' (0.02370) of olefin as perfluoro-1-heptene was determined in the purified sample. It is assumed that the olefins present as impurities in perfluoroheptane have approximately the same molar extinction coefficients as perfluoro-1-heptene.
Table I. Molar Extinction Coefficients at 26.3" C. for the System Perfluoro-1-heptane in Perfluoroheptane
X
.
Wave Length, m p
.01
0
Extinction Coefficient, 6
226 230 236
x-x
X
Figure 4. Curves for Perfluorotripropylamine (Undiluted Sample) at 25" C.
tane. That Beer's Ian is followed is seen from Figures 5. hveiage molar extinction coefficients calculated from the data of Figure 5 are given in Table I. Values obtained for the moqt dilute 5olution (0.0070 mole liter-1) of added olefin are high and are not inc~luded in the calculation of the average extinction coefficient, since the error in concentration becomes appreciable, because of the presence of residual olefin in the purified heptane. It ~ ~ o u be l d desirable to make these measurements :it shorter wave lengths, where the curves tend to a maximum. However, the instrumental error below 220 mp makes this inadrisnble. For the estimation of purity, the 226-mp curve was used as a compromise between decreasing sensitivity of the inqtrument and in-
18.9 15.9 9.6
In order to expand this analytical method to other fluorocaibons, it becomes necessary to obtain a tiTical olefin impurity of that system. Of particular interest is the cyclohexane system, where a msvimum is observed in the commercial samples at 260 mr .
Bases other than potassium hydroxide have been investigated as substitutes in the purification procedure. Lithium hydroside, sodium hydroxide pellets, and calcium oxide are not as effective. I t appears that the major impurities present in perfluorinated liquids are hydrogen-containing polyfluoro compounds. The hydrogen-containing molecules are converted to perfluorinated olefins by dehydrofluorination and the resulting olefins are degraded by the potassium hydroxide pellets in this purification procedure. Two posqible alternative mechanisms for the initial step in this degradation, to form highly reactive allylic ketones, ai?:
1. The olefin may undergo an addition of a molecule of water
( 1 )eventually followed by chain cleavage. Similarly, alcohols will
add t o fluorinated olefins in the presence of alkoside ion:
F F IiOH
-+
(-CF,-CF=CF-)
(-(Xiz
HZO
I
1
-F
I
-HF II
I
+(-CF=C-C-)
H d
2.
1
H OH
P
(-CF-C-C-) I
-HF --+
-&-c--)
KOH
II
+etc.
0
The allylic fluorine atoms may be susceptible to hydrolysis: F
F
Perfluoro-1-heptene itself was shown to undergo complete decomposition in the presence of potassium hydroxide pellets at 135" C., combined fluorine being quantitatively converted t o fluoride ion. ACKNOWLEDGMEXT
The author wishes to express his indebtedness to the late Leon hlcCulloch for helpful diqcussions and to Julia Senko for the absorption spectra. LITERATURE CITED
(1) Coffman, D. D., Raasch, 11. S.,Rigby, G. W., Barrick, P. L. and Hanford, IT-. E., J . Org. Chem., 14,747 (1949). (2) Hals. L. J., Reid, T. S., and Smith, G. H., J . Am. Chem. Soc., 73,
CONCENTRATION ( rnoles/liter )
Figure 5. Concentration Curves for Perfluoro-lheptene i n Purified Perfluoroheptine at 26.3" C.
4054 (1951). (3) AIcCulloch, L., unpublished data. (4) Oliver, G. D., Blumkin, S., and Cunningham, C. W., J . Am. Chem. Soc., 73, 5722 (1951). RECEIVED for review Augiist 18, 1953. Accepted October 30, 1953.
