V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5
875
APPLICATIONS
In addition to the applications included in the evaluation of accuracy and internal consistency of the method, one more application is given in Table XVII to provide a better idea of the usefulness of the method in the study of composition of lubricating oils. The table gives the analysis of saturate concentrates from medium-viscosity lubricating oils from various crude sources. The data show that although the noncondensed cycloalkanes and cyclopentyl-cyclohexyl ratio remain fairly constant, the alkane content drops from a high of 26% in the Pennsylvania saturates to a low of 3% in the California saturates while the condensed cycloalkanes vary from a low of 23% to a high of 44%, respectively.
ect 42, and especially to R. R. Schiessler for the loan of many of the pure hydrocarbons employed in this work. The help of RI. L:4ndrB, H. J. Cannon, C. K. Hines, and Jan Samson is also gratefully acknowledged. LITERATURE CITED
Brown, R. 8.,ANAL.CHEM., 23, 430 (1951). Brown, R. A . Doherty, W., and Spontak, J.. Consolidated Engineering Corp., Mass Spectrometer Group, Report 84, 1951. (3) Friedel, R. A., A p p l . Spectroscopy, 6, 24 (1952). (4) Lipkin, ill. R., and Kurtz, S. S., Division of Petroleum Chemistry, 100th Meeting, ACS, Detroit, Mich., September 1940. (5) Lumpkin, H. E., Thomas, B. FV., and Elliott, A , , ~ N A L .CHEM., (1) (2)
24, 1389 (1952). (G)
O'Seal, 11.J., and Wier, T. P.. Ibid., 23, 830 (1951).
ACKNOWLEDGMENT
The authors wish to express their appreciation to the Advisory Committee of the American Petroleum Institute Research Proj-
RECEIVED for review August 2 , 1954. Accepted February 7, 1955. Presented a t the Second Annual Meeting of ASTM E-14 Committee o n Mass Spectrometry, May 1954.
Analysis of Fluorinated Polyphenyls by Mass Spectrometer PAUL BRADT and FRED L. MOHLER National Bureau o f Standards, Washington,
D. C.
A mass spectrometric method has been used to investigate the molecular weight and chemical composition of some fluorinated polyphenyls. The polymers were evaporated from a tube furnace directly into the ionization chamber of a mass spectrometer and mass spectra recorded as the furnace temperature w-as increased step by step. Mass spectra of polymers made from p dibromotetrafluorobenzene showed molecules of formula (CeF&+Brz with n ranging from 3 to 6. Polymers made from the diiodo compound gave ions with as many as eleven phenylene rings with or without one or two iodine atoms attached to the chain. RIolecule ions are predominant in these mass spectra; this simplifies the interpretation of results in terms of molecular weight distribution.
T
H E mass spectrometric analysis of heavy nonvolatile compounds by the use of a heated reservoir on the high pressure side of the inlet leak has become a standard procedure ( 1 , 5 ) , and mass spectra of mixtures containing hydrocarbons with as many as 45 carbon atoms (molecular weight approximately 630) have been published. I n exploratory research there are some advantages in evaporating molecules or thermal degradation products from a small tube furnace directly into the ionization chaniher of a mass spectrometer. This arrangement is more flexible, as the furnace can easily be adapted for use under a wide range of conditions and it is a simple matter to clean out nonvolatile residues. There is an important difference b e b e e n the two techniques. Degradation products from a tube furnace enter the ionization chamber after relatively few collisions and mag include radicals and other unstable configurations. Molecules from a reservoir are the products in thermal equilibrium a t the reservoir temperature and pressure. The application of the tube furnace technique to a study of pyrolysis products of polystyrene has been described, including some experimental details (2). EXPERIMENTAL PROCEDURE
Hellmann ( 3 ) has synthesized chains of fluorinated phenylene rings by heating p-dibromotetrafluorobenzene or diiodotetrafluorobenxene in the presence of hot copper. The products are granular light gray solids, presumably of the structure Br-CEFd-
CsF4-C6F4-Br, and the corresponding compounds terminated by iodine rather than bromine. This solid material is partially soluble in benzene and was separated into two fractions on this basis. These polymeric materials were submitted for mass spectrometric study, to determine the number of phenylene rings in the material and to investigate the chemical composition and thermal degradation. The samples were placed in a small metal or glass sample holder in a tube furnace which extended to the inlet port of the ionization chamber of a 60" Nier-type mass spectrometer ( 4 ) . A thermocouple in contact mith the sample holder measured the temperature of evaporation. The mass spectrum was recorded with a pen recorder by varying the magnetic field. Exceptionally heavy ions were encountered and the mas9 range was extended by lowering the ion-accelerating voltage from 2500 volts to less than 500 volts. The resolving power under these conditions was less than 200 and there is an uncertainty of nearly 1% in the mass scale. However, these compounds give mass peaks which are widely separated on the mass scale and the identification of the ions is probably reliable. In the case of the bromine compounds the triple isotope structure of the molecular ion3 containing two bromine atoms and the double structure of fragment ions containing one bromine atom were conspicuous in spite of incomplete resolution and this aided in identifying the ions. The procedure was to increase the sample temperature a t a slo~vrate until the sample began to evaporate, and hold the temperature constant while the spectrum was recorded. The temperature was increased step by step and the spectrum recorded at each increment in temperature. The rate of evaporation did not remain constant over the time required to record the spectrum, and after 4 or 5 hours insulating films formed in the ionization chamber and the evperiment had to be interrupted to clean the electrodes. RESULTS
Table I gives the larger mass peaks in the polymerized bromine compounds. The first column identifies the ion, the second column gives the molecular weight (the median value is given for polyisotopic ions), and the third column gives the mass spectrum of the fraction soluble in benzene. The compound began to evaporate a t about 100' C. and the spectrum was recorded at 1 2 i " C. This is an unusual spectrum, for all fragment ions
ANALYTICAL CHEMISTRY
876 Table I. Ion
illass Spectra of Polymers of CBFIIBIZ BenzeneSoluble Fraction, 127' C. 9.0 5.0 4.0 19 2.3 10.0 8.5 7.8 22 100 5.0 6.9
m/e 296 326'/2 336 376 450 524 604 653 672 752 820 900 968 LO48
... ...
BenseneInsoluble Fraction, 260' C. 6.1
...
...
0.7 12.0 3.2
... ... ...
3.6 26 100 7.3 6.9
intensities at six evaporation temperatures from 208' to 431' C. More than twenty records \$ere made in all and the experiments were interrupted twice to clean out the ionization chamber. Evaporation began at 208" C. and three small mass peaks \\ere observed, as listed in the third column. hIost of the evaporation was in the temperature range 276" to 409" C. and at 431 C. the ion current was small. The temperature was increased to 550" C., but only impurity peaks weie found. -1small peak at mass 85 is probably SiFa+ from silicon tetrafluoride. After the experiment a black residue comparable in bulk to the original sample remained in the sample tube. There was no evidence of melting.
Table 111. Mass Spectra of Polymers of C6F412
of low molecular weight are of very low abundance. This polymer seems to be predominantly molecules with four phenylene rings, ( C6F4)4Br~. The fraction insoluble in benzene evaporated in a narrow range of temperature from 200' to 266' C. and with increase in temperature t o 400" C. there was no further evaporation. There was a dark residue left in the copper sample tube and it seems probable that evaporation was terminated by a chemical reaction with the metal. The fourth column gives the mass spectrum of the vapor from the insoluble fraction a t 260" C. The molecule with five phenylene rings is predominant. It is probable that heavier molecules were present in the solid material and that a chemical reaction prevented evaporation of the less volatile compounds. The diiodotetrafluorobenzene polymers were evaporated from a glass sample holder t o reduce the possibility of chemical reaction. The fraction soluble in benzene began to evaporate at 170" C. andTable I1 lists the more abundant ions in the spectrum of the material evaporated a t 189' C. This material seems t o be predominantly a mixture of molecules with four and five phenylene rings. The largest ion peak containing five rings is (CeF&I+, while the largest peak containing four rings is (CBF~)~IZ + and the same difference is seen in the doubly charged ions. The ratio of the ions containing two, one, and no iodine atoms depends on the evaporation temperature, but this effect is more conspicuous in the mass spectra of the insoluble fraction. Table 11. Mass Spectra of Polymers of CeF& (Soluble fraction) Ion
m/e 296 355l/z 370 423 4331/* 497 571 592 698 719 740 846 867 888 994 1015
Rel. Int. at 189' C. 1.7 2.7 4.6 9.5 11.0 1.3 2.2 4.0 2.0 29.0 46.0 86.0 100 4.3 15.0 0.7
The insoluble fraction evaporated over a wide range of temperature and as the temperature was increased there was a marked change in the spectrum. Data on the more abundant heavy ions are given in Table 111. The first column identifies the ions, which are grouped according to the number of phenylene rings in the ions rather than molecular weight. The table omits doubly charged ions and some small peaks involving loss of fluorine from the molecules and there are some small unidentified peaks. There is also an Iz+ peak of relative intensity ranging from 2 to 9% of the maximum peak. Columns 3 t o 8 list relative
(Insoluble fraction) M 01. Temperatures of Evaporation, C. Iona Weight 208 276 309 331 408 592 31 x 4 100 ... 740 .. 10'2 867 xsl 7.9 994 , . 3 i 5.4 xsl, 21 24.4 888 100 XS 1015 . . 88 73 XSI 1142 100 .. 9.1 XeI2 .. 1036 50 16 xi 1163 28.8 0 9 XI1 1290 11.1 Xi12 1184 7.6 XS 1311 1.3 XSI 1438 0.4 XaI2 1332 Xe 1459 XI1 1586 ... ... XlIZ 1480 ... ... XlO 1607 ... ... ZlOI 1734 XlU12 1755 XI11 a Fluorinated phenylene ring, CsF4, denoted by X.
431
xs
4
The relative intensities of ions SJZ, X,I, and X, changed rapidly with increasing temperature. X, was the most abundant ion at lower temperatures and X,JZmost abundant a t higher temperature. This indicates that these molecules and radicals are present in the vapor state and that X, is somewhat nioie volatile than X,I2. The evaporation temperature also increased rapidly as the number of phenylene rings increased, as is to be expected. The molecules with six rings were most abundant at 2i6' C. and molecules with eight rings were most abundant at the highest temperature. CO?CLUSIONS
The volatile perfluorophenyl polymers give simple mass spectra in which molecule ions are most abundant, and fragment ions involving breaking of the polymer chains were not observed. The lighter ions disappear almost completely at higher temperatures. Thus, the constitution of the vapor at a given temperature is indicated in a qualitative manner by the mass spectrum. The soluble fractions of the bromine and iodine compounds seem to be completely volatile and the composition of the vapor is probably indicative of the composition of the solid. The insoluble fractions contain polymers with a wide range of molecular weights and evaporation temperatures. At the higher temperatures degradation and other chemical reactions stop the evaporation. The molecular lyeight of the vapor is not characteristic of the solid material and it is not surprising that molecules nith more than eleven rings are not observed in the vapor state. I t is evident that the long chains do not degrade by breaking into shorter chains, but it appears that they become carbonized. The spectra indicate that radicals of formulas X, and S,I must be present in the vapor and, presumably, in the solid material. The evolution of Iz accounts at least in part for the production of these radicals. It is possible that chains of phenylene rings that have lost both the terminal iodine atoms form stable molecules by forming closed loops. h novel feature of this research is the high range of molecular
V O L U M E 27, NO. 6, J U N E 1 9 5 5 n eights encountered. Heretofore the heaviest published mass bpectra did not extend much beyond molecular weight 600. Apart from the obvious limitation in resolving power and mass nieasurement in a n instrument designed for molecular weights of 350 or less, no difficulties were encountered in extending the mass innge above 1700. The success of the experiment depends on the unusual properties of the material. T h e thermal stability of the molecules is so great that chains with as many as eleven phenylene I ings evaporate without degradation and the stability of the ions I. sufficient t o make the molecule ion the predominant mass peak i n the spectrum. These circumstances make it possible t o identify qualitatively all the different molecules and radicals liqted in Table 111. I n most mixtures of heavy and light molecules fragment ions from heavy molecules can mask the lighter
a77
molecules in a spectrum and it is necessary t o know the spectra of the pure compounds t o derive an analysis. LITERATURE CITED (1) Ani. Petroleum Inst., “API Catalog of Mass Spectral D a t a , ” Research Project 44, 1949 t o d a t e . (2) B r a d t , P a u l , Dibeler, V. H., and 3Iohler. F. L., J . Research N u t l .
Bur. S t a n d a r d s , 50, 201 (1953). (3) Hellmann, Max, Bilbo, A . J., a n d P u m m e r , W. J., J . Am. Chem. Soc.. s u b m i t t e d for publiration. (4) S i e r . -1.O., Rea. Sci. Instr., 11, 212 (1940). ( 5 ) O’Xeal, 31. J., a n d K i e r , T. P., ;Is.~L. CHEY.,23, 830 (1961).
RECEIVED for review November
4, 1954. Accepted February 17, 1955. Work performed as part of a research project on high temperature-resistant polymers sponsored by the Ordnance Corps, Department of the .Irmy.
Direct Spectrophotometric Determination of Uranium in Aqueous Solutions R. G. CANNING and P. DIXON Geological Survey Laboratories, Department o f Mines,
A rapid, direct method was required for estimating uranium in sulfate solutions containing uranium, vanadium, chromium, and rare earths in concentrations of 1 to 2 grams per liter of the respective oxides, titanium up to 10 grams per liter of titanium dioxide, and ferrous and ferric iron up to a total of 40 grams per liter of ferric oxide. Existing chemical methods were too lengthy, while rapid fluorimetric and colorimetric methods were not sufficiently precise. A two-component spectrophotometric method was developed, utilizing the reduction of uranium and vanadium by ferrous phosphoric acid solution. sulfate in 40 volume Estimations may be completed in 1 hour with reasonable precision. Using a Hilger Uvispek spectrophotometer, standard deviations in precision of &lqo were obtained over the range 0.5 to 5.0 grams per liter of uranium oxide in the presence of other components in the concentrations mentioned above. Standard deviations in accuracy were better than 2’70 over the same range. Concurrent with the determination of uranium, a less accurate estimation of vanadium is possible.
P
RELIMINARY examination of the absorbance curves of all the known components of the solutions under investigation showed that the most feasible method of determining uranium directly was to make use of the strong absorbance peak of the u ~ a n o u sion a t a wave length of 660 mfi. It mas necessary, therefore, to produce two solutions-one a reduced solution and the other an unreduced reference solutionfrom the sample in such a way that the components, other than uranium, remained unaltered in the two solutions. A large number of conditions of reduction and oxidation were studied to this end. One method for obtaining specific reduction of uranium was found. This was the copper-catalyzed reduction by hydroxylamine in hot 40 volume% phosphoric acid solutions, but the method was discarded because of the slow rate of reduction. I t \vas found that ferrous sulfate in hot 40 volume % phosphoric acid reduced uranium and vanadium rapidly and completely from the uranium(V1) and vanadium(1T’) t o the uranium( IS‘) and vanadium(II1) valence states, respectively, without affecting the other components of the solutions, and this was the procedure finally adopted. The reduced solutions xere
South Australia perfectly stable over long periods. The characteristics of the absorbance curves of hexavalent and tetravalent uranium and tetravalent and trivalent vanadium made it possible to apply a two-wave-length method for determining uranium in the presence of vanadium, using wave lengths of 660 and 700 mp. h satisfactory reference solution was obtained in the same strength of phosphoric acid by treating with hydrogen peroxide to oxidize all components fully, boiling to destroy peroxides of vanadium and titanium, then treat’ing mith sodium sulfite t o destroy any remaining peroxide and to ensure that no false absorbance readings were caused by oxygen evolution in the solution while in the spectrophotometer cells. -1final modification M-as to treat both solutions with hydrogen peroxide, the excess of which was then boiled off. In addition to oxidizing all components, this destroyed oxidizing agents such as chlorate and permanganate, which could interfere indirectly by consuming some of the ferrous sulfate reducing agent. A small addition of solid sodium sulfite was then made t o both solutions. Ferrous sulfate was added to one solution to form the reduced solution and an equivalent volume of sulfuric acid was added t o the reference solution, in order to preserve the same acidity in each solution. REAGESTS
Hydrogen peroxide, 30% 17,/v. Phosphoric acid, 90% w./w. Sulfuric acid, lA\7. Ferrous sulfate, analytical reagent grade, 0.5.U in 1N sulfuric acid. Sodium sulfite, analytical reagent grade, 4PP4R4TUS
-1Beckman Model DU spectrophotometer, with 1-em. Corex cells, was used for developmental work and the method was finally applied on a Hilger rvi3pel; ,spectrophotometer using 1cm. and 4-em. quartz cells. T o other special equipment x-as used. PROCEDURE
A sample volume of 20 ml. or lejs is selected where possible to yield between 10 and 50 mg. of uranium oxide. This amount of uranium gives absorbance readings between 0.1 and 0.6 in 4em. cells. Two samples are pipetted into beakers and diluted with water,
