causing variations in the degree of interaction. However, attempts to correlate the relative intensities with the Hammet u substituent constants failed, indicating that the influence is complex and not merely due to the relative electron donating or withdrawing properties of the substituent. The constancy of the integrated absorptivities of the aromatic aldehydes which extend over both bands of the doublet is in accord with our previous discussion. The constancy of the integrated absorptivities of the low frequency doublet band of the aliphatic aldehydes is somewhat surprising in the light of the relative intensity results obtained for the aromatic aldehydes. The probable explanation lies in the very simiIar structural environment of the aldehydic group
in the series of aliphatic aldehydes. It is possible that substituents on the alpha or beta carbon atom can strongly influence these results. This possibility has not been investigated. In applying group type absorptivities to the determination of aldehydes in the presence of other organic compounds, a base line procedure is always used and contributes greatly to the improved accuracy. The method of drawing this base line is illustrated in Figure 2. The sample was an oxygenated material and on analysis was found to contain 43.0 mole % aldehyde, 10.6 mole % ’ alcohol, and 0.4 mole % ’ acid. The need for a base line technique arises from general background absorption. Acids, in particular, contribute to this background absorption because
of their intense hydrogen bonded hydroxyl absorption in this region. LITERATURE CITED
(1) Eggers, D. F., Lindgren, W. C., ANAL. CHEM.28,1328 (1956). (2) Evans, J. C., Bernstein, H. J., Can. J . Chem. 34, 1083 (1956). (3) Herzberg, G., “Infrared and Raman Spectra of Polyatomic Molecules,” p. 266, Van Nostrand, New York, 1945. (4) Pinchas, S., ANAL. CHEM. 27, 2 119551. ( 5 j - 1 6 2 , 29,334 (1957). (6) Pozefsky, A., Coggeshall, N. D., Ibad., 23, 1611 (1951). (7) Saier, E. L.. Hughes, R. H.. Ibid.. 30,513(1958).’ ’ (8) Saier, E. L., Cousins, L. R., Basila, M. R., J . Phys. Chem. 66,235 (1962). ’
-
RECEIVEDfor review January 5, 1962. Accepted April 4, 1962.
Mass Spectrometric Determination of the Ratio of Branched to Normal Hydrocarbons Up to C,, in Fischer-Tropsch Product A. G. SHARKEY, Jr., J. L. SHULTZ, and R. A. FRIEDEL Pittsburgh Coal Research Center, Bureau of Mines, U . S. Department o f the Interior, Pittsburgh, Pa,
b A method is described for the determination of carbon number distribution and ratio of branched to normal paraffins in mixtures consisting primarily of saturated hydrocarbons. Separation of the normal and branched paraffins, CI1 to C18, is carried out using a Molecular Sieve technique similar to the elution method described by O’Connor and Norris. Mass spectra obtained before and after Molecular Sieve separation are compared using an internal standard, thus eliminating the need for recovering the normal paraffins from the sieve material. Calibration data for the branched-chain paraffins are obtained from fractions of Fischer-Tropsch synthesis product. Carbon number distribution data, CI1 to CB, and the ratio of branched to normal components, CII to C18, were obtained for the hydrocarbons in a Fischer-Tropsch synthesis product following hydrogenation. Agreement between determined and calculated values for normal paraffins, C, to C18, indicates the validity of Molecular Sieve methods in a range where the desired pure compounds are not available for synthetic blends.
T
Fischer-Tropsch synthesis is being investigated by the Bureau of Mines as a means of converting coal to hydrocarbons. Gaseous and liquid HE
826
m
ANALYTICAL CHEMISTRY
hydrocarbons from the Fischer-Tropsch synthesis have been studied by several investigators including Anderson, Friedel, and Storch (@, Weitkamp, et d . (14), Bruner (4), and Gall and Kipping (8). While carbon number distribution data for the hydrocarbons have in certain instances been obtained up to C ~ Oinformation , concerning the ratio of branched to normal hydrocarbons in Fischer-Tropsch product is not available above Ca. The desired information can be obtained for the paraffis plus olefins in the product if hydrogenated material is analyzed. The purpose of this investigation was to derive methods for determining carbon number distribution data, and the ratio of branched to normal paraffins, in hydrogenated fractions of Fischer-Tropsch synthesis product. Theoretical chain branching schemes for the hydrocarbons in Fischer-Tropsch product have been proposed by Anderson, Friedel, and Storch, and others (2, 6, 15, 16). The carbon number distribution for the hydrocarbons is also predicted by these schemes. Previous investigations have shown that these chain branching schemes adequately describe the synthesis product up to Cs ( 2 ) . For these comparisons, the individual isomers for the C3 t o CShydrocarbons were determined. Several mass spectrometric methods for determining the ratio of branched
to normal paraffins have been described, In all of these methods, certain assumptions have been made concerning fragmentation patterns and sensitivity factors for the branched paraffis. In the first investigation of this type, O’Neal and Wier assumed that the pure compound, 3-ethyl tetracosane, was representative of the isoalkanes in petroleum waxes ( l a ) . Brown et al. “felt that an average isoparaffin would dissociate so as to lose 43 mass units,” and therefore, used peaks corresponding t o (molecular weight -43 mass units) for isoparaffin determinations (3). Ferguson and Howard described a method for determining the is0 t o normal paraffin ratio in gasoline range petroleum ( 5 ) . The above authors used thermodynamic equilibrium data of Prosen, Pitzer, and Rossini to weight individual cs to c8 isoparaffins and obtain representative sensitivity factors a t each of these carbon numbers ( I S ) . Using Molecular Sieve techniques, average isoparaffin sensitivities were also obtained for the CS to Cl1 paraffins in representative gasolines. I n this instance, Ferguson and Howard had to assume that a representative fraction of isoparaffis was obtained by the Molecular Sieve technique, as very few pure isoparafis above Clo are available for synthetic blend determinations. O’Connor and Korris described a complete analytical scheme for char-
M'S. 1.R
Hyd!openotian MS (170'10 3 2 3 . C ond 323'10 390'C
A- - *
I r O C t ~ O n S only/
7 -
MI5
Add k-methylnaphtholene
I
I
~ o i e c u i a r sieve column
M 5.
1
i
Molecular sieve
COiYmn
nique (FIA) was also applied to a portion of the 170" to 323" C. and 323" to 390" C. fractions after hydrogenation. Drop quantities of the saturates were collected from FIA for spectrometric investigation. Results obtained by this separation method were consistent with those from the acid extraction technique. Carbon number distribution data were then obtained for the paraffinic hydrocarbons in all three fractions. Prior to Molecular Sieve separation, approximately 10% by volume of amethylnaphthalene {vas added as an internal standard to a portion of the saturate concentrate collected from FI-4. When examining fractions containing Clo paraffins, Decalin n a s used as an internal standard to avoid interference a t mass 142. Removal of normal paraffins by the hIolecular Sieves resulted in a reduction in parent peak intensities. Thus, comparison of mass spectra obtained before and after sieve treatment, following adjustment to bring internal standard peaks to the same intensity, permitted deteiminations for normal and branched paraffins. Mass Spectrometric Calibration Data. Mass ions corresponding to the molecular weights of the paraffins were used. Appropriate calibration data covering the mass range of interest were needed for both the normal and branched-chain paraffinic structures (Figure 2 ) . As pure normal paraffins are available t o (332, data vere easily obtained for the normal compounds (curve 1). The problem with the branched compounds consisted not only of obtaining the proper pure compounds (very few available above C,) but also of weighting the calibration data. TKOapproaches were used to determine the sensith-ities shown in Figure 2: The branched components in hydrogenated FischerTropsch product were isolated by distillation and Molecular Sieve techniques and sensitivities for the total branched paraffins Cl0 to CZodetermined from actual product (curve 4); data from fractions of branched paraffins consisting primarily of single carbon num-
tigation, Molecular Sieve techniques were used to obtain the ratio of branched to normal p a r a f i s in the range C11 to Cu in hydrogenated fractions of FischerTropsch product. The product was produced a t 265" C. using an ammonia synthesis catalyst composed of Fe304, MgO, and KzO. The synthesis was carried out with lHz:lCO gas a t 21.4 atmospheres. After hydrogenation and removal of the oxygenates, the remaining product consisted of the same major types of hydrocarbons found in petroleum : parafKns, naphthenes, and monoaromatics. Information derived from this investigation concerning the use of Molecular Sieves should therefore be applicable to liquid fractions of petroleum origin. While the purpose of this investigation was to determine the adequacy of the chain branching schemes in describing the distribution of Clo to C20synthesis product, agreement between theoretical and experimental data also indicates the validity of N o lecular Sieve separation in this range.
323' - 39O'C r330°C residue
M 5.
I
M.S.
Figure 1 . Separation and analysis of > 170" C. fraction of Fischer-Tropsch synthesis product
acterizing the compound types in the light gas oil (100" to 650" F.) fraction of petroleum (11). Molecular Sieves were used in determining total percentages of normal paraffins and straightchain olefins; however, carbon number distribution data were not obtained. Excellent results were reported on synthetic blends prepared from the limited number of pure compounds available. For the isoparaffins, 2methyldecane was the highest molecular weight compound included in the blends. Using their method, n-pentane must be refluxed through the sieves for approximately 100 hours if the normal paraffins up to Cz2are to be recovered for a carbon number distribution determination by gas chromatography or other method. Branched-chain hydrocarbons in Fischer-Tropsch synthesis product consist primarily of mono- and dimethylsubstituted compounds. While this is a less complex mixture than found in petroleum, the methyl branched compounds, particularly those having methyl branching near one end of the alkyl chain, should cause the most difficulty with separations by Molecular Sieve material. I n the present inves-
EXPERIMENTAL PROCEDURE
Separation and Sample Preparation. Fractions of the product investigated boiled in the ranges 170' to 323"C., 323" to 390" C., and >390° C. residue. These fractions included all paraffins and olefins having more than 10 carbons. The 323" to 390" C. fraction was intermediate between the liquid and residue product and was the smallest of the three fractions. The flow diagram, Figure 1, gives the sample handling and preparation for the spectrometric investigation. Hydrogenation was carried out a t 185" C. for about 16 hours on Raney nickel catalyst. The three fractions were then investigated by infrared and found to be free of olefins. Oxygenated material was removed from the three fractions by sulfuric acid extraction, and infrared determinations showed these samples were free of hydroxyl and carbonyl groups. The fluorescent indicator tech-
1 Norm01 porolfins ( i - p u r e compoundS1 2 Weighted overoge of t o t o l p o r a f f i n r 3 Senrtttwlier lor branched poroflinr extrapolated from Cg nnd lower carbon numbers 4 S e n s ~ t w t i e tf o r bronched p o i o l l ~ n s determined f r o m Producl
5 400
O - S ~ n g I e c o r b a n number c o n c e n t r a t e s 0-170- 3 2 3 * c troctian
--
' 10
1 '0
12
Figure 2. product
14
16
18
20
22
24 26 28 CARaON NUMBER,n
30
32
34
36
38
40
Sensitivity data for paraffins in Fischer-Tropsch
12
I 14
1
1
,e.
ia
l
l
l
/
I
22 29 CARBCU NUMBER,n
'
26
ZB
30
20
I 32
%
Figure 3. Carbon number distribution for paraffins in hydrogenated Fischer-Tropsch product B. p.
> 170'
C.
VOL 34, NO. 7, JUNE 1962
827
Table 1.
Analysis of Hydrogenated Fractions of Fischer-Tropsch Product
(B.p. >170" C.)
Volume, %
Volume fraction of total b.p. > 170" C. Oxygenates Naphthenes Paraffins Carbon Number 10 11 12 13 14 15 16 17 18
323-90' C.
>390" C.
36.8 20.6 2.4 77.0
13.3 35.8 1.6 62.6
49.9 34.6 16.4 49.0
100. 29.6 9.3 61.1
Carbon Xumber Distribution for Paraffins 3.7 12.1 11.7 11.1 10.3 8.9 7.0 4.8 2.9 1.8 1.1 0.71 0 43 0.27 0.20
19
20 21 22 23 24 25 2 (5 2i 28 29 30 31 32 33 34 35 36 37 38
170-323' C.
Composite," b.u. >170" C.
0.02 0.28 1.3 3.4 6.3 8.4 9.7 9.4 8.1 6.0 3.9 2.7 1.5 1.1 0.50
+
0.25
1.P 4.9 4.7 4.3 4.0 3.6 3.2 2.7 2.3 2.1 1.9
3.1 3.1 3.1 3.2 3.0 2.7 2.3 2.3 1.9 1.8 1.4 1.5 1.4
1.4 1.5 1.4 1.4 1.4 1.3 1. o 0.90 0.80 0.82 0.63 0.60 0 50
.iromatics higher molecular weight paraffins 8.0 Determined from peak summations, based on volume fractions. S o t total Clo; some Clo also appears in fraction boiling below 170" C.
3.9
5
m.;,CI3to C14, were combined with data from a broader fraction in establishing wrve 4 . Sensitivity factors based on lata for the mono- and dimethyl sub\titilted compounds, major components ,n Fischer-Tropsch product, were extrapolated from C9 and lower carbon numbers (curbe 3). Good agreement is shown between sensitivity factors obtained by the two methods. Nore highly branched compounds having ioiver sensitivity factors \yere not considered in deriving curve 3. It is therefore reasonable that curve 4 gites slightly lower sensitivity factors than curve 3. Theoretical weighting factors for combining normal and total branched paraffins were used in obtaining sensitivities for the total paraffin a t each carbon number (curve 2) (2). Molecular Sieve Technique. Three Molecular Sieve techniques nere investigated: Simple mixing of the sample and sieve material (9), the vacuum evaporation method developed by Nelson, Grimes, and Heinrich (IO) and an elution technique developed by 828
ANALYTICAL CHEMISTRY
Table II. Determination of n-Paraffins in Hydrogenated Fischer-Tropsch Synthesis Product
(B.p. >170° C.) Volume Per Cent of Total Paraffin at Each Carbon Number Appearing as n-Paraffin Carbon Predicted Number Experimental (a) 74.1 70.2 66.6 62.1 58.2 54.0 50.7 47.9
Briefly, the technique is as follows. Between 0.2 and 0.5 ml. of sample containing about 10% by volume of amethylnaphthalene is charged t o the column (type 54-powder), followed by approximately 8 ml. of isopentane that has previously been put through a Molecular Sieve column to remove any traces of normal components. The material collected 1s then stripped of most of the isopentane-with approximately 25% isopentane remaining in the final mixture. Recovery of the normal components from the sieve material is not necessary, as changes in the intensities of the molecular ions and internal standard after sieve treatment can be used to calculate the normal paraffins. Synthetic mixtures were prepared to determine that Molecular Sieves are effective in removing normal paraffins from actual Fischer-Tropsch product, and that during the stripping operation only isopentane and not portions of the sample or internal standard are removed. A blend n a s prepared consisting of approximately 40% n-Cla and the remainder 170' to 323" C. product. The n-CI6 was completely removed by sieve treatment. A second mixture was prepared consisting of 0.3 ml. of 170" to 323" C. product (including approximately lOy0 CYmethylnaphthalene as internal standard) and 8 ml. of isopentane. After stripping the isopentane, the same mass spectrum was obtained as before addition of the isopentane. RESULTS
O'Connor and Norris (If). The elution technique, described completely in ( I I ) , gave a better separation of normal and branched components (above Clo) in synthetic mixtures and was therefore used in characterizing the
11 12 13 14 15 16 17 18
170" to 323" C. and 323" to 390" C. fractions.
71.3 68.4 65.7 63.1 60.6 58.2 55.9 53.8
A summary of the carbon number distribution data for the three fractions is shown in Table I. Paraffin-naphthene type-analysis results for total percentagw of these two structural types are also given ('7). The naphthene content is -2% in the first two fractions and then rises to 16% in the >390" C. residue. Values for the oxygenates were obtained by summing the infrared functional group determinations for the hydroxyl and carbonyl group-containing compounds. In these fractions of hydrogenated product there is a trend toward increased oxygenate and decreased paraffin content with increasing carbon number. Determinations on the >390" C. residue were made to G8,the highest carbon number with readable peaks. The composite analysis of the total (last column) was based on a summation of peak intensities in the three fractions, weighted according to the volume fraction of each, rather than on a summation of percentages shown for the three fractions. This was done to eliminate any errors that might occur by applying weighted sensitivity factors if either branched or normal material were concentrated in the low- or high-boiling
3
4
5
6
'
8
9
82
IO
I3
I4
15
C4R30h NUMBER, n
Figure 5. Variation in proportion of n-isomer with carbon number for paraffins logarithmic plot
I
,
I8
20
I
I
12
16
14
22
24
CARBON NUMBER, n
Figure 4. Carbon number distribution for paraffins in hydrogenated Fischer-Tropsch product 6. p.
> 170" C.
ends of the fractions. Figure 3 shows the volume per cent of paraffins a t each carbon number from CI1 to Ca4for the hydrogenated product. Values from C2*to Caaare similar. Table I1 summarizes the deterniinations for normal paraffins from Cll through CI8 using l\lolecular Sirve separations. Predicted values according to the chain gram-th scheme of Anderson, Friedel, and Storch (discussed later) are also shown ( 2 ) . The carbon number distribution for normal paraffins ( C , to C20) in the 170" to 323" C. fraction was also determined by a n independent mass spectrometric method (courtesy, Atlantic Refining Research Laboratory), This method is based on a matrix operation involving over TOO coefficients (3). Results shown in Table I11 are in good agreement considering the complexity of the mixture. For normal parafins >5y0 values obtained by the t n o methods show differences of less than 15% of the amount present. Hoaever, determinations for Cz0 and higher paraffins made by sieve methods and the method described in (3) have shown discrepancies. DISCUSSION OF RESULTS
Anderson, Friedel, and Storch derived the following expression to describe the carbon number distribution for FischerTropsch product ( 2 ) C$
log(&/F,)
where
=
= kF,an-2
n log a
+ log(k/a*)
(1)
(2)
number of moles containing (n)carbon atoms F , is a function of small (f), a constant representative of the #n
=
extent of branching ( k ) and (a) are constants As a plot of log (&/Pn) us. (n) should be linear (Equation 2), this expression can be used to examine experimental data. Figure 4 is a plot of this type showing the agreement between the present data for total paraffins and predicted values based on a branching factorf = 0.045. Values for normal paraffins determined in this investigation by the combined Molecular Sieve-mass spectrometric technique agree with the isomer distribution predicted by Anderson, Friedel, and Storch. Determinations made by an independent method (Table 111) also support the validity of Molecular Sieve separations for paraffins to approximately C20. Predicted values for the normal paraffins shown in Table I1 are based on a chain growth calculation that considers only mono- and dimethyl branched isomers ( 2 ) . The calculation of the total branched paraffin a t each carbon number above eight is not feasible using methods described in the literature. Contributions by other branched structures ill result in the predicted value for the total branched paraffins being low and the normal isomers high. However, branched paraffins other than monoor dimethyl have been found only as minor components in previous investigations (14). The trend for predicted values for the normal paraffins to be increasingly larger than the experimental values, from CI4 to CIS (Table 11), possibly indicates that contributions from the other branched isomers become significant at higher carbon numbers. Results obtained on Fischer-Tropsch product produced at 265" C. with an iron catalyst are also consistent IT ith results reported by Gall and Kipping under similar synthesis conditions (8). Gall and Kipping found in their study of the Cs to Ca paraffinic fraction of synthesis product that the proportion of branched-chain isomers increases regularly TT ith synthesis temperature, that is, the (f) factor increases with in-
creasing synthesis temperature Lxtrapolating the Cs to Cs data obtained by Gall and Kipping to higher rn3?bon numbers (Figure 5 ) indicates that, for 265' C. operation, the paraffin fr:t~~tiiln should consist of approximatr-'T, 64: normal paraffins at CI4 1 J , investigation gave a value normal paraffin a t (214. For product obtained with an i 'I catalyst, Anderson et d.have 5111 that predicted values for isomer ci!, tribution, usingf = 0.115 in thrir chbiii branching scheme, show excellrnt agv: ment with certain experimental m111t However, the experimcsntal c-l.ita 4 Weitkamp (14) and Bruner (4)US ti for this comparison Rere obtain( r under high temperature qynthesis cor ditions. The product mith nhicaiWeitkamp dealt was prcduct,d a t 320' ( and the product 13rurLer a n a l y z d is described as being produced hctnrc I 290" and 370" C. Thcw datn from 1iii; temperature operations R I ~ sistent with the data Gd! :til 1 1 obtained at 315" C. (T'igurr -7 i carbon number distribution dath, :in also the ratio of branched tc 11~~111 'I' hydrocarbons determind 10 (drlion numbers higher than in prex iou, i n it
Table 111. Normal Paraffin Distribution for Fischer-Tropsch Synthesis Product (B.P. 170-323" C.) Volume_ _Per _ _ Cent Mass AIolecular spectroSieve-m:iss metric matrix
Carbon Sumber 0
10 11 12 13 14 15 16 17 18 19 20
spectrnmetric
calculation
1 6 3.2 13.3 11.9 10.0 9.1 7 .3
5 3 3.5
VOL. 34, NO. 7,JUNE 1962
(3) 1G 3.2 13.0 10.5 9 ,2 8 3 7 6 5.8 4 0
829
vestigations, tend to support the work of Gall and Kipping in the temperature variation of isomer distribution. Data shown in Figure 4 imply that one value for (a) and (f) in the chaingrowth scheme will not apply over the entire molecular weight range and a change in these constants must be made at approximately Cm. Anderson has noted that a similar plot of Weitkamp’s data is linear for the range Cz to Ci4 with a = 0.6185 and f = 0.115 (1). The deviation found in the plot of the Weitkamp data is in the same direction as found in the present investigation. Anderson also noted that carbon number distribution data obtained for synthesis product produced with a cobalt catalyst gave a linear plot from Cz to C20 with a = 0.836 and f = 0.035. Thus, for the lower branching factor (f), the chain-growth scheme was found to apply over a much wider carbon number range. The present data are h e a r from a t least CI2to C20and are more in line with the
data for product from cobalt catalyst. This would indicate that the chaingrowth scheme adequately describes the product from low temperature operations, but not the extensively branched product obtained at approximately 300’ C. Perhaps other terms to describe cyclization or the role of oxygenated compounds are required. ACKNOWLEDGMENT
The authors acknowledge the helpful discussions with R. B. Anderson. Charles Zahn prepared the distillation fractions and Anthony Logar carried out the Molecular Sieve separations. LITERATURE CITED
(1) Anderson, R. B., “Catalysis,” vol. IV, chap. 3, P. H. Emmett, ed., Reinhold, New Yorlr, 1956. ( 2 ) Anderson, R. B., Friedel, R. A., Storch. H. H.. J . Chem. Phw. 19. 313 (1951): (3) Brown, R. A,, Skahan, D. J., Cirillo,
V. A.. bfelpolder, E’. W.,AKAL.CHEM. 31, 1531 (1959). (4) Bruner, F. W.,lnd. Eng. Chem. 41, 2511 (1949). ( 5 ) Ferguson, W. C., Howard, H. E., AXAL.CHEM.30,314 (1958). ( 8 ) Friedel, R. A,, Anderson, R. B., J . Am. Chem. Soc. 72, 1212 (1950). ( 7 ) Friedel, R. A., Logar, A. F., Jr., Shultz, J. L., Appl. Spectroscopy 6 , 24 (1952). (8) Gall, D., Kipping, R. J., J . Inst. Petrol. 44. 243 (1958). (9) Larson,’ Lewis P.,’Becker, Harry C., AZIAL.CIIEM.32, 1215 (1960). (10) Kelson, K. H., Grimes, M. D., Heinrich, B. J., Ibzd., 29, 1026 (1957). (11) O’Connor, John G., Norris, Matthew S.,Ibid., 32, 701 (1960). (12) O’Seal, M. J., Jr., Wier, T. P., Jr., Ibid., 23, 830 (1951). (13) Prosen, E. J., Pitzer, K. S., Rossini, F. D., J . Research h’atl. Bur. Standards 34,403 (1945). (14) Weitkamp, A. W., Seelig, Herman S.,
Bowman, Norman J., Cady, William E.,Ind. En5. Chem. 45,343 (1953). (15) Weller, S., Friedel, R. A,, J . Chem. Phys. 17,801 (1949).
(16) Ibid., 18, 157 (1950). RECEIVEDfor review January 4, 1962. Accepted March 28, 1962.
Identification of the Chemical Form of Radioactive Ions at Submicromolar Concentrations R. W. HENKENS and D. R. KALKWARF Hanford laborafories Operation, General Electric ,Paper electrophoresis was used to separate and identify the chemical forms of radioactive ions a t concentrations of less than 10-12M. Identifications were made by comparing the free-solution mobilities of these ions with those calculated from the limiting ionic conductances of known forms. The identifications were confirmed b y comparison of the electrophoretic migration of known and unknown ions under identical conditions.
S
of the by-products of the atomic energy industry are dilute solutions of radioactive ions which, after disposal, enter into various environmental processes. The routes taken by the ions in these processes will depend on their chemical forms. Methods are available for establishing the identity of many radioactive isotopes, however, most of them can exist in several chemical forms in aqueous solution. Knowledge of these forms is necessary for understanding the uptake of radioisotopes by biological systems, and for studies of methods of their removal. For example, Hogan and Eagle (8) found that the binding of different chemical forms of arsenic OME
830
ANALYTICAL CHEMISTRY
Co., Richland,
Wash.
by body tissues differed up to several orders of magnitude. Use of ordinary methods for identifying chemical forms was not possible since the radioactive ions were present in concentrations far below those required for analysis. Concentration of the radioactive ions by methods such as evaporation could not be used because of the chemical-form changes which occur during these processes. At a given temperature and ionic strength, the free-solution mobility of an ion is a constant, characteristic of chemical form. It can be estimated from values of ionic conductance, such as those given by Landolt-Bornstein ( I d ) . Determination of free-solution mobilities offers a means of identifying chemical forms through comparison of the experimental values with those estimated from ionic conductances. I n the present work, paper electrophoresis was used to separate the radioactive ions without sample concentration. Their free-solution mobilities were calculated from their apparent mobilities in the paper. The chemical-form identifications of the ions were confirmed by comparison of their electrophoretic migrations with those of known ions run under identical conditions.
THEORY OF MOBILITY DETERMINATIONS
The mobility of an ion may be defined as its velocity, relative to the solvent, when the potential gradient is unity. While the free-solution mobility, U , is a characteristic constant for a given ion, the apparent mobility, Ua, of a visible zone of the ion on a piece of paper moistened with a background electrolyte depends on many factors. Some of these factors are discussed by Bailey and Yaffe (1). Under conditions of constant velocity and uniform potential-gradient and where there are no specific interactions of the ion with the paper or background electrolyte (6, IO, I S ) ,
(g)
The factor ’, where 4‘ is the average distance which an ion must travel in the paper to move between points separated by a straightline distance, .t, corrects for the increased distance an ion has to travel in the channels of the paper. This factor is a function of the absorbance (the volume of liquid present per gram of paper) ( 5 ) ) and perhaps the type of paper. The term Cc is a constant which corrects for electroosmotic flox of t h e
