60-
w v)
z P 0
40-
W v)
a a
W
n
5 20W
U I
'e - 0 - I
Figure 7, and are in good agreement (but 6x in excess) with the calculated quantity of water released by conversion to the monohydrate. As the monohydrates lose water only above 400 "C (7), and we were unable to heat above 350 "C, no further water loss was detected. Thin Film Samples (Dioctyl Sebacate). Various areas were sampled from a large section of aluminum sheet having an electrostatically-applied coating of dioctyl sebacate. Adjacent sections from this sheet yielded coating weight values, using this E G D technique, of 0.59 pg/cm2 with a precision of =t1 %. Areas sampled from non-adjacent sections yielded values that differed by i10%. Figure 8 shows typical E G D curves, for the dioctyl sebacate samples, obtained using the sample chamber diagrammed in Figure 2. The bulk sample, contained in a glass tube within
A Study of the Influence of Silicone Grease on Spectrophotometric Methods for the Determination of Fluoride J a n Tu3 Research Institute f o r Animal Nutrition, Feed Science and Technology, PohoTelice, Czechoslouakia
DIFFUSION METHODS of the separation of fluoride are of very great importance for the determination of fluorine in biological and other materials. They involve liberating fluoride ions by mineral acids and their absorption in trapping solutions of alkali. As diffusion vessels, polyethylene bottles (1, 2), Conway dishes (3, 4 ) , multicell trays (3,polyethylene stoppers (6), special dishes with two square chambers (7), and plastic Petri dishes (8) are used. In most diffusion methods, silicone grease is used to seal diffusion vessels in order to prevent any leakage of fluoride ( 4 , 5 , 7 , 9 , 10). Polyethylene bottles and stoppers only need not be sealed; nevertheless, sealing bottles with wax was also recommended (2). Recently, Taves (10) found that fluoride passes into trapping solutions in the form of methylfluorosjlane if silicone grease is used for sealing; the diffusion of hydrogen fluoride was supposed earlier. In the presence of the simpliest silicone, hexamethyldisiloxane, the separation of fluoride is much more rapid. Therefore, a faster diffusion method for the separation of fluoride was proposed (8). Fluoride was liberated by 25% perchloric acid in the presence of a solution of hexa(1) L. Singer and W. D. Armstrong, ANAL.CHEW,26, 904 (1954). (2) R. J. Hall, Analyst, 88, 76 (1963). (3) F. J. Frere, Microchem. J . , 6, 167 (1962). (4) H. W. Wharton, ANAL.CHEM., 34, 1296 (1962). (5) C . R. Nicholson, ibid., 38, 1966 (1966). (6) J. Baumler, Chimin, 18, 218 (1964). (7) J. A. Wheatherall and J. A. Hargreaves, Adcunces in Fluorine Reserrrch and Dental Curirs Pretention, 4, 181 (1966). (8) D. R. Taves, Tuluntn, 15, 969 (1968). (9) D. R. Taves, Nature, 211, 192 (1966). (10) D. R. Taves, APU'AL. CHEM., 40,204 (1968). 352
a
ANALYTICAL CHEMISTRY
methyldisiloxane in 6N hydrochloric acid. The separation was carried out at 25 "C for 2 or 6 hours, depending upon the volume of sample analyzed. Otherwise, the separation of fluoride by diffusion takes place for at least 24 hours at much higher temperatures (usually 60 "C) (2, 4 , 5). Thus, the use of silicone grease enables quicker separations of fluoride, but there was the possibility of the interference of components of silicone grease and their breakdown products, with subsequent spectrophotometric determinations of fluoride (8, IO). It was desirable, therefore, to study the influence of silicone grease (and the products of its decomposition) o n various spectrophotometric methods for the determination of fluoride. After the separation in diffusion vessels usually sealed by silicone grease, fluoride was determined by lanthanum alizarin complexonate (3), by indirect methods with zirconiumSPADNS (4,5-dihydroxy-3-(p-sulfophenylazo)-2,7-naphthalene disulfonic acid) ( 4 , 5 , 7, I / ) , and with zirconiumEriochrome Cyanine R (12, 13). Also, an indirect spectrofluorimetric method with thorium-morin was used after the separation of fluoride by diffusion (8, 9, 14). Only the spectrophotometric methods mentioned were studied because the fluorimetric method using thorium and morin proved to be unaffected by silicone (8, 14). In the present work, other (11) C. W. Chang and C. R. Thompson, Microchem. J . , 8, 407 (1964). (12) L. Greenland, Anal. Chitv. Acfa, 27, 386 (1962). (13) L. Singer and W. D. Armstrong, Anal. Biochem., 10, 495 (1965). (14) D. R. Taves, Tnlntfta, 15, 1015 (1968).
Pg
F-
0
0.25 0.5 1 .o 1.5 2 3 4 6
A
Table I. Determination of Fluoride by Alizarin Complexone (ACP,b Absorbance X lo3 La-AC in acetone-water media (16, 17) La-AC in water media (3) Ce(II1)-AC in water media (15) B dev C dev D dev A B dev C dev D dev B dev C dev D dev A
0
0
...
...
0
...
0
...
0
...
0
...
0
...
0
0
0
0
0 +1
2 6 12 18 30 41 56 66 80
0
0
0
0 7 0 15 -1 27 +1 40 0 54 -2 80 - 2 107 + l 160 0 215 +1 265
... . . . . . . . . . . . . . . . . . .
3 0 3 0 3 0 3 7 +2 9 + 1 8 0 6 0 12 0 11 -1 12 12 $1 16 0 17 $1 17 18 -1 31 $1 28 -2 30 29 0 44 0 44 0 41 41 $1 71 - 1 73 $1 56 55 100 0 99 - 1 68 67 -2 $1 128 + 2 126 0 80 80 0.5-10 pg (3, 15) and 0.25-10 pg (16,17). Determinations were made in 5 replicates. A. Untreated fluoride B. Diffused fluoride C. Fluoride 2 ml of the solution of silicone grease D. Fluoride 100 pg of silicon as potassium silicate
3 3 8 10 12 12 16 17 30 29 44 44 72 73 8 100 98 126 127 10 a Ranges of method:
0
+l -1 0 -1 -1 0
-1 0 0
+l 0 0 0
-2 0
3 5 13 17 28 39 57 68 81
0 7 16 27 41 54 80 106 160 214 266
0 0 +l 0 $1 0 0
0 7 15 28 39 54 81 -1 106 0 159 - 1 215 + 1 265
0 0 0 $1
0 7 14 27 -1 39 0 56 +1 79 -1 107 -1 160 0 217 0 266
0 0 -1 0 -1 $2 -1 0 0 +2 +1
+ +
methods for the determination of fluoride were studied: further determinations with alizarin complexone (&In, and indirect determination of fluoride with thorium and xylenol orange (18), which proved to be a very desirable method. The reason for this study was to determine the extent to which spectrophotometric methods for the determination of fluoride are influenced by silicones and their breakdown products. If there were any interferences, chances for using this important diffusion method for the separation of fluorine would be limited. EXPERIMENTAL
Apparatus. Diffusion multicell trays with polyethylene sheets (Kendall Co., Res. Center, Barrington, Ill.) were used for the separation of fluoride by diffusion. Spectrophotometric determinations were made with Spektrofotometr SF-4A (UdSSR) using 1-cm cells. Determinations according to Nicholson (5) were made in 5-cm cells. Reagents. For study of the zirconium-SPADNS system, reagents were used according to Wharton (4) and Nicholson (5); for zirconium-Eriochrome Cyanine R according to the modified method using this reagent (19); and for thoriumxylenol orange system reagents according to RezBE and Ditz (18). Reagents, including alizarin complexone, were prepared according to Belcher, Leonard, and West (15), Frere (3), and Yamamura, Wade, and Sikes (16), with the replacement of cerium by lanthanum as recommended in a following paper (17). Reagents for the separation of fluoride by diffusion were the same as used by Nicholson (5). High vacuum silicone grease (Lukosan M 14, Synthesia, Kolin, CSSR) was used. The grease is a homogenous mixture of methylsilicone fluid and the aerogel of silica. The solution of the grease was prepared by gently boiling 1 g of the grease in 200 ml of 25 perchloric acid. Immediately before using, portions of this solution were neutralized with concentrated sodium hydroxide solution and then made up t o double the original volume. (15) R. Belcher, M. A. Leonard, and T. S . West, J. Chern. Soc., 1959, 3577. (16) S. S . Yamamura, M. A. Wade, and J. H. Sikes, ANAL.CHEM., 34, 1308 (1962). (17) M. A. Wade and S . S.Yarnamura, ibid., 37,1276 (1965). (18) Z..l?ezbE and J. Ditz,Z. Anal. Chern., 186,424(1962). (19) s. Magregian, ANAL.CHEM., 26, 1161 (1954).
The solution of potassium silicate was prepared to contain 1 mg of silicon per ml. Standard Fluoride Solution. 211.1 mg of reagent grade sodium fluoride was dissolved in water and the solution was diluted to 1 liter. The solution contained 0.1 mg of fluoride per ml and was stored in a polyethylene flask. Procedure. Standard curves for the determination of fluoride were prepared over the range 0-10 pg of fluoride. Great attention was devoted to low concentrations of fluoride. Procedures were the same as in the papers cited. Solutions were prepared in 5-ml flasks, with the exception of Wharton's method (4), for which 10-ml flasks were used. The influence of the solution of silicone grease was studied by additions of 2.0, 1.0, and 0.5 ml of neutralized solution of the grease to standards. The influence of silicon was examined by additions of 10-100 pg of silicon as potassium silicate to standards of fluoride. Fluoride from standards was also diffused in multicells lubricated with silicone grease at 60 "C for 24 hours (5). Before diffusion, small pieces of silicone grease (about 0.3 g) were added to solutions studied. RESULTS AND DISCUSSION
During studies of the influence of silicone grease, its breakdown products, and potassium silicate on the spectrophotometric methods, it was found that determinations using alizarin complexone are by no means influenced by all substances mentioned above, even if higher concentrations were used. The values of absorbance for various concentrations of fluoride were the same for untreated standards and for fluoride with substances examined (Table I). The finding that silicone grease does not interfere in the lanthanum alizarin complexonate method in acetone-water media (16, 17) is especially important because this procedure seems t o be the best of the alizarin complexonate methods. For the modification of zirconium-SPADNS methods suitable for simultaneous analysis of larger numbers of samples (3, it was found that silicone and other substances examined d o not interfere (Table 11.). Small depressions appeared for less than 1.5 pg of fluoride on standard curves when the solution of silicone grease was added, but these depressions were due to a small decolorizing effect of sodium perchlorate; this was proved by experiments with the solution of sodium perchlorate only, which gave the same depressions as the solution of silicone grease. VOL. 41, NO. 2, FEBRUARY 1969
353
Table 11. Determination of Fluoride by Zirconium-SPADNS
Absorbance X l o 3 Zr-SPADNS (5) FA B dev CC dev D dev A 1048 +2 1046 -2 0 1048 0 264 1050 1008 -2 248 0.25 1010 +2 1012 1011 $1 975 0 972 -3 0.5 975 977 +2 230 898 0 895 -3 195 1.0 898 900 +2 0 851 851 853 +2 158 1.5 852 +l 796 0 797 $1 120 2 796 798 +2 -1 705 0 55 705 704 3 707 +2 4 652 +2 652 0 654 653 +1 5 553 -2 6 555 +3 ... 558 556 +1 480 483 $3 ... 8 +2 478 -2 482 10 415 412 -3 416 +1 417 +2 ,,, The ranges of methods: 0.1-10 pg ( 5 ) and 0.25-4 pg (4). Determinations were made in 5 replicates. c The decolorization by NaC104 was subtracted. A. Untreated fluoride B. Diffused fluoride C . Fluoride 2 ml of the solution of silicone grease 100 fig of silicon as potassium silicate D. Fluoride
B
265 250 226 198 158 122 53 3
... ... ...
System5.b
Zr-SPADNS ( 4 ) dev Cc dev +1 262 -2 +2 251 +3 -4 232 +2 $3 196 +1 0 159 +1 $2 120 0 -2 52 -3 -2 4 -1
...
... *..
D 266 248 233 192 156 121 56 8
... ... ... . . . . . . . . . ... . . . ...
dev +2 0
$3 -3 -2 +1 +1
+3
... ... ...
0
+ +
Table 111. Determination of Fluoride with Zirconium-Eriochrome Cyanine R and Thorium-Xylenol Orange. $ b
Absorbance X lo3 Zr-Eriochrome Cyanine R (19) pgFA B dev Cc dev -2 867 0 0 867 865 1 837 835 -2 837 0 2 817 +2 817 0 819 0 3 804 804 805 +1 -2 4 800 798 802 +2 6 175 +2 777 776 +1 8 740 740 0 738 -2 10 700 697 -3 701 $1 = The ranges of both methods: 0.7-10 p g . b Determination were made in 4 replicates. c The decolorization by NaC104 was subtracted. A. Untreated fluoride B. Diffused fluoride C. Fluoride + 2 ml of the solution of silicone grease D. Fluoride + 100 p g of silicon as potassium silicate
D 868 837 818 802 799 775 743 698
Similar results were obtained for Wharton's modification of the zirconium-SPADNS method ( 4 ) (Table 11). Therefore, this method is not influenced by silicone grease and its breakdown products. Nevertheless, it seems that the more sensitive, rapid, and accurate procedure according t o Nicholson (5) will be preferred t o this method, especially since measurements are not carried out against the solution of the dyestuff. When methods for the determination of fluoride by zirconium-Eriochrome Cyanine R (19) and by thorium-xylenol orange (18) were studied, it was found that both methods are much less sensitive than determinations with alizarin complexone or zirconium-SPADNS. Only about 0.7 pg of fluoride or more may be exactly determined by these methods. Moreover, n o strictly linear dependence of the decrease of absorbance on fluoride concentration was found for both methods, but both methods are uneffected by diffusion in the presence of silicone grease, by the solution of silicone grease, or by potassium silicate (Table 111). Small depressions on standard curves in the presence of the solution of the grease were due t o the decolorizing effect of sodium perchlorate, as 354
ANALYTICAL CHEMISTRY
dev +1 0
+1 -2 -1 0
$3 -2
A 685 645 610 580 560 542 526 490
B 685 648 607 582 560 542 528 492
Th-xylenol orange (18F dev dev 683 -2 0 +3 +2 647 -3 +1 611 580 0 +2 0 561 +1 541 -1 0 526 0 +2 490 0 +2
D
685 644 612 577 562 542 526 493
dev 0
-1
+2 -3 +2 0 0
+3
was found for the zirconium-SPADNS system. Both methods are suitable for determinations of larger amounts. The method using xylenol orange will probably be preferred t o the zirconium-Eriochrome Cyanine R method owing to negligible absorbance of the dyestuff at the working p H (18). Results obtained in this study proved that it is possible t o use silicone grease for sealing diffusion vessels without any precautions. The comparison of standard curves obtained with and without diffusion seems to be the best evidence for the suitability of silicone grease. Our results are in good agreement with results obtained when the zirconium-SPADNS system was used after diffusion of fluoride in multicell trays was checked for the effect of silicone grease; n o influence of silicones was found (20). Results also warrant the use of silicones for promoting diffusion of fluoride. The addition of silicone grease directly t o the acid before diffusion caused larger amounts of silicones (including hexamethyldisiloxane) to pass t o the trapping solution; however n o
(20) C. R. Nicholson, private communication.
SPADNS method (after the diffusion in multicell trays) was found (20).
interference with photometric methods was found. These conclusions are in the accordance with findings of Singer (21), who found no interference of hexamethyldisiloxane with the zirconium-Eriochrome Cyanine R method (13). Obtained results are valid in general, because the composition of the silicone grease used is almost the same as that of high vacuum silicone grease (Dow-Corning Co., Midland, Mich.) often used for sealing diffusion vessels. Also, no influence of Dow-Corning silicone grease on the zirconium-
Diffusion methods using silicone grease for sealing diffusion vessels proved t o be reliable for the separation of fluoride and also for determination of the fluoride isolated. Diffusion methods, especially in combination with very selective alizarin complexonate methods, enable sensitive, rapid, and selective determination of fluoride in a wide variety of materials.
(21) L. Singer, private communication.
RECEIVED for review May 28, 1968. Accepted September 27, 1968.
Identification of Surface Functional Groups on Active Carbon by Infrared Internal Reflection Spectrophotometric James S. Mattson, Harry B. Mark, Jr.,l and Walter J. Weber, Jr.’ Water Resources Sciences, Department of Chemistry,l and Department of Cicil Engineering,= Unicersity of Michigan, Ann Arbor, Mich. 48104
PREVIOUS EFFORTS (1-3) to obtain infrared spectra for carbon black, activated carbon, and coal have involved transmission measurements on prepared KBr pellets or Nujol mulls. Although some information regarding structure of the bulk materials has been gained in this manner. such techniques have proved rather unsatisfactory for identification of the surfaces of carbon materials because of invariably poor resolution. Ergun ( 4 ) has shown that the extinction coefficient of graphite in the infrared spectral range is very high, approaching that of a metal. The average extinction coefficient, k , is about 0.66 in the visible region, and little variation is observed through the short wavelength region of the infrared ( 4 ) . Common organic compounds or functional groups exhibit extinction coefficients approximately two orders of magnitude smaller than that of graphite, and therefore transmit sufficient radiation to give infrared spectra of reasonable contrast. From the present spectral studies of active carbon, it appears that the bulk extinction coefficient of this material is approximately that of graphite. On this basis it is reasonable to conclude that light cannot be transmitted through particles of carbon unless they are extremely thin, in fact, light of 5-p wavelength will decay to 1 of its initial value after passing through 3.7 of graphite. Thus, infrared light incident upon most microcrystals of active carbon will be totally absorbed, unless it hits at a sufficiently grazing angle to allow the light to be reflected from the particle. In view of the magnitude of the extinction coefficient for active carbon it would appear that the attempts reported in the literature to obtain transmission spectra of this material in KBr or Nujol have actually involved measurement of a complicated type of mixed transmission and reflection spectrum. This type of spectrum results from a combination of forward scattered radiation and radiation which misses the particles entirely, as illustrated in Figure 1, as well as radiation which (1) V. A. Garten, D. E. Weiss, and J. B. Willis, Aust. J . Chem., 10,
RADIATION
I*
CARBON
MICROCRYSIALS
J Figure 1. Schematic diagram of the forward scattering process obtained when strongly absorbing carbon particles are included in KBr or Nujol, and infrared light is transmitted through the matrix
passes through extremely small particles. The resulting spectrum might be best referred to as a “diffuse reflectance” spectrum. This approach to examining the surface of a material such as active carbon is complicated by the fact that light losses due to scattering are inversely proportional to the fourth power of the wavelength, producing huge scattering losses at shorter wavelengths. Internal reflection spectrometric (IRS) ( 5 ) techniques allow all of the incident light to interact with the sample, without losses due to scattering (6). Thus it is possible to obtain spectra of high contrast and resolution with which to examine the nature of surface structures of active carbon. The general theory behind the use of IRS has been discussed by Harrick (6, 7). The present study has involved the use of IRS to
295 (1957). (2) J . K. Brown, J . Chem. Soc. (London), 1955, 744. (3) R. A. Friedel and J. A. Queiser, ANAL.CHEM., 28, 22 (1956). (4) S,. , Ergun, in “Chemistry and Physics of Carbon,” Vol. 3, Philip L. Walker, Ed., Marcel Dekker, Inc., New York, 1968, pl> 45-119.
( 5 ) J. Fahrenfort, Spectrochim. Actu, 17, 698 (1961). (6) N. J. Harrick and N. H. Riederman, Spectrochim. Acta, 21, 2135 (1965). (7) N. J. Harrick, “Internal Reflection Spectroscopy,” Interscience, New York, 1967. VOL. 41, NO. 2, FEBRUARY 1969
355
