minutes and rebalance the conductance bridge by rotating the decade rheostat. 6. When a stable blank (usually corresponding to about 10 p g 3f S) is obtained, calibrate the apparatus by adding known quantities of sulfur in the form of beryllium sulfate or of a certified ferrous metal standard to the crucible charge. Proceed as in Step 5 . Plot decade reading against sulfur content. 7. Weigh approximately 0.2 grams of the powdered ceramic onto the flux in the crucible. Stir well with a platinum wire before combusting. (Finely powdered PzO, Tho2, etc., present a serious inhalation hazard and should be handled with adequate precautions.) Use calibration curve to determine the sulfur content. RESULTS AND DISCUSSION
Some typical results obtained on a number of high fired beryllium oxide samples are shown in Table I. For each sample the variance is somewhat specific, probably because of the different conditions under which the ceramics were prepared, The variances are greater than those which can be obtained with iron or beryllium sulfate standards and this suggests that total release of sulfur is not always achieved from the ceramics, As the ratio of Be0 to the iron and tin flux is increased the variance of the results increases and the workingratio of0.2:2:2(ingrams)forBeO:Fe:Snwaschosenasa reasonable compromise for a material low in sulfur. For ceramics of higher sulfur content, it is advantageous to decrease the ratio of ceramic to flux. Since no standard samples of beryllium oxide containing
certified amounts of sulfur were available, it was not possible to prove that the figures obtained represent the true sulfur content of the material. However, as shown below, it is expected that interference due to other elements would be negligible. The results obtained by the combustion method were also found to be almost 30z higher than those obtained by the colorimetric method ( I ) , which seems to be the only other practical method available. They are, therefore, thought to represent a better recovery of sulfur. Although the method has so far only been used on BeO, Tho2,UOn,and ferrous metals, it is believed to be of very general applicability to inorganic materials low in halogen. Interferences. Any material giving rise to appreciable amounts of water vapor could cause condensation to occur in the lines between the furnace and the absorption cell. Sulfur dioxide dissolving in such condensed water would lead to low recoveries. All the powdered ceramics should therefore be thoroughly dried before combustion. Carbon, even when present in large excess, has no effect on the conductivity of the solution but material producing more acidic gases could cause positive interference. Chlorides and nitrates were observed to cause such interference when present in amounts comparable with the sulfur content. It is felt, however, that these interferences do not detract from the usefulness of the method when applied to high-fired ceramics since these generally contain negligible quantities of such volatile acid impurities.
RECEIVED for review August 7,1967. Accepted September 26, 1967.
Effect of Silicone Grease on Diffusion of Fluoride Donald R. Taves Department of Radiation Biology and Biophysics, University of Rochester School of Medicine and Dentistry, Rochester, A? DIFFUSION METHODS for the separation of fluoride from biological samples involve the passage of gaseous fluoride, presumed to be in the form of HF, from an acid medium to an alkaline trapping solution ( 1 ) . Silicone grease is widely used to seal the diffusion dishes to prevent leakage of fluoride. The assumption that this is the only function of silicone grease had to be tested because the use of substitutes for the silicone grease resulted in very low recoveries of fluoride. To avoid the possibility that the substitutes were absorbing the diffusing fluoride or preventing diffusion by forming a film on the surface of the solutions, experiments were run without a sealant with similarly low results. However, silicone grease or silicone liquid placed on the middle of the lids, where it could not act as a sealant, was associated with markedly increased rates of fluoride diffusion. These preliminary findings suggested that silicone was contaminating the acid and catalyzing the escape of fluoride from the solution or combining with the fluoride and diffusing over to the trapping solution. When the silicone grease was placed in the usual sealing position it was only 1 to 3 mm distance from the acid, and contact was likely. However, when the silicone grease was placed toward the center of the lid, in a nonsealing position, this mechanism was not likely and raised
(1) L. Singer and W. D. Armstrong, Anal. Biochern., 10, 495 (1965).
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the possibility of a volatile fraction in the grease. A volatile component in the silicone grease could contaminate the trapping solution directly so that it would make it difficult to see a relationship between the amount of fluoride and the amount of silicone diffusing from the acid. Hence, experiments to determine this relationship were designed in which the silicone was introduced directly into the acid. Spectrophotometric fluoride reagents have not been proved to be unaffected by the presence of silicone or its breakdown products, so it was desirable to use radioactive fluoride, the measurement of which cannot be influenced by the silicone. EXPERIMENTAL
Carrier-free fluoride-18 was obtained from Western New York Nuclear Research Center, Inc., Power Drive, Buffalo, N. Y. In all of the experiments reported here the counting was finished by 23 hours and standards were counted within an hour of the samples so that large corrections for decay could be avoided. The only gamma emitting radioactive contaminant that has been detected, E2Br (2), (half life 35.7 hours) constituted less tnan 3 % of the counts at 23 hours based on a half-life for 18Fof 109.7 minutes (3). Uniformly (2) C. C . Thomas, J. A. Sondel, and R. C. Kern, Infern. J. Appl. Radiation Isotopes, 16, 71 (1965). (3) C. H. Carlson, L. Singer, D. H. Service, and W. D. Armstrong, Ibid., 4, 210 (1959).
I BOILING STARTED
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0 TIME (MINUTES) Figure 1. Loss of radioactive fluoride from 67N HCI by boiling with silicone liquid (100 cs) carbon-14 labelled Dow-Corning 200 Silicone fluid, 350 centistokes, was obtained from the Dow-Corning Co., Midland, Mich. The activity was 1 mCi per gram. Silicone grease and nonradioactive silicone fluids, 0.65 and 100 centistokes, were obtained from the same source. The former is the simplest silicone, hexamethyldisiloxane. Radioactive silicone was counted in a liquid scintillation counter with Bray's solution (4). Care was taken to make the blanks and standards comparable to the particular samples being counted because the aqueous solutions result in some quenching depending on quantity, salt concentration, and pH. The diffusion dishes were made of nonwettable polystyrene, SP-7004, obtained directly from Falcon Plastics, 5500 West 83rd Street, Los Angeles, Calif. The diameters of the rings were 35 and 60 mm. The outer compartment has a radial septum which was broken out to allow the contents to flow around the chamber when the dish was placed on a rotary shaker (room temperature only). The Gyrotory Shaker, Model G-25, was obtained from the New Brunswick Scientific Co., New Brunswick, N. J. The pertinent feature is a 0.5inch diameter circular motion run at about 80 cpm. In experiments 1 through 3 of Table I, 200 nmoles of fluoride per dish were thoroughly mixed with the 18Fprior to contact with the acid. In the other experiments no carrier was added but 1 to 10 nmoles of fluoride were present as a contaminant. A sealant was used only in the experiments in Table 11, parafilm (American Can Co.), and in one of the three curves labeled in Figure 1, silicone grease was used. A prior experiment showed a 4 % (k2z standard deviation) loss of '*F using the parafilm sealant in 5 samples. In all of the experiments, regardless of the type of sealant or lack of sealant, the lids were held in place with 3 pounds of lead. (4) G . A. Bray, Anal. Bioclzeem., 1, 279 (1960).
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TIME (HOURS) Figure 2. Room temperature diffusion of radioactive fluoride from silicone-treated 2.9N HCI to 1 ml of O.1N NaHC03 without sealing compared to nontreated 2.9N HCl with silicone grease as a sealant and a control which had nontreated 2.9N HCI without sealing Preparation of silicone-treated HCl involved placing 1 ml of 100-centistokes silicone fluid on 400 ml of 6N HCl in a 600-ml glass beaker and boiling gently. Approximately 3 pmoles of contaminating fluoride present in the acid were labelled by the addition of 18F prior to the addition of the silicone, so that the loss of fluoride could be followed. Samples of the acid were discharged from a pipet into an excess of concentrated KOH to prevent further evolution of fluoride. The radioactive silicone treatment of HCl was carried out on one-tenth of the above scale. The treated HCl solutions were cooled and, without allowing the solution to wash the side of the beaker, siphoned off by means of a squeeze bottle with the tube reversed. The surface film of silicone was displaced by blowing air out of the squeeze bottle as the tube was passed into the liquid. RESULTS
As shown in Table I, the presence of a silicone enhanced the recovery of fluoride by as much as a factor of 10. The fluoride left in the acid of the controls of experiment 2 was measwed, and when added to that in the trapping solution, was found to account for 99.8% (* 1.3 standard deviation) of the original quantity. Because of the different amounts of materials, the only reliable comparison that can be made between the effect of the various silicone preparations is that the more volatile liquid (0.65 centistokes) is much more effective. Figure 1 shows that at least 99% of the fluoride can be removed from HCI by gently boiling it with 1 ml of silicone fluid VOL 40, NO. 1, JANUARY 1960
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Table I. Effect of Silicone on Fluoride Diffusion“ Experiment Type of siliconeb Amount, mg 18Fin Alkali, 1 Grease lO(1-300 67 f 8.66 Control None 7.5 f0.6 2 Liquid (100 cs) 7 12 f 0 . 9 Control None 6 . 8 f 0.5 67 f 2 . 6 3 Liquid (0.65 cs) 7-20 two hours at 55°C from 4 ml 2 5 z HCIOl to 0.5 ml 2.5N NaOH except for experiment 3, which was 25 minutes. b Silicones were spread on lids not closer than 5 mm to point of contact with the dish, so that none of the dishes was sealed. e f Relative standard deviation, 5 samples in each group except for experiment 3, 6 samples.
Table 11. Relationship of Fluoride to Diffusion of Radioactive Silicone“ nmoles F 0 100 lo00 nmoles (CH3)*Si0- 46 f 12 (5) 158 f 8 (3) 698 f 38 (4) a Diffusion from 3N HC1 to 1 ml 0.1N NaHC03 at room temperature for 23 hours.
(100 centistokes for 2 hours). The final sample count was 0.5x of the initial sample, both corrected for background and decay. In the absence of a silicone, fluoride was lost more slowly than HCl when the solution was boiled, tending to concentrate the fluoride in the remaining acid. Figure 2 shows that the silicone-treated HCl, when used in the diffusion dishes without any sealant or additional silicone, enhanced the rate of fluoride diffusion at room temperature at least 10-fold. Table I1 shows that there is a direct relationship between the amount of silicone moving from the acid to the trapping solution and the amount of fluoride present. The radioactive silicone was introduced into the HC1 by boiling them together and then carefully separating the acid, avoiding the surface film of silicone. The concentration of the silicone remaining in the acid, calculated as the silyl monomer (CH3)8O-, was 3.7mM initially and 3.lmM after 11 days. When this treated acid (7000 nmoles (CH3)2SiO-) plus 100 nmoles of fluoride were placed in the diffusion dishes (room temperature for 23 hours), the radioactivity in the trapping solution minus the controls was equivalent to 112 nmoles of silyl monomer. When 1000 nmoles of fluoride was present, the excess was 650 nmoles. In two of these latter dishes and two controls, the acid solution was left in place and fresh trapping solution was placed in the centers, and they were allowed to diffuse for another 23 hours. After the second diffusion, the dishes that had contained the fluoride showed the same amount of radioactivity in the trapping solution as did the controls, f10 %. DISCUSSION
Methylfluorosilanes rather than hydrogen fluoride appear to be the important gaseous-diffusing species under the conditions commonly employed for the diffusion of fluoride. The evidence for this is the very low rate of diffusion in the absence (5) U. S. Dept. of Agriculture and Tenn. Valley Authority, “Superphosphate: Its History, Chemistry and Manufacture,” U. S. Gov. Printing Office, Washington, D. C., 1964, pp. 222-229.
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of a silicone and the close association between the amount of radioactive silicone found in the trapping solution and the amount of fluoride being diffused (Table 11). The ratio of silicone (expressed as the dimethylsilyl radical (CH3)&O-) to fluoride was 1 : 1 when the amount of fluoride was low and 0.7:l when the amount of fluoride was high. The dimethylsilanes, in contrast to the trimethylsilanes, combine with fluoride in a ratio of either 1 :1 or 0.5 :1, so it would be expected that the ratio would range from 1 :1 to 0.5:1, depending on the relative concentrations of the two silanes and fluoride in the acid. (Viscous silicone fluids contain mainly the dimethylsilyl groups.) An alternative explanation for the decrease in ratio with large amounts of fluoride is incomplete diffusion of the fluoride, but this would not alter the basic conclusion and appears unlikely, because there was no excess silicone found when the remaining acid was rediffused. The very low rates of fluoride diffusion in the absence of a silicone cannot be ascribed to loss because it was demonstrated that the fluoride was still in the acid (experiment 2). The increase in concentration of fluoride in boiling 6-7N HCl is not surprising in light of the constant boiling temperatures of the two acids. The constant boiling temperature of H F in HzOis given as 120°C and that of HCI as 110°C by the 45th edition of the “Handbook of Chemistry and Physics” (reference 5 gives the value for H F in HzO as 112.4”C). The rapid loss of fluoride from boiling HCl in the presence of a silicone (Figure 2) can be explained on the basis of the formation of a compound with silicone that is either volatile or sticks to glass. However, because in the other experiments a volatile compound must have been involved, presumably this is also true of the temperature needed for boiling aqueous HCl. It is possible, however, that some heavier fluorine-substituted silicone breakdown products might escape as a gas particularly at the higher temperatures. Introduction of silicone directly into the acid has not been previously used in methods of fluoride diffusion. However, experiment 1 indicates that a sufficiently volatile fraction was available from the silicone grease to provide the necessary silicone for enhanced fluoride diffusion under usual diffusion conditions. The breakdown of silicone polymers by concentrated acids is well known (6) and there is evidence that this occurs to some extent in dilute acids (7), particularly H F (8). The more rapid diffusion of the resulting methylfluorosilanes (and possibly also methylfluorochlorosilane or methylfluorosilanol) from aqueous solution as compared to H F can be reasonably accounted for by the marked degree to which H F reacts with itself and with water to form hydrated complexes (5).
Finding that silicone breakdown products enter the trapping solution means that methods employing silicone grease as a sealant should be checked for possible interference in the analysis of fluoride. RECEIVED for review November 14, 1966. Accepted September 18, 1967. Paper based on work performed under contract with the U. S. Atomic Energy Commission at the University of Rochester Atomic Energy Project and has been assigned Report No. UR-49-769. (6) C. Eaborn, “Organosilicone Compounds,” Academic Press, New York, 1960, pp. 264, 318. (7) W. Noll, H. Steinback, and Chr. Sucker, Kolloid-Z. Z . Polymere, 211, 98 (1966). (8) C. Eaborn, “Organosilicone Compounds,” Academic Press, New York, 1960, p. 457.
