Defermination of Carbon and Fluorine in Highly Fluorinated Subsfances H.E. FREIER, B. W. NIPPOLDT, P. B. OLSON, and D. G. WEIBLEN Minnesota Mining and Manufacturing co., St. Paul, Minn.
In a general method for the determination of carbon and fluorine in organic compounds containing a high percentage of fluorine, a combustion procedure is used in which the sample is burned with moist oxygen in a quartz tube. Gases, liquids, and solids can be handled. The fluorine is determined by a n arid-base titration of the hydrofluoric acid formed and the carbon, which is simultaneously converted to carbon dioxide, is absorbed by Ascarite. Carbon and fluorine can be determined in a large variety of compounds with a relative error of less than 1%.
has been modified. With these modifications of the apparatus and procedure, carbon and fluorine have been determined successfully in a large variety of fluorinated materials including the compounds which gave erratic results originally. APPARATUS
The equipment listed is advised where a permanent setup capable of handling a variety of samples is desired (see Figure 1). Standard pressure regulator, A , giving sufficient pressure to allow an oxygen flow rate of approximately 25 ml. per minute. Gas sample bulb B, of IO-ml. capacity, 7/10 standard-taper joints, is used for ail gas samples and low boiling liquids. Two gas bubble traps, C, filled with water which serve as flow rate meters as well as a source of water vapor. A movable electrically heated furnace, D,3 to 5 em. in length, and operating a t 800" to 900" C., is used to vaporize the liquid and solid samples. Fused quartz combustion tube, E, with body length of a proximately 650 mm. and outside diameter of 11 111111. The tug, is equipped with a side arm a t the entry end and a 10/30 male joint a t the exit end (available from The Arthur H. Thomas Co., Philadelphia, Pa.). The joint is sparingly greased with Celvacene heavy vacuum grease. The section of the combustion tube within the hot part of the main furnace, F, is packed alternately with quartz chips and rolls of platinum gauze freshly etched with aqua regia. The packing used consists of four sections of platinum gauze and three sections of quartz chips. The section of platinum a t the entrance end protrudes out of the furnace for l to 2 em. An 8-cm. silver gauze roll is placed in the section of the combustion tube within furnace N. This serves to remove interfering sulfur compounds and halogens other than fluorine from the gas stream. Lindberg high temperature micro furnace, F , operating at 1100" to 1200' C. (available from The Arthur H..Tho,mas Co.). A 10-em. electrically heated furnace, N , which is kept a t 350" C. Spray trap and modified Grote absorber with a medium frit, G . Ideally, this should be made of quartz, but borosilicate glass has been found to be satisfactory. This has been modified by using a standard-taper joint which facilitates removal of the acidic material from the absorber (Figure 2). The entrance end of the absorber is so constructed that the condensed water vapor will drain into the absorber and not form a pool of water near the standard-taper joint. Concentrated sulfuric acid trap, H . Anhydrone absorber, I . Standard-taper microabsorption tubes, J, K, filled with -4scarite. Tube J is the carbon dioxide absorption tube andIK is used as a tare. Guard tube containing Anhydrone, L. Standard llariotte bottle assembly, JI.
F
LUORINE-containing organic compounds have become of general interest and importance. A method more universally applicable than those available was needed for the simultaneous determination of carbon and fluorine in these materials. The first problem encountered is that of decomposing the material so as to convert the fluorine into the ionic form. Belcher and Goulden (1) in their review article have conveniently arranged these methods into five main groups-combustion in oxygen, combustion in other gases, alkali metal fusion, fusion with alkalies, and decomposition in solution. Decomposition by Combustion in oxygen appeared to be the most suitable and the least objectionable in regard to handling of reagents. Of the various combustion methods available for the determination of fluorine (1-10) the only methods Rhich appeared applicable nere those of Teston and RlcKenna ( I O ) , Milner ( 7 ) , and Clark (3). Teston and McKenna ( I O ) reported a semimicromethod for the simultaneous determination of carbon, fluorine, and chlorine. Their method of determining fluorine does not, however, give correct fluorine values in hydrogen containing compounds. llilner ( 7 )described a procedure in which the sample is burned in a stream of moist oxygen, utilizing a platinum tube. This method is reported to be useful for compounds boiling a t about 60" C. or higher. Uilner's procedure with various modifications was used successfully in this laboratory. Several disadvantages in addition to the high cost of the platinum tube were soon realized. Controlled vaporization of the sample is difficult when the sample cannot be observed. Maintaining an even flow of steam without increasing the flow rate of the sample through the tube is bothersome. The over-all size of the apparatus is relatively large and the length of time per analysis is somewhat long for routine analyses. The quartz tube micromethod of Clark ( 3 ) for the determination of fluorine only was then investigated. I t was faster and less costly than Alilner's platinum tube method, but yielded low results Lvith certain types of compounds. -4study of this method was made in an attempt to apply it to the types of compounds which gave erroneous results. The proposed procedure differs from Clark's in several important respects. First, a simultaneous carbon determination has been incorporated in the procedure. Second, owing to difficulties encountered in decomposing many compounds, the temperature has been raised from 900" C. to about 1150' C., and a small amount of moisture has been introduced into the combustion tube. Third, the Grote absorber has been modified slightly, and modifications have been made in the apparatus to facilitate handling of gas samples. Fourth, the acid-base titration
-
REAGE-TS
Sodium hydroxide, O . O l S , carbonate-free, stored in polyethylene bottle equipped with an automatic buret. Alkali remaining in glass buret after a titration is discarded and not allowed to return to the bottIe. Phenolphthalein solution, 1% in ethyl alcohol. Saturated mannitol solution, reagent grade.
-0.
Figure 1. Combustion Apparatus
146
V O L U M E 27, NO. 1, J A N U A R Y 1 9 5 5
147
PROCEDURE
Samples of 8 to 10 mg. are used. Solids are weighed i n p l a t i n u m boats, liquids in glass capillaries, a n d g a s e s either directlv in the ma I bulb or the weight is calculated by the method of taking gas aliquots. The usual oxygen purification train is used, and the oxygen flow rate is adjusted to a proxim a t e l y 25 mP. p e r minute. The joint connecting the combustion tube with the modified Grote absorber is cooled with a stream of air. Glass c a p i l l a r i e s f o r liquid samples are sealed on both e n d s b e f o r e weighing. The liquid is frozen by immersing one end of the capillary in liquid air. The other Figure 2. Standard-Taper end is snapped off and Joint both pieces are quickly introduced into the combustion tube. The solid or liquid sample is sloivly vaporized into the hot portion of the tube, taking about 15 to 20 minutee for combustion with a 20-minute sweeping time. In the case of a gas sample, the apparatus is so arranged that the main flow of oxygen bypasses the sample bulb and only one bubble every 2 seconds goes through the bulb. After about 10 minutes, the ratio of oxygen goin through to that bypassing the bulb is increased until finalfy the entire flow of oxygen is directed through the bulb. After about 40 minutes the Ascarite tubes are removed and weighed. Cooling of the joint between the combustion tube and modified Grote absorber is discontinued. This allows the joint to warm up so as to drive any condensed moisture into the absorber. When the joint is dry and free of acid, the oxygen flow is turned off, the absorber is removed, and its contents are transferred to a 125-ml. flask. To remove all acid from the absorber, the titration is started by adding a few drops of phenolphthalein and small amounts of standard O O l N sodium hydroxide directly to the absorber. After the latter is scrubbed with a swab made of cotton on a platinum wire, the washing is added to the 125-ml. flask. This process is repeated until 1 drop of alkali in the absorber remains pink to phenolphthalein. The titration is continued by titrating the combined solutions in the flask with standard 0.01N sodium hydroxide. Before the end point is reached, the solution is boiled a few minutes on the acid side to remove carbon dioxide absorbed from the atmosphere, is quickly cooled, and the titration is completed to the permanent pink color of phenolphthalein. The titer of the alkali is determined by running a series of determinations for fluorine on fluorine-containing organic compounds whose purity is rvell established. Purified samples of perfluorobutyric acid; polytetrafluoroethylene; 1,l-dihydroperfluorobutyl acrylate; or perfluoropropane have been used. In order to check the “mannitol increment” occasionally, as discussed below, 10 nil. of neutralized saturated mannitol solution are added after the end point has been reached. The titration is then continued to a reappearance of the pink phenolphthalein color. The carbon is weighed as carbon dioxide, which is absorbed by the -4scarite. A carbon dioxide blank is determined, using the same volume of oxygen over the same period of time. The blank is usually about 50 y or lees.
aommW \ r
CaFs
DISCUSSION
The decomposition of a fluorocarbon sample in a quartz tube, in the presence of oxygen and a t elevated temperatures, can be represented as in Equation 1for perfluoropropane.
0 2 A
- 2SiFl + 3C02
(1)
4HF
(2)
+ Si02
In Clark’s ( 3 )procedure, as in the present one, the silicon tetrafluoride is absorbed by hydrolysis in a Grote absorber containing water according to Equation 2. The hydrogen fluoride can then be titrated with sodium hydroxide, using phenolphthalein as the indicator. Clark postulated that some of the fluorine was tied up as the weak acid, monohydroxyfluoroboric acid which was formed from the reaction of silicon tetrafluoride or hydrogen fluoride with boron in a Grote absorber.
+ H20 HBFaOH BFa + H F HBFi HBF, + H10 e HBFsOH + HF BFa
(3)
A
(4)
A
(5)
Accordingly, he added mannitol after reaching the first end point and then titrated to a second end point. The difference between the two end points he assumed represented the monohydroxyfluoroboric acid present. Hence, he added three times this value to the first titration volume to obtain the volume of base equivalent to the fluorine present. The experience in this laboratory is in agreement with that of Clark’s, in that there is definitely a mannitol increment using the procedure described. Furthermore, if the base is standardized acidimetrically, fluorine values are low if no increment is added and near-theoretical values can be obtained by adding three times the mannitol increment to the titration volume. Nevertheless, there is some question as to the validity of this use of the mannitol increment. In attempting to reduce the increment to a minimum, the following was observed. The 0.OliV alkali on standing slowly picks up boron from the glass. Very little boron appears to result from combustion in the quartz tube-most of it comes from the absorber. As was shown by Clark ( 4 ) and also found in this laboratory, the pH of the colorimetric equivalence points before and after adding mannitol are not the same. Borate ion a t concentrations which might be found in the Grote absorber does not affect the titration of hydrofluoric acid. The mannitol increments from a large number of individual determinations were small (about 0.10 nil. of 0.01N sodium hydroxide); in the range of 1 to 10 mg. of fluorine, the variation was no greater than the titration error The use of the mannitol increment decreases the precision of the titration, since three times the error in its determination is added to the error in ascertaining the titration volume. Hence an empirical standardization of the alkali, eliminating the mannitol increment, was adopted. This simplifies the titration and a t the same time increases the accuracy. It is advisable to run reference samples periodically to ensure that the conditions have remained constant. In the course of deciding on the best conditions for the quantitative decomposition cf highly fluorinated materials, it became apparent that both temperature and moisture are important factors.
Table I. Sample C8Fis
CALCULATION FOR PER CENT FLUORIBE
[Titration volume (ml. of standard alkali)] X [titer of alkali (mg. of F/ml. of NaOH)]/sampIe weight in mg. X 100 = OJoF
+ 2Si02 + + 2H20
SiF4
CaEs
Comhustion at 900’ C. without Water Vapor Theoretical Values Zl.S%C 78.1% F 19.l%C 80.9% F
Values Found Sample Sample A B 20.0%C 19.5%C 64.5% F 63.5% F 17.9%C 16.8%C 73.3% F G8.2% F
Low in Ratio of C / F Sample Sample A B 1/4.4 1/3.8 1/3.9
1,3.4
Table 11. Decomposition of Tetrafluoromethane at Various Temperatures in Presence of Water Vapor ~~~~~~~t~~~
c. 1000 1200
Observed Values % C % F 0.4 2.1 0.6 3.1
Theoretical Values % C %F 13.6 86.4
ANALYTICAL CHEMISTRY
148
Table IIT.
Table V.
Effect of Moisture in Decomposing Some Compounds at l l O O o C.
Compound C8Fl8
Without hloisture 19 5 % C 63.5% F
With h4oisture 22 0% c 77.7% F
Theoretical 21.9% c 78 1 % F
CsF1eO
21 l % C 62 8% F
23.1% C 72.9% F
23 l % C 73.1% F
General Table of Results
Cornpound
Table IV. Effect of Temperature in Presence of Rloisture Compound C~FMOZ
Found a t 1000° C. 21.8% C 64.2% F
Found a t 11000 c. 22.4%C 70.0% F
Found a t 12000 c .
... ...
Theoretical 22.2% c 70.4% E
... ...
18.1%c 54.5%F
22.2% c 69.8% F
22.2% c 70.4% F
Following are some results obtained on a few compounds analyzed under various conditions which will serve to demonstrate these effects. I t was observed in the analysis of perfluoro-octane and perfluoropropane a t 900" C. with no water vapor present that both carbon and fluorine results were low in a ratio of approximately 1 carbon to 4 fluorines (Table I). This would lead one to believe that the results were probably low owing to the formation of tetrafluoromethane. If this were formed during the combustion, one would expect to obtain low carbon and fluorine values since tetrafluoromethane does not, under the conditions of the analysis, decompose quantitatively a t even higher temperatures and in the presence of Ivater vapor (Table 11). An effort was then made to isolate and identify the breakdown products formed in the combustion to see whether tetrafluoromethane could actually be found in the gas stream from compounds which did not yield quantitative results under the conditions used. The exhaust gases from a typical run of perfluoro-octane and a cyclic perfluoro ether (c-CeFlzO) were collected and identified by infrared spectroscopy. I n both cases with decomposition at 900" C. and no water vapor present, tetrafluoromethane was identified in the exhaust gases. It must be realized, however, that other breakdown materials might be found in the exhaust gases if the sample is introduced into the hot part of the tube too rapidly. For example, when the cyclic perfluoro ether ( c - C e F d ) was volatilized in about one fourth the normal time, carbon monoxide and hexafluoroethane were produced in addition to tetrafluoromethane. From these observations it appears that if one could prevent tetrafluoromethane from forming during the combustion, quantitative results might be obtained. Several investigators have reported that the addition of moisture aids in the decomposition (1, 8). Since tetrafluoromethane was realized with compounds containing no hydrogen, the introduction of hydrogen-containing material, such as water, should alleviate this difficulty. Table I11 shows the results obtained on two compounds which demonstrates the effect of the addition of moisture into the combustion tube. Of course, repeated runs without water vapor gave variable results. The percentages listed are averages of several individual determinations. The addition of moisture alone is not always sufficient to decompose the compound quantitatively, and an increase in temperature as well is required. I n the case of c-C&?&, 1100" was sufficient to yield quantitative results whereas cyclic perfluorobutyl ether gave low carbon and fluorine results a t 1100" C. but yielded quantitative results a t 1200" C. (Table IV). No one set of conditions is necessarily essential to decompose the various fluorine-containing compounds quantitatively. Polytetrafluoroethylene gives correct carbon and fluorine values a t 900" to 1000" C. with no water vapor present. On the other hand, several of the cyclic ethers were quantitatively decomposed only after temperatures of 1200" C. were used in the presence of
:sFi
ZCaF;
Table VI.
Carbon, % Calcd. Found 22.8 22.7 24.0 24.0 22.4 22.2 22.6 22.7 21.1 21.1 21.8 22.0 24.1 24.0 23.9 24.0 22.0 22.0 31.2 31.3 24.2 24.2 33.1 33.1 22.2 22.2 23.4 23.6 18.5 18.6 12.2 12.2 18.1 18.0 27.5 27.4 43.3 43.6
Fluorine, % Calcd. Found 77.2 77.4 76.0 75.8 77.4 77.4 77.0 76.7 73.3 73.4 75.5 75.0 56.9 57.0 72.2 71.8 57.9 57.8 49.8 49.5 69.0 68.6 52.4 52.3 61.6 61.3 68.5 68.0 53.9 54.0 44.9 44.8 40.6 40.7
Reproducibility of Results from Ten .4nalyses of 1,l-Dihydroperfluorobutyl Acrylatea
By Analysis % C % F
Theoretical 7OC
Average a
33 1 52.3 33 07 kO.05 ~ 0 . 1 ~ Obtained by three analysts using two combustion trains
% F
52 36
water vapor. By using the apparatus and procedure as described, a large variety of compounds has been analyzed for carbon and fluorine ivith a relative error of 1% or less (Table V). The reproducibility of this method is demonstrated by the results from ten analyses of 1,l-dihydroperfluorobutyl acrylate (Table VI) Most of these and many other research samples were supplied by the Central Research Department, IIinnesota Mining and Manufacturing Co. SULFUR-CONTAINING COMPOUNDS
Most sulfur compounds present no particular problem H ith the usual procedure. However, in the case of certain sulfur compounds in which the sulfur is in a low state of oxidation, both the carbon and fluorine results were high To overcome the high fluorine values it has been found advantageous to have one of the sections of quartz chips (in the hot zone of the furnace) impregnated n ith vanadium pentoxide. Vanadium pentoxide has long been used as a catalyst for the oxidation of sulfur to sulfur trioxide. At these temperatures, fluoride ion is not held by vanadium. In addition, the carbon values on this type of sulfurcontaining compound were high, probably because some acidic material other than hydrofluoric acid was swept out of the
Table VII. Summary of Results and Conditions Used for .4nalyzing Certain Sulfur Compounds
c.
Compound (CaF;)zSz (CaFi)zSa
(CaF;)pS? (CaF7)zSi
Calcd. 66.2 61.4
17.9 16.6
Per Cent Fluorine Found With With general Vz05, in packing packing 69.5 66.0 65.6 61.2 Per Cent Carbon With With VzO? in general packing packing 18.9 18.8 19.8 21.7
Found With VzOa in packing with KJInO4 t r a p 18.0 16.7
V O L U M E 2 7 , N O . 1, J A N U A R Y 1 9 5 5 absorber. To correct this difficulty, a small bubble trap filled with acidic potassium permanganate was inserted in the train between the modified Grote absorber and sulfuric acid trap. Table VI1 gives a summary of the results for tm-o sulfur-containing compounds analyzed under various conditions. INTERFERENCES
Metal ions which retain fluorine a t the operating temperature of the furnace will give low fluorine results. I n the case of nitrogen compounds, acidic oxides of nitrogen are formed, so that the fluorine cannot be determined by an acid-base titration. Quantitative carbon values are, however, obtained. Several phosphorus-containing compounds have been successfully analyzed for both carbon and fluorine.
149 LITERATURE CITED
Belcher, R., and Goulden, R., Ind. Chemist, 27, 322, 323 (1951). Calfee, J. D., et al., J . Am. Chem. Soc., 62, 267 (1940). Clark, H. S., ANAL.CHEM., 23, 659 (1951). Clark, H. S., and Rees, 0. W., Illinois State Geol. Survey, Rept. 169 (1954). Grosse, A . V., et al., J . Phys. Chem., 4 4 , 275 (1940). Hubbard, D. 31., and Henne, A. L., J . Am. Chem. soc., 56, 1078 f -1934). - - -I \
Milner, 0. I., ANAL.CHEM.,22, 315 (1950). Rickard. R. R., Ball, F. L., and Harris, W. W.. I b i d . , 23, 919 (1951). Schumb, W. C., and Radimer, K . *J., I b i d . , 20, 871 (1948). Teston, R. O’D., and McKenna, F. E., Ibid., 19, 193 (1947). RECEIVED for review hlarch 29, 1954. .4ccepted October 6, 1954. Presented in part before the Division of Industrial and Engineering Chemistry a t the 124th Meeting of the AMERICAN CHEMICAL SOCIETY,Chicago, Ill
Interference of Sulfhydryl Groups in Analysis oi Urinary Mercury and Its Elimination 1. M. WEINER and OTTO H. MULLER Department o f Physiology, State University o f New York, Upstate Medical Center, Syracuse, N. Y. The mercury in the Sal>-rgan-cysteinecomplex, which contains a mercury-sulfur bond, was found to be more difficult to determine quantitatively than either the mercury in Salyrgan or inorganic mercury. The same difficulty was encountered in the analysis of mercury in the urine of dogs treated with Salyrgan. The urinary mercury is believed to exist in a form analogous to the Salyrgan-cysteine complex. Minor changes in Kozelka’s method for mercury analysis made this procedure satisfactory for the quantitative determination of mercury in these sulfur containing compounds.
H E usual procedure for studying the applicability of an anaT i , ’t ical method to biological material is to analyze for known amounts of a particular substance added to tissues or body fluids. However, in the intact animal such a substance may be converted to a form which cannot be duplicated in dead tissue, and which might complicate the analysis. This was found to he true in the case of mercury excreted in urine, in contrast to mercury added to urine. I n earlier experiments performed in this laboratory Jvith a variety of analytical procedures, recoveries of mercury were poor. The authors suspected the deficiency was due to the loss of mercury through volatilization and, therefore, turned to an ingenious method by Kozelka (b), which actually takes advantage of this volatility. The authors found it to be adequate for standard solutions of either inorganic mercury or Salyrgan (mersalyl), an organic mercurial. In the course of studies on the excretion of mercury after the administration of Salyrgan to dogs, simultaneous determinations of urinary mercury were made with Kozelka’? method and the polarographic method. Results obtained by these two methods showed considerable discrepancies which had not been present in the control experiments. The reason for this was suspected to be the formation, in vivo, of a compound considerably more resistant to digestion than Salyrgan itself. The nature of this compund was suggested by further polarographic study ( 5 )to be RHgSR’, where R represents the organic part of the Salyrgan molecule and -SR’ represents a small, unidentified sulfhydryl compound. T o test this hypothesis an analogous compound was prepared from Salyrgan and cysteine, and analyzed by Kozelka’s method. This compound was found to be more difficult to analyze, but this difficulty can be surmounted by relativelv minor changes in the original method.
The interference by sulfhydryl moieties is not noticeable when solutions of mercurials are added to urine, because urine does not usually contain enough neutral sulfur to form an appreciable amount of the mercury complex. In this respect, Simonsen (3) implied that mercury determinations in urine were unsatisfactory if the urine contained much protein. METHODS
I n Kozelka’s original method, 100 ml. of urine is concentrated and then digested with 50 ml. of concentrated sulfuric acid, 20 grams of ammonium sulfate, and 1 gram of copper sulfate in a Kjeldahl flask fitted to a condenser which ends in a water trap. Any mercury that is not distilled over during the digestion procedure is carried over as a complex chloride by a stream of chlorine gas and heat. The mercury in the distillate is determined colorimetrically with dithizone. This method was used by the authors with the following modifications. Kozelka did not specify the time necessary for digestion nor the rate of chlorine flow. The time of digestion and the time of chlorine flow were therefore varied while the rate of chlorine flow was controlled a t approximately I50 ml. per minute with a differential water manometer as an indicator. Urine from animals treated with Salyrgan contains enough mercury to allow analyses to be made on 0.1 to 3 ml. of urine. Therefore, the procedure for concentrating solutions was eliniinated. In addition to the previously mentioned chemicals, 2 grams of sodium chloride were added to the digestion mixture to simulate the chloride content of concentrated urine. Instead of an Evelyn colorimeter a t 470 mp, a Beckman DU spectrophotometer was used a t 480 mp, which is closer to the absorption maximum of niercury dithizonate ( 2 ) . The grade of dithizone (Eastman, white label) used did not require purification ( 4 ) , but some difficulty was experienced with the carbon tetrachloride in which it was dissolved. The dithizone solution waa tested by subjecting it to the same procedure as in the analysis-that is, 25 ml. of dithizone solution was washed twice with 50-ml. portions of 9 N ammonium hydroxide and the absorption of the solution a t 480 mfi determined. Different brands and even different lots of the same brand of carbon tetrachloride (all marked “suitable for dithizone test”) gave different blank readings. -Moreover, a given solution increased in absorbance from day to day. Redistillation of the carbon tetrachloride did not remedy this. As a consequence daily dithizone blanks were run in addition to the usual reagent blanks, which were obtained a t the beginning of each series of analyses. In the authors’ experience the extraction of mercury is best made from less than 75 ml. of total fluid, distillate plus waeh water. Because the most critical step in the procedure is the extraction of all the mercury containing distillate and wash water with exactly 25 ml. of dithizone solution, even minute losses through
