Quantitative Determination of Methyl Mercaptan, Dimethyl Disulfide, and Dimethyl Sulfide in a Gas Mixture Following Separation of Mercaptan by Mercuric Cyanide and Absorption of DisulJide and Sulfide in Benzene WILLIAM SEGALI AND ROBERT L. STARKEY Department of Microbiology, Agricultural Experiment Station, Rutgers University, New Brunswick, N. J.
Studies were undertaken to determine the sulfur products of microbial decomposition of organic sulfur compounds. Evidence was obtained that volatile sulfur products such as methyl mercaptan and alkyl sulfide were produced from methionine. No satisfactory methods were available for the quantitative determination of these products, formed slowly during the decomposition process. The method described provided a means of quantitatively determining methyl mercaptan, dimethyl disulfide, and dimethyl sulfide occurring as mixtures in gas. The gas was passed through mercuric cyanide and
I
N a study of the transformation of methionine by niicroorgan-
isms i t was found that all of the sulfur of methionine was converted to organic sulfur products that volatilized from the culture solution. The use of 3% mercuric cyanide and 4% mercuric chloride as trap solutions for the absorption and precipitation of these volatile sulfur products as mercury derivatives, as described indicated that methyl mercaptan by Challenger and Charlton (4), (methanethiol) and dimethyl disulfide, and possibly dimethyl sulfide, were present. These reagents were inadequate for the quantitative separation and determination of the highly volatile sulfur compounds because the mercury derivatives could not be analyzed accurately. A review of the literature revealed no method for the quantitative determination of a mixture of methyl mercaptan, dimethyl disulfide, and dimethyl sulfide, slowly volatilizing from a bacterial culture solution over a period of several days. The methods either could not be applied to the conditions of the bacterial system, or could not be applied to a gas mixture of these three types of compounds. Some parts of methods used in the petroleum industry for the determination of mixtures of organic sulfur compounds in hydrocarbon solutions ( 1 , 2, 5, 6,11) were found to be adaptable to the conditions of a constant gas flow of volatile sulfur' products. Indeed, the analytical procedure of Bell and Agruss (Z), whereby organic sulfides are dissolved in benzene and determined in this solvent, n as incorporated in the adopted procedure. APPARATUS AND SEPkRATION PROCEDURE
An air stream containing the volatile products released from the culture solution in the 250-ml. Erlenmeyer flask, A (Figure l ) , was drawn at the rate of approximately 5 liters per hour through 175 ml. of 4% mercuric cyanide in absorption bottle B and 50 ml. in test trap C, n hich absorbed the mercaptan. Then it passed through 175 ml. of benzene in absorption bottle D, which absorbed the dimethyl sulfide and dimethyl disulfide. The reflux condenser, E, was cooled with iced water, 5, to keep the temperature between 5' and 15 ' C. and prevent appreciable loss of benzene (loss was from 10 to 15 ml. a day). The iced water jacket was insulated with a 40-mm. layer of asbestos, 6. The jacket was 1 Present address, Public Health Research Institute of the City of S e w York, ?;en. York, ?u'. P.
benzene. Methyl mercaptan was removed by the mercuric cyanide, whereas dimethyl disulfide and dimethyl sulfide were removed by the benzene. The immediate value of the method was the solution of the difficult problem of determining the volatile products of methionine decomposition. Similar organic compounds are encountered in products of the brewing industry, in petroleum products, in sewage, and in various other substances undergoing microbial attack. The method should find application where quantitative determination of mercaptans and alkyl sulfides is needed.
diained a t 7 and iced water was added as needed. Even when the room temperature was 30" C. the temperature of the water did not rise above 15 C. in an 8-hour period. The glass wool in the condenser, 4, was kept saturated with benzene by slowly releasing the solvent from the separatory funnel, F , and this served to trap any of the sulfides that were not collected in D. The stopcock of the separatory funnel was lubricated n i t h silicone lubricant. Attached to the end of the separatory funnel by benzene-resistant plastic tubing M as glass tubing reaching part way into the glass wool. From the condenser the air stream was drawn through absorption bottle G, which contained 175 ml. of 3% mercuric chloride that served as a test trap to collect any mercaptan, dimethyl disulfide, or climethyl sulfide that passed through the system. I n no case were any of these products detected in this solution. O
The refluv system was required because of the volatility of benzene. In tests made Tyithout the reflux condenser but with the benzene in the absorption bottle kept a t 5" C. to prevent ewessive loss by volatilization, there n as incomplete absorption of the sulfur products. Before the solution of mercuric cyanide vas adopted as the absorbent for mercaptan, tests n-ere made in which standard 0.05 N silver nitrate n a s used as the absorbent. This solution preceded absorption bottlc G, and B and C nere eliminated. Part of the methyl mercaptan was recovered in the benzene and part in the solution of silver nitrate. .4fter absorption, the excess silver nitrate was determined by titration with standard ammonium thiocyanate, ferric alum being used as the indicator. The use of mercuric cyanide to absorb the methyl mercaptan was preferable because a standard solution was not required, rind the standard silvei nitrate x a s unstable. Furthermore. the d v e r precipitate of methyl mercaptan is fine, clings to the gl,rw, and decomposes readily with no distinct melting point, hereas the precipitate formed by mercuric C T anide, mercuric dithiomethoxide, can be readily collected, dried, and weighed, and has R specific melting point of 175' C . Mercuric cyanide is inert to dimethyl disulfide and dimethyl sulfide, and the solution absorbed all of the mercaptan from the air stream. I t was, therefore, unnecessary to test for mercaptan in the benzene absorbent or to remove mercaptan from the solvent before making the determinations for the sulfides.
1645
ANALYTICAL CHEMISTRY
1646 REAGEh-TS
Reagent grade chemicals were used throughout. The methyl mercaptan, dimethyl disulfide, and dimethyl sulfide were Eastman chemicals. Ferric alum indicator. Eighty grams of ferric alum were dissolved in 400 ml. of 4 '2' nitric acid and made to 800 ml. A41coholicplumbite. A solution of sodium hydroxide (25%) was saturated with lead monoxide and filtered, and 100 ml. of the clear filtrate were mixed with 100 ml. of ethyl alcohol (95%). METHODS
The methyl mercaptan was absorbed in the solutions of niercuric cyanide. Tests for the methyl mercaptan were made separately on the contents of containers B and C. The precipitate of mercuric dithiomethoxide was filtered off on Xo. 42 Whatman filter paper, washed, and dried in vacuum over calcium chloride to constant weight. The filtrate and washings were collected in a 200-d. volumetric flask. The amount of methyl mercaptan was calculated from the weight of the precipitate, assumed to be mercuric dithiomethoxide, (CH3S)*Hg. A portion of the precipitate was recrystallized from ethyl acetate three times and the melting point as well as the mixed melting point with authentic mercuric dithiomethoxide was determined.- -The compound turns greenish brown at 160" C. and melts at 1 4 0 C. with decomposition. A portion of the precipitate was transferred to a boiling flask provided with a ground-glass stopper carrying a separatoi y funnel and a tube leading to an absorption vessel containing standard 0.01N iodine, which in turn was connected to a similar vessel containing standard 0.01N sodium thiosulfate. Concentrated hydrochloric acid (25 ml.) was added from the separatory funnel and the solution was heated and boiled for 20 minutes, the liberated mercaptan being absorbed by the iodine. The amount of mercaptan was calculated from the decreaae in iodine by thiosulfate titration, and was also estimated indirectly by measuring the cyanide released by reaction of methyl mercaptan with the mercuric cyanide to form mercuric dithiomethoxide (8). The filtrate and washings from the precipitate of mercuric dithiomethoxide were made to 200 ml., and 50-ml. aliquots were treated with 5 ml. of 0.3S mercuric nitrate. 5 ml. of 4 5 nitric acid, and 1 ml. of ferric alum indicator, and titrated with standard 0.1X ammonium thiocyanate ( 7 ) . The mercurir nitrate reacts with the cyanide released from the mercuric cyanide in the absorbing solution by the mercaptan and forms mercuric c j anide. The mercury of the residual mercuric nitrate was titrated. Because mercuric cyanide is much less dissociated than mercuric thiocyanate, the results are unaffected by mercuric cyanide. The method was also verified by titration of solutions of mercuric cyanide to which measured amounts of methyl mercaptan had been added, and by the gravimetric and iodometric determinations of the mercury precipitates. Mercuric cyanide proved to be a particularly desirable reagent for absorption of methyl mercaptan, for it was possible to determine mercaptan, after ahsorption by gravimetric and iodometric determinations of the precipitate and by the titrimetric determination on the residual solution. I t was not necessary to use a standard solution of mercuric cyanide in the absorbing vessels. Dimethyl disulfide and dimethyl sulfide 1%ere determined on the benzene absorbent. The general pattern of the analytical scheme of Bell and Agruss ( 2 ) was folloxed, but modifications of Faragher, Morrell, and Monroe ( 6 ) and of Ball ( 1 )were adopted. Condenser E was washed v ith benzene into the absorption bottle, D,and the benzene was made to 200 ml. in a volumetric flask. At this point two methods of analysis were used to determine dimethyl disulfide and dimethyl sulfide. Scheme A. To a 100-ml. aliquot in a 250-ml. Erlenmeyer flask were added 25 grams of polydered zinc and 50 ml. of glacial acetic acid. The flask was immediately attached t o a reflux condenser equipped at the top with a trap containing 20 ml. of standard O.0jAVsilver nitrate diluted to 50 mi. and the solution was heated and refluxed for 3 hour$. The dimethyl disulfide was thereby reduced to methyl mercaptan. After cooling, the flask was removed and the contents and washings were transferred to a 500-ml. separator!- funnel. The water layer was drawn off and discarded and the benzene layer was washed with distilled water until the washings were no longer acid to litmus, after which the benzene was filtered and the volume made to 200 ml.
Determinations were made on SO-nil. aliquots for methyl mercaptan formed from the dimethyl disulfide, according to the method of Borgstrom and Reid ( 3 )modified by Malisoff and Anding (9). The aliquot was transferred to a 250-ml. glass-stoppered flask and an excess (usually 25 ml.) of 0.05Nsilver nitrate, 10 ml. of ferric alum indicator, and 3 ml. of 95% ethyl alcohol were added. After being vigorously shaken, t,he solution was titrated with standard 0.05N ammonium thiocyanate to the end point, which was a salmon-pink color of the water layer. The amount of mercaptan that reacted with the silver nitrate was calculated from the value obtained by subtracting the titration value of excess silver nitrate from that of the original quantity of silver nitrate added.
F i g u r e 1. Apparatus for Trapping and Separating Volatile Organic S u l f u r C o m p o u n d s A . 250-ml. Erlenmeyer culture flask with 100-ml. solution m e d i u m 1. Drying tube with cotton to free incoming air of microorganisms 2. Perforated bulb of aeration tube 3. Cotton to protect culture solution from air oontamination B . Gas-absorption bottle with mercuric cyanide for absorption of mercaptan C. Tebt trap with mercuric cyanide D . Gas-absorption bottle with benzene for absorption of dimethyl disulfide and dimethyl sulfide E . Condenser cooled with iced water 4. Condenser tube filled with glass wool 5. Bath of iced water 6. Asbestos insulation 7. Waste line to drain condenser jacket, with screw clamp F. 60-ml. separatory funnel for addition of benzene G . Gas-absorption bottle with test trap of mercuric chloride
The reactions are as follows:
+
+
RSH Ag?;Oa = IISO:, RS-Ag 4 AgNOs SHASCS = XHISO3 AgSCN
+
+
3
The red color develops with appearance of excess ammonium thiocyanate. A similar determination was made on the condenser trap solution of silver nitrate. The combined results provided the value for methyl mercaptan, from which the value for dimethyl disulfide was calculated. A portion of the reduced and washed benzene solution was used to determine the melting point of the mercury salt of the methyl mercaptan derived from the dimethyl disulfide; the solution was treated with mercuric cyanide and the resulting precipitate was separated, purified, and tested in the same manner as the precipitate of mercuric dithiomethoxide derived from the methyl mercaptan in the absorbing solution of mercuric cyanide. Determinations were made on 50-ml. aliquots for dimethyl sulfide. The aliquot was transferred to n 500-ml. separatory funnel
V O L U M E 25, NO. 11, N O V E M B E R 1 9 5 3 and 25 ml. of 10% silver nitrate acidified with nitric acid to below pH 2.0 were added to precipitate mercaptans formed by reduction of dimethyl disulfide. After being shaken vigorously, the water layer was drawn off and the benzene washed until the washings were neutral and free of silver. The benzene was filtered and transferred to a 500-ml. separatory flask containing 50 ml. of distilled water and an excess (10ml.) of saturated bromine water (IO). The mixture was shaken for 10 minutes, after which 10 ml. of 15% potassium iodide was added and the mixture was shaken again. Sodium thiosulfate ( 0 . 1 N )was added to react with the iodine, and the solution was shaken and the water layer drawn off and recovered. The benzene was washed with three 50-ml. portions of water, and the washings were added to the original water solution. The total amount of solution was titrated with 0.1N potassium hydroxide, methyl red being used as indicator. The hydrobromic acid which was titrated is formed by the following reaction:
RzS
+ BrP = R2SBr2 + =HzO RZSO + 2HBr
A portion of the benzene solution treated with silver nitrate to remove the mercaptan formed from the disulfide served aa a source of material for melting point determinations representative of the mercury derivative of dimethyl sulfide. To obtain the material, the benzene waa shaken with 3% mercuric chloride, filtered, washed, and recrystallized from hot benzene three times. The melting point aa well as the mixed melting point with an authentic specimen of 2( CH3)2S.3HgC12 was determined. This compound has a melting point of 158' C. Scheme B. When dimethyl sulfide was present in the mixture of volatile products in the air stream, and when silver nitrate was used to react with the mercaptan formed from the dimethyl disulfide, low values for dimethyl sulfide were obtained according to Scheme A and the following procedure proved to be preferable. Bell and Agruss ( 2 ) did not report low values for this procedure. The starting material was the benzene in which the alkyl sulfide and disulfide were dissolved. A 100-ml. portion of the benzene solution was reduced with powdered zinc and acetic acid as in Scheme A, and the benzene was washed and the benzene portion recovered as before. For the determination of dimethyl sulfide, a 50-ml. aliquot of the benzene was transferred to a 250-ml. separatory funnel and shaken with 25 ml. of alcoholic plumbite ( 6 ) . The RTater fraction was removed, the benzene again treated with alcoholic plumbite, the water fraction removed, the benzene washed with water, and the benzene finally filtered and recovered. The dimethyl sulfide was determined on the benzene fraction by the bromine method described above.
1647
If no dimethyl sulfide was found, a 50-ml. aliquot was treated with standard silver nitrate and titrated with standard ammonium thiocyanate according to the procedure for the reduced disulfide under Scheme A. If dimethyl sulfide was found, dimethyl disulfide was determined as follows: To a 50-ml. aliquot of the benzene absorbent in a 250-ml. glass-stoppered Erlenmeyer flask were added 20 grams of pulverized mercurous nitrate (monohydrate) ( I ) and the mixture was shaken for 10 minutes. The solid, which removed the sulfide as R2S.x Hg(N0&, was filtered off and washed with benzene, and the filtrate was added to the original benzene filtrate. The benzene was washed with three 50-ml. portions of distilled water in a separatory funnel, after which it was treated with powdered zinc and acetic acid to reduce the disulfide, and then treated with silver nitrate and titrated with ammonium thiocyanate according to the procedure for reduced disulfide under Scheme A. EXPERIMENTAL
The procedures were tested on mixtures of known quantities of pure methyl mercaptan, dimethyl sulfide, and dimethyl disulfide. Because of flammability, toxicity, and volatility, special methods were used in sampling. Rubber gloves were worn and all work was carried out in a hood in a well ventilated room. The tips of the glass-sealed bottles were broken off and samples of the liquids were withdrawn by I-ml. syringes with long needles. The density of dimethyl sulfide is 0.8458 a t 21 C. and that of dimethyl disulfide is 1.057 a t 16" C. The liquids were brought to these temperatures by water baths before sampling. By reason of the low boiling point of methyl mercaptan (5.96" C.), its bottle was cooled in a bath of solid carbon dioxide, and the syringe used in sampling was cooled to below 0' C. There may have been some error in measuring the methyl mercaptan, for the density value of 0.896 a t 0 " C. was used. The liquids were added to 50 ml. of distilled water in a 250-ml. Erlenmeyer flask with the tip of the needle below the surface of the water. After the compounds were added. the flask was closed with a cork stopper covered and sealed with collodion. The flask occupied position A in Figure 1. To simulate the conditions of the tests with bacterial cultures, the air stream was drawn through the system for 3 days a t a room temperature of approximately 28" C. Determinations were made in duplicate on solutions containing 0.1-ml. uantities of each of the three liquids. An untreated control sJution was also tested. The resulta of the determinations carried out according to Schemes A and B are summarized in Table I.
According to Scheme A, the values for dimethyl disulfide were somewhat too high and those for dimethyl sulfide were too low. This may have been due to reaction between dimethyl sulfide and silver nitrate used to determine and remove methyl mercaptan resulting from the reduction of dimethyl disulfide. The systematic analysis by Table I. Determinations of Known Quantities of Methyl Mercaptan, Scheme B gave satisfactory results for these comDimethyl Disulfide, and Dimethyl Sulfide pounds. When only methyl mercaptan and di( I n solutions containing O.l.-inl. quantities of each compound)5 methyl disulfide nere present, Scheme A was prefAnalytical Procedure erable because the procedures were simpler. When Titrimetric b Gravimetric Iodometric dimethyl sulfide was also present, Scheme B gave sr, of % of % of more accurate results. S, mg. theoretical S, mg. theoretical S.mg. theoretical The results for methyl mercaptan were essenMethyl mercaptan (calculated S content of material used, 59.7 mg.) tially the same in both schemes of analysis. Slow Scheme A 58.2 97.5 57.7 96.6 56.5 94.6 57.4 96.1 57.1 95.6 56.0 93.8 osidation by air was found to convert between 2 94.1 95.0 56.2 Scheme B 56.8 95.1 56.7 and 3% of the methyl mercaptan to dimethyl di55.9 93.6 57.2 95.8 56.5 94.6 sulfide under the conditions of the experiment, thus Dimethyl disulfide (calculated S content of material used. 72.0 mg.) tending to give somen-hat ]OR value3 for methyl Scheme A 14.4 103.3 mercaptan and high values for dimethyl disulfide. 101.9 13.4 70.8 98.3 Scheme B The results of gravimetric and iodometric deter69.4 96.4 minations of the mercuric preripitates of methyl Dimethyl sulfide (calculated S content of material used, 43.7 mg.) mercaptan showed good agreement with those obScheme A 38.8 88.8 tained by the titrimetric procedure, but they were 39.6 90.7 somen-hat low. Scheme B 41.9 95.9 42.3 96.8 Most of the results were within 5% of the 0 Determinations on blanks of distilled water gave zero valuss in all owes. theoretical, which was adequate for the purposes b Results obtained by treatment of filtrate from the mercuric cyanide absorbent with mercuric nitrate a n d titration with ammonium thiocymate. for which the procedures were t o be used. The degree of accuracy of the values seems to be
1648
ANALYTICAL CHEMISTRY
very good, in view of the volatility of the compounds and the technical difficulties involved in sampling and in the analyses. I n no case did precipitate form in the trap solution of mercuric chloride (G in Figure l), thus indicating that none of the mercaptan and sulfides failed to be absorbed by the mercuric cyanide and benzene. Melting point determinations of the mercuric derivatives confirmed the identity of the methyl mercaptan, dimethyl disulfide, and dimethyl sulfide. LITERATURE CITED
(1) Ball, J. S.,U. S. Bur. Mines, Rept. Invest. 3591, 1 (1941). (2) Bell, R. T., and Agruss, hI. S., IND.ENG.CHEM.,ANAL.ED., 13, 297 (1941). (3) Borgstrom, P., and Reid, E. E., Ibid., 1, 186 (1939).
Challenger, F., and Charltori, P. T., J . Chem. Soc., 1947, 424. Claxton, G., and Hoffert, W. H., J . SOC.Chem. I n d . , 65, 333, 341 (1946). Faragher, If-. F., Morrell, J. C., and l l o n t oe, G. S.,I n d . Eng. Chem., 19, 1281 (1927). Kolthoff, I. M., and Sandell, E. B.. “Textbook of Quantitative Inorganic AnalyBis,” Kew YOIk. lIacmillan Co., 1948. Kolthoff, I. N., and Stenger, V. A , , “Volumetric Analysis,” Vol. 11, h-ew Tork, Interscience Publishers, 1947. Rlalisoff, W. AI., and Anding, C . E., Jr., ISD. ENG.CHEM., ANAL.ED.,7, 86 (1935). Sampey, J. R., Slagle, K. IT., and Reid, E. E., J . A m . Chem. Soc., 54, 3401 (1932). Seyfried, W. D., Chem. Eng. S e w s , 27, 2482 (1949). RECEIVED for review blarch 7, 1953 Accepted August 17, 1953. Journal Series Paper, N. J. Agricultural Experiment Station, Rutgers University, The Stat? University of Xew Jersey. Department of Microbiology. Investigation supported by a grant proi,icled by the Texas Gulf Sulphur Co.
Determination of Traces of Hydrogen Fluoride in Inert Gases D. L. RlANNING AND J. C. WHITE Analytical Chemistry Division, Y - I 2 Plant, Oak Ridge ,Yational Laboratory, Oak Ridge, Tenn. A method has been developed for the determination of traces of hydrogen fluoride in inert gas sweepings. The gases are passed through a dilute solution of boric acid and the increase in conductivity of the solution, which is produced by the reaction between boric acid and hydrogen fluoride to form a complex strong acid, is measured. The relationship between conductivity and concentration of hydrogen fluoride was linear for the range 0 to 1 mg. of hydrogen flnoride. As little as 10 micrograms of hydrogen fluoride can be determined by this procedure. The method is particularly valuable for industrial application when no other ionizable gases are present.
solution is :I strong acid (of the aanie order as hydrochloric acid), infinit,ely stronger than either boric acid of hydrofluoric acid (K1 a t 25’ C.) ( 2 ) . Hence, a solution of boric acid = 6.89 X will show a distinct increase in conductivity when hydrogen fluoride is added. -4study was made to adapt this reaction to the det,ermination of t,races of hydrogen fluoride in inert gas sweepings. EXPERIMENTAL
T
IIE detection and determination of traces of hydrogen fluoride in inert gases have been of recent interest’. The ideal method for this type of determinat,ion is one that could be conducted a t the site of the gas outlet, be relatively simple to accomplish, provide a rapid and accurate analysis, and be sufficiently sensitive to determine the gas in concentrations of the order of a few parts per million. Such limitations tend to Pliminate the known colorimetric methods for fluoride. -4 proposed method which would meet these criteria is a conductometric measurement of a scrubbing solution through which the gases have been passed. Possible solutions which would serve as scrubbers include solutions m-hich, contain cations that form slightly soluble fluoride salts such as calcium, barium, and lead, or solutions which contain an ion that forms complexes wit,h the fluoride ion. Water itself, because of t,he great solu1)ility of hydrogen fluoride in water, could be used as a scrubber. Boric acid, H,B03, was considered a promising scrubhing solution in this instance. A solution of boric acid has several inherent advantages with respect to ultimate conductometric determination; it is slightly ionized in water ( K = 6.4 X at 25’ C.) (1) and would hence have a specific conductance of the same order as water, and it reacts rapidly with fluoride ion to form a soluble complex acid. The reaction can be written: &Boa Wamser
+ 3HF +HBF30H + 2H20
(1)
(4) reports that monohydroxyfluoboric acid in aqueous
A standard, dilute solution of hydrofluoric acid x a s used to provide hydrogen fluoride. A stock solution was prepared by diluting 1 ml. of 48% hydrofluoric acid to 1000 ml. and stored in a polyet,hylene bottle. This solution was standardized by a conductometric titration against standard alkali solution. Known amounts of hydrogen fluoride were taken from this stock solution by appropriate dilution. The scrubbing solutions were prepared from a stock solution of boric acid which contained 10 grams of boric acid, C.P. reagent grade, dissolved in 1 liter of freshly boiled, distilled water. This solution v-as not standardized, but its concentration calculated to give 0.016-$1 boric acid. The experiments were conducted by adding from a buret knoFn amounts of hydrofluoric acid to 100 ml. of boric acid and nieasuring the conductivity of the solution after each addition. The final volume of solution in all cases was 110 ml. Conductivity measurements were made with a Leeds and Northrup conductivity bridge using a “dip-type” cell with a cell constant of 0.1 cm-1. Five concentrations of ecrubbing solutions were used: 0.16X, 0.10M, 0.01M, 0.0016M, and 0.0003M. No precautions were taken to regulate the temperature. The temperature control of the conductivity bridge was set a t the temperature of the dip cell. Inert gases, which were suspected to contain hydrogen fluoride, were sampled by passing the gases through the scrubbing solution of boric acid and then through a wet-test meter to record its volume. For continuous monitoring of the gas stream, the conductivity cell was located directly in the gas outlet line; for periodic monitoring, the scrubbing solution was removed and its conductance measured. Borosilicate glass containers were used and showed no evidence of etching. RESULTS A X D DISCUSSION
A tabulation of the results is given in Table I, which shows that the conductivity of the boric acid scrubhing solution increases n.ith the addition of hydrofluoric acid. As expected, the conductivity decreases with decreasing concentration of boric acid. .4larger increase in conduct,ivity is produced by the addition of 1007 of hydrofluoric acid to a solution of boric acid t,han
