ANALYTICAL EDITION
408
sult. I n transferring the ignited residue from silica to porcelain, the solution was not filtered, but the potassium chloroplatinate was dissolved in hot water after weighing, so that the net weight was obtained. The aliquots were evaporated on the water bath, fumed off over Gilmer heaters a t a low temperature, and then placed in the electric furnace a t the temperature and for the period stated. It was found that 2 minutes in the electric furnace removed practically all of the ammonia, when various types of dishes were used. Even where the lowest visible red heat was used for 2 minutes only, the maximum amount of ammonia found was 0.2 mg. It may be noted that the Alternative Method (1) for potash specifies: “Ignite over a free flame beIow a red heat until all volatile matter is driven off .” Early in these determinations it was noted that after ignition small amounts of potash might sometimes be left undissolved in the silica dishes. Unusual care was used in dissolving the ignited residue, particularly in regard to the time. All of the dishes were tested for potash by boiling out with hydrochloric acid, and amounts of potash varying from none to 0.4 mg. of potassium chloroplatinate were found. These amounts were included with the amounts reported. In one case where a high temperature was used, 1.0 mg. of potassium chloroplatinate was noted. The ignited residue, although somewhat slow in dissolving, can be readily dissolved in a reasonable time, especially if the ignition has been made at a low temperature. Five minutes were found to be ample for solution before transferring to porcelain. I n ignition where phosphates are present, the phosphoric
Vol. 3, No. 4
acid may replace in part the sulfuric acid which is more volatile at high temperatures. Low-temperature ignition prolongs the life of silica dishes and reduces the amount of silica loosened from the dishes. I n this series of analyses, the maximum amount of silica noted in the precipitated potash was 3.2 mg. where a high temperature was used with triple superphosphate mixtures. The usual amount of silica varied, with low-temperature ignition, from none to 0.4 mg. Dishes deteriorate more rapidly with triple superphosphate mixtures. All mixtures containing acidulated phosphates normally contain phosphates in the potash solution. Ignition residues containing triple superphosphate melt at a very low temperature, forming a glassy instead of a crystalline or granular residue. Although few duplicate analyses are shown, no exception to low results with high-temperature ignition was obtained. As the only object of ignition is to remove ammonia and organic matter, the results with low-temperature ignition are probably nearest the true potash content. It is suggested that low-temperature ignition be stressed in any future study of potash methods for mixed fertilizers, and that adequate time be given for complete solution of the ignited residue. Literature Cited (1) Assocn. Official Agr. Chem., Methods, p. 15 (1925);also references cited below. (2) Bible, C. M., J . Assocn. Oficial Agr. Chem., 8,420-3 (1925). (3) Fraps, 0.S.,Ibid., 9, 192 (19261. (4) Haigh, L. D.,Ibid., 10, 220-3 (1927). (6) Haigh, L. D.,I b i d . , 11, 219-20 (1928). (6) Kerr, A. P.,Ibzd., 8, 419-20 (1925).
Practical Methods of Detecting and Estimating Methyl Chloride in Air and Foods With a Description of a Modified Combustion Method’ Mathew J. Martinek and Wm. C. Marti BUREAU OP LABORATORIES AND RESEARCH, CHICAGO DEPARTMENT OX HEALTH, CHICAGO,
Comparative studies of available field tests for methyl chloride in air showed the copper gauze-flame test to be the most efficient and rapid. A study of laboratory methods for the qualitative detection of methyl chloride in miscellaneous samples resulted in developing a practical combustion method which proved satisfactory. It is sensitive to about 50 parts of methyl chloride per million parts of air. For quantitative work, a modified combustion method
ILL.
was devised in which the methyl chloride gas, diluted with air, is oxidized, and the halogen products of combustion3. e., the hydrochloric acid gas and the free chlorine and its oxides-are absorbed in a solution of sodium carbonate and arseniousacid. The resultant chloridesare precipitated with silver nitrate and titrated by a very sensitive modified Volhard method, the accuracy of the method being about 50 parts per million. It is applicable to various types of samples, including air, food materials, urine, etc.
. .. .. HE recent occurrence of poisonings and fatalities from
T
methyl chloride gas used in mechanical refrigerating systems in Chicago (2) made it necessary to seek practical tests for the rapid detection of this gas and to develop and perfect accurate quantitative methods for its estimation in air and miscellaneous substances. I n this report the subject will be treated under two headings, field methods and laboratory methods, with special attention to the development of a modified combustion method. Field Methods
A method commonly used by service men for detection of gas leaking from refrigerating systems is to coat the pipes f
Received May 26, 1931.
with a film of soap, using a shaving brush and shaving stick to produce a lather, Wherever a leak occurs there will be a formation of bubbles. This test is useful but is not suitable for detecting leaks in inaccessible places. In refrigerating systems containing a mixture of methyl chloride and sulfur dioxide, the test used is to pass an open bottle of ammonium hydroxide along the pipes, the formation of a white cloud indicating a leak. I n small concentrations where the gas cannot be detected by odor, the copper flame test for organic halogens was found to be the most sensitive and convenient. The following apparatus has been adopted as an official field test by the Inspection Bureau of the Chicago Department of Health: A small alcohol lamp (Figure l), fitted with a cone of copper gauze, is constructed as follows: A wide-
October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
mouth bottle, A , is stoppered with a cork, B, through which is inserted a glass tube, C, for holding a wick, F. Over this is suspended a copper gauze cone, E, held in place by a wire support, D. For operation the lamp is partially filled with alcohol and the wick ignited. Ordinarily the lamp will burn with a pale blue flame but, if brought into the presence of organic halides, it burns with a distinct blue-green flame. The test is sensitive to about one part per 100,000. The lamp should not be used where large quantities of the gas might be present. The lower limit of flammability is 8.1 per cent of the gas to 91.9 per cent of air by volume (3). Laboratory Methods
The detection of methyl chloride by dissolving it in water and then refluxing it with freshly precipitated silver oxide in order to convert it to silver chloride was tried and found unsatisfactory. Hydrolysis of the methyl chloride to form methanol by use of strong alkali or potassium acetate with alkali also proved unsuccessful unless boiled for a very long time u n d e r high p r e s s u r e . Hydrolysis and oxidation of the methyl chloride to formaldehyde, and then testing for the latter by Rimini's method ( I ) , likewise proved unsuccessful because the formaldehyde could never be produced in the low c o n c e n t r a t i o n s handled. Figure 1-Lamp Used in Official Test The method of Roka and Fuchs (d), of Chicago D e p a r t - in which methyl chloride dissolved in m e n t of Health alcohol is heated with potassium iodide in a sealed tube for 3 or 4 hours a t 60" C. and the resultant methyl iodide distilled and precipitated as silver iodide, was also discarded since the reaction was not sensitive to minute amounts of methyl chloride. After these failures, a combustion method was devised based on the oxidation of methyl chloride in the presence of a large excess of air, according to the following equation: 2CHsC1
409
The above combustion method for qualitative tests was used as a basis for a quantitative method, suitable modifications being made to assure sufficient accuracy. The specifications of the apparatus (Figure 2) are as follows: A 4-fOOt (10.16-cm.) quartn tube, A , having an inside diameter of '/4 inch (0.63 cm.), is inserted through two Hoskins electric furnaces in series, B and B', the temperature of which is regulated by means of two resistance units, C and C'. The inlet end of the tube is connected to a measuring pipet, in which methyl chloride for analysis is placed. This pipet is connected to a tower containing soda lime and a cotton filter to free the air entering the apparatus from chlorides. The outlet end of the combustion tube is connected to two absorption bottles, F , arranged in series, having an outlet tube of fritted glass with pores measuring 100 to 120 microns. This arrangement breaks the gas up into very fine bubbles and assures very good absorption. These bottles are connected to a gas meter, G (wet type), which serves as a means of measuring the volume of air used and of regulating the rate of flow. The air is drawn through the apparatus by means of an electric vacuum pump, but other suction apparatus may be used. Cylinders containing a good grade of commercial methyl chloride manufactured by the Roessler & Hasslacher Chemical Company and obtained in the open market were used in developing the method. This grade of methyl chloride was analyzed in the laboratories of the National Board of Fire Underwriters, and had a purity of 99.6 per cent and a water content not over 0.026 per cent (S). The gas before use was dried over soda lime and Dehydrite, and all precautions, such as drying the mercury surface and the buret, were used to ensure a dry sample. Stopcock grease recommended by the Bureau of Standards was used (6).
+ 302 = 2C02 + 2H20 + 2HC1
This reaction is not exactly correct as it has been ,shown later that chlorine and oxides of chlorine are also formed. The resultant hydrochloric acid gas is absorbed in a 2 per cent silver nitrate solution. The presence of methyl chloride is indicated by a cloudy precipitate of silver chloride. The apparatus used in making this test consists of an electric furnace through which passes a quartz combustion tube whose outlet is connected to an absorption bottle containing the silver nitrate solution. For testing air samples, the furnace is heated to approximately 1000" C., and the inlet end of the tube is connected to a 200-cc. Thoerner gassampling bottle placed in a vessel of mercury. The gas is passed through the furnace by displacement and by suction applied at the outlet end. To exclude chlorides or free hydrochloric acid, which may be present in the air used for flushing out the sample, a soda-lime tube and cotton filter are introduced. A distinct precipitate of silver chloride was formed in samples containing 50 parts of methyl chloride per million parts of air. The test is sensitive to all organic halogens, and therefore the source of the sample should be known before any conclusion is reached as to the presence of methyl chloride.
Figure 2-Apparatus for Combustion Method
For measuring the methyl chloride accurately an allglass microburet of 5 cc. capacity and calibrated to 0.01 cc. was constructed (Figure 3). It is connected to a compensator, A , and the whole is immersed in a water jacket. The remaining outlet of the three-way stopcock, B', is fused to a glass dilution chamber, D. This chamber has two tubes, E and E', fused into it, tube E leading from the top of the chamber, and tube E' reaching to the bottom. Stopcock B is a three-way stopcock, one opening of which is connected, by means of tube C, to the outside air. The buret is operated in the following manner: The gas is adjusted on both sides of the mercury in the compensator manometer to atmospheric pressure by opening the manometer cock, F , and removing the lower buret cock plug, B'. Plug B' is replaced, manometer cock F is closed, and the top of the mercury meniscus is marked in the manometer tube. Lower buret cock B' is turned away from compensator, and stopcock B is opened through C. Buret leveling bulb G is raised until the mercury is a t C. The sample is connected a t C, and about 5 cc. are drawn in by lowering the leveling bulb G.
ANALYTICAL EDITION
410
L
This gas is discharged into the outer air in order to avoid contamination of the laboratory air. This should be repeated several times. Next approximately 5 cc. of sample are drawn into the buret and cocks B and B’ are closed. With the leveling bulb so adjusted on the ring support that the mercury in the bulb is approximately level with the mercury in the buret, lower buret cock B’ is turned to communicate with the compensator. If the gas sample is not a t atmospheric pressure, the mercury will rise or fall in the compensator m a n o m e t e r . The buret leveling bulb should be raised or lowered carefully until the mercury in the manometer is brought back exactly to the mark, and then the lower buret cock should be turned away from the compensator. The gas sample in the buret is now a t atmospheric press u r e , a n d t h e volume can be read. Barometric pressure and temperature G of the water in the jacket are noted. All volumes are calculated to standard conditions of 760 mm. and 0” C. The buret is then connected to the combustion train at E and to the drying tower
A
at The E’. temperature of the furnaces is Figure 3--Micro- allowed to reach about 1000” C., and buret for Measuring the absorption bottles, F , containing Methyl Chloride Ac25 cc. of 0.005 N silver nitrate, are curately a t t a c h e d . S u c t i o n is s t a r t e d and so regulated that the meter registers l/2 cu. ft. (0.014 cu. m.) of air per hour. By raising the leveling bulb G, opening B and partially opening B’ to communicate with chamber D, the sample is very slowly passed into the chamber, where it is constantly diluted with dry air before it enters the furnaces, After 1 hour, the absorption bottles are disconnected, and the silver chloride filtered off through a Gooch crucible. The precipitate is washed with small amounts of distilled water and the filtrate acidified with nitric acid and titrated with 0.005 N sulfocyanate, using ferric ammonium sulfate as indicator, according to Volhard’s method (6). The results obtained are shown in Table I, and were too low to be satisfactory. Table I-Results with Silver Nitrate Used a8 Absorbent Gas USED GASRECOVERED ERROR cc. Gram Gram % 4.92 4.47 4.18 4.84 4.76
0.00982 0 00893 0.00840 0.00966 0.00907 I
0.00930 0.00822 0.00759 0.00930 0.00871
5.2 8.0 0.6 3.7 3.8
At this stage of the experiment, tests indicated Chat either all the methyl chloride had not been changed to hydrochloric acid, that the acid was not completely absorbed by the silver nitrate, or that free chlorine or oxides of chlorine were being formed. By repeating the combustion and passing the gases of combustion into a solution of starch and cadmium iodide, the development of a blue color indicated the presence of free chlorine or oxides of chlorine. Substitution of a solution of sodium hydroxide did not remedy the trouble. This necessitated the use of reducing agents strong enough to change the chlorine and its oxides to hydrochloric acid. After trying several possibilities, an alkaline solution of sodium arsenite was found to give satisfactory quantitative results. This solution was made by dissolving 0.2 gram of arsenious oxide in 100 cc. of a 1.0 per cent solution of sodium carbonate. The best results are obtained by using the following pro-
Vol. 3, No. 4
cedure: The gases of combustion are absorbed in two frittedglass absorption bottles, each of which contains 20 cc. of the alkaline sodium arsenite solution. After 1 hour, the absorption bottles are removed, and the contents washed into a 100-cc. volumetric flask to which 5 cc. of 3 to 1 nitric acid have been added. Twenty-five cubic centimeters of standard 0.1 N silver nitrate are added, and the whole is diluted to the 100-cc. mark. The flask is stoppered, well shaken, and the precipitate allowed to .settle. An aliquot portion (25 cc.) of the filtered solution is titrated with 0.05 N potassium sulfocyanate, using a 5-cc. microburet capable of delivering a 0.01-cc. drop. A crystal of ferric ammonium sulfate is used as indicator instead of the saturated solution. By this method a sharp end point results, and the volume of the solution titrated in no case exceeds 40 cc., whereas in the earlier method the volume amounted to as much as 300 cc. Table I1 shows some of the values obtained. The largest error did not exceed 0.5 per cent, and the test proved sensitive to 50 parts of methyl chloride in one million of air. By the use of a platinum needle attached to the microburet, making it possible to split drops, the accuracy of the titration was greatly enhanced. Table 11-Results
cc. 4.65 4.77 4.70 4.47 4.96 4.95 4.96
with Arsenious Oxide and Sodium Carbonate Used as Absorbent GASUSED GASRECOVERED ERROR Gram Gram % .. 0.00878 0.00901 0.00880 0.00857 0.00977 0.00969 0.00970
0.00879 0.00898 0.00884 0.00853 0.00973 0.00967 0.00965
0.1 0.4 0.4 0.4 0.4 0.2 0.5
For routine testing, the procedure is as follows: The sample is collected in a 200-cc. Thoerner gas-sampling bottle by mercury displacement. One opening of the bottle is connected to the inlet end of the combustion train, and the Sample is slowly passed into the heated furnace tube, displacing the sample in the Thoerner bottle with mercury. The train is then cleared with chloride-free air, this air being passed through a t the rate of l/z cu. ft. (0.014 cu. m.) per hour. After 1 hour, the absorption bottles are removed, the titration is performed as stated in the previous paragraph, and the amount of methyl chloride present is calculated from the chlorides found by the titration. For the determination of the methyl chloride in a sample of foodstuffs, urine, or similar material, the sample is placed in ana Erlenmeyer flask equipped with a two-hole rubber stopper covered with tin foil. Glass tubes pass through these holes, the inlet tube passing to the bottom of the flask and the outlet tube passing through only to the bottom of the stopper. This flask is immersed in a hot-water bath, and air is drawn through the warmed sample. This air is very slowly passed through the combustion train at the same rate as for the test on air samples. From the amount of chlorides found the quantity of methyl chloride in the sample is calculated. Blank tests were run on the sample by passing the air from the heated flask directly into the alkaline arsenite solution, thus determining any free hydrochloric acid or chlorine in the sample. Literature Cited (1) Allen, “Commercial Organic Analysis,” 6th ed., Vol. I, p. 327, Blakiston, 1930. (2) Kegel, McNally, and Pope, J . A m . Med. Assocn., 98, 363-8 (1929). (3) National Board of Fire Underwriters, Bull. 1418, 25 (August 26, 1926). (4) Roka and Fuchs, Z . anorg. Chem., 71, 381 (1927). (5) Shepherd, Bur. of Standards Res. Paper 226, 142 (1931). (6) Sutton, “Volumetric Analysis,” 11th ed., p. 150, Blakiston, 1924.
