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A rapid thermal conductivity microanalytical method for combined oxygen determination
MICROCHEMICAL
JOURFAL
A Rapid
8,389-394
(1964)
Thermal Conductivity Method for Combined
Microanalytical Oxygen
Determination R. N. Boos Merck
Sharp 13 Dohme Research Laboratories, Division Rahway, New Jersey
of Merck
ti Co., Inc.,
Received September 29, 1964 INTRODUCTION
One of the difficult problems in elemental analysis has been the direct determination of oxygen. Elving and Ligett published a comprehensive review of the literature in 1944 including 99 references (4)) and concluded that the methods then available were not wholly satisfactory. Many publications concerning the direct determination of oxygen have appeared during the past two decades (1-12) giving improvements on or variations of the Schiitze method (8). Zimmerman (12) adapted Schiitze’s procedure to the microscale, and Unterzaucher (10) vastly improved upon it. The voluminous literature is indicative of the difficulties encountered which included the elimination of air inclusion during the insertion of the sample into the pyrolysis tube, a study of the temperature required for the quantitative conversion of CO2 to CO, the type of carbon essential to this conversion, the type of furnace that could withstand the high temperature, the purification of the carrier gas, and the effects of the various pyrolytic gases: such as hydrogen, ammonia, hydrogen sulfide, carbon disulfide, and carbonyl sulfide. Unterzaucher (12) reported, too, that compounds containing phosphorus and fluorine not only produced high results but had a harmful effect on the tube packing so that subsequent determinations were unsatisfactory. The proposed procedure is rapid, accurate, and adaptable to automation. It involves the pyrolysis of the sample at 1120°C over pelletized carbon with the subsequent measurement of the resulting carbon monoxide by thermal conductivity. The method has been in use in this laboratory for several years, and, in spite of the great variety of organic compounds
390
R. N. BOOS
analyzed, there has been no interference from the various pyrolytic gaseous products noted in the literature, with the exception of fluorine and phosphorus-containing compounds. EXPERIMEKTAL
Apparatus Swagelok No. 316 with g-inch plug. Furnace: Hevi-Duty (Hevi-Duty Electric Co., Milwaukee! Wisconsin). Thermocouple: Platinum-rhodium. Temperature controller: Lindberg No. 291 connected through a mercury switch. Gow-Mac four-filament thermal conductivity cell in an insulated box
z s c01 REF. SAMPLE
FIG. 1.
Complete apparatus
heated at 100°C with a model 8658 Harrison Power Supply to power the bridge at 140 mA. Texas Instrument Servo-Riter Integrating Recorder (model PWSNLMVB-OS-A16-AT-R-Xl) set at 10 mV and 12 inches per hour. Chromatographic column: No. 11-134-33, Fischer Scientific Co. (containing activated molecular sieve 13X ) .
THERMAL
DETliRMIr\TATION
OF
391
OXYGET
Sates
Water at room temperature was pumped through the condenser. \Vhen cold water was used, the results were high due to water condensation on the sample holder, as it was removed from the pyrolysis tube for the insertion of the sample. -411of the ?;o. IO,‘30 joints were sealed with Kronig cement. The stainless steel tubing connections were made by insertion into 2 mm i.d. capillary tubing, and were made gas-tight by drawing in melted Krijnig cement under slight vacuum (Fig. 3).
FIG.
2. Quartz sample holder.
, _a KRijNlG
2mm I.D. CAPILLARY TUBING FIG.
3.
CEMENT
STAINLESS STEEL TUBING
Stainless steel tubing connections.
The helium was passed over Dehydrite and Ascarite prior to its admission to the system. In order to prevent the shifting of the packing when the direction of the gas stream is changed, indentations were made in the quartz tube at the exit end of the furnace. After pressing quartz wool against the indentations, pelletized carbon is added to a height of 7 inches. The packing was then heated for 2 hours at 112O’C and was cooled to room temperature under helium. Vibration of the quartz tube in a vertical position settled the carbon packing, and a quartz wool plug was inserted. Indentations in the quartz tube were then made as close to the quartz wool as possible with a very small oxygen-hydrogen flame. When the apparatus is being used, the furnace is kept at 1120°C and when not in use, it is kept at 900°C. The helium flow is never changed
392
R.
N.
BOOS
but is kept at 20 ml per minute measured at the “sample-out” the thermal conductivity cell.
tube of
Reagents Pelletized carbon black: A. H. Thomas Co., Philadelphia, Pennsylvania. Dehydrite. Ascarite. Helium: The Matheson Co. Procedure The sample is weighed into a platinum boat or a quartz capillary by using approximately 1 mg for 30-60y0 and up to 3 mg for smaller quantities of oxygen. With the two stopcocks (Fig. 1) adjusted for the reverse flow of helium in the pyrolysis tube, the Swagelok is removed, and the sample holder is taken from the pyrolysis tube with the aid of a strong magnet (Alnico Button Magnet; l-inch diam., vs-inch high). A cotton glove is worn when handling the holder. The boat or quartz capillary is inserted into the sample holder and the latter is replaced into the pyrolysis tube. The Swagelok is replaced but not tightened for 1 minute to permit the displacement of air. While holding the Swagelok in place, the two stopcocks are adjusted to allow the helium to pass through the pyrolysis tube and on into the thermal conductivity cell. The Swagelok is then tightened with the fingers; a wrench is unnecessary. Approximately 3 minutes are required for the recorder pen to return to the baseline. The integrator is turned on, and, with the magnet, the sample holder is moved rapidly to the center of the furnace. After 2 minutes the sample holder is withdrawn into the condenser area of the tube so it will cool sufficiently for the next sample. In the case of nitrogen-containing compounds, a peak of varying height is sometimes observed shortly after the sample holder has been withdrawn. Approximately 3 minutes later, the carbon monoxide peak appears. As soon as the recorder pen returns to the baseline and the integrator pen levels out, the next sample may be inserted. Known compounds are analyzed in order to determine the number of counts on the integrator curve per microgram of oxygen. The unknown compounds are then calculated from this constant. Oxygen-free organic compounds have never produced a carbon monoxide
THERMAL
DETERMINATION
303
OF OXYGEN
peak, hence there is no blank correction. Fluorine-containing compounds, however, yield high results as would be expected, due to reaction with the quartz tube.
The proposed method herein described permits the complete determination of oxygen in organic compounds in 15 minutes and can readily be adapted to a completely automatic procedure. The recorder is generally set on the 10 mV range for l-3 mg samples. .\lthough not recommended for routine work due to the instability of the baseline and the extra care involved, a few samples in the 100 ug range have been analyzed successfully by using the 1 or 2 mV range. Pyrolysis tubes and packing have been used for many months before replacement became necessary. In addition to the samples listed in Table 1, a variety of research TABLE
1
$7 Oxygen
Valine
Sample (mp)
Theory
Found
0.506
27.3
27.4 27.2 27.3 52.3
1.052
3.024 1.104 0.921
Mannitol
52.7
51.8
52.2
1.076
Potassium
biphthalate
1.340
31.3
1.872
Anisole Cortisone
2.070 1.843 1.926
31.2
14.8 22.2
1.334
Guanosine Codeine
0.841 1.636 2.122 1.835
31.2
28.2 16.0
14.9 15.1 22.3 22.4 28.2 28.1 15.8 16.1
compounds containing ester, ether, amide, nitro, sulfamyl, diphenyl ether, ketone, aldehyde, acetyl, hydroxyl, and sulfonyl groups have been successfully analyzed. SUMMARY A method is proposed for the microdetermination of oxygen in organic compounds based upon a modified Unterzaucher pyrolysis under helium and the measurement
394
R.
N.
BOOS
of the resulting carbon monoxide by thermal conductivity, The sample is pyrolyzed under helium at llZO”C, and the oxygen is converted to carbon monoxide over carbon. The resulting gases are passed over Dehydrite for the removal of ammonia, over Ascarite for the removal of acid gases such as HX and H,S, and through a 6%.foot 13X molecular sieve chromatographic column before entering the thermal conductivity cell. The chromatogram was recorded on a Texas Instrument Co, Servo-Riter recorder equipped with a full-scale integrator. The unknown compounds were compared with a known compound used as a standard. REFERENCES 1.
ALUISE, V. A., HALL, R. T., STEATS, F. C., ,~ND BECKER, W. W., Oxygen in organic compounds. Direct microdetermination by the Unterzaucher method. Anal. Chem. 19, 347-351 (1947). 2. ALUISE, V. A., ALBER, H. K., CONWAY, H. S., HARRIS, C. C., JONES, W. H., AND SMITE, W. H., Direct determination of oxygen in organic compounds. Anal. Chem. 23, 530-533 (1951). 3. DUNDY, M., AND STEHR, E., Determination of oxygen in organic materials by a modified Schiitze-Unterzaucher method. Anal. Chem. 23, 1408-1413 (1951). 4. ELVING, P. J,, AND LIGETT, W. B., Determination of oxygen in organic compounds. Chem. Revs. 34, 129-156 (1944). 5. HOLOWCHAK, J., AND WEAR, G. E. C., Direct determination of oxygen in organic compounds. Anal. Chem. 23, 1404-1407 (1951). 6. KIRSTEN, W., Sources of error in the determination of oxygen by the method of Schiitze-Unterzaucher. Mikrochem. ves Mikrochim. rlcta 34, 151-152 (1949). 7. KORSHUN, M. O., Microanalytic determination of oxygen in organic compounds. Zavodskayu Lab. 10, 241-245 (1941). 8. SCH~TZE, M., Die direkte Bestimmung des sauerstoffs in Zinkoxyd. 2. anal. Chem. 118, 241-245 (1939-40). 9. SHEFT, I., AND KATZ, J., Direct determination of oxygen in organic compounds. Anal. Chem. 39, 1322-1325 (1957). 10. UNTERZAUCHER, J., Die mikroanalytische Bestimmung des Sauerstoffes. Chem. Ber. ‘73B, 391-404 (1940). II. UNTERZAUCHER, J,, Crystallized anhydrous iodic acid as oxidizing agent in the micro-oxygen determination in organic compounds, Mikvochim. Acta 1956, 822-835. 12. ZIMMERMANN, W., Mikroanalytische Ausfiihrungsform der direkten Sauerstoffbestimmung in organischen Substanzen. 2. Anal. Chem. 118, 258-263 (1939).
JOURFAL
A Rapid
8,389-394
(1964)
Thermal Conductivity Method for Combined
Microanalytical Oxygen
Determination R. N. Boos Merck
Sharp 13 Dohme Research Laboratories, Division Rahway, New Jersey
of Merck
ti Co., Inc.,
Received September 29, 1964 INTRODUCTION
One of the difficult problems in elemental analysis has been the direct determination of oxygen. Elving and Ligett published a comprehensive review of the literature in 1944 including 99 references (4)) and concluded that the methods then available were not wholly satisfactory. Many publications concerning the direct determination of oxygen have appeared during the past two decades (1-12) giving improvements on or variations of the Schiitze method (8). Zimmerman (12) adapted Schiitze’s procedure to the microscale, and Unterzaucher (10) vastly improved upon it. The voluminous literature is indicative of the difficulties encountered which included the elimination of air inclusion during the insertion of the sample into the pyrolysis tube, a study of the temperature required for the quantitative conversion of CO2 to CO, the type of carbon essential to this conversion, the type of furnace that could withstand the high temperature, the purification of the carrier gas, and the effects of the various pyrolytic gases: such as hydrogen, ammonia, hydrogen sulfide, carbon disulfide, and carbonyl sulfide. Unterzaucher (12) reported, too, that compounds containing phosphorus and fluorine not only produced high results but had a harmful effect on the tube packing so that subsequent determinations were unsatisfactory. The proposed procedure is rapid, accurate, and adaptable to automation. It involves the pyrolysis of the sample at 1120°C over pelletized carbon with the subsequent measurement of the resulting carbon monoxide by thermal conductivity. The method has been in use in this laboratory for several years, and, in spite of the great variety of organic compounds
390
R. N. BOOS
analyzed, there has been no interference from the various pyrolytic gaseous products noted in the literature, with the exception of fluorine and phosphorus-containing compounds. EXPERIMEKTAL
Apparatus Swagelok No. 316 with g-inch plug. Furnace: Hevi-Duty (Hevi-Duty Electric Co., Milwaukee! Wisconsin). Thermocouple: Platinum-rhodium. Temperature controller: Lindberg No. 291 connected through a mercury switch. Gow-Mac four-filament thermal conductivity cell in an insulated box
z s c01 REF. SAMPLE
FIG. 1.
Complete apparatus
heated at 100°C with a model 8658 Harrison Power Supply to power the bridge at 140 mA. Texas Instrument Servo-Riter Integrating Recorder (model PWSNLMVB-OS-A16-AT-R-Xl) set at 10 mV and 12 inches per hour. Chromatographic column: No. 11-134-33, Fischer Scientific Co. (containing activated molecular sieve 13X ) .
THERMAL
DETliRMIr\TATION
OF
391
OXYGET
Sates
Water at room temperature was pumped through the condenser. \Vhen cold water was used, the results were high due to water condensation on the sample holder, as it was removed from the pyrolysis tube for the insertion of the sample. -411of the ?;o. IO,‘30 joints were sealed with Kronig cement. The stainless steel tubing connections were made by insertion into 2 mm i.d. capillary tubing, and were made gas-tight by drawing in melted Krijnig cement under slight vacuum (Fig. 3).
FIG.
2. Quartz sample holder.
, _a KRijNlG
2mm I.D. CAPILLARY TUBING FIG.
3.
CEMENT
STAINLESS STEEL TUBING
Stainless steel tubing connections.
The helium was passed over Dehydrite and Ascarite prior to its admission to the system. In order to prevent the shifting of the packing when the direction of the gas stream is changed, indentations were made in the quartz tube at the exit end of the furnace. After pressing quartz wool against the indentations, pelletized carbon is added to a height of 7 inches. The packing was then heated for 2 hours at 112O’C and was cooled to room temperature under helium. Vibration of the quartz tube in a vertical position settled the carbon packing, and a quartz wool plug was inserted. Indentations in the quartz tube were then made as close to the quartz wool as possible with a very small oxygen-hydrogen flame. When the apparatus is being used, the furnace is kept at 1120°C and when not in use, it is kept at 900°C. The helium flow is never changed
392
R.
N.
BOOS
but is kept at 20 ml per minute measured at the “sample-out” the thermal conductivity cell.
tube of
Reagents Pelletized carbon black: A. H. Thomas Co., Philadelphia, Pennsylvania. Dehydrite. Ascarite. Helium: The Matheson Co. Procedure The sample is weighed into a platinum boat or a quartz capillary by using approximately 1 mg for 30-60y0 and up to 3 mg for smaller quantities of oxygen. With the two stopcocks (Fig. 1) adjusted for the reverse flow of helium in the pyrolysis tube, the Swagelok is removed, and the sample holder is taken from the pyrolysis tube with the aid of a strong magnet (Alnico Button Magnet; l-inch diam., vs-inch high). A cotton glove is worn when handling the holder. The boat or quartz capillary is inserted into the sample holder and the latter is replaced into the pyrolysis tube. The Swagelok is replaced but not tightened for 1 minute to permit the displacement of air. While holding the Swagelok in place, the two stopcocks are adjusted to allow the helium to pass through the pyrolysis tube and on into the thermal conductivity cell. The Swagelok is then tightened with the fingers; a wrench is unnecessary. Approximately 3 minutes are required for the recorder pen to return to the baseline. The integrator is turned on, and, with the magnet, the sample holder is moved rapidly to the center of the furnace. After 2 minutes the sample holder is withdrawn into the condenser area of the tube so it will cool sufficiently for the next sample. In the case of nitrogen-containing compounds, a peak of varying height is sometimes observed shortly after the sample holder has been withdrawn. Approximately 3 minutes later, the carbon monoxide peak appears. As soon as the recorder pen returns to the baseline and the integrator pen levels out, the next sample may be inserted. Known compounds are analyzed in order to determine the number of counts on the integrator curve per microgram of oxygen. The unknown compounds are then calculated from this constant. Oxygen-free organic compounds have never produced a carbon monoxide
THERMAL
DETERMINATION
303
OF OXYGEN
peak, hence there is no blank correction. Fluorine-containing compounds, however, yield high results as would be expected, due to reaction with the quartz tube.
The proposed method herein described permits the complete determination of oxygen in organic compounds in 15 minutes and can readily be adapted to a completely automatic procedure. The recorder is generally set on the 10 mV range for l-3 mg samples. .\lthough not recommended for routine work due to the instability of the baseline and the extra care involved, a few samples in the 100 ug range have been analyzed successfully by using the 1 or 2 mV range. Pyrolysis tubes and packing have been used for many months before replacement became necessary. In addition to the samples listed in Table 1, a variety of research TABLE
1
$7 Oxygen
Valine
Sample (mp)
Theory
Found
0.506
27.3
27.4 27.2 27.3 52.3
1.052
3.024 1.104 0.921
Mannitol
52.7
51.8
52.2
1.076
Potassium
biphthalate
1.340
31.3
1.872
Anisole Cortisone
2.070 1.843 1.926
31.2
14.8 22.2
1.334
Guanosine Codeine
0.841 1.636 2.122 1.835
31.2
28.2 16.0
14.9 15.1 22.3 22.4 28.2 28.1 15.8 16.1
compounds containing ester, ether, amide, nitro, sulfamyl, diphenyl ether, ketone, aldehyde, acetyl, hydroxyl, and sulfonyl groups have been successfully analyzed. SUMMARY A method is proposed for the microdetermination of oxygen in organic compounds based upon a modified Unterzaucher pyrolysis under helium and the measurement
394
R.
N.
BOOS
of the resulting carbon monoxide by thermal conductivity, The sample is pyrolyzed under helium at llZO”C, and the oxygen is converted to carbon monoxide over carbon. The resulting gases are passed over Dehydrite for the removal of ammonia, over Ascarite for the removal of acid gases such as HX and H,S, and through a 6%.foot 13X molecular sieve chromatographic column before entering the thermal conductivity cell. The chromatogram was recorded on a Texas Instrument Co, Servo-Riter recorder equipped with a full-scale integrator. The unknown compounds were compared with a known compound used as a standard. REFERENCES 1.
ALUISE, V. A., HALL, R. T., STEATS, F. C., ,~ND BECKER, W. W., Oxygen in organic compounds. Direct microdetermination by the Unterzaucher method. Anal. Chem. 19, 347-351 (1947). 2. ALUISE, V. A., ALBER, H. K., CONWAY, H. S., HARRIS, C. C., JONES, W. H., AND SMITE, W. H., Direct determination of oxygen in organic compounds. Anal. Chem. 23, 530-533 (1951). 3. DUNDY, M., AND STEHR, E., Determination of oxygen in organic materials by a modified Schiitze-Unterzaucher method. Anal. Chem. 23, 1408-1413 (1951). 4. ELVING, P. J,, AND LIGETT, W. B., Determination of oxygen in organic compounds. Chem. Revs. 34, 129-156 (1944). 5. HOLOWCHAK, J., AND WEAR, G. E. C., Direct determination of oxygen in organic compounds. Anal. Chem. 23, 1404-1407 (1951). 6. KIRSTEN, W., Sources of error in the determination of oxygen by the method of Schiitze-Unterzaucher. Mikrochem. ves Mikrochim. rlcta 34, 151-152 (1949). 7. KORSHUN, M. O., Microanalytic determination of oxygen in organic compounds. Zavodskayu Lab. 10, 241-245 (1941). 8. SCH~TZE, M., Die direkte Bestimmung des sauerstoffs in Zinkoxyd. 2. anal. Chem. 118, 241-245 (1939-40). 9. SHEFT, I., AND KATZ, J., Direct determination of oxygen in organic compounds. Anal. Chem. 39, 1322-1325 (1957). 10. UNTERZAUCHER, J., Die mikroanalytische Bestimmung des Sauerstoffes. Chem. Ber. ‘73B, 391-404 (1940). II. UNTERZAUCHER, J,, Crystallized anhydrous iodic acid as oxidizing agent in the micro-oxygen determination in organic compounds, Mikvochim. Acta 1956, 822-835. 12. ZIMMERMANN, W., Mikroanalytische Ausfiihrungsform der direkten Sauerstoffbestimmung in organischen Substanzen. 2. Anal. Chem. 118, 258-263 (1939).