(11) Giddings, J. C., J . Chem. Ed. 35, 588 (1958). (12) Gow-Mac Instrument Go., Madison, N. J., BulE. TCTH-6-59. (13) Grant, D. W., Vaughan, G. A., J. A p 1. C h m . IO, 181 (1960). (14) ausdorff, H. H., Chemiker-Ztg. 81, 392 (1957). (15) Klinkenberg, A., Sjenitzer, F., Chem. Eng. Sci. 5,258 (1956). (16) Messner, A. E., Rosie, D. M., Argabright, P. A., ANAL. CHEM. 31, 230 (1959).
ti
(17) Phillips, C., “Gas Chromatography,” p. 63, Academic Press, New York, 1956. (18) Schmauch, L. J., AKAL.CHEM.31, 225 (1959). (19) Scott, R. P. W., ANAL.CHEM.35, 481 (1963). (20) . . Scott. R. P. W.. Hazeldean. G. S. F.. in “Gae Chromatography-i960,” R: P. W. Scott, ed., p. 144, Butterworth, Washington, D. C., 1960. (21) Tandy, R. K., Lindgren, F. T.,
Martin, W. H., Wills, R. D., ANAL. CHEM.33, 665 (1961). (22) ZhukhovitskiI, A. A,, Turkel’taub, N. M., Zavodskaya Lab. 24, 796 (1958); C. A . 54 2408% (1960). RECEIVEDfor review April 8, 1963. AcceptedOctober 16,1963. Reference toa company or product name is made to facilitate understanding and does not imply endorsement by the U. S. Bureau of Mines.
Gas Chromatographic Determination of Microgram Amounts of Carbon in Sodium Metal T. G. MUNGALL, J. H. MITCHEN, and D. E. JOHNSON Chemical Research & Development, Ethyl Corp., Bafon Rouge, l a .
b An accurate and sensitive method is described whereby as little as 10 p.p.rn. total carbon in sodium can be determined. It involves combustion of the sample at llOOo C. in a mixture of nitrogen and air to convert elemental and carbonate carbon to COZ. The C02 is collected at liquid nitrogen temperature and measured by a modified gas chromatographic procedure.
B
the high purity requirements for sodium used as a coolant in the operation of atomic reactors, a need has developed for a sensitive method for the determination of micro amounts of carbon in sodium metal. Present methods for the determination of carbon in small quantities are generally restricted to substances which contain a high percentage of carbon (1) or which require a relatively large sample (2) when the carbon content is small. One of the recently published methods (4), in which a small sample of sodium-potassium alloy is analyzed for total carbon, requires a n elaborate scrubber system for the combustion gases. The sampling and analytical methods described below have greatly simplified the determination of carbon in sodium and have proved to be more satisfactory than the more elaborate procedures previously reported. ECAUSE OF
and through two Ascarite-filled U-tubes in series (Figure 1). The nitrogen dilution of air is necessary t o prevent excessive accumulation of liquid oxygen in the COZ trap. COMBUSTION.Since a temperature of l l O O o C. is required for the combustion, the tube furnace must be in good condition. The quartz combustion tube is protected from sodium oxide attack by a removable quartz sleeve. A quartz wool plug is placed just downstream of the sleeve but still within the hot zone of the furnace to prevent sodium oxide migration into the cooler end zone where it might recombine with COZ. A loose quartz wool plug is also used in the downstream end of the protective sleeve. The sample boat is also of quartz. No lubricant is used on the combustion tube joints to preclude carbon contamination from this source. COz COLLECTION.A U-tube trap of 7-mm. borosilicate glass tubing immersed in liquid nitrogen is used to collect the COZ (Figure 1). It is necessary to pack the exit arm of the tube with glass wool to prevent loss of COY as an aerosol. The ball joints and stopcocks on the trap are lubricated with silicone grease.
EXPERIMENTAL
Apparatus. The analytical apparatus consists of four sections: gas purification, combustion, COz collection, and COz measurement. Figure 1 shows the assembled apparatus except for the gas chromatograph used for COz measurement. GAS PURIFICATION. Cylinder air and nitrogen are purified by passing the mixed gases over copper oxide at 950’ C.
70
ANALYTICAL CHEMISTRY
Figure 1, 1. 2.
3. 4.
5. 6.
CO:, MEASUREMENT. A PerkinElmer Model 154 gas chromatograph or its equivalent is used for the CO? measurement. It is adapted for use with the COZtrap as shown in Figure 2. Essentially, the sample loop is externally tied into the helium stream between the flow regulator and the column. The lines are brought outside the instrument case with 1/8-inch copper tubing. The loop, bypass, and %way stopcock are all 1-mm. bore borosilicate glass capillary. The chromatographic column is l / 4 X 26 inch stainless steel tubing packed with 30- to 60-mesh silica gel. The column is operated at 40’ C. with a helium flow of 50 cc. per minute. On the Perkin-Elmer Model 154 the detector is set at 4.5 volts and a n attenuation of 4 is used. SODIUMEXTRUDER. Samples are extruded from stainless steel tubes by means of the device shown in Figure 3. It was assembled from an Imperial flaring tool and a Jacobs drill chuck. When the limit of thread travel is reached the chuck is loosened, retracted, and retightened. Then another extrusion, as long as the threaded section, can be made. Procedure. SAMPLING. The sodium samples are taken in 3/s-incli diameter
Combustion apparatus
Flowmeters for nitrogen and air Quartz tubes, 1 inch o.d., 1 5 inches long, “/M inlet cap, ‘”5 ball joint outlet Copper oxide wire packing High temperature tube furnaces Ascarite-filled U-tubes Quartz wool plugs
7.
Quartz protective sleeve, 3/a-inch o.d., 9 inches long Quartz sample boat 9. COn trap, 7-mm. borosilicate glass U-lube 8 inches high, ‘”5 male ball joints, 2-mm. capillary stopcocks 10. liquid nitrogen Dewar 1 1 . Water bubbler 8.
u5
8
Figure 2. system 1. 2. 3.
4. 5.
8
Gas chrclmatograph inlet
Helium from gas chromatograph regulator Helium to gas chromalograph column 3 - W a y stopcocks Glass ball joints COz trap
stainless steel tubes. T o prevent possible carbon coritamination the tubing is degreased with a n appropriate solvent, dipped in chromicsulfuric acid cleaning solution, washed with distilled water, and ignited in a muffle furnace overright a t 500" C. The molten sample iii drawn into the tube by vacuum and allowed t o cool rapidly t o room temperature. The sample for combustion is extruded from a convenient length of the sodium-filled tube directly into a tared, preignited boat. Tho first half inch of sodium extruded is (discarded because of possible contaminu5on. All extrusion operations are conducted in a nitrogen-filled dry-box. During weighing and transfer out side the dry-box the sample is handled in a nitrogenflushed weighing bottle. ANALYSIS. The equipment should be preignited at l l O O o C. for several hours to remove traces of carbon or carbonates from the :ombustion tube, sleeve, and boat. This is conveniently done as an overnight operation with a slow flow of air through the tube. Extra sleeves and boats may be preignited in a muffle furnace under similar conditions. They must then be stored and handled in such E L manner that no dust, lint, or organic matter can contact them. After preignition 1he apparatus is ready for a blank determination. The conditions for the biank simulate as closely as possible those later used in the sample combustion. Set the gas purification furnace a t 950" C. and regulate the nitrogen flow at 120 cc. per minute and the air a t 50 cc. per minute.
Table 1. Gas Chromatograph Calibration Data
Carbon, pg. 3.0 7.4 9.9 14.8 23.0
Peak area, sq. mm.
129 354 472 708 1044
Place the boat and sleeve in position, with the combustion furnace a t room temperature. Connect the COZ trap t o the apparatus as shown in Figure 1 and flame i t with a small burner to expel moisture. Allow about 5 minutes for the purified gases t o sweep out the trap, then immerse the trap in liquid nitrogen to a point about '/4 inch below the stopcocks. Heat the combustion furnace rapidly to llOOo C. and allow the combustion to proceed for approximately 1 hour a t this temperature. Close both stopcocks on the COI trap and remove it from the system while it is still immersed in the liquid nitrogen. Connect it to the inlet system of the chromatograph with the helium carrier gas flowing through the bypass. Switch the helium flow through the COz trap (still in the liquid nitrogen) and wait until a steady baseline is recorded on the instrument. This will occur when most of the trapped oxygen has been swept out of the COZ trap. (A small amount of oxygen always remains which cannot be removed from the trap with helium as long as the trap remains a t liquid nitrogen temperature.) Remove the liquid nitrogen bath. The COa will vaporize very rapidly without any need for warming and will enter the column essentially as a plug. The COa will be separated from oxygen and other gases and will appear on the recorder chart as a sharp, measurable peak. Normal blanks are about 1 pg. of carbon. If a higher value than 2 rg. is obtained, the blank determination should be repeated until a lower, reproducible blank is obtained. For the sample combustion, prepare the apparatus and COz trap and set the gas flows as described for the blank determination. After following the sampling procedure described earlier, with the combustion furnace a t ambient temperature, rapidly transfer the sample boat into the quartz sleeve in the combustion tube and reconnect the gas inlet. Flush the system about 5 minutes with the purified gases. Place the liquid nitrogen bath under the GOz trap and turn on the combustion furnace. Allow the combustion to proceed for approximately 1 hour after the furnace has reached llOOo C. Determine the COZ in the trap as described for the blank run. Correct the micrograms of carbon found for the blank, and calculate p.p,m. carbon in the sample.
Table II.
Replicate Analyses of Sodium
Carbon found P.p.m.
Sample h., g.
rg.
0.556 0.553 0.315 0.487 0.579 0.530
15 18 12 17 13 19
27 33 38 35 22 36 Av. 32
Std. dev. = f 6 . 1 Rel. std. dev. = 19%
Table Ill. Accuracy Determinations with Carbon Standards
Added, Found, Dev., rg.
rg.
PI&
Carbonate 23 23 Standard (23 23 pg. C per ml. 23 solution) Graphite 62 Standard 172 (12.57 mg. 244 C per g.jl 30 Leco carbon standard (30pg. 30 C per capsule) 30
26 19 22 19
+3 -4 -1 -4
63 175 248
+1 $3 $4
23 24 30 34 29 28 25 23
-7 -6 0 $4 -1 -2 -5 -7 Av. 3 . 5
30 30 30 30 30
RESULTS AND DISCUSSION
Carbon dioxide and atmospheric air were used for calibration of the chromatograph. Simulating combustion operating conditions, COZ was recovered from this series of known samples and determined as described above. Good agreement was obtained b e h e e n all of these standards, and an area factor was obtained for COn expressed as carbon. The sensitivity of the COz measurement is shown by the data in Table I. Since an attenuation of 4 was used, much more sensitivity is available if desired.
M
Figure 3. 1. 2. 3.
4.
Sodium extruder
S/&ch diameter X 1 2 inch long steel rod with s/la-inch diameter X */8 inch long head Jacobs chuck, IB cap. 0 to l / 4 inch Imperial flaring tool No. 193-FA '/,-Inch diameter X 3*/4 inch steel rod handle
VOL. 36, NO. 1, JANUARY 1964
71
or manometric measurements of the Table IV.
Additive Na2COs Graphit e
co*.
Analysis of Sodium with Added Carbon Micrograms of carbon
Sodium wt., g. 0 . 294a 0.309" 0.313" 0.230b
In sodium 9 10
10 31 33 31
In
additive 23 23 29
Total 32 33 39 105 72
74
39 0.224b 119 150 4 Carbon content 32 p.p.m. (av. of previous analyses). Carbon content 137 p.p.m. (av. of previous analyses). 0.244b
To establish the precision of the method and to determine the carbon level in a sodium sample to be used for standard additions, replicates were burned with the results shown in Table 11. The relative standard deviation of this series with approximately 0.5-gram samples was *19%,. To establish the accuracy of the method, a series of tests was conducted on carbon standards alone (Table 111) and then in the presence of the sodium on which the carbon level had been obtained (Table IV). Quantitative recovery of the added carbon was obtained. Sodium carbonate, graphite, and "Leco" carbon standards were employed. The latter may be obtained from K&L Scientific Co., Columbus, Ohio. I n view of the good accuracy demonstrated, it is possible that the
Found 36 33 42
103 77 155
Dev. +4
0
+3 -2 +5
s5
precision observed reflects to some degree the fact that impurities are not necessarily uniformly distributed in sodium. Larger samples might improve precision but would be more difficult to handle in the combustion. It is felt that the precision and accuracy obtained are quite satisfactory for monitoring the carbon level in molten sodium systems. -4dry combustion method for carbon analysis was chosen for this study because it provides a simple and accurate method for the determination of total carbon. Low blanks are obtained and it provides, via gas chromatography, a positive identification of the COz. Oxides of nitrogen are observed in the gas chromatogram but are resolved by the silica gel column. They would be a potential interference in conductometric
Stoffer and Phillips ( 3 ) pointed out that the dissociation of sodium carbonate begins around 700" C.; however, the pressure does not exceed l mm. of mercury until about 950" C. It was found in the recovery experiments that a temperature of 1100" C. is needed for complete recovery of carbon from sodium carbonate in the time allotted for this analysis. Combustion tube mortality is lower when quartz protective sleeves are employed. A new quartz sleeve and boat are used with each analysis since the old one is destroyed by the hot sodium oxide. It has been found useful to rinse the combustion tube periodically with 1:l HC1 solution and then with distilled water to remove any sodium oxide that has accumulated during combustion of the samples. LITERATURE CITED
(1) Ayers, Cora W., Belcher, R., West, T. S., J. Chem. SOC. (London), 1959
2582. (2) Pepkowitz, Leonard P., Porter, John T., AYAL.CHEM.28, 1606 (1956). (3) Stoffer, K. G., Phillips, J. H., Zbid., 27, 773 (1955).
(4)Vogel, Alfred M., Quattrone, Joseph J., Zbid., 32, 1754 (1960).
RECEIVEDfor review M a y 16, 1963. Accepted September 30, 1963. Division of Analytical Chemistry, 145th Meeting, ACS, S e w York, N. Y., September 1963.
Apparent Conformational Changes of Liquid Phases in Gas Liquid Chromatography CHIADAO CHEN and DIONE GACKE Department o f Biochemisfry, Northwestern University Medical School, Chicago 7 7, 111. The brief heating at high temperature (-200" C.) of diatomite packed columns coated with certain liquids just before use at low temperature (
