the fats and oils analyzed, the accuracy and precision of measuring the glycerol and fatty acid contents, and allowed the expression of these contents on absolute terms not dependent on estimations based on internal standards. The same criteria should be applicable to samples which are pure or nearly pure mono- or diglycerides. Average molecular weights calculated from the amount of glycerol determined by analysis and the weight of sample analyzed are included in the fourth column of Table I1 along with standard deviations. Because they were calculated from glycerol content, these molecular weights were no more precise or accurate than duplicate values from which they were calculated. However, the confidence placed in these values should be greater than those determined from saponification numbers because of being able t o first establish whether or not the sample is pure triglyceride. Calculations. Some minor components present in trace amounts were not included in Table I because their identities were not definitely established. Thus, tallow contained mi-
nute amounts of materials with retention times expected for tetradecenoate (CJ, pentadecanoate (&), hexadecadienoate (CJ and heptadecanoate (Cl,) while lard contained a trace amount of material corresponding to a retention time expected for heptadecanoate (C17). However, estimations were made for these trace components and included in values in the first and third columns of Table 11. Area.to-pmole relationships were obtained for each methyl ester and I P G (2) from the chromatograms of an appropriate mixture of standard methyl esters and I P G dissolved in methanol. These relationships were then used to calculate the amounts of fatty acids and glycerol in the fats, oils, and triglyceride mixtures. All samples analyzed containel the same amount of internal standard (methyl caprate). If the area of the internal standard changed by more than i 2 % , injection of the standard solution of methyl esters was repeated and new unit areas calculated: however, this was necessary only occasionally. Increments from 10 t o 50 111. were chro-
matographed to ensure linearity of response of the detector over the range of concentration being analyzed. ACKNOWLEDGMENT
The authors thank F. L. Kauffman, Swift and Co., Chicago, Ill., for supplying fats and oils and E. J. Eisenbraun, W. D. Gallup, and Ralph Matlock for helpful suggestions. LITERATURE CITED
(1) Hornstein,
Irwin, Alford, J. A., Elliott, L. E., Crowe, P. F., AKAL. CHEW32, 540 (1960). ( 2 ) Mason, Michael E., Waller, George R., Ibzd., 36, 583 (1964) (3) Tinoco, Joan, Shannon, Angela, Miljanich, Peter, Lymah, Richard L., Okey, Ruth, Anal. Biochem. 3, 514 (1962). (4) T-orbeck, Marie L., Mattick, Leonard R , Lee, Frank A Pederson, Carl S., A N A L . cHEM. 33, i k i 2 (1961). RECEIVEDfor review June 14, 1963. Accepted November 7, 1963. This work was supported in part by grants from the Southwest Peanut Research Foundation and Corn Products Institute of Nutrition. The major author is a National Institutes of Health Trainee Fellow.
The Determination of Total Carbon and Sodium Carbonate in Sodium Metal SILVE KALLMANN and ROBERT LIU Ledoux & Co., Inc., Teaneck, N. J.
b The total carbon content of sodium metal is determined b y low-temperature ignition of the sample in an argonoxygen mixture, treatment with dilute sulfuric acid, and measurement of the liberated COS in a conductometric Leco Apparatus. For the determination of the sodium carbonate, steam introducted into the combustion chamber by argon gas decomposes sodium metal with the formation of sodium hydroxide. Again, COz is liberated b y dilute sulfuric acid and measured conductometrically.
P
a n hEC Round Robin program covering the determination of carbon in sodium metal indicate wide discrepancies in results obtained by the participating laboratories (6) and emphasize the lack of reliable procedures. The literature covering the determination of various forms of carbon in sodium metal is very scanty. Stoffer and Phillips (8) published a procedure based on the ignition of a 100-mg. sample in a stream of oxygen. The carbon in the sample is converted to carbon dioxide which RELIMINARY DATA Of
590
ANALYTICAL CHEMISTRY
is then absorbed in a n AscariteDrierite micro absorption bulb. The combustion tube is made of quartz and the section containing the sample is externally heated to 950' C. It is claimed that sufficient heat is evolved to prevent combination of the carbon dioxide with the alkali oxides. An asbestos plug serves to confine the oxide fumes within the combustion area and prevents their being carried to the cooler parts of the apparatus. When this procedure was tested in this laboratory on sodium samples containing about 100 p.p.m. of carbon, recoveries of COz varied, but never exceeded 50%. The reason for the incomplete recovery of COz may be attributable to its interaction with sodium oxide, either in or outside of the combustion area. FYhile the geometry of the apparatus used in this laboratory varied somewhat from that recommended b y Stoffer and Phillips, it is felt that even under ideal conditions, the reaction of C o n with ?;azo is difficult to prevent, thus causing low results. It has recently also been pointed out (4) that while dissociation of sodium carbonate begins around
700" C., the vapor pressure does no exceed 1 mm. of Hg until about 950' C. and a temperature of 1100' C. is needed to recover completely carbon from sodium carbonate in 1 hour of combustion time. The original method has been improved by using sensitive manometric ( 2 ) or gas chromatographic (4) measuring devices. A procedure for free carbon in sodium which employs !vet combustion has also been described (5, '7). The sodium sample is cut into small pieces, which are then dissolved, one by one, in a small volume of water under a stream of nitrogen. The resulting solution is neutralized with sulfuric acid until an excess of the acid is present. The mixture is evaporated to near dryness, and then Van Slyke combustion fluid (9) is added. Cpon heating, the carbon is oxidized to carbon dioxide. Hot perchloric acid has also been used for the oxidation of the carbon, and the COz, after being coldtrapped, can be measured by manometric methods, by gas chromatography, or by titration of unreacted Ba(OH)2. Recently a method was recommended based on the solution of the sample in
Pop CKn
SUPRYLIING WIRE
TO LECO ANALYZER
\','?&METAL
\
OR GLASS TUBING
w"& '4
,SODIUM
\d
METAL
ZlKONIUM
CRUCIBLE
STOP COCK
METAL
Figure
1.
Reaction tube
an ethanol-water mixture and evolution of the COz by treatment with dilute y gas chromasulfuric acid, followed l ~ its tographic measurement ( 1 ) . PRINCIPLE OF SUGGESTED PROCEDURE
While there is a pojsibility that carbon exists in sodium metal as hesasodium hexacarbonyl [through the interaction of carbon monoxide with molten sodium at a temperature of 250' to 380' C. ( S ) ] , the investigation reported here is limii ed to the determination of total ca-bon and carbon present as sodium carbonate. In the absence of the hvpothetical hexasodium hexacarbonyl, it can be assumed that the difference between the total carbon and the carbonate carbon represents the free or graphite carbon content of the sample. For the determinaiion of total carbon the sample is converted by low temperature ignition in a zirconium crucible in a stream of purified argonoxygen mixture (passed through CuO a t 700" C. then riscarite). Irrespective of the form in which the carbon is present in the sample, the former is converted to COZ which reacts with the surrounding sodium oxide to form sodium carbonate. The cooled melt is dissolved in the reaction tube in water, always under protection of argon, then is acidified with dilite sulfuric acid which liberates the C'OZ. The COZ is passed through a sulfuric acid cleaner, then into a conductometric measuring unit containing barium hydroside solution. The change in the conductance
of the solution is used as a measure of the carbon content of the sample. For the determination of the carbonate content the sample is reacted in a zirconium crucible with moist, COZfree argon until completely converted to sodium hydroxide, Any sodium carbonate in the sample remains unchanged. Upon completion of the reaction the crucible is again flooded, first with water, then with dilute sulfuric acid, and the liberated COz is introduced into the conductometric unit where it is measured. EXPERIMENTAL
Apparatus. Details of the apparatus are shown in Figures 1 and 2. The most important unit is the Vycor reaction tube, details of which appear in Figure 1. It is placed into a n inert gas glove box where the sample is introduced. The reaction tube is then closed and connected to the rest of the assembly which during the above operations is maintained under a n argon atmosphere. Procedure. DETERMINATION OF TOTAL CARBON. Sample Preparation. The sample size is determined by cutting off and weighing (while inside the glove box) a convenient size of glass or stainless steel tubing containing 3 to 5 grams of sample. (The exact weight is determined by weighing the empty tubing a t the end of the operation.) If the sample is contained in steel tubing, the latter is carefully degreased on the outside and then is placed into the inert gas glove box. The tubing containing the sample is introduced into the glass funnel of the reaction chamber. (See Figure 1). The latter is closed a t both ends, removed from the dry bos and installed into the assembly (see Figure 2) which is maintained under an argon atmosphere. The part of the assembly leading from the reaction chamber to the conductometric unit remains disconnected during the following operation : Reaction of Sample with Oxygen. The stopcocks of the reaction tube are opened allowing the purified argon to pass through the reaction chamber and STOP
into the air. While maintaining the flow of argon, the reaction chamber is heated in the vicinity of the portion of the glass funnel which holds the tubing containing the sample, until the sodium melts and flows into the zirconium crucible. After a short cooling period the argon is slowly replaced by purified oxygen, (CuO at 700' C., followed by Ascarite) which enters the purifying system via a Y connection. The reaction tube is then heated gently until the sodium begins to ignite. The heat and amount of oxygen are adjusted in such a way that the reaction will take place at the lowest possible temperature. As the reaction begins to subside, the flow of oxygen is increased and that of the argon decreased until the sample is finally ignited in pure oxygen. (The ignition temperature controlled by the argon/oxygen ratio and the amount of external heat supplied has visually been estimated to be about 600' C.). When the reaction is completed the reaction chamber is allowed to cool. Liberation of COz. Freshly boiled water is placed into a gas wash bottle and is further purified by passing argon through it for a few minutes. The gas wash bottle is introduced into the assembly (see Figure 2) and the threeway stopcocks are turned in such a way that the argon will push the water into the reaction tube. The latter is now connected to the rest of the assembly so that the emerging argon passes through sulfuric acid and then into the conductometric Leco *lpparatus. A second gas wash bottle containing 6JI sulfuric acid (previously boiled and further purified by passing argon through it) is now introduced into the assembly as is indicated in Figure 2 and the three-way stopcocks are arranged in such a way that the argon will push the dilute acid into the reaction tube. The amount of total acid must be sufficient to acidify the solution and the total volume such t h a t it covers the zirconium crucible and the glass funnel. The reaction tube is now heated until the solution gets sufficiently hot to allow the argon to push the COz into the conductometric unit. Measurement of COz. The final measurement of the COz is carried out
COCK
LECO
ANALYZER
""'7
A Figure 2.
Schematic illustration of apparatus for determinationof total carbon and sodium carbonate in sodium metal VOL. 36, NO. 3, MARCH 1964
e
591
Table I.
Recovery Study of Total Carbon Determination
Carbon, pg. Sample wt., grams 3.85 4.25 4.05 4.45 3.75 4.50 4.25 4.70 4.20 3.85 4.35
Exp. so.
1 2
3 4 5
6 7
8
9 10 11
... ...
12 13 14 15 16 17 20 a
b
150 180 170
125" 200" 250" 300" 3500" 7500" 10000" 55c 15OC 47" 93" 135c 255d 515d 230d 450d
188
168 154 174
...
4.30 4.15 3.95 ...
172 166 158 ...
3.95 4.00
158 160
Added
... ... ... ...
...
...
18 19
Present in Xa
...
Found, total 154
Relative error,
70
375 405 446 3510b 7720b 10085* 51 155 210 268 280 250 525 400 603
-1.3 -3.6 -9.4 -4.3 +0.9 -0.9 -7.3 $3.3 -4.1 +3.5 -4.4 -2.0 +1.9 $3.0 +1.1
Carbon added in form of sucrose solution. Carbon determined gravimetrically by absorption in Ascarite. Carbon added in form of an Aquadag suspension. Carbon added in form of graphite powder.
Table II.
Sample wt., grams
Recovery Study of Sodium Carbonate Determination
Added
Na2C03, aa carbon, pg. -~ Present in Na
50 75 100 300 3500 7400
26 26 23 28 26 26
3.80 . -~
4.35 4.25 4.25 3.75 4.65 4.40 4.30
Found 23 26 70 97 120 345 3550" 7380a
Relative error, 70 -7.9 -4.0 -2.5 $5.2 +0.7 -0.6
Carbon determined gravimetrically by absorption in Ascarite.
in the usual way by recording the change in conductance of the barium hydroside solution. The exact weight of the sample used in the above test can now be established b y removing the glass or steel tubing, drying, and weighing. I t should be noted that the manipulations described above do not change the weight of the stainless steel tubing by more than a few milligrams. DETERhlINATION O F SODIUM CARBOXATF. A sample weight of 3 to 5
grams is introduced into the reaction tube as previously described. The reaction chamber is connected to the apparatus and the sodium is heated, always under the protection of argon, until the sodium flows into the zirconium crucible. The argon is now allowed to pass through hot water and sufficient steam is thus introduced into the reaction tube until the sample is completely converted to sodium hydroxide. The final steps of the procedure are identical with those described for the determination of the total carbon; viz, water and then dilute sulfuric acid are introduced into the reaction chamber and the liberated COe is determined conductometrically.
592
0
ANALYTICAL CHEMISTRY
content between 3 and 5 pg. and the sodium used in subsequent tests, after correction for the blank, indicated a carbon content of 40 p.p.m.-(Experiments 1 to 4). The procedure \vas verified by introducing sucrose solution into the zirconium crucible, drying the crucible, and introducing about 4 grams of sodium metal in exactly the same way as described in the procedure. (Experiments 5 to 11). To prove that the procedure achieves quantitative ignition of graphite carbon, weighed portions of dilute suspensions of Aquadag were evaporated in zirconium crucibles and the carbon content determined with and without sodium. (Experiments 12 to 16). As a further check of the method, small amounts of 300-mesh graphite (micro-balance) were added to the zirconium crucible and the carbon determined with and without the addition of sodium. (Esperiments 17 to 20). In Experiments 12, 13, 17, and 18, the combustion chamber was connected to the conductometric apparatus during the ignition, since no sodium oxide was present which would absorb the CO, formed. Results are shown in Table I. The results in Tables I and I1 show that the suggested procedures yield results with satisfactory precision and accuracy \%-hendetermining 10 to 2000 p.p.m. of total carbon or carbonate carbon in sodium metal. The maximum error of 9.4% in Experiment 8 still is tolerable for all but the most exacting requirements. Preliminary tests indicate that the methods suggested above are equally applicable to the other alkali metals requiring only minor modifications in manipulations.
DISCUSSION
The ignition temperature of the sodium is not critical and no loss of C 0 2 need be anticipated with temperatures below 700' C. This was verified by igniting samples of sodium metal to which 300 pg. of carbon were added at varying temperatures while the reaction chamber was connected to the conductometric unit. This is not surprising in view of recent work (4). It can also be explained by the gettering action of the fine aerosol of sodium oside existing in the cool part of the reaction tube a t the time of ignition. Although a conductometric unit was used to determine the carbon and carbonate content of sodium metal, the authors see no reason why the senqitivity of the method could not be improved by incorporating a gas chromatographic unit. RESULTS
Blank determinations in the absence of sodium, but incorporating all steps of the procedure, yielded a carbon
LITERATURE CITED
(l)TBradley,H. L., Moore, C. R., paper h o . 47, 7th Conference on Analytical Chemistry in Suclear Technology, Gatlinbure. Tenn.. 1963. (2) Herringron, J., t.K. Atomic Energy iluthority, A WRE Rept. 0-62/62. (3) Miller, H. C. (to E. I. du Pont de Semours & Co.). U. S.Patent 2,858,194 . . (Oct. 28, 1958)." (4) Mungall, T. G., Mitchen, J. H., Johnson, D. E., ASAL. CHEM.36, 70 llQfi41 ,-" - -,.
(5) Pepkowitz, L. P., Porter, 11, J. T., Zbzd.. 28,1606 (1956). (6) Pursel, C. A., E. S. Atomic Energy Commission, private communication, Argonne, Ill, "Summary of Results," October 1963. (7) Steinmetz, H., Minushkin, B., Xuclear Development Associates, White Plains, N. Y., Rept. 2154 (1961). (8) Stoffer, K. G., Phillips, I. H., ANAL. CHEM.27,773 (1955). (9) Van Slyke, D. D., Folch, J., J . Biol. Chem. 136, 509 (1940). RECEIVEDfor review March 26, 1963. Accepted Xovember 19, 1963. The work reported here was in part financed by the Sational Aeronautics and Space Administration.
