To evaluate the applicability of the proposed method to the materials of interest, a number of samples and controls containing rare earths in the presence of alumina were rendered soluble by fusing with potassium pyrosulfate. The solutions were aliquoted and the rare earths determined in ident’ical aliquots by both the volumetric method and the corivent,ional oxalate method precipitation - gravimetric (Table 111). The gravimet,ric results showed poor repeatability and were consistently higher than the volumetric results which, on the other hand, were quite repeatable. The high values were due t,o the presence of coprecipitated potassium, aluminum, and sulfate. When precipitates were dissolved and titrated, however, results were generally lower than those obtained on direct’ titration (last column, Table 111). This led to the suspicion that not only were the gravimetric results in error because of contamination of rare earth oxalates but that some of the rare earth escapes precipitation. This was confirmed when typical filtrates from the oxalate precipitation were treated with nitric acid to destroy the oxalate and t i h t e d for rare earths. The amount found ranged up to 3 mg. of rare earth oxides-from 5 to 10% of the total present. I n subsequent work it has been found that acidit,y and time of standing as well as the presence of other ions are factors in determining values obtained by the oxalate precipitation
method, in agreement with reports of others on the difficulty in obtaining complete precipitation of pure rare earth oxalates (15). I n analyzing rare earth-alumina formulations, samples are decomposed by either acidic or alkaline fusion, depending on what other elements are present. I n all cases, the excess fluxing agent should be kept to a minimum to reduce the neutral salt effect. Where either type of flux may be used, alkaline fusion followed by solubilization of rare earths with nitric or hydrochloric may be preferable to fusing with alkali pyrosulfate because of the more unfavorable effect of high concentrations of sulfate on the end point. Some success has been achieved by fusing with ammonium bisulfate and continuing the heating to volatilize the excess salt, but more care is required with this reagent to avoid spattering. If rare earth mixtures are analyzed, the titration data can be converted accurately to total rare earths only by use of a weighted average equivalent weight. This will have to be calculated from the relative percentages of the individual rare earths in the samples of interest. ACKNOWLEDGMENT
The authors thank J . E. Bowen who provided data that helped clarify difficulties with the oxalate precipitation method and made helpful suggestions on the manuscript.
LITERATURE CITED
(1) Anderegg, G., Nageli, P., Muller, F.,
Schwarzenbach, G., Helv. Cham. Acta
42, 827 (1959).
(2) Cheng, K . L., Chemist-Analyst 47, 93 (1958). 13) . , Chernikov. Y. A.. Tramm. R. S.. Pevsner, K.’ S., Zavodsk. Lab.‘ 26, 921 (1960). (4) Fritz, J. S., Richard, M. J., Karraker, S. K., ANAL.CHEM.30, 1347 (1958). (5) Gupta, A. K., Powell, J. E., Talantu 11, 1339 (1964). (6) Hildebrand, G. P., Reilley, C. N., ANAL.CHEM.29, 258 (1957). (7) Jablonski, W. Z., Johnson, E. A,, Analyst 85, 297 (1960). (8) Kinnunen, J., Wennerstrand, B., Chemist-Analyst 46, 92 (1957). (9) Korbl, J., Pribil, R., Ibid., 45, 102 (1956). (10) Korbl, J., Pribil, R., Emr, R., Collection Czech. Chem. Commun. 22, 961 (1957). 1) Lyle, S. J., Rahman, M., Talanta 10, 1177 (1963). 2) Munshi, K. N., Dey, A. K., ChemistAnalyst 53, 105 (1964). 3) Prajsnar, D., Chem. Anal. ( W a r s a w ) 8 , 71 (1963). 4) Pribl, R., Vesely, V., Talanta 10, 899 (1963). (15) Vickery, R. C., “Chemistry of the Lanthanons,” Chap. XII, Academic Press, New York, 1963; “Analytical Chemistry of the Rare Earths,” Chaps. I1 and IV, Pergamon Press, Oxford, 1961. 0. I. I M ~ S.J. G E D A N S K Y ~ Research Dept. Socony Mobil Oil Co., Paulsboro, N . J. Present address, United States Rubber Co., Research Center, Wayne, N . J.
Determination of Trace Carbon in Metals by Nonaqueous Titration SIR: Little use has been made of nonaqueous titrimetric procedures for the determination of carbon dioxide. Patchornik and Shalitin (3) have described a titration of carbon dioxide in benzylamine and applied it to biological samples. Grant, Hunter, and Massie ( 2 ) used dimethyl formamide as a solvent and used a titrimetric procedure for carbon in limestone, dolomite, organic compounds, and a high-carbon steel. Blom and Edelhausen ( I ) have shown that the continuous titration of carbon dioxide in acetone with a methyl alcohol-pyridine solution of sodium methoxide using thymol blue indicator is a sensitive and accurate procedure for the determination of carbon dioxide in flowing systems. They used it for determining carbon in organic compounds. White ( 4 ) showed that the end point drift in this system was caused by the presence of water and
excess methanol in the solvents and successfully applied the titration to microgram quantities of carbon derived from organic compounds and carbonates. These titrimetric procedures require a relatively low flow rate of oxygen (30 to 60 ml. per minute) better suited to the determination of carbon in nonrefractory, easily combusted materials. Preliminary experiments confirmed the work of White (4)and it was felt that if this procedure could be applied to the determination of carbon dioxide using higher oxygen flow rates and inductionfurnace combustion techniques, a simple and accurate method for determining trace carbon in a wide variety of materials would result. EXPERIMENTAL
Apparatus. T h e incoming oxygen was precombusted over copper oxide and passed through ascarite and
anhydrone. Coiled stainless steel tubing was used for flexible connections. M n 0 2 traps were used on both sides of the Leco induction furnace for absorbing oxides of sulfur and nitrogen. A second copper oxide furnace is used after the induction furnace. Figure 1 is a sketch of the titration cell showing the buret attachment. I t is a gas washing bottle of about 250-ml. capacity (Houston Glass Fabricating Co., Houston, Texas). A small hole is blown in the shoulder or side of the bottle and a short length of glass tubing attached and fitted with a rubber syringe cap. A 19-gauge 4-inch stainless steel hypodermic needle is used as a buret tip. I t fits into a 5- or 10-ml. microburet with detachable tip (Kimble Glass Co.). An ascarite filled drying tube can be placed on the funnel of the buret with a rubber stopper. Sleeves made of Teflon (Du Pont) are used to seal the standard tapered 29/42 caps to the tops of the cells, VOL. 37, NO. 7, JUNE 1965
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Table 1.
Determination of Carbon in N.B.S. Standard Sample 1668
Lon-carbon stainless steel, 0.0191~oC Ca Cb taken, found, C, Drift, pg. p.p.m. ml./min. erg. 19 7 21 5 208 0 022 19 4 194 0 020 19 1 19 2 189 0 024 18 9 38 5 38 3 190 0 026 38 5 38 3 190 0 026 38 0 38 9 195 0 022 75 2 188 0 024 76 2 76 6 75 2 187 0 028 192 0 020 77 0 77 3 116 6 15 4 189 0 026 12 8 186 0 028 115 6 11.8 185 0.026 115 2 a Sample wc ghts ranged from 100 through 600 mg. h O . O O 5 M titrant used.
Two of these cells may be connected in series to check the completeness of carbon dioxide absorption, or they may be easily rearranged so that one may be used to generate a standard amount of carbon dioxide for titration in the second. Leco So. 528-35 crucibles for heavy duty use and Leco KO. 528-42 porous covers were used throughout this investigation. Reagents. Satisfactory results were obtained using ACS reagent grade acetone and methanol without further drying. ACS reagent grade pyridine was distilled just before preparing solutions of sodium hydroxide. Sodium hydroxide in pyridine, 0.02 and O.O05M, was prepared and standardized just before use from a stock solution of 0.2.11 sodium hydroxide in ‘methanol. XCS reagent grade NaHCOs was used as a standard. The indicator was a 0.1% solution of thymol blue in methanol. G. Frederick Smith electrolytic iron, Leco granulated tin, copper rings, and especially prepared hInOz were used. Procedure. STAKDARDIZATION O F ~ O D I C M HYDROXIDE: SOLUTION. The titration cells are arranged so t h a t COz is generated and flushed out of cell S o . 1 into cell S o . 2. P u t 30 ml. of 10% H 2 S 0 4 into cell S o . 1 and attach the outlet of this cell to cell No. 2 through a small (10 x 1.5 cm.) drying tube containing silica gel. Add 150 ml. of dry acetone and 30 drops of thymol blue indicator to cell Yo. 2. Connect the purified oxygen from the induction furnace to the inlet of cell KO. 1. Adjust the oxygen flow rate to about 1 liter per minute and pretitrate with the appropriate NaOH solution to the final blue color of the indicator. The blue color of the indicator will gradually drift back to the yellow at a rate which must be predetermined and corrected for. This amounts to about 0.006 to 0.008 ml. of 0.02M XaOH per minute or 0.024 to 0.030 nil. of 0.005.11 S a O H per 934
ANALYTICAL CHEMISTRY
minute. After the system has been flushed out with oxygen as indicated by stable drift rates, an appropriate concentration of standard NaHCOa is allowed to enter cell S o . 1 from a buret. The CO, is titrated continuously as it is sbept into cell No. 2 by the oxygen without allowing the entire solution to turn yellow a t any one time. The cell is flushed out for 8 minutes and then the drift is checked for 5 minutes. The time required for the titration is recorded and the drift-rate correction is made. BLANKING.For the most precise results, the crucibles were individually prefired by the following procedure. A copper ring was placed in the bottom of the crucible. Two grams of iron and 2 grams of tin were added in alternating 1-gram layers. h porous cover was placed on the crucible and the variac control set a t 70%. The crucible was placed in the furnace, the oxygen flow set a t 1 liter per minute, and the furnace turned on. When the plate current meter indicated 400 ma., the variac control was slowly 101%ered over 5 minutes until the crucible gave only a faint red glow. The furnace was then turned off and the crucible allowed to cool in the closed furnace for 10 minutes. Residual carbon after a mock loading was not detectable. S A m L E s . The sample is added and the crucible is covered and returned to the furnace and the furnace is purged for 5 minutes with oxygen (1 liter per minute). The titration cell is prepared as in the standardization step and then the furnace is turned on and the variac turned up until the sample “fires.” The variac is adjusted so that the plate current is held a t around 350 ma. The CO, from the sample is continuously titrated as in the standardization step. The sample is burned for 10 minutes and then a drift is determined for 5 minutes. The drift-rate correction is made from the recorded titration time as before. DISCUSSION A N D RESULTS
Among the variables related to the use of the nonaqueous titrimetric procedure, the most important is the gradual drift of the end point. Our values are somewhat higher than those reported by White ( 4 ) . However, the end points are sharp to 0.01 to 0.02 ml. of 0.005.11 reagent and the drift varies only slightly from run to run. The usual variation in drift is from 0.002 to 0.004 ml. per minute, uhich for a 10minute run using 0.00531 titrant would amount to a difference of only 1 to 2 pg. of carbon a t most. The absolute rates are about 0.028 ml. per minute for the 0.005.11 titrant and 0.010 ml. per minute for the0.02.11 titrant. Of course, accui ate results require the aetermination of the drift after each run. Five minutes are sufficient for this determinatioh. Typical drifts are included in Table I.
Figure 1 .
Sketch of titration cell
The drift rate depends, to some extent, on the amount of CO, that has been absorbed by the acetone. The acetone is usually replaced after three or four carbon determinations. A marked increase in the end point drift rate resulted if the MnO, trap for oxides of sulfur and nitrogen became exhausted. A large increase occurred if traces of acetone came in contact with the r\lnOa. This trap was replaced about once a week. When two cells are connected in series the frit of the second cell overpressurizes the first cell and this results in an excessive aniount of acetone diffusing up the buret tip. The backpressure also makes the buret drain slowly and hence it is difficult to continuously titrate the CO, as it is swept into the cell. Even under these conditions, two samples containing 150 and 90 pg. of carbon, respectively, and burned under oxygen flowing a t 1.2 liters per minute showed only 3 pg. of carbon appearing in the second cell. Crucibles which have been prefired in the induction furnace will refire even though no additional flux or conducting sample has been added. Once the flux in the crucible has reached the molten state, even for the second firing, it will remain in this state for over an hour if the power is regulated to keep the plate current to around 300 to 400 ma. If the copper ring was not present in the original flux mixture, a second addition of about 1 gram of iron is necessary to refire the crucible. Slow cooling of the crucible after the prefiring is necessary because, otherwise, the crucible may fail during the subsequent combustion step. The titrant is relatively stable. However, as the volume of titrant in a flask decreases, the change in titer with time becomes pronounced unless precautions are taken to prevent CO, from coming
in contact with the titrant. Also, in the 0.005M solutions, a precipitate begins to form after a few days nhich tends to interfere with proper drainage of the buret. For these reasons a fresh dilution from the 0.231 stock solution was made and restandardized each day. S.13.S. standard sample 166B has been run using the S a H C 0 3standardization procedure and crucibles which have been prefired in the induction furnace. Table I gives the results on this sample for several runs of different sample sizes. The sensitivity of the dilute titrant is about + 1 p g . of carbon. The 0.02Jf titrant is used a h e n total carbon was expected to exceed 200 pg. I t is comparable to the more dilute t i t r a n t and no unusual problems were
encountered in its use. The sensitivity was about +4 pg. of carbon. The general procedure works well for other samples and has been used for determining trace carbon in such metals as uranium, beryllium, and boron, as well as various alloys and ceramics. Sometimes extra flux or accelerator must be added to a prefired crucible to obtain proper burning characteristics and ensure complete combustion. I n this case the blank on the added material must be determined. The high oxygen flow rate, the continuous visual observation of the CO, as it is titrater', and the simplicity and sensitivity of the detection system afford a procedure well suited for trace carbon determinations in diverse ma-
terials and for investigational procedures for determining the proper conditions for the induction furnace technique. LITERATURE CITED
(1) Blom, L., Edelhausen, L., Anal. Chzm. Acta 13, 120 (1955). (2) Grant, J. A,, Hunter, J. A,, Mamie, W. H. S , Analyst 88, 134 (1963). (3) Patchornik, A,, Shalitin, Y., ANAL.
CHEM. 33, 1887 (1961). (4) White, D. C., Talanta 10, 727 (1963). WALTERG. BOYLE,JR. FREDERICK B. STEPHENS WILLICVI SUNDERLAND Lawrence Radiation Laboratory University of California Livermore, Calif. WORKperformed under the auspices of the U. S. Atomic Energy Commission.
Flash Exchange Method for Quantitative Gas Chromatographic Analysis of Aliphatic Carbonyls from Their 2,4Dinitrophenylhydrazones SIR: Separating, identifying, and quantitatively determining the amounts of 2,4-dinitrophenylhydrazones ( D S P ' s ) in complex mixtures have been the subject of many techniques and investigations. I n one of the more promising methods, described by Ralls (8),the D S P ' s of volatile carbonyl compounds were pyrolyzed with CYketoglutaric acid, and the liberated carbonyl compounds were flashed into a gas chromatograph and separated on 307, Carbowax 2011 or 30Y0 LAC-446 columns. Ralls suggested that the method could be made quantitative, and, subsequently, Stephens and Teszler described a modified procedure in which formaldehyde D N P mas added to the mixture of derivatives in an effort to aid the "flashing" of liberated carbonyls into the gas chromatograph (10) equipped with a thermal conductivity detector. Standard deviations were reported for acetaldehyde D S P (*7 pg.) and propionaldehyde D S P ( k 5 pg.) over a range of 30 to 250 pg. of derivative with a recoverability of 104.3 f 0.57, of acetaldehyde. This procedure was extended to include acetone, isobutyraldehyde, 2-butanone, isovaleraldehyde, and valeraldehyde DXP's (1s) and applied to their quantitative determination in the steam distillates of various tobaccos. Further work in this area (f 2 ) was subsequently reported; honever, large variations in peak area were noted in addition to orcasional new unexplained peaks and it was felt that the method as described ( I d , I S ) did not provide the quantitative data
required. T o that end, a rigorous investigation of the various parameters involved in the analysis was initiated, using flame ionization detection. EXPERIMENTAL
Apparatus. An F and 11 Model 300 gas chromatograph equipped with a Model 1609 flame ionization detector a n d a l-mv. recorder was used. Helium, hydrogen, and oxygen flow rates were precisely controlled by Kupro Model 2SA very fine metering valves (Suclear Products Co., 15635 Saranac St., Cleveland, Ohio). Oven temperature was maintained a t 70" C., with the detector block at 175" C. a n d injector port at 165" C . A helium flow rate of 47.5 ml. per minute a t a column pressure of 15 p.s.i. was employed and hydrogen and oxygen flow rates were 80 and 350 ml. per minute, respectively. Of the large number of liquid substrates examined, a 16-foot X '/*-inch column of 8y0XE-60 and 127, QF-1 on Chromosorb P gave superior separation of the carbonyls of interest. However, when this column was heated above 125" C. and then returned to the operating temperature of 70" C., the retention times of the carbonyls decreased. Equilibrating a t 70' C. resulted in a gradual return to the original retention times (approximately 48 hours). This effect has been previously attributed to conformational changes occurring in liquid substrates, particularly XE-60 ( 2 ) . Materials. All carbonyl compounds were obtained from commercial sources and purified by repeated fractional distillation until gas chromatography indicated better t h a n 99% purity.
The 2,4 - dinitrophenylhydrazones (DNP's) were prepared by the method of Johnson (4) and each derivative was recrystallized several times from at least three different solvents. Melting points agreed with previously published constants (6). Standard mixtures of the D S P ' s were prepared by accurately weighing the derivatives of acetaldehyde (0.10000 gram), propionaldehyde (0.07030 gram), acetone (0.07030 grain), isobutyraldehyde (0.05620 gram), 2 - butanone (0.05620 gram), isovaleraldehyde (0.04750 gram), and valeraldehyde (0.04750 gram). These were then dissolved in reagent grade carbon tetrachloride and the solution was added to 2.0000 grams of Celite. Evaporation of the carbon tetrachloride with stirring, followed by oven drying (110" C,), gave the master analytical sample. Arbitrarily, seven replicate analyses were performed on several levels of sample prepared by mixing 1, 2, 4, or 6 mg. of the master D N P standard with sufficient Celite to give 8 mg. of sample. This provided five samples with the following representative ranges of carbonyl weight (micrograms) : acetaldehyde 8.42 to 64.32; propionaldehyde 7.36 to 56.22, acetone 7.35 to 56.14; isobutyraldehyde 6.89 to 52.61 ; 2butanone 6.89 to 52.61 ; isovaleraldehyde 6.59 to 50.37, valeraldehyde 6.59 to 50.37. The butyraldehyde D S P was separately weighed (5.25 mg.) and made up to 5.00 ml. with reagent grade carbon tetrachloride. This solution was freshly prepared every third day. Procedure. I n general, 8 mg. of t h e DX'P mixture was accurately weighed, 50 p l . of the butyraldehyde D X P VOL. 37, NO. 7, JUNE 1965
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