ANALYTICAL CHEMISTRY
362 .4 typical rate curve for a 1 to 16 chloroacetic-acetic acid mixture is shown in Figure 1. Here, the initial rapid reaction with chloroacetic acid is essentially complete in 8 minutes and the balance of the reaction displays the pseudo-unimolecular kinetics expected for the excess acetic acid. The total acid consumption in the first 8 minutes is easily determined colorimetrically and the major problem is to compute the amount consumed by each component of the acid mixture. Letting [DDhl] represent the diphenyldiazomethane concentration and taking A and B as acetic and chloroacetic acids, respectively, the differential form of the rate equation in the first 8 minutes is
[DDhIl [.fB([B])]
(2)
where f.4( [A]) and f ~ [B]) ( are unknown functions which depend on the respective concentrations of A and B and symbolize the fact that the kinetic orders are not exactly integral in the acid concen trations. Because acetic acid is present in large excess, its concentration does not change appreciably throughout the rate run and, consequently, . f ~ [A]) ( is constant and equal to the pseudo-unimolecular rate constant lZIA obtained from the rate curve ( I ) after 8 minutes, if the reasonable assumption is made that chloroacetic acid does not catalyze or inhibit the reaction between acetic acid and the diazo compound. During the first 8 minutes, the consumption of.of diphenyldiazomethane by acetic acid is given by the expression
(3) The complex kinetics of the chloroacetic acid reaction (N-hen the acid concentration changes markedly) prevent mathematical evaluation of [DDM]and integration of Equation 3. However, graphical integration may be used, since instantaneous [DDLI] values can be obtained from the rate curve. Such graphical integration was carried out by multiplication of kl,, obtained from the slope of the rate curve after 8 minutes, by the observed concentrations of the diazo compound a t 0.26-minute intervals over
Table 111. Determination of Chloroacetic Acid in Mixtures w-ith Acetic Acid by Reaction with Diphenyldiazomethane in Benzene at 30.0" C. % C l F k % H in Total Time. [DDnf] Acid Mixture HOdc t, Concn. &fin." [DDM]; [DDXI]; HOAcd Calcd. Found 4.8 0.00138 4.7 0.0514 8 0.00927 0.00624 9.0 0.00152 9.0 0.0514 9 0.00927 0.00481 9.0 9,s 0.0514 9 0.00920 0.00428 0.00150 9.0 8.6 0.00898 0.00432 0.00167 0,0514 9.5 22.9 0.00095 22.9 0,00971 0.00390 0.0257 14 a Time required for essentially all chloroacetic acid t o react. 6 Initial diphenyldiazomethane concentration, mole/l. C Diphenyldiazomethane concentration a t time t , mole/l. d Diphenyldiazomethane calculated t o have been consumed by acetic acid up t o time t , mole/l.
the first 8 minutes to give a set of instantaneous acetic acid-diphenylcliazomethane reaction rates. The diazo compound concentrations were computed from the optical density readings and the separately determined molal extinction coefficient (e = 103) a t 526 mfi in benzene. The instantaneous reaction rates were plotted against time and integrated graphically from 0 to 8 minutes. The chloroacetic acid concentration was then obtained as the difference between the calculated initial diphenyldiazomethane concentration and the sum of the concentration a t 8 minutes, and the concentration change corresponding to the acetic acid reaction in the first 8 minutes as obtained from the graphical integration. The results of five runs of this type using several chloroacetic acid concentrations are summarized in Table 111. The agreement between calculated and found is generally good, with a median accuracy of about 2%, despite the rapidity of the initial reaction. LITERATURE CITED
(1) Roberts, J. D., McElhill, E. A , , and Armstrong, R., J . A m . Chem. Soc., 73, 2923 (1949). (2) Roberts, J. D., and Watanabe, W., Ibid., 72, 4869 (1950). (3) Roberts, J. D., Watanabe. W., and McMahon, R. E., Ibid., 73, 760 (1951). (4) Ibid., p. 2521. RECEIVED April 30, 1951. Supported by the program of research of the Atomic Energy Commission under Contract No. AT(30-1)-905.
Determination of Carbon, Oxygen, and Sulfur in Copper Vacuum Fusion Analysis Using the Mass Spectrometer W. M. HICK4JI Westinghouse Research Laboratories, East Pittsburgh, Pa.
T
H E vacuum fusion method of analysis has been used extensively by the steel industry for a number of years. Murray and Ashley ( 7 ) and Sesbitt and Henderson (8) are among those describing methods for carbon determination in steel. Included in the many reports on oxygen determination are Vacher and Jordan (9), McGeary, Stanley, and Yensen ( 6 ) , and Alexander, Murray, and Bshley ( 1 ) . I n vacuum fusion analysis the system generally consists of a furnace chamber for conveying the element eought into a particular gas and an analytical system for obtaining pressure-volume measurements on the desired gaseous constituent. These systems necessitate the transfer of a small quantity of gas through a lengthy path, and separation by either condensation or chemical absorption. The mass spectrometer can be used to eliminate the analytical system in the conventional vacuum fusion apparatus. Use of this instrument for gas analysis has been described by Hipple ( 5 ) , Brewer and Dibeler ( 4 ) , and Washburn, Wiley, and Rock ( I O ) .
I n analyzing a gaseous sample with this instrument, the partial pressure of each constituent is determined independent of the total pressure. This condition is maintained in most analytical mass spectrometers over a useful pressure range of 1000. Throughout this range measurable ion currents are obtained ahich are a linear function of the pressure of the constituent being sought. This instrument is calibrated a t a knonn pressure with pure samples of the gases obtained in the vacuum fusion process. Partial pressures of the desired constituents in the unknown sample can be readily determined. The mass spectrometer is extremely useful not only in making the analyses, but also in following the development of a suitable vacuum fusion method. It not only yields a measure of the quantity of gas, but a t the same time permits positive identification of the molecular mass. The mass spectrometer pumping system was used to evacuate the furnace. This reduced the construction of the vacuum fusion apparatiis to that of a furnace chamber and minor glass connec-
V O L U M E 2 4 , NO. 2, F E B R U A R Y 1 9 5 2
363
This work w-as undertalren to develop simple and rapid quantitative methods of determining carbon, oxygen, and sulfur in copper in concentrations as low as 0.0001 weight %. Rapid vacuum fusion methods for determining carbon and sulfur in copper in the range 0.0050 to 0.0001 weight yo were developed using the mass spectrometer. A simple system for determinining oxygen in copper in the range 0.04 to 0.02 weight 70 was disclosed. Little difficulty is seen in extending with the suggested refinements. The mass specthis determination to 0.0001 trometer serves to replace the analytical train associated with the conventional vacuum fusion apparatus. It not only provides more rapid gas analyses, but also enables positive identification of molecular ~tructure. Its sensitivity surpasses the conventional vacuum fusion apparatus and thus provides a means for extending this method of analysis to lower concentrations.
tions. The attachment of the vacuum fusion apparatus directly to the mass spectrometer eliminatcd the transfer of the gaseous sample to a separate container, thus avoiding extensive delajbetween time of collecting sample and making the analvsis. This technique minimizes absorption and contamination risks. CARBON DETERMIiYATION
The furnace system employed for the determination of carbon in copper was similar to that described by Bever and Floe ( 3 ) . The determination of the carbon content involved the transformation of the carbon into carbon dioxide, measurement of the, pressure and volume of the gas obtained, and calculation of thv weight percentage of carbon wing the known weights of the copper samples. A schematic diagram of the system used, excluding the mass spectrometer is shown in Figure 1.
c _
m
ATTMOSP
W4XED GROUND
Figure 2.
R F COIL-
Figure 1.
-
System for Carbon Determination
The outer Alundum crucible was 3.5 inches long and 1.25 incht.,i in diameter with a wall thickness of 0.125 inch. The inner Alundum crucible waa 3.25 inches long and 0.625 inch in diameter with a 0.60-inch well. Magnesia powder was used as a packing material between the crucibles and for insulating the outer crucible from the borosilicate glass envelope. At the start of each run the inner crucible contained 10 to 20 grams of copper and approximately 5 grams of cupric oxide. Weighed samples to be analyzed were stored in the sample tube. These numbered not more than six and weighed approximately 10 grams each. The physical shape of the sample was found to he unimportant. Preparation of the sample included degreaPing, etching in nitric arid, and rinsing in distilled water. Initially the system was thoroughly degassed by heating the copper and copper oxide to 1300' to 1400' C. for approximately 1 hour. A thermocouple gage indicated when the degassing was completed. The low vapor pressure of copper a t its melting point prevented the escape of any appreciable amount of the cop-
Linearity Test for Carbon in Copper
per from the crucible during the high temperature treatment. The copper vaporieed from the bottom of the crucible, condensed near the top of the crucible, and flowed back to the reservoir. Following degassing the temperature was lowered to 1150' C. Prior to introducing the copper sample, a background determination was made under the same conditions as for analyzing a sample. Gas from the furnace system was collected in the freezeout volume by condensation with liquid nitrogen for 10 minutes and then analyzed with the mass spectrometer. During the c.01lection of all gas samples, stopcock 1 was closed, so that the gas sample passed over the cupric oxide, which converted any carbon monoxide present into carbon dioxide. The cupric oxide was maintained a t 350" C. Gas collected was analyzed for water, sulfur dioxide, and carbon dioxide. The background and sample gases were high purity carbon dioxide as measured with the mass spectrometer. This is in agreement with Bever and Floe's tiiscussion of their results. HolT-ever, it was experimentally verified that sulfur dioxide was rapidly absorbed by the cupric oxide at 350" (1. and that this accounts for the absence of sulfur. The mass spectrometer measurement of the carbon dioxide gave the prcsqure in a standard volume, rhich consisted of the system enc1oi;ed by stopcocks 2, 3, and 4. From the pressure, volume, arid temperature of the carbon dioxide, the carbon background was determined. The carbon concentration in a weighed copper sample was evaluated in the same manner, taking into account the background correction. The background correction used was the average of that before and after treating the sample. It amounted to less than 0.000025% carbon by weight for a 10-gram sample.
ANALYTICAL CHEMISTRY
364 Figure 2 shows the results of a linearity test made on the method. This particular copper has a high carbon content due to high temperature treatment in a graphite crucible. Table I gives typical results for three Chilean copper samples. The average carbon content approximates 0.0001 weight %. This figure agrees with the solubility data of Bever and Floe for the refining temperature, which is in theneighborhood of 1100" C. ATMOSPHERE
4
STANOARD PYREX JOINT
W A X E D SEAL
GRAPHITE CUP INCONEL TUBE$
@
COPPER S A F L E
Figure 3.
Hg MANOMETER
System for Oxygen Determination
A second method of determining carbon in copper, which required complete oxidation of the copper at approximately 1600"C., was tried and discarded. The carbon results were erratic and higher than those obtained by the described method. It is believed this method fails because of reaction of the molten copper oxide with the refractory retaining crucible. The crucible is a source of carbon in addition to the copper sample.
To obtain the weight percentage of oxygen in the copper by the method described, it was necessary to obtain experimentally a number of factors. The volumes of the furnace and manometer were measured by comparison to a standard volume. In order to calculate the weight of oxygen in the gas obtained, it was necessary to know the ratio of pressure in the total system a t room temperature to that with the furnace at 1150" C. Measurements of this factor using argon, helium, and carbon monoxide gave the room temperature pressure as 84% of the pressure with the furnace a t 1150' C. This factor was used to correct the pressure reading throughout this work. In addition, it was necessary to make a background correction for the furnace. This was due to absorbed gas on exposure of the system to atmospheric pressure when introducing the sample. A number of background runs gave this pressure correction as 1 mm. of mercury in the standard volume when measured with the same procedure as used for analyzing the sample. This background correction was made for all analyses. The pressures obtained for the typical samples analyzed were in the range 20 to 30 mm. of mercury. It is common practice in vacuum work to admit an inert atmosphere rather than air to the system to enable faster recovery of the vacuum. The helium used gave this result and thus a more reproducible background as well as shortening the time of cooling. The results of the described method have been compared with results obtained on a standard vacuum fusion system for steel operatedat 1650" C., and a system employing conversion of the
OXYGEN DETERMINATION
Sample weight aad preparation for the oxygen determination were the same as for the carbon analysis. The system shown in Figure 3 was used for the oxygen determination.
Table 1. Carbon Content of Three Chilean Copper Samples Sample No.
Carbon by Weight,
P.P.M.
1
1.3 1.0
2
1.1 0.8 0.7 1.0
3
-_
~
2
0
Prior to making any determinations the quartz tube and carbon crucible were degassed at 1200' C. for 15 to 20 minutes. The power was then shut off from the oscillator and the Inconel tube removed. After the system had cooled for 2 to 3 minutes, tank helium was admitted to the furnace a t atmospheric pressure. This hastened the cooling in the absence of oxygen. When the system had cooled to near room temperature, heat was applied to the waxed joint and the uartz tube was freed. The carbon cup was removed, the weighe%sample placed in it, and the system reassembled in as short a time as possible. The furnace was then evacuated as indicated by a thermocouple gage, followed by closing stopcock 1. The sample tem erature was raised to 1150' C. for 10 minutes. Deoxidation of tRe copper by carbon and carbon monoxide was completed in this time, as evidenced in Figure 4. Stopcock 1 was opened and the pressure read on the manometer with the furnace a t 1150" C. Analysis of the sample with the mass spectrometer revealed high purity carbon monoxide. Having established this, it was then possible to eliminate the mass spectrometer, and for the high oxygen concentration being studied to use the mercury manometer for pressure measurements. The standard volume used consisted of the furnace and the manometer. Initial degassing of the furnace system was necessary only a t the start of each day's work. The system permitted analysis of 15 to 20 samples per 8-hour day.
4 6 TIME (MIN.)
8
IO
Figure 4. Carbon Monoxide Pressure from Heating of Copper in Carbon Boat at 1150" C. in Isolated System as Function of Time
Table 11.
Oxygen Content of Some Chilean Copper Samples
Sample No. 1
Oxygen,
Weight % 0.0210 0.0215 0.0215 0,0220
2
0,0235 0.0240 0.0245 0.0240 0.0230
3
0.0260 0.0250 0.0260
V O L U M E 2 4 , NO. 2, F E B R U A R Y 1 9 5 2 oxygen into water by means of hydrogen. The results of all three methods were in good agreement. Typical results for some Chilean copper samples are shown in Table 11. It is felt that a system capable of detecting 0.0001% oxygen could be made by the addition of a few refinements. These would include a sample introduction system which would permit thorough degassing in order to reduce the background, use of the mass spectrometer for measuring the pressure of the particular gaseous constituent desired independent of other gases, and possibly conversion of the carbon monoxide into carbon dioxide a s in the carbon determination t o enable easy concentration of t h e gas into a small volume which would yield a large working pressure for the mass spectrometer. SULFUR DETERMINATION
In the development of a method of determining sulfur in copper, a number of different techniques were tested. A method was desired that would convert the sulfur into a stable gas a t room temperature in order that the mass spectrometer could be used to determine the quantity of sulfur rapidly. Hydrogen treatment of the copper above the melting point was disregarded because of the slow reaction of hydrogen with the sulfide (8). Vacuum melting for a reasonable length of time was found to remove only a part of the sulfur as sulfur dioxide which could be analyzed. Complete oxidation of the copper required high temperatures, a large source of high purity oxygen, and a means of handling the molten oxide. An oxidized Inconel surface could be used for holding the molten copper oxide, but in spite of decarbonization of the Inconel a t 1000°C. for 16 hours in wet hydrogen, it released large quantities of carbon when in contact with the molten oxide. This plus other difficulties seen in analyzing a large number of samples by such a method caused its abandonment. The method which was developed and used was t h a t of converting the sulfur into sulfur dioxide. It was found that the addition of an oxide coating to the copper sample followed by heating to 1100' C. yielded recovery of the sulfur as sulfur dioxide. In Figure 5 is shown a schematic diagram of the system used. STANDARD PYREX JOINT WAXED SEAL
CLAY BOAT
I
R raracl
,I
4TMOSPHERE
4 2 Rf F
QUARTZ TUBE
365 After the gas had been transferred to the standard volume, sto cock 1 was closed and the li uid nitrogen was removed from freeze-out arm. The p a r t i 3 pressure of sulfur dioxide in this standard volume was measured with the mass spectrometer and the necessary calculations were made.
2;
The described method of removing sulfur from the copper by the addition of oxygen is similar to that used in the refinery. In refining Chilean copper, air blown through the molten copper to increase the oxide content results in the removal of the sulfur. In analyzing samples, the low partial pressure of sulfur dioxide above the copper facilitates complete removal of the sulfur. The sulfur removal is complete in that a repeat analysis a t a higher temperature yields sulfur approaching the background and lower limit of sensitivity for the system, which is approximately 0.0000250/, sulfur for a 10-gram copper sample. Table 111.
Sulfur Content of Some Chilean Coppers
Sample No. 1
Sulfur, P.P.M. by Weight
10 12 10
2
9
10 10 12
3 ~~~~
A number of the variables examined in analyzing copper for sulfur were found not t o be critical. The quantity of oxygen was varied from 10 to 20 cc. with no effect on the results. No purification of tank oxygen was found necessary, as is required in complete combustion analysis. The temperature was relatively unimportant, so long as it was above the melting temperature of copper. The time of heat treating (30 minutes) could probably be shortened, but certainly it is sufficiently long for the concentrations encountered. The coating of copper in the quartz tube did not affect the results and the same quartz tube was used for more than a month. The clay combustion boats have an average life of about six samples. The reproducibility of the method in the range 0.0010% is of the order *15% for the particular samples studied. The magnitude of this spread to be associated with variable sulfur content of the samples and errors of measurement is difficult to determine. Some typical results are presented in TO MASS SPECTROMLTER Table 111. DISCUSSION
INCONEL TUBE
Figure 5.
System for Sulfur Determination
Ten-gram samples prepared as described above were used. The clay combustion boat was degassed prior to its first usage. A weighed sample was placed in the boat and the quartz tube attached by means of the waxed joint. The system was evacuated and the ~ - generator f set into operation to yield a temperature of a proximately 700" C. After the system had been degassed a t t&s temperature as indicated by a thermocouple gage, it was isolated by stopcock 3. Approximately 15 cc. of tank oxygen were admitted through stopcock 2. Liquid nitrogen was then placed on the trap to maintain a low partial pressure of sulfur dioxide above the copper. The cop er was heated to a temperature of 1100" C. for 30 minutes. Txe quartz tube was then cooled and the noncondensed gas exhausted from the system. The condensed gas was then transferred from the trap to the freeze-out arm using liquid nitrogen. Throughout the analyses the volume isolated by stopcocks 1, 2, and 3 was used as a standard volume.
The usefulness of the mass sDectrometer in the \\ field of vacuum fusion analysis has been shown. TO Hg PUMP In the extension of this method of analysis to smaller concentrations, the mass spectrometer could be the tool for supplying analytical information. It provides one of the most sensitive means of gas detection and a t the same time enables identification of the molecular mass. As ordinarily used, the mass spectrometer provides an ion current measurement for a particular mass to charge ratio that is proportional to the number of atoms or molecules of the constituent passing through a given volume per unit of time. To measure extremely small quantities of gas, it may be beneficial to provide a means of integrating this ion current. The total sample would then be pumped through the niass spectrometer and the f i dt would be a measure of the total quantity of the particular constituent where i is the ion current for the particular m/e value and t is the time. For calibration purposes, where normally a larger quantity of gas is used than obtained from the vacuum fusion process, this method permits extrapolation over the two variables of pressure and time rather than only pressure. The lower limit of detection of a vacuum fusion apparatus using a mase
366
ANALYTICAL CHEMISTRY
spectrometer as the analytical tool will not be determined by the sensitivity of the mass spectrometer until the present limits have been decreased by several orders of magnitude. It is to be expected that the limitations will be due to other factors than that of detecting the small quantity of gas. ACKNOWLEDGMENT
The author wishes to acknowledge the many helpful suggestions given by C. C. Hein, hlagnetics and Solid State Phpics Department, in carrying out this program. His experience with copper problems greatly aided the development of the described techniques. J. K. Stanley, Magnetics and Solid State Physics Department, was most helpful in supplying detailed information of the vacuum fusion method as applied to steel.
Bassett, W. H., and Bedworth, H. A , Trans. Am. Inst. Mining Met. Engrs., 73, 784 (1926). Bever, M. B., and Floe, C. F., Ibid.,166, 128 (1946). Brewer, A. K., and Dibelcr, V. H., J . Research ‘Vutl. Birr. Standurds, 35,125 (1945). Hipple, J. h.,J . Applied Phys., 13, 551 (1942). McGeary, R. K., Stanley, J. K., and Yensen, T. D., Trans. A m . SOC.Metals, 42,900 (1950). Murray, W. M., and Sshley, R. E. Q., ISD. EXG.CHEX.,ANAL. ED., 16,242-8 (1944). Nesbitt, C. E., and TIeiidersoii, James, ANAL. CHEM.,19, 401 (1947). Vacker, H.C., and Jordan, Louis, J . Research Natl. Bur. Standurds, 7,375 (1931). Washburn, H. W., Wiley, H. F., and Rock, S. AT., ISD. ENG. CHEM.,ANAL.ED.,15, 541 (1943).
LITERATURE CITED (1) Alexander, Leroy, Murray, TV. M., and Ashley, S. E. Q., ANAL.
CHEM.,19,417 (1947).
KzcEIvED
February 10,1951
Analytical Applications of the Polarography of Molybdenum MARVIS G. .JOHNSON AND REX J . ROBINSON, C’niuersity of F’ashington. S r a t t l e 5, Wash. Previous reports on the polarographic behavior of molybdate solutions exhibit an unsatisfactory lack of agreement. Investigations by the authors have shown that the polarographic characteristics of molybdate solutions are modified by the presence of nitrate ions. The previous discrepancies may be attributed to the catalytic polarographic reduction of nitrate. In the absence of nitrate, reduction of molybdate in 0.1 M sulfuric acid solution occurs in three steps with half-wave potentials of $0.06, -0.29, and -0.60 volt us. S.C.E. The reduction
I
NVESTIGATIONS of the polarography of molybdenum in weakly acidic solutions have been reported by Uhl ( I O ) , Hokhshtein ( 2 ) ,and Holtje and Geyer (3). These authors agreed that molybdate ion is polarographically reducible only in solutioni more acidic than about p H 6 , though in specific details their eyperimental results and conclusions were not in close agreement. During an investigation by the authors of the polarographic Iiehavior of molybdate in acidic solutions, it was noted that the presence of nitrate led to an increase in the reduction current, which was attributed to catalytic nitrate reduction. Kolthoff, Harris, and Matmyama (6) reported a siniilar catalytic nitrate I eduction activated by uranyl ion. The conclusions of Uhl and Hokhshtein, who used nitric acid 111 establishing the acidity of their molybdate solutions, are invalidated by the catalytic polarographic reduction of nitrate ion by molybdenum(II1). Holtje and Geyer, who used sulfuric acid in their investigations, were able to characterize the polarographic reduction steps of molybdate and to postulate a reduction mechanism, but, unfortunately, their results were not of quantitative value, possibly because of a very rapid capillary dropping I ate (13.2 drops per 10 seconds). In this paper the characteristics of the polarographic reduction waves of acidified molybdate solutions, with or without nitrate present, are described and procedures are outlined for the polarographic determination of molybdate and nitrate.
current is proportional to the molybdate Ooncentration in the range 0.02 to 1 millimolar. Either nitrate or molybdate may be quantitatively determined by utilizing the enhancement of the reduction current a t -0.75 volt us. S.C.E. due to the presence of nitrate. Procedures are described for the polarographic determination of molybdate in the concentration range 4 X lo-’ to 4 X lo-‘ M and for the determination of nitrate in the range 2 X lo-‘ to 5 X M. The method is particularly suitable for estimation of low concentrations of molybdate.
equipped with a potassium chloride bridge which could be immersed directly in the solution in the polarographic cell. The mercury reservoir design was that suggested by McReynolds ( 7 ) . The capillary characteristic, m 2 / 3 t 1 / 6 , was 1.30 mg.2/3 sec. - 1 i 2 . REAGENTS
The sodium molybdate was purified by the method of Yagoda aiid Fales (11). All chemicals used in the investigation were of
s VOLTS vs. S.G.E.
APPARATUS
The polarogra hic data were obtained with a Heyrovsk9 micropolarograph, bodel calibrated by the method of Kolthoff and Lmgane (6). The saturated calomel reference electrode w-as
x,
Figure 1. Typical Polarogram of Sodium Molybdate in 0.1 M Sulfuric Acid 8.75 x 10-6 ,W sodium molybdate, 0.1 M sulfuric acid, 0.2 M sodium aulfata
