for ascorbic acid in the rate equation suggests that the heteropoly undergoes initially a 2-electron reduction. Higher reduction stages are undoubtedly possible (15, 16) but would not be observed in a study of the initial reduction process. The data in this study indicate that hydrogen ion did not directly affect either the rate of the reduction step or the amount of reduced material formed in the initial reduction step. The hydrogen ion concentration is important, however, because it affects the amount of 12-MPA produced and thus has an indirect effect on the rate of reduction. It is likely that hydrogen ion is directly involved in subsequent reduction steps (15). N o heteropoly compounds have ever been prepared that contain more than twelve coordinating groups for each central group. The high molybdate dependency observed in our study strongly suggests a dimeric heteropoly species containing one bismuth and one phosphate as central groups, coordinated with an 18-polymolybdate structure. Dimeric phosphate heteropoly compounds with molybdenum to phosphorus and
(15) M. T. Pope and G. M. Varga, Jr., Znorg. Chem., 5, 1249 (1966). (16) M. T, Pope and E. Papaconstantinou, ibid., 6, 1147, 1152 (1967).
tungsten to phosphorus ratios of 18:2 are well known (16-20) and it seems plausible to suggest that a similar dimer was formed in our experiments. The phosphate dimers consist of the same basic units that make up 12-molybdo- and 12tungstophosphoric acid, therefore many of the solution properties of the dimer are similar to those of 12-MPA and 12-WPA. The data we have presented represent the first direct evidence for the existence of mixed dimeric heteropoly acids. The results of this study have helped clarify the role of bismuth in heteropolymolybdate systems and suggest several potentially useful methods for determining trace amounts of bismuth with increased speed and selectivity. Further research is presently being conducted along these lines. RECEIVED for review November 1, 1968. Accepted January 2, 1969. Presented at the 156th National Meeting, ACS, Atlantic City, N.J., September, 1968. (17) B. Dawson, Acta Crystallogr., 6, 113 (1953). (18) H. Wu, J. Biol. Chem., 43, 189 (1920). (19) G. A. Tsigdinos, Ph.D. Dissertation, Boston University, Boston, Mass., 1962. (20) E. A. Nikatina and 0. N. Sokolova, J. Gen. Chern. USSR Eng. Transl.. 24, 1271 (1954).
An Analyzer for the Dynamic Microdetermination of Carbon, Hydrogen, Nitrogen, Sulfur, and Oxygen George Dugan and V. A. Aluise Research Center, Hercules Incorporated, Wilmington, Del. 19899
An apparatus is described for the microdetermination of carbon, hydrogen, nitrogen, sulfur, and oxygen. The technique is based on the uncatalyzed, dynamic, flashcombustion of the sample in an oxygen/helium atmosphere at 1060-1080°C in a quartz tube. Retention and separation of the combustion gases, NO,, CO,, SO,, and H20,are accomplished by using chromatographic columns in liquid nitrogen. Measurement is by thermal conductivity. The precision and accuracy of the carbon, hydrogen, and nitrogen analyses are comparable with those obtained with commercial analyzers and classical methods. The preliminary results for sulfur are encouraging. A C, H,N, and S analysis requires 16 minutes. With carbon in the combustion tube at1120 “C, oxygen can be determined in 9 minutes as carbon monoxide, with no change in trapping, separation, or measurement. The precision and accuracy are equal to those obtained with slower pyrolysis methods.
SIGNIFICANT PROGRESS in the development of automated instruments for elemental microanalysis has only been achieved in the past five or six years following the initial stimulus provided by a group of microanalytical chemists at an International Symposium on Microanalytical Techniques in 1961 ( I ) . One of the first successful commercial analyzers was based on the work of Walisch ( 2 ) . Other commercial analyzers appeared a few years later. The majority of the currently available instruments have been described recently by Francis (3). In general, they are based on the rapid combustion principle of Ingram (1) International Symposium on Microchemical Techniques, Penn-
sylvania State University, University Park, Pa., August 13-18, 1961. (2) W. Walisch, Chem. Ber., 94, 2314-2327 (1961). (3) H. J. Francis, ANAL.CHEM.,36, (7), 31A (1964).
( 4 , 5 ) , and operate under either static or dynamic conditions. Only one of these analyzers, based on a design by Francis (6), employs the empty combustion tube method first proposed by Belcher and Spooner (7) and later improved by Belcher and Ingram (8). All of the rest employ tubes packed with oxygen donors or catalysts to ensure the complete combustion of the sample. A variety of methods has been proposed for the separation and determination of the combustion products (6,9-13). Most of these methods involve the use of selective absorbents (6, 9-12). In one commercial analyzer, the combustion products are separated by a chromatographic technique, first proposed by Duswalt and Brandt (13), and soon after reported by Maresh et al. (14, 15). In two commercial analyzers designed for carbon and hydrogen only, the combustion products are determined by weighing. In others, the products are deter(4) G. Ingram, Analyst (London), 86, 411 (1961). ( 5 ) G. Ingram, in “Microchemical Techniques,” N. Cheronis, Ed., Vol. 11, John Wiley, New York, N.Y., 1962, pp 495-526. (6) H. J. Francis and E. J. Minnick, Microchem. J., 8, 245 (1964). (7) R. Belcher and C. E. Spooner, J. Chem. Soc., 1943, 313. (8) R. Belcher and G. Ingram, Anal. Chim. Acra., 4, 118 (1950). (9) J. T. Clerc, R. Dohner, W. Salter, and W. Simon, Helv. Chin?. Acra., 46, 2369 (1963). (10) W. Simon, P. F. Summer, and G. L. Lyssy, Microchern. J., 6, 239 (1962). (11) W. Walisch, Trans. N.Y. Acad. Sci., 25, 693-704 (1963). (12) G. M. Grant and M. L. Teft, Microchem. J., 10,236 (1966). (13) A. A. Duswalt and W. W. Brandt, ANAL.CHEM., 32,272 (1960). (14) C. Maresh, C. E. Sundberg, R. A. Hofstader, and G. E. Gerhardt in “Microchemical Techniques,” N. Cheronis, Ed., Vol. 11, John Wiley, New York, N.Y., 1962, pp 387-396. (15) 0. E. Sundberg and C . E. Maresh, ANAL.CHEM.,32,274 (1960).
VOL. 41, NO. 3, MARCH 1969
495
INTEGRATOR RECORDER
DETECTOR
Y-
Oxygen is determined by dynamic flash-pyrolysis using a tube containing carbon at 1120 "C in an atmosphere of helium, utilizing the water gas reaction. The carbon monoxide formed is retained and separated in the same trapping system and is also measured by thermal conductivity. The apparatus has been in continuous routine operation in the authors' laboratory for 8 months with exceptionally little downtime and has proven to be simple to operate, easy to maintain, and very versatile in the analysis of a wide variety of research samples. EXPERIMENTAL
Figure 1. Apparatus for microdetermination of C, H, N, S, and 0 a-Electric timer &Solenoid c-Cross pattern fine metering valve &Gas inlet tube e-O-ring joint f-Quartz combustion tube g-Indentations in combustion tube h-High temperature combustion furnace &-Quartz wool j-Reduction furnace k-Copper &Copper oxide m-Swagelok connections n-Needle valve o-Toggle valve p-Carbowax column q-By-pass tube r and &-Three-way valves s-Molecular sieve column u-Constant differential-type flow controller v-Micro adjustable restrictor w-Florator and soap film flowmeter x-Rotary gas sampling valve y-Calibration gas mixture 70% N, and 30% C 0 2
mined by using thermal conductivity,either by manual measurement of the peak heights or areas under the peaks, or by electronic integration (11). In the opinion of the authors, none of the commercially available instruments meets the requirements for a completely satisfactory analyzer. One of the most important requirements is the ability to effect complete combustion of a wide variety of samples without the use of special oxidation aids. Problems associated with blanks and nonuniformity of reagents are thus eliminated. Other desirable features include a low required frequency of standardization, long combustion furnace life up to 1125-1150 "C and minimization of potential sources of leaks-e.g., use of solenoid valves to hold gases under pressure. The authors have designed, constructed, and tested a semiautomated apparatus that fulfills all of the above requirements. It can be used for the simultaneous determination of carbon, hydrogen, nitrogen, and sulfur in organic compounds. The use of a separate combustion tube with carbon at 1120 "Calso permits the direct determination of oxygen. In the simultaneous determination of carbon, hydrogen, nitrogen, and sulfur, the apparatus employs dynamic, flashcombustion of samples at 1060-1080 "C in an atmosphere of 4 0 z oxygen in helium. The combustion gases formed are subsequently retained and separated in two cold traps, one containing a chromatographic packing of Carbowax 20M on Teflon (DuPont) to retain C o n , SOz,and HzO, and the other Molecular Sieve 5A to retain N2. Sequential heating of the traps releases the trapped gases which then are measured by means of a thermal conductivity detector. 496
ANALYTICAL CHEMISTRY
Apparatus. Figure 1 is a schematic diagram of the main components of the analyzer. Swagelok fittings are used to join the component parts of the analyzer through which gas flows. In the determination of C, H, N, and S, helium is used as the carrier gas and a 40% mixture of oxygen in helium is used for the combustion of the sample. In the determination of oxygen only helium is used. The helium and oxygen are delivered from cylinders (at 40 to 50 psi) to separate pressure regulators, Fisher Type 67441 (Fisher Governor Co., Marshalltown, Iowa). These may be connected directly to the two-stage regulator on the cylinder. One of the Fisher regulators is adjusted to deliver helium at 10 psi to a pair of special controls which combine to give the constancy of gas flow required for the quantitative measurement of the desired products by thermal conductivity. The first of these controls (u,Figure 1) is a Constant Differential-Type Flow Controller (Model 63 BU-L, Moore Products Co., Philadelphia, Pa.), and the next ( u , Figure 1) is a Micro Adjustable Restrictor (Lockwood and McClorie, Inc., P.O. Box 113, Hatboro, Pa.). A second Fisher regulator is adjusted to deliver oxygen at 20 psi to the CrossPattern Fine Metering Valve (c, Figure l), Part No. 4MX (Nuclear Products Co., 15635 Saranac Road, Cleveland, Ohio 44110). This valve has a micrometer screw which permits precise control of the oxygen flow rate through one of its ports without affecting the helium flow rate through the other port. The amount of oxygen supplied through this valve for combustion of the sample is the same for each sample and is monitored by an electric timer, a, used to actuate a three-way solenoid valve, b, (No. V5D6300S, normally closed, Skinner Electric Valve Division, New Britain, Conn.). The helium flow rate is monitored by a suitable Florator (Fisher Porter Co.) and measured accurately with a soap-film flowmeter attached to the Florator at the exit end of the system at w. Figure 2 shows the construction details of the high temperature furnace which was designed in the authors' laboratory and which fulfills the requirements for the flash combustion of the sample without the use of oxygen donors or catalysts. It is capable of continuous operation up to 1125-1150 "C for a period of 3 to 4 years, as demonstrated over a period of 21 years. The furnace features a heating core (Englehard Industries, Inc., 113 Astor St., Newark, N.J.) made by winding an alundum refractory tube (Norton Co., Worcester, Mass.) with 30.3 feet (26 grams) of 80% platinum-2Oz rhodium resistance wire, The windings are spaced closer near the ends of the tube to compensate for heat losses at the ends of the furnace. The heating core is supported in the center of the stainless steel or brass furnace shell (Whitehead Metals, Inc.) by two opaque quartz end plates. Diatomite (Johns-Manville Co., Philadelphia, Pa.) is used as insulation between the core and the furnace shell. The current input to the wire is 8 A and the current carrying capacity of the wire is 11 A. The resistance of the furnace is 13.8 ohms in the cold state and 40 ohms at 1150 "c. A 3S-inch section of fine quartz wool is used in the hot part of the combustion tube (Figure 3). The hot quartz wool serves as a baffle and provides the additional high-temperature contact surface needed to complete the combustion of any stray fragments in the rapidly moving gas stream. A section of 30 to 60 mesh copper, k , (Figure 1) 7% inches
16 GAGE STAINLESS OR BRASS
POWDERED DIATOMITE INSULATION
Protection Tube
BRASS SUPPORTS END PLATE
NORTON REFRACTORY TUBE NOTE: Shell To Be 6" Brou Tube 1/8" Wall NOTE: All Screws to be 2-56 Round Head Bmrr Mochine Screws
Figure 2.
High temperature combustion furnace
long, is used for the reduction of nitrogen oxides and removal of excess oxygen (see also Figure 3). To prepare the copper, the oxide is first mufled one hour at 900 "C and then heated to dull red in a heavy-wall quartz tube while aspirating vapors of methanol through the heated copper oxide. Heating is stopped and aspiration is continued until the copper is cool, in order to remove all traces of hydrogen. This technique ensures the reduction of the inside of the copper oxide particles as well as their outer surface and accounts for the greater reductive capacity (80 to 90 samples) of the copper prepared in this manner. For the determination of oxygen, the combustion tube is packed (Figure 4) with an 8-cm section of pelletized carbon (16) [Carbon Black-Wyex Compact, pelletized, approximately 30 to 80 mesh, for use in the microdetermination of oxygen in organic compounds by the thermal decomposition method, (Arthur H. Thomas Co., Philadelphia, Pa.)]. Before placing in the tube, the carbon is heated several hours at 800 to 900 "C. This sinters the carbon and thereby avoids channeling during use at 1100 to 1120 "C. The carbon section is followed by an 11-cm section of quartz chips, 8 to 20 mesh (Arthur H. Thomas Co.). This serves to fill the void in the combustion tube between the carbon and the 16-cm section of Ascarite (Arthur H. Thomas Co.) which is used for absorption of any acidic gaseous products. A piece of 52-mesh platinum gauze is used to keep the carbon section from channeling during use. Quartz wool plugs, 3 to 4 mm long, are used between the various sections as shown in Figure 4. (16) V. A . Aluise, H. K. Alber, H. S. Conway, C. C. Harris, W. H. Jones, and W. H. Smith, ibid., 23, 530 (1951).
SIAINLESS S l l t l MAGNET WIRE
A 1%
I8 GAUGE
VUTINUM
PLATINUM WIRE
BOA1
=&-5.1/2--.u t
S A M P L E INJECTOR
BOAT CARRIER fOR SLMPLE BOAT
Figure 3.
I N QUAI12 1UIE
e , INDtNlAlIONS
WOOL
I-Sh''
COWOl OXIDE
Y.
Combustion tube and sample injector
, ' *
26"
INDENTATION IN QUARTZ TUBE
O-RING
Figure 4.
PLATINUM GAUZE
\\/ QUARTZ WOOL
Tube packing for determination of oxygen VOL. 41,
NO. 3, MARCH 1969
497
The cell used to measure the thermal conductivity of the combustion gases contains a pair of Veco Thermistors No. 2796, Type A140 (Victory Engineering Corp., 136 Springfield Ave., Springfield, N.J.). The cell is located in a vapor-jacketed chamber (17). Ethanol is used as the refluxing liquid so that the cell is maintained at about 78 "C, as determined by the temperature of the vapors in the chamber. The sample is weighed in a disposable aluminum foil boat (13 rnrn long x 5 mm wide X 5 mm deep, Cat. No. 7556-B80, Arthur H. Thomas Co.). The use of aluminum foil sample boats under rapid combustion conditions raises the temperature of the sample, at the instant flashing occurs, above the ambient temperature at this point. The holder for the aluminum sample boats is made from thin ( 5 mil) platinum sheet to keep the heat capacity low. The platinum holder is welded to one end of No. 18 B&S gauge platinum wire which is folded at the other end to fit snugly inside a magnetic steel coil (400 series stainless). This device (Sample Injector, Figure 3) is used to move the sample into the combustion zone of the furnace and to remove it after 30 seconds by means of a magnet. The use of platinum in the construction of the sample injector prevents accumulation of deposits that are detrimental to the life of the combustion tube. This feature makes it worthwhile to reduce the spent (oxidized) copper after 80 to 90 analyses so that the same tube may be used over again for 400 to 500 analyses. Another feature of this apparatus is the use of a rotary gas sampling valve, x , Figure 1 (Perkin-Elmer Corp., Norwalk, Conn.) for sampling a reproducible volume of gas so that a standard gas mixture (calibration gas mixture, 30% COS70% Nz, The Matheson Company, East Rutherford, N.J.) can be used to determine the overall performance of the absorption columns, the sensing elements, the recorder, and the integrator. The combustion products, COZ,SOz, and H 2 0 , are retained on column p , which contains 20 wt% Carbowax 20M on Haloport F, 30-60 mesh (Hewlett Packard, F&M Division, Avondale, Pa.). Nitrogen and carbon monoxide (when determining oxygen) are retained on column s, which contains Molecular Sieve 5A, 40-50 mesh. These columns are made from %-inch copper tubing 11 inches in length and the ends are equipped with Swagelok fittings. Column p and the Swagelok fittings between In and Y are wound with 21 feet of asbestos insulated nichrome wire (B&S gauge No. 26) to permit heating for removal of retained COz, SOz, and HzO. Another 21-foot length of the same wire is wound around the 12-inch length of %-inch copper tubing, q, the Swagelok fittings, and T-connectors used to by-pass the molecular sieve column, s. The latter is wound with 12 feet of the same wire. The temperature of the Carbowax column, p , is set at 130140 "C, and that of the by-pass tube, q, at 110-115 "C. The molecular sieve column, s, is allowed to warm to room temperature for nitrogen determination and heated continiously to 55-60 "C for oxygen determination. The power supply to the Carbowax column, p , and the by-pass tube, q, is kept on continuously. Diversion of the gas stream from the molecular sieve column to the by-pass tube is done by means of two Republic No. 310-3s D, three-way valves, r and t (Louis H. Hein Co., 1151 Matson Ford Rd., West Conshohocken, Pa.). The recorder used is a Brown Electronik, Continuous Balance Unit, Model 153, pen speed one second, and chart speed 15 to 60 inches per hour (Minneapolis-Honeywell Regulator Co., Brown Instrument Division, Phila., Pa.). The recorder signal is fed to an Electronic Integrator, Model PX-592 (Ridgefield Instrument Group, Schlumberger Corp., Ridgefield, Conn.). The bridge circuit is a Wheatstone bridge with fine and coarse zero adjustment, stepwise attenuation from 1 X to 1OOOX and indicating meters in volts and milliamperes. The power supply is 12 V. (17) E. Bennett, S. Dal Nogare, L. W. Safranski, and C. D. Lewis, ibid., 30, 898 (1958). 498
ANALYTICAL CHEMISTRY
I
BRIDGE ATT. INTEG. ATT.
I
70
I
8
I
I
I
8
I
I
I
1W
70
300
16
8
Figure 5. Typical chromatogram (1.113 mg of sulfanilamide) Nitrogen = 16.27%, carbon hydrogen = 4.86%
=
41.84%, sulfur
=
18.62%, and
Procedure. Weigh from 0.5 to 2 mg of sample in the disposable aluminum sample boat, which is crimped before taking the final reading. Place the aluminum boat in the platinum holder of the sample injector, which contains some quartz wool to prevent fusion of the aluminum boat to the holder during combustion. In this way the platinum holder can be used for several thousand analyses without replacement. Before placing the sample injector into the combustion tube, turn the handles of the three-way valves, r and t , toward each other. In this position the valves divert the helium flow through the by-pass tube. Raise the handle of the reverse flush toggle valve, 0, (open position) and disconnect the helium inlet tube, d, immediately from the combustion tube at the O-ring joint, shown in e , Figure 1. This joint is sealed to the quartz combustion tube by a borosilicate to quartz graded seal. (Cat. No. LG-10861, Labglass, Inc., Vineland, N.J.). This allows helium to escape through the open end of the combustion tube and thus minimizes entry of air when placing the sample injector in the combustion tube (platinum sample holder end 5 to 6 cm from the combustion furnace). After reconnecting the helium inlet tube to the combustion tube, immediately lower the handle of the toggle valve (closed position). In 1% to 2 minutes, deflections of the recorder pen will be noted, indicating equilibration of the pressure surges in the system cia the by-pass tube. Turn the handles of valves Y and t to their original position to divert the helium through the molecular sieve column. Again deflections of the recorder pen will occur indicating equilibration of the pressure surges in the system, this time ria the molecular sieve tube. The effect of these surges on the analysis is thus avoided. At this point, use procedure (A) for the simultaneous determination of carbon, hydrogen, nitrogen, and sulfur, and procedure (B) for determination of oxygen. (A) SIMULTANEOUS DETERMINATION OF CARBON, HYDROGEN, AND SULFUR.Place liquid nitrogen traps under the NITROGEN, Carbowax and molecular sieve columns, set the timer, a, for 50 seconds and turn it on. This energizes the solenoid valve, b, and admits oxygen to the combustion tube through the fine metering valve, c, at a rate previously calculated and adjusted to provide a mixture of 4 0 z oxygen in helium. After 33 seconds use a magnet to move the sample injector so that the sample holder is in the center of the high-temperature furnace, where flash combustion of the sample takes place under highly favorable dynamic conditions. At the end of 50 seconds, the timer de-energizes the solenoid valve. This closes the oxygen supply port to the combustion tube and
Table I. Carbon, Hydrogen, Nitrogen, and Sulfur Results on Standard Compounds % Element calcd. % Element found (E) n C H N S C H N Compound 13 79.05 5.59 15.25 79.09 5.53 15.38 Azo benzene 13 58.53 4.09 11.34 58.54 4.09 11.38 Nicotinic acid 10.44 71.09 6.71 10.36 3 71.10 6.79 Acetanilide 2 56.77 4.72 8.62 56.65 4.75 8.26 p-Chloroacetanilide 1 45.11 3.76 6.53 44.89 3.77 6.54 p-Bromoacetanilide 2 59.86 3.59 60.00 3.60 p-Fluorobenzoic acid 4 51.47 8.55 40.09 51.40 8.63 39.97 Hexamethylenetetramine 2 16.87 7.11 19.72 16.90 7.09 19.71 Ammonium oxalate 2 39.81 6.47 40.00 6.71 Dextrose 2 68.75 4.98 68.84 4.95 Benzoic acid 2 42.03 4.82 16.28 41.84 4.68 16.27 Sulfanilamide 7 30.02 5.31 11.60 26.36 29.99 5.03 11.66 Cystine Pooled Standard Deviation, Sp 0.25 0.80 0.14 0.28 Root Mean Square Errora 0.22 0.80 0.18 0.27 a
Calculated from all individual deviations from the known value, p . RSME
opens the exhaust port which is set to bleed 4 cc of oxygen per minute to the atmosphere. After an additional 30 seconds, remove the liquid nitrogen trap from the molecular sieve column. With the magnet, withdraw the sample injector and holder containing the spent aluminum boat from the furnace. Zero the recorder pen and set the attenuations for the bridge circuit and integrator to keep the nitrogen peak and its integrated value on scale. This peak is obtained in 3% to 4 minutes after the liquid nitrogen trap is removed. Turn the valves r and t as before to divert the gas stream through the by-pass tube, q , and remove the liquid nitrogen trap from the Carbowax column, p . Reset the bridge circuit and integrator attenuations to keep the peaks and integrated values representing carbon, sulfur, and hydrogen on scale. The carbon dioxide peak is obtained in about 1% minutes after the nitrogen response and followed in about 1% minutes by the sulfur dioxide peak. Approximately 1% minutes after the sulfur response (as the Carbowax column heats to 140 "C), the water peak is obtained (Figure 5). A complete analysis takes 16 minutes. (B) DETERMINATION OF OXYGEN. For the determination of oxygen, the tube used for C, H, N, and S is replaced with one containing carbon (Figure 4) and conditioned, preferably overnight at 1110-1120 "C in a stream of helium. After placing the sample in the combustion tube and equilibrating the system uia the by-pass tube and the molecular sieve column, as described above, place a liquid nitrogen trap under the molecular sieve column and use the magnet to move the sample boat into the combustion furnace. After one minute, remove the liquid nitrogen trap from the molecular sieve column and withdraw the spent aluminum sample boat from the combustion furnace. Zero the recorder and set the attenuation. In about 4 minutes a peak for CO will be obtained. Nine minutes are required for a determination. (C) CALIBRATION. Prepare calibration curves using known compounds with appreciable spread in the amount of each element present. The plot of integrator response cs. the weight of each element present is a straight line relationship. RESULTS AND DISCUSSION
The results obtained from 53 analyses of 12, high purity standard compounds are summarized in Table I. The mean values show excellent agreement with the theoreticalpercentages. Pooled standard deviations for C, H, N, and S are also given in Table I. These were calculated in conventional fashion from the 11 small sets of data (excluding p-bromoacetanilide). No values were rejected. In the authors' experience, the levels of
=
S
26.69
Pxl
precision obtained for C, H, and N compare favorably with those obtained in realistic evaluations of classical methods (18) and commercial analyzers. The precision of the sulfur values is less satisfactory. However, these are preliminary results and should be improved with further refinements in technique. The closeness of agreement between the pooled standard deviation and the root mean square error shows that no bias exists in the determination of any of the four elements. The analysis of difficultly combustible samples without the use of solid catalyst was accomplished by flash burning of the sample under dynamic conditions at 1060-1080 " C . The gaseous products pass over quartz wool which is at the same temperature. This hot quartz wool acts as a baffle and provides additional high temperature contact surface needed to complete the combustion of stray fragments in the gas stream. The efficiency of the combustion system was demonstrated by the analysis of two special compounds. Table I1 contains the data obtained from the analysis of 1.5-mg samples of a nickel salt of a substituted pyridazine. Satisfactory results were obtained. When this sample was analyzed for nitrogen using the Dumas technique and for carbon and hydrogen using the Pregel method, low and variable results were obtained. Table I11 compares the data obtained from the analysis of a nitrogencontaining polypropylene by several different analyzers. A closure of 99.76% was obtained with 1-mg samples using the authors' analyzer. Figure 5 is a typical chromatogram showing peaks for nitrogen, carbon, sulfur, and hydrogen, while Figure 6 illustrates the peaks obtained for oxygen. The nitrogen is measured as Nn after reduction of its oxides in the reduction furnace at 650 "C. When calibration curves are established for nitrogen, little or no day-to-day curve shift is observed. Nitrogen values are not dependent on preliminary conditioning of the instrument. Low nitrogen results can be obtained if the copper used to reduce NOn to N z is spent. At the same time, hydrogen values will be high because no separation of H 2 0 and NOz is obtained. The carbon is measured as COS. Carbon values are influenced to some extent by equilibrium effects in the Carbowax column. The first run of the day yields a slightly low carbon result, which is attributed to the adsorption of C o non Carbo-
(18) F.
w. Powers, IND.ENG.CHEM.,ANAL.ED., 11, 660 (1939). VOL. 41, NO. 3, MARCH 1969
499
Table 11. Precision of Replicate‘ Carbon, Hydrogen and Nitrogen Results on Difficultly Combustible Research Sampleb Calcd. values, % Observed values, % C H N C H N 58.74 4.26 14.28 59.17 4.18 14.53 59.62 4.37 14.05 59.76 4.71 14.03 59.40 4.48 14.25 58.98 4.40 14.19 59.11 59.47 59.23 59.18
4.36 4.54 4.38 4.24
14.00 14.12 14.08 14.37
59.00 58.86
4.31 4.30
14.37 14.23
Mean, 2 59.21 4.40 14.19 Standard deviation, s 0.31 0.13 0.13 a Determined in sequence in three groups on three different days over five days. b Monohydrate of nickel salt of substituted pyridazine. Table 111. Comparison of Results Obtained on Nitrogen-Containing Polypropylene Polymer by Various Analyzers‘ Pregla No. lb No. 2c No. 3d Authors’ analyzere 77.20 77.50 79.60 75.60 78.17 Carbon 7.48 7.38 7.29 7.24 7.11 Hydrogen 12.30f 12.30f 12.70 12.06 12.45 Nitrogen 2.03 2.03 2.03 2.03 2.03 Oxygenu % Closure 99.01 99.21 101.62 96.93 99.76 a Average of 4 C and H determinations in authors’ laboratory. b Average of 4 C and H determinations by commercial unit in authors’ laboratory. c Average of 2 C, H, and N determinations on commercial analyzer by manufacturer. d Average of 4 C, H, and N determinations on commercial analyzer by manufacturer. e Average of 2 C, H, and N determinations in authors’ laboratory. f Average of 4 N determinations in authors’ laboratory (Dumas). 0 Average of 2 0 determinations in authors’ laboratory (Unterzaucher). wax 20M. Calibration shifts for carbon may be greater than for nitrogen or hydrogen. The hydrogen is measured as HzO. The first run of the day gives a significantly low result. There is a negative blank of about eight micrograms for hydrogen which indicates this amount is needed to maintain equilibrium, due to some retention of water in Carbowax and on the walls of the copper tubing. This equilibrium is maintained throughout the day and excellent results for water are obtained with very little day-to-day calibration curve shift. Good peak quality for water is obtained by using a highly loaded packing of 2 0 x Carbowax 20M on Teflon (DuPont), and heating the column t o 140 “C. If this column temperature is not allowed to exceed 150 “C., and no oxidation of the Carbowax occurs, column life is greater than 1000 analyses. The sulfur is measured as SOn. The reaction of SO2 and Cu is a temperature dependent reversible reaction in which CuzS and CuzO may be formed (19). Optimum conditions which favor the formation of SOnand drive the reaction quantitatively t o the left are currently being investigated. Mixing the copper with C u 2 0 in the reduction furnace is believed to be part of the solution. The final optimum temperature which favors the formation of SOz is yet to be determined. This temperature should be maintained t o i 2 “C. The temperature of the reduction furnace is currently 650 “C. Some of the difficulty in sulfur results is attributed t o temperature variations in the furnace. Better control should improve precision. The formation of SO1 by the reaction of SOz with molecular oxygen although thermodynamically favorable is extremely (19) A. Butts, “Copper,” Reinhold Publishing Corp., New York, N.Y., 1954, p 235. 500
ANALYTICAL CHEMISTRY
c I
‘I
6.40
1.262 mg
1.414 mg
1.205 rng
1.044 mg
1.010 rng
0.941 rng
IBRIDGE ATT. = 50,
INTEGRATOR ATT. = 16
Figure 6. Carbon monoxide peaks for replicate oxygen determinations on dehydroabietic acid slow in the absence of catalyst (20). There is no evidence of reaction of SOz HzO when they are trapped together on the Carbowax column at the temperature of liquid nitrogen. The calibration curve shows that there is a negative value of about 0.08 mg for sulfur, which indicates a n equilibrium effect similar to water. Low results for sulfur are obtained until about 0.4 mg of sulfur is passed through the system. At this level equilibrium is maintained. The determination of sulfur simultaneously with carbon and nitrogen has been reported by
+
(20) F. Ephraim, “Inorganic Chemistry,” Interscience Publishers, Inc., New York, N.Y., 1949, p 577.
Pennington and Meloan (21). However, their method was applied only to liquid organic compounds. The presence of elements such as bromine, chlorine, and fluorine does not interfere with the analysis or change the calibration factors. These materials are effectively removed by reaction with the copper in the combustion train. No extraneous peaks were noted when samples containing halogens were analyzed. However, in the combustion of samples containing iodine, high values were obtained for carbon, hydrogen, and nitrogen on the second and subsequent runs. It was observed at this point that there was a decrease in flow of helium from 32 to 31 ccjmin. This was attributed to the formation and fusion of cuprous iodide (mp 605 "C) in a part of the 1-cm section of copper which was allowed to extend inside the combustion furnace at this end, where the temperature was about 700 "C. The helium flow was sufficiently restricted to change the response of the thermal conductivity cell. The simplest solution to this problem is not to allow any of the copper to extend inside the combustion furnace, so that it will not be heated appreciably above 600 "C, which is the temperature near the end of the reduction furnace. Because of the equilibrium and adsorption effects described above, the apparatus must be properly conditioned prior to calibration by burning a 2- to 3-mg sample of an organic material containing the elements that are to be determined. The oxygen content of a sample is determined by conversion to carbon monoxide using carbon in the combustion tube at 1100-1 120 'C in a helium atmosphere. Under these conditions the oxygen in the sample is converted to CO. This is trapped (21) S. Pennington, and C. E. Meloan, ANAL.CHEM., 39,119 (1967).
in the molecular sieve and measured by thermal conductivity as described by Meade (22). Because of its low freezing point (-207 "C) CO is not retained on the Carbowax but passes to the molecular sieve column where its retention is about 4 minutes from the time of removal of the liquid nitrogen trap. Because of their higher freezing points, other gases such as H2S (-82.9 "C), CS? (-111.5 "C), and COS (-138.2 "C), if present, will be retained and separated from the CO by the Carbowax column in liquid nitrogen. Figure 6 shows peaks obtained for carbon monoxide. A small positive blank of about 0.012 mg is obtained for oxygen. This is attributed partly to impurities in the helium and partly to air introduced with the sample. In the latter case 2 or 3 minutes reverse sweeping with helium before closing the sample-introduction end of the combustion tube would reduce this source of blank. In the determination of oxygen a standard deviation of 0.14 was obtained when dehydroabietic acid was used as the standard. The precision and accuracy of the results are equal to those obtained with the slower pyrolysis methods in current usage. Nine minutes are required for a determination. ACKNOWLEDGMENT
The authors gratefully acknowledge the contribution of James F. Carre who made many of the analyses reported here. RECEIVED for review October 17, 1968. Accepted December 23, 1968. (22) C. F. Meade, D. A. Keyworth, V. T. Brand, and J. R. Deering, ibid.,39, 512 (1967).
Study of Groves' Method for Determination of Ferrous Oxide in Refractory Silicates Elsie M. Donaldson Mineral Sciences Dicision, Mines Branch, Department of Energy, Mines and Resources, Ottawa, Canada
Groves' method for determining ferrous oxide in refractory silicates (by decomposing samples by fusion with sodium metafluoborate in a platinum boat in an inert atmosphere) is unreliable. Reduction occurs during fusion because of the tendency of iron compounds, particularly oxides, in the molten state to dissociate to lower oxidation states and metallic iron under the experimental conditions of high temperature and oxygen-free atmosphere used for sample decomposition. This effect increases in the presence of platinum and leads to low ferrous oxide values for samples containing predominantly iron(ll), and to high values for samples containing mostly iron(ll1) or moderate amounts of both iron(l1) and iron(ll1).
RECENT studies of pleochroism in some refractory iron-bearing silicates (tourmaline and cordierite) ( 1 ) have suggested that this phenomenon is related to iron(I1) -,iron(II1) electronic interaction. Because a relatively accurate knowledge of the iron(I1) content of the above minerals was essential to these studies, the present investigation was undertaken to assess existing methods for determining ferrous iron in refractory (1) G. H. Faye, P. G. Manning, and E. H. Nickel, Amer. Mineral., 53, 1174 (1968).
materials and to determine their applicability to the samples under consideration. Most of the methods for determining ferrous iron in silicates and refractory materials, described in a recent review by Schafer ( 2 ) , are not applicable to tourmaline and cordierite because they involve acid-dissolution procedures which are unsuitable for decomposing these minerals. However, several methods by Rowledge (3), Hey (4), and Groves ( 5 ) involving fusion of the sample were considered of interest in the present work because most acid-resistant silicates can usually be completely decomposed by fusion with a suitable flux. Rowledge's method involves heating a mixture of sample (staurolite, tourmaline, axinite, garnet) and flux (sodium metafluoborate) in a sealed borosilicate glass tube to 900 "C in a sand bath. Hey applied this method on a micro scale but used an evacuated tube to avoid difficulties arising from the oxidation of iron(1I) by air enclosed in the tube. Groves used the same flux but minimized air oxidation by fusing in a carbon dioxide (2) H. N. S. Schafer, Analyst, 91, 755 (1966). (3) H. P. Rowledge, J. Proc. Roy. SOC. West. Aust., 20, 165 (1934). (4) M. H. Hey, Mineralog. Mag., 26, 116 (1941). (5) A. W. Groves, "Silicate Analysis," 2nd ed., George Allen and Unwin Ltd., London, 1951, pp 181-6. VOL. 41, NO. 3, MARCH 1969
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