Automatic Combustion Apparatus for Determination of Sulfur and Halogen THEODORE
T. WHITE, CARL J. PENTHER, P H I L I P C. TAIT, AND Shell Development Co., Emeryville, Cdg.
In order to achieve the advantages of automatic operation, an automatic quartz tube oomhustion apparatus has been developed for the determination of sulfur and halogens in organic materials. The sample is vaporized in a stream of nitrogen and combustion occurs in the high temperature zone of the combustion tube, where a stream of oxygen is injected. A thermooouple, located just beyond the point at which oxygen is introduced, senses the heat generated by the combustion of the sample vapors and controls the amount of heat that is applied to cause vaporization of the sample. The apparatus reduces the training which must be given the operator, eliminates improper combustions resulting from errors in judging the volatility of samples, and automatically burns each sample at the maximum safe rate. An operator oan complete 15 to 25 analyses per 8-hour day with virtually no possibility of losing an analysis heoause of improper combustion.
FRANCIS R. BROOKS
tion is controlled in this manner until the heater remains on for a 5-minute period, after which control is transferred to 8 2-minute program controller. This removes the sample heater from the amplifier oirouit and maintains it a t full heat, canses oxygen to be passed over the sample boat to burn out m y carbonaceous residue and, at the end of 2 minutes, sounds a h a z e r and places the apparatus in a stand-by condition for the next sample. The desired constituents are absorbed from the gas stream and measured in the usual manner (1). DESCRIPTION O F APPARATUS
A photograph of the apparatus is shown in Figure 1. The cabinet is 23 inches long, 12.5 inches deep, and 11 inches high. There is no bottom, and cooling is promoted by raising the cabinet above the table with rubber feet and providing louvres a t the top of the rear panel. The front panel is of Bakelite with the upper section a t a 60' algle to make meter reading and switch manipulation more convenient. Construction of the quartz combustion tube, its mounting, and furnace are shown in Figure 2.
A shattemroof elsss guard nratects the overator from quartz.
T
HE q u a r k tube combustion method ( 1 ) is widely used in the
petroleum industry for determining sulfur and halogens in organic materials. I n this method the sample, contained in a combustion boat, is vaporieed in a stream of air by gradually applying heat t o the sample boat. This technique requires the constant attention of the operator, and his experienceand training are important factors in achieving properly controlled vaporization of the sample. In attempts to design an apparatus which would allow more rapid combustion and would facilitate the combustion of volatile materials, a unit was constructed in which the sample was vaporized in a stream of nitrogen and the vapors were mixed with oxygen within the heated zone of the combustion tube. The observation that an intense flame resulted in the zone of admixture of oxygen and sample vapors suggested the possibility of Using the flame temperature as a measure of the rate of sample vaporization. In preliminary tests a thermocouple was located in the flame and the temperature indicated by a pyrometer was used to guide the rat0 a t which heat was manually applied to vaporize the sample. The direct relationship found between the flame temperature and sample vaporization rate suggested that the former could be used as the basis for automatic control of the latter. As a result of further investigation along these lines, an apparatus was developed which provides completely automatic sample combustion. Combustion is carried out in a horizontally mounted quartz tube, a portion of which is maintained a t 1000' C. The sample is placed in the cool zone of the tube and rapidly heated by applic* tion of full line voltage to a bare wire coil on the outside of the tube. Sample vapors are swept into the heated zone by a stream of nitrogen and mixed with a large excess of oxygen, a t which point they burn with an intense flame. The rate of sample vaporization is controlled by a thermocouple located in this flame. The thermocouple, by means of an amplifier and relay circuit, switches the heater off and turn8 on a cooling air blast, or switches the heater on m d turns off the air blast, as the flame temperature goes above or below the control point. Vaporiaa-
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carries the vacuum'supply to the topabsorber, so that the idnnection to the glassware can be made with a short piece of flexible tubing. The left 9 inches of the front panel mounts the control unit, which can be independently withdrawn from the cabinet by means of the handle on the low- panel. Those electrical components not directly associated with the control unit, such as the tram-
Figure 1. Apparatus
1664
V O L U M E 25, NO. 11, N O V E M B E R 1 9 5 3
Inlet Assembly
/Platinum-Iridium Platinum-Iridium Oxygen TubeThermocouple ,Quartz Sleevino
former and switch for furnace temperature control, millivoltmeter for furnace temperature indication, program timing unit and relays, switches, fuses, pilot lights, and the starting pushbutton, are mounted on the upper right 14-inch section of the front panel. This unit is also fitted with panel handles and plug-in electrical connections which permit it t o be easily withdrawn from the cabinet for servicing. The gas flow indicators and control valves are located on the vertical section of the front panel. Solenoid valves and copper tubing interconnecting the components of the gas flow system are located behind this panel and below the unit containing the furnace controls. All gas and electrical connections are a t the rear of the cabinet. Gas Flow System. The gas flow system is shown in Figure 3. Nitrogen and oxygen a t 5 pounds per square inch are introduced into the rear of the apparatus. Oxygen is introduced into the heated zone of the combustion tube through the long platinum-iridium tube, which also serves as one leg of the control thermocouple. Rotameters with self-contained needle valves are used to regulate the flow of oxygen and nitrogen. Vacuum is adjusted, using the panel-mounted vacuum control valve, so that it slightly more than accommodates the combined nitrogen and oxygen flows. At proper adjustment, a slow stream of air enters the system through the glass bubbler. Automatic application of vacuum and cooling air and selection of gases are achieved through the use of solenoid valves. A toggle-action valve permits the vacuum solenoid to be by-passed so that vacuum can be applied to the absorbers during transfer of the absorbing solution. Combustion Control. -4 viiring diagram of the combustion control system is shovn in Figure 4. When the start button is momentarily depressed, relay 1 is energized and is locked in through cam 1 of the program controller (Type &IC4 with .212 gear, Industrial Time Corp., Newark, N. J.). The start button circuit receives power through the amplifier power switch, which ensures operation of the control amplifier before the sample can be heated. The contacts on relay 1 connect power to the sample heater under control of relay 2 and amplifier relay 3, and to the vacuum, nitrogen, and secondary oxygen solenoids. The vacuum and secondary oxygen are under control of relay 1, while the nitrogen solenoid is under the further control of program controller cam 1. The sensitive element is a platinum-platinum-iridium thermocouple formed between a No. 22 gage platinum wire and a fingerlike extension of the platinum-iridium oxygen supply tube (see Figure 2). The electromotive force from the couple a t the control temperature is balanced by an equal voltage of opposite polarity obtained from a local battery circuit. The local battery
corn Transire Sil-0-Cel
Combustion Heater bust ion Heater
1665 Aluminum Heat Shield
circuit is standardized by setting the voltage across the divider network with coarse and fine rheostats to 1 volt, as indicated by a small panel meter. A separate adjustment is provided for setting the operating flame temperature. The difference voltage between the reference circuit and the thermocouple is converted to alternating current by a Brown vibrating converter and coupled to a two-stage, high-gain R-C coupled amplifier by a Brown input transformer. The miniature amplifier tube, a dual triode Type 12AX7, and all of the amplifier components shown within the dotted line on the wiring diagram are mounted on a heavy brass plate which is suspended from the chassis by four rubber shock mounts to protect the circuit from vibration-induced transients. The output of the amplifier is capacitively coupled to a Type 2D21 miniature thyratron, in the anode circuit of which is a double-pole, double-throw relay (relay 3) which supplies power to the sample-cooling air solenoid valve in its normal position and power to the sample heater and to the timing relay in its energized position. A signal-limiting germanium diode is included in the input circuit of the thyratron to prevent undesired relay action from large amplitude signals. The sensitivity is such that clean-cut relay operation is obtained for an input signal change of approximately 4 microvolts. The direct-current anode voltage source for the amplifier must be free of voltage fluctuations. -4separate small transformer and selenium rectifiers in a voltage doubler circuit, followed by an R-C filter and two Type OA2 voltage regulator tubes, provide the required stability. When relay 3 is energized, by the reference voltage being greater than the electromotive force generated by the control thermocouple (a condition resulting when no flame exists), the sample heater and time-delay relay receive power and the cooling air solenoid is de-energized. The sample starts to vaporize when heated, and the vapors are carried into the hot zone of the combustion tube by the nitrogen stream. As the vapors enter the hot zone they are mixed with oxygen and a flame results. The control thermocouple is located in the flame and quickly generates an electromotive force which, when it exceeds that of the reference electromotive force, shuts off the sample heater and time-delay relay and turns on the distributed air blast which rapidly cools the sample below the vaporization temperature. Because the amplifier relay 3 connects the heater in its energized position, the control amplifier is “fail safe,” applying cooling air on electron tube failure. When the flame subsides, the thermocouple output decreases below the control value, resulting in the energization of relay 3, which shuts off the cooling air and starts a new heating and timing cycle. This action is repeated until a heating cycle lasts 5 minutes. The timing device by which completion of the sample vaporiration cycle is determined is a Cramer Type TC-5M time-delay relay. This unit consists of a synchronous motor geared down to
ANALYTICAL CHEMISTRY
1666
m
drive a cam which actuates a switch. A spiral spring is wound up as the cam turns and returns the cam to the starting position when the energizing current to the motor is interr u p t e d . Preliminary t e s t s showed t h a t a l l m a t e r i a l s n o r m a 1 I y encountered were completely vaporized or reduced to a carbonaceous residue when a 2-minute time delay was used and a 5-minute time delay w a s a r b i t r a r i l y selected to provide a large m a r g i n of safety. At the termination of the 5-minute heating period the delay relay energizes relay 2 and the program controller motor. Relay 2 locks in under control of relay 1 and removes the sample heater control from amplifier relay 3 and places it on full heat under control of relay 1. When the program controller starts its 2-minute cycle, it locks in under control of cam 3 while cam 1 switches power from the nitrogen solenoid to I_.__ St-’c t h e o x y g e n solenoid. This Figure 3. Block Diagram of Automatic Quartz T u b e Combustion Apparatus stops the nitrogen flow over the sample and substitutes oxygen, which burns out any carbonaceous residue remaining in the sample boat. S e a r cause of the difficulty in exactly fixing the position of the the end of the 2-minute program controller interval, cam 2 thermocouple with relation to the flame. The proper setting of momentarily energizes a warning buzzer and opens the locking the rheostat is determined by burning a 1-gram portion of circuit on relay 1. A few seconds later cam 3 opens the program mineral oil in the apparatus and adjusting the rheostat during controller motor circuit, which restores the entire apparatus to starting status. Had the air switch been closed, sample cooling combustion so that the heating and cooling periods of each conwould have commenced at the end of the program cycle and trol cycle are approximately equal and combustion is completed continued until the operator manually opened the switch or in I7 to 20 minutes. started another run. Sample Heater and Furnace. The sample heater consists of PROCEDURE 34 feet of No. 18 Chromel d wire, wound on a mandrel so that it fits loosely on the quartz tube. The combustion furnace eleIf sulfur is to be determined, introduce 30 nil. of 1.5y0 hydroment is wound on an Alundum tube, also of Chromel A wire (KO. gen peroxide solution into the lower (primary) absorber and 10 22), approximately 40 feet wound a t 22 turns per inch for 1 inch ml. into the upper (secondary) absorber. (While 3% hydrogen on each end, n-ith the balance of the 73/8-in~hlength wound at peroxide solution has been employed by others and is satis18 turns per inch. This tube is insulated with fireclay brichs, factory, the 1.5% solution has adequate reserve capacity for any bored out to receive it, and the bricks are housed in a Transite materials that might be encountered and there seems to be no box which is further insulated with a spaced aluminum shield. need for use of a stronger reagent.) If halogen is to be deterThe furnace can be adjusted to the operating temperature, mined, introduce 25 ml. of 2.5% sodium carbonate solution and approximately 1000° C., for line voltages from 105 to 125 volts. 10 ml. of 6% hydrogen peroxide into the primary absorber, A radio-type filament transformer is used as an autotransformer and place 10 ml. of water in the secondary absorber. Insert the and a 5-position rotary switch permits line voltage, or line voltsample boat into the combustion tube and slide it approximately age &5 and 10 volts, to be connected to the furnace.
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Quartz and Glass Parts. The quartz combustion tube is shown in Figure 2. The tube is 535 mm. in total length; 440 mm. is 25 m m .in outside diameter and the remainder is 12 mm. in outside diameter. A baffle, with a central hole 7 mm. in diameter, is located 310 mm. from the entrance end so as to be just inside the furnace. The oxygen supply tube is 2.0 mm. in inside diameter X 2.5 mm. in outside diameter and 340 mm. long and, with its thermocouple, is placed a short distance through this hole. A second baffle, with a central hole 5 mm. in diameter, is located 375 mm. from the entrance end to promote complete mixing of the combustion gases. The combustion tube contains no packing. The absorbers, spray traps, and absorber adapters are identical with those described previously ( 1 ) . The 2-foot-long brass pipe which extends the vacuum line to the top of the glassware aerves as a condenser which takes out droplets passing the glass trap and is provided with a vent which is used in the operations involved in transferring the absorbing solution. In addition, a metal trap with baffles is connected in series with the line and traps out any remaining water. The trap is provided with a convenient drain cock for emptying. Adjustment of Operating Flame Temperature. The temperature a t which the thermocouple generates sufficient electromotive force to actuate the control circuit is determined by a rheostat (see Figure 3) located on the front panel. rln adjustable rather than a fixed operating temperature is emplol-ed be-
Table I. Determination of Sulfur and Halogen with Automatic Quartz T u b e Apparatus Sulfur or Halogen, o/o Material Carbon tetrachloride Gamma-benzene hexachloride hlonoohlorinated paraffin wax Monochlorinated paraffin wax white oil Sample A Sample B Sample C 4.4-Dibromodiphenyl ethei
Present 92.2
o-Iodobenzoic acid m-Iodotoluene Flowers of sulfur
51.2 58.2 100 0
+
Diethyl sulfide
73.2 17.3
6 05 0 64 0 13 48 9
35.6
Benzyl disulfide 3-Sulfolanal
. 2263 . 06
Phenyl sulfide Phenyl sulfoxide
17.3 15.9
Benzyl sulfide Sample A Sample B Sample C
+ white oil
2.52 1.00 0.21
Found 92 0 72 8 17 3 6 0.1 0 69 0 12 48 5 48 6 51 7 58 6 98 9 99 1 35 5 36 0 35 6 26 1 23 2 23 3 17 4 16 0 16 0
2 50 0 98 0 21
V O L U M E 25, NO. 11, N O V E M B E R 1 9 5 3 to the center of the sam le heating section of the tube; replace the inlet cap. Depress t i e start button and set the flow of oxygen a t 2.5 liters per minute, and the flow of nitrogen a t 400 ml. per minute, and adjust the vacuum to draw a small stream of air through the bubbler. Combustion is completely automatic and the buzzer will sound to indicate completion of the combustion. The apparatus automatically resets and the cooling air blast turns on when the combustion cycle is complete. When the combustion cycle is completed, disconnect the quartz tube from the adapter. Close the adapter with a ball joint plug, turn on the vacuum by-pass valve, and allow vacuum to be applied to the absorber. Turn off the vacuum valve and slowly bleed air into the system through the air inlet valve, allowing the liquid to be drawn from the secondary absorber into the primary absorber. As the pressure equalizes, remove the spray trap, rinse it. and collect the washings in the secondary absorber. .Wer all the liquid has been drawn into the primary absorber, rinse the secondary absorber, using not more than 20 ml. of water. Finally, remove the secdndary absorber and wash the ground joint, both inside and out. Remove the stopper from the adapter and rinse both the stopper and adapter with a small quantity of water, collecting the washings in the primary absorber. For the determination of sulfur, in the absence of other acid-forming elements such as halogens, titrate the sulfuric acid collected in the abmrber with standard caustic solution; for the
1667 determination of halogen, titrate the halide ion content of the absorbent by the F’oihard method ( 1 ) . DI SCU SSIOh
The aliove method and apparatus have been applied to a great variety of materials; Table I shows results typical of those obtained for samples containing known amounts of sulfur or halogen. These data indicate that an accuracy of 99% or better may be expected for materials containing more than 5 % sulfur or halogen. In the lower concentrations, sulfur or halogen can generally be determined to =k0.03%. The sampling techniques employed with the automatic unit differed somewhat from those used with the manually operated apparatus (1). Solids or heavy liquids were weighed directly into open boats. Liquids which tended to “creep” from the combustion boat were delivered from a small weighing pipet to a boat placed partially inside the combustion tube and the boat was then quickly inserted into the tube. Sample tubes with capillarv stems were found to be unsatisfactory for sampling volatile liquids because the liquid was expelled too rapidly troni
7EMP€GATU#€ CONTROL UN/T
Figure 4.
Wiring Diagram of Automatic Quartz Tube Combustion Apparatus
ANALYTICAL CHEMISTRY
1668 the tube, resulting in incomplete or too rapid combustion. This difficulty was overcome by placing the sample in size 00 gelatin capsules which were half-filled with 100- to 200-mesh silica gel. The silica gel served to adsorb the sample and, thus, to reduce its vapor pressure; without the silica gel, sample vapors escaped from the capsule. The capsule containing silica gel was weighed, the sample dripped onto the silica gel from an eyedropper, the capsule capped, and reweighed, and the loaded capsule placed in a porcelain boat. The capsule was then punctured with a pin and the boat quickly inserted into the combustion tube. As gelatin contains nitrogen, a small amount of nitric acid was formed during combustion of the capsules, necessitating a correction when sulfur was measured by titration of sulfuric acid. The correction, approximately 0.6 ml. of 0.06 N sodium hydroxide solution, was established by titrating the acid formed when empty capsules were burned in the apparatus. The capsules were sufficiently uniform in weight, so that the value of the correction did not vary significantly. The time required for combustion varied from 6 minutes for 0.2-gram samples to 20 minutes for I-gram samples. These times are considerably less than those required with the manually operated apparatus for comparable samples. The automatic operation permits the operator to weigh samples or to titrate absorber liquids while combustion is proceeding; and because of
the rapidity of the combustion, an operator can complete from 15 to 25 determinations in an %hour working day. This productivity is, in effect, further increased relative to the manually operated apparatus because no determinations are lost with the automatic unit as a result of incomplete combustion. Although the control and operating circuits of this unit are fairly complex, the apparatus has required very little maintenance service when used daily for many months. The principal maintenance operations have involved occasional replacement of the quartz tube and the small battery which serves to balanc: the thermocouple e.m.f. ACKNOWLEDGMENT
The authors are grateful to E. D. Peters for his counsel in the early stages of the development work and to E. J. Agazzi for his aid in conducting the experimental work. LITERATURE CITED
(1) Peters, E. D., Rounds, G. C., and Agazzi, E. J., AXAL.CHEM.,24, 710 (1952). RECEIVED for review J u n e 1, 1053. Accepted August 21, 1953. Presented before t h e Divisions of Analytical and Petroleum Chemistry, Sympoaium on Automatic Analytical Methods in t h e Petroleum Industry, a t t h e 124th Meeting of t h e AMERICAN CHEMICAL SOCIETY, Chicago, Ill.
Extraction of Heteropoly Acids Application to Determination of Phosphorus COE WADELIN'
WITH
31. G. MELLON, Purdue University, Lafayette, Znd.
A study was made of liquid-liquid extraction to evaluate its applicability to separations of heteropoly acids. The efficiency of the extractions was measured by means of ultraviolet spectrophotometry. Molybdophosphoric acid is the most readily extracted of the acids tested. A method is recommended for determining phosphorus in steel utilizing extraction of molybdophosphoric acid with a 2070 by volume solution of I-butanol in chloroform. The method was successfully applied to National Bureau of Standards steel samples containing up to 1.4% manganese, 0.3% silicon, 1.0% chromium, or 0.2% vanadium.
S
TUDY of heteropoly acids has led to useful methods for the determination of phosphorus ( 6 ) , arsenic ( 5 ) ,silicon (Y),and germanium (16), among other things. The methods cited are similar in that the element being determined is the central atom of a heteropoly acid in which molybdate is the coordinated group. A common drawback of these methods is that the presence of any of the central-atom elements other than the one being determined causes interference of such magnitude that the concentration of the interferer can be only a fraction of that of the desired constituent without seriously impairing the accuracy of the results. Since the methods under consideration are used for determinations of traces, this means that the interferers must be completely absent when the measurements are made. Interference by soluble silica is especially common, as aqueous solutions in contact with glass are contaminated in a short time. Using the molybdovanadophosphoric acid method ( l 7 ) , one can measure 10 p.p.m. of phosphorus in the presence of 50 p.p.m. of arsenic or 500 p.p.m. of silicon. I n several instances selective extraction of heteropoly acids has been used to effect the separations necessary to overcome interferences. Consequently, a systematic study of liquid-liquid ex1 Present address, Research Division. Goodyear Tire and Rubber Co., Akron 16, Ohio.
traction was undertaken to evaluate the general applicability of this type of separation. APPARATUS AND REAGENTS
Absorbance measurements were made with a Cary spectrophotometer, Model 10-11M. The wave-length scale was calibrated frequently with a mercury arc using the lines a t 280.4 and 313.2 mp. The photometric scale was calibrated with cobalt blue, copper green, selenium orange, and carbon yellow glasses issued by the National Bureau of Standards. The slit width of the instrument gave a half-intensity band width of approximately 0.1 mp throughout the spectral region used. A matched pair of 1-cm. quartz cells with ground-glass stoppers, supplied by the American Instrument Co., was used. The organic solvents used for extraction were of technical grade. All other chemicals were of reagent grade. Aqueous solutions were stored in hard-rubber or polyethylene bottles to prevent contamination by silica. GENERAL STUDY OF EXTRACTION OF HETEROPOLY ACIDS
Literature Survey. Liquid-liquid extraction has been used to effect separations of heteropoly acids in a variety of applications. For example, Jacobs extracted excess tungstophosphoric acid with 1-pentanol after using the acid as a protein precipitant (19). Wu extracted molybdophosphoric acid into ethyl ether as a means of purifying the acid ( S I ) .
