V O L U M E 2 8 , N O . 2, F E B R U A R Y 1 9 5 6 ti ,ction products which had lieen analyzed by the standard proc d u r e s given hy Scott f121. These data are given in Table 11. LITERATURE CITED (1) Baggott, E . R., and Willcocks, R. G . W , , AnaZyst 80, 53-64
(1955). ( 2 ) Bryson, A%., and Lenzrr. S.,Ibid., 78, 299 (1953). ( 3 ) Dakin, H. D., Analyst 39, 273 (1900). (4) Hague, J. L., hlaezkomske. E. E., and Bright, H. A , , J . Research Nutl. Bur. Standards 53, 353-8 (1954).
( 5 ) Hillebrand, W. F., and Lundell, G. E . F., “Applied Inorganic Analysis,” pp. 256-5. Wiley, Yew York, 1953. (1;) Ibid., pp. 422-3i.
233 (7) Jentisch, D., and Frotscher, I., Z . anal. Chem. 144, 17 (1955). (8) Kraus, K. .I., a n d Moore, G. E., J . Am. Chem. SOC.75, 1460 (1953). (9) Kraus, K. A , , Kelson, F., and Smith, G. W., J . Phys. ChenL. 58, 11 (1954). (10) Miller, C. C., and Hunter, J. A , A n a l y s t 79, 483-92 (1954). (11) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” pp. 623-6, Interscience, S e w York, 1950. (12) Scott, W.W., “Standard Methods of Chemical Analysis,” pp. 1054-68, Van Kostrand, New York, 1939. (13) Welcher, F. J., “Organic Analytical Reagents,” vol. 3, pp. 555-6, Van S o s t r a n d , S e w York, 1947. RECEIVED for review M a g 17, 1955.
.-iccepted November 10, 19.5;.
Continuous Analysis of Trace Amounts of Oxygen in Gases F. W. KARASEK, R . J. LOYD, D. E. LUPFER, and E. A. HOUSER Research and Development Department, Phillips Petroleum Co., Bartlesville, O k l a .
In many chemical processes, a knowledge of t h e oxygen c o n t e n t of gases is necessary. Although laboratory methods of adequate acciiracy for d e t e r m i n a t i o n of trace a m o u n t s of ox>gen exist, t h e changing characteristics of p l a n t processes m a k e a continuous measurem e n t desirable. A continuous oxygen analyzer of t h e colorimetric-differential photometer type suitable for p l a n t service h a s been devcloped. Oxygen in t h e sample gas continuously oxidizes a reagent pumped i n a closed cycle, producing a color change proportional t o t h e a m o u n t of oxygen present. T h e oxygen-sensitive reagent, a n alkaline m l u t i o n of sodium anthraquiiione2-snlfonate, is continuoilsly reduced t o a deep red color 1))- passing i t o \ e r a zinc-mercury amalgam. Barrierla>-er photocells i n a direct c u r r e n t bridge circuit nieasiire t h e differential ahsorptivitv between t h e conipletely reduced. red reagent a i d t h e partially oxidized reagent of lighter color. T h e bridge is contini~oiisl>balanced k)?- a motor-driven potentiometer in :I null-balance type servosvstem. Angular position of this potentiometer is telenietered t o a s t a n d a r d IO-mv. recorder. T h e i n s t r u m e n t m a y be employed t o nionitor trace a m o u n t s of ox?-gen i n a n y gas which does n o t rapidly destroy t h e reagent. T h e i n s t r u m e n t is conLained i n a housing designed t o be explosion-resistant in Class I, Group (7, locations. T h e full scale sensitivity of t h e analyzer is adjustable from 0-50 t o 0-500 p.p.m. by volume of oxygen.
I
has been employed in a continuous osygen analyzer suitable for plant use in ranges below 1000 p.p.m. PRINCIPLE OF OPERATION
Operation of the analyzer is based on the quantitative change in color of the reduced alkaline reagent solution when it is brought in contact with oxygen-bearing sample gas. The red, reduced dihydrory form is oxidized by molecular oxygen t o the colorless diketo form. The oxidation reaction is rapid; hoivever, the gas and liquid must be brought intimately in contact under conditions such that all of the oxygen is reacted. The reaction is reversed Iiy the reducing action of zinc amalgam PO that the reagent returns t o the original red, dihydroxy form (.3). Operation of the analyzer is illustrated schematically in Figure 1.
4 i_iLlQUlO
GAS VENT
I
S CARRYISG out rnciny (.hemica1reactions, especially those
involving polymerizations, an accurate knowledge of osygen c~oiic~entration is desired. In cxontinuous plant processes the resiiltx of laboratory control analysis are often obtained long aftm the required operating corrrctions should have been made. -111 instrument which provides continuous information would fac,ilitat,e control of the plant process. Several laboratory methods for the ana lyse^ of trace quantities of oxygen in gases ( 1 , 2, 5-11) were studied i i the ~ basis for design of a continuous instrument. -4chemical eystem first studied by Feiser ( 4 ) and later used by Brady (3) shon-ed the most promise. This system, based on the reversible reactions OH
OH
0 II
t)
___
G A S DISCHARGE_
OVERFLOW
PLIFIER
I
I
GAS INLET C D N fROLLED
FLOW
PY-PAS PUMP
Figure 1. Schematic diagram of oxygen analyzer
Starting a t the top of the regenerator, the reagent passes over the amalgam, becoming completely reduced t o a deep red color. From the bottom of the regenerator, the reagent is accurately metered by a bellows-type positive-displacement pump equipped with check valves. At a flow rate around 30 mi. per minute, the solution passes through the reference cell, where the optical transmittance of the completely reduced reagent is measured by the reference photocell. In the reactor, the reagent comes in contact
234
ANALYTICAL CHEMISTRY
with sample gas accurately metered by a flow controller and becomes lighter in color by an amount proportional to the oxygen content of the sample. The partially oxidized reagent is then
bypass p;mp on the Fegenerator column s e r k the d i a l purpose of assisting in the reduction of the reagent and removing gas which accumulates below the wire screen supporting the amalgam.
I n Figure 3, the instrument is viewed from the left rear, with the explosion-resistant bell removed. The right side of the vertical mounting plate supports the oloctronic components including the amplifier, bridge potentiometers, and source power transformer. On the left side of the mounting plate may be seen the servo motor, photometer, and reactor column. All gas, liquid, and e l & r i d connections to the mounting plate components are mule through the front panel. On the bottom panel are mounted the sample gas flow controller and rotameter, regenerator, bellows pumps, vent gas condensate trap, and associated valves.
Two barrier-layer photocells, bridge c mrvo motor comprise a null-balance tyF amplifier produces power of the correct pk the servo motor to drive the indicator pote net direct current voltage amass the bridg relative angular position of the indicator p< t o the oxygen content of the sample gas. Each potentiometer in the bridge is a tl. The second section of the indicator poten telemeter its angular position to a stmr
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dismantling t,he instrument.
igure 2.
The position of the span ~___.._._. ___._I.____ ..I___" _ _ _ .uiu.tivity of the bridge by determining the ratio of the indicator potentiometer resistance t o that of the remainder of the bridge. Adjustment of the zero potentiometer changes the balance point of the indicator by introducing resistance in one bridge arm and by removing resistance from the other arm. The employment of a differential type photometer minimizes the effects of changes in Source intensity, ten the solution ages. DESCRIPTION O F T H E PLANT INSTRUMENT
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Firnure 2 is shown a froni view d t h e piant instrument. Allelec-
equipped with multithm dials mounted on th; front panel, which is protected by a. removable door not shown in the illustration. A gas flow rotameter is momted on the lower panel, where it is readable from the front of the instrument.
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interfering components and reduced to a pressure of from 8 to 15 pounds per square inch gage before entering the analyzer a t the flow controller. Gas flow rates between 200 and 300 cc. a minute are used. The gas passes through the controller and rotameter and into the reactor column, whore it comes in contact with re
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.
liquid, oxygen-6ee effluent gas Erom the reactor column flan-s through the top portion of the regenerator before being passed t o vent. REAGENTS
Indicator reagent. Dissolve 0.125 f 0.005 gram of sodium anthraquinone-2-snlfanate in 1.5 liters of dist,illed water.
235
V O L U M E 2 8 , NO. 2, F E B R U A R Y 1 9 5 6 After the sulfonate has completely dissolved, add 25 @. of 5% sodium hydroxide and make up to a volume of 2.00 liters rnth distilled water. The reagent deteriorates under the in8ueFce of light, especially direct sunlight, and should be stored in painted (black or aluminum) bot.tles.
’
011
iz
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w
0
3 6 9 P S I DIFFERENTIAL ACROSS LEAK
Figure 5.
through a capillary le& into a flawing oxygen-free gas stream. standard The leak is a Voceo standard capillary giving 7.7 X ce. per second flow. It is manufactured by Vacuum-Electronics Engineering Go., New Hyde Park, Long Island, N. Y. This apparatus is shown schematicdy in Figure 4. The oxygen-free stream is obtained by passing a gas through a scrubber column containing fairly concentrated sodium anthraqu!none-%suIfonate solution which is continuously reduced by being pumped over zinc amalgam. It is oonvenlent to use the process gas one intends to analyze 8.8 a source of calibrating gas during calibration in the plant. Prepurilied nitrogen is suitable for use in laboratory checks of the instrument. Both these gases usually have oxygen contents below 100 p.p.m., which ensxes complete oxygen removal in the scrubber. The maximum allowable eonoentration of oxygen in a suitable calibrating gas depends upon scrubber reagent strength and rate of gas flow. The flowing calibrating gas is next dried in a Drierite tube and reduced to a pressure of 10 pounds per square inch gage by means of a pressure regulator. Compressed air, regulated to various pressures above 10 pounds per square inch gage, is applied to the opposite end of the cqillary leak. The amount of air which bleeds through the leak into the flowing gss will depend upon the prwsure differentin1 across the leak.
12
Capillary leak calibration curve
Zinc amalgam. Stir the required amount of 20- to 30-mesh granular eine with a saturated.solution of mercuric chloride far 4 minutes until the zinc has become uniformly dark gray in calor. Pour off the excess mercuric chloride solution and wash with 15% hydrochloric acid until the amalgam is uniformly bright. Drain off the hydrochloric acid and wash with distilled water until B silver nitrate test indicates complete removal of chlorides.
“,z 20
/
Figui
Calibration of the emillarv leak is accompli$ned by liquid dis-
~~
Figure 6. Analyzer calibration ourve
When the amalgam coating has become exhausted, it may be reactivated with 15% hydrochloric acid as indicated %hove. Scrubber reagent far removal of oxygen from calibrating gas. Make up in same manner as indicator reagent, except for the use of 0.625 gram of sodium anthraquinone-2-sulfonate. CaLIBRATION TECHNIQUE
The analyzer is calibrated with a stream of t,he gas containiilg known conc.entrations of oxygen.
I n order toobtainknown andvariable concentratiansof oxygen, a device was develoued in nhich B, calculated amount af air is bled
inch gage is Gaintained above th; outer surfaci of thebrineby a dead-end service pressure regulator. Compressed air, regulated to various pressures, is applied to the capillary leak, md’the air which bleeds through the leak a t each pressure differential is collected over B period of sevcral hours and mensired in the graduated tube.
A typical leak cdibrstian curve is shown in Figure 5. Unless it becomes clogged with foreign material, a leak will hold its for very long periods. Calibrations have heen reproduced after a leak has been in use for a year. Reproducibility of calibration is within *2%. The gas from the calibrator is passed into the analyzer at a constant flow rate of from 100 to 300 cc. per minute, depending upon the oxygen content of the process stream ‘to he analyzed. The zero point for the analyzer is set with oxygen-free gas obtained as indicated above. The data present in the leak calibrs; tion curve are then used to set the instrument span adjustment to the desired f u l h c d e recorder reading. Far example, to have a 0 to 50 p.p.m. calibration, a differential of 12 pounds per square inch air pressure is plaoed acrosB the capillary and then the elec-
236
ANALYTICAL CHEMISTRY
0~
~~~~~~
__
.~~~~~~~~ .. . -.I/ ...~ ~.
sure differentials across the capillmy leak. A tyoical calibration curve appears in Figure 6.
Calibration of the instrument. may be ehnnged by m y of three variables: gas flaw rate, reagent flow rate, and electrical sensitivity. Each has it8 particular limitittions. The maximum gas flaw rate is dependent upon producing complete reaction of oxygen with the reagent in the reactor column; reagent flow rate is limited by desired response time; and electrical sensitivity is limited by signal to noise ratio. I n Figure 7 is shown a front view of the calibrating device. Gas and air connections are made a t the left side of the ealibrator. The controls for the pressure regulators are above the corresponding pressure gages. Figure 8 is a rear view of the calibrs, tor d h the door removed. The three glass chambers contain, from left to right, Drierite, regenerator, and scrubber solution.
difficulty through acceleration of the formation of rubbery deposits and through interferenoe with normal action of the polymerization catalysts. The main portion of the oxygen contamination was found to OCC.UP in the butadiene charged t o the prace~s and therefore the sample for the oxygen analyzer was taken from the butadiene charge line, Pressure of the butadiene sample was dropped from 100 t o 10 pounds per square inch gage by means of a vaporizer regulator. A trap was installed in the sample line to remove entrained liquids, mainly styrene, and the acidic gases and styrene vapor were removed by an Ascarite sorubber column. In the early operation of the analyzer in this installation, the reagent was found t o deteriorate after about 24 hours of eontinuous operation. This situation, first suspected of being due t o a Diels-Alder oondensrttion of the oxidized reagent and butadiene, was later traced to the presence of oarbon disulfide in the sample. The reaction of carbon disulfide with the sodium hydroxide of the rcagcnt solution results in the formation of sodium earhonnt,e and sodium thiocarbonate. Yellow thiocarbonate interferes with the photometric measurements and also reacts t o liherat,e hydrogen sulfide. A basic Carbitol (diethylene glycol monoethyl ether) scrubber was found t n iemove the carbon disulfide and results in an extension of tho useful life of the reagent of a t least 2 to 3 weeks. LITERATURE CITED
(1) Binder, K., and Weinland, R. F.. Ber. 46, 255 (1913). (2) Binder. K.. and Weinland, R. F.. Ges Whrld 59, 125 (1913). (3) Brady, L. J . . ANAL.CHEX.20, 1033 (1948). (4)Feiser, L. F., J . Am. Chem. Sac. 46, 2639 (1924). (5) nand, P. G. T.,J . Chem. Soc. 40, 1402 (1918). (6) Mohr. F., Z. anal. Che’hem. 12, 138 (1873). (7) R i d 4 S.,and Burgess, W. T.. Analyst 34, 193 (1909). (8) Sham. J . A,. IND.ENG.CHEY.,.ANAL. Eo. 14, 891 (1942). (9) White. H. A,, J . Chem. d l e l . .TIi8birw Soc. S. Af~ica 18, ?9Z (1918). (IO) Winkler, L. W.. Ber. 21, 2843 (1888). (11) Window. E. H., and Liebhafsky. H. :A,. IND. EN“.CHEK, ANAL.Eo. 18, 565 (1946).
Figure 8.
Exposed view of calibration device
R ~ c s i v r ofor review April 4. 1955. .Arabinose and >Fucose” [ANAL.CHEM.,27, 1998 (1955)l wlyxose should be added to the list of sugars mentioned in footnote ’.
L. M. WHITE In the subject index, page 204i, the 10th line under Sugars should read L-fucose instead of L-fructose.
