Identification of mixtures of phenolic antiosidants is usually not possible. However, quantitative analysis can be accomplished with mistures when the components are qualitatively known. This is done by the use of simultaneous equations. I n the c a w of strongly colored alcohol extracts, an aliquot of the extract diluted nithout the addition of coupling reagent is used in the reference cell and the differential absorption curve is obtained.
ACKNOWLEDGMENT
The author gratefully acknowledges the work of D. N.Pregler who carried out much of the esperimental work described herein. LITERATURE CITED
(1) Hilton, C. L., Rubber Age, .V. Y . 84, 263 (1958). ( 2 ) Sawakowski, A. C., ANAL. CHEM. 30,1868 (1958).
( 3 ) Sewell, J. E., U.S.Rubber Co. General Laboratories, I'nssaic, S . J., unpublished pa er, 1952. ( 4 ) Wadelin, I\-,) i l N . I 1 . . C I I E ~ I 28, .
8,
1530(1956).
( 5 ) Zijp, J . W. H., Rubber Chem. and Technol. 30, 705 (1957).
RECEIVEDfor review September 24, 1959. Accepted November 23, 1959. Division of Analytical Chemistry, 136th Meeting, A4CS, Atlantic City, 3. J., September 1959. Contribution No. 186 from the Research and Development Department, E'. S.Rubber Co.
A Continuous Gas Titration Analyzer for Fluorine C. W. WEBER Technical Division, Oak Ridge Gaseous Diffusion Plant, Union Carbide Nuclear Co., Oak Ridge, Tenn.
bA reliable automatic fluorine analyzer has been developed to meet the need for precise continuous monitoring and control of complex gas systems. Based upon pneumatic detection of the reduction in molar flow as fluorine reacts with sulfur dioxide, the analyzer is precise to f1 % fluorine. Excellent on-stream performance is achieved in the presence of uranium hexafluoride, hydrogen fluoride, and oxygen. With proper selection of reagents and reactor conditions, the principles of the instrument could b e applied to the analysis of other gases.
I
chemicals industry, increasing use is being made of elemental fluorine. I n many continuous processes such as the fluorination of uranium tetrafluoride to produce uranium hesafluoride, a need has existed for the control of fluorine in feed and/or effluent streams. Relatively few satisfactory methods exist for the determination of gaseous fluorine. Busch et at. (14) have reviewed the development of methods for this corrosive element and have discussed the problems introduced by interferences. Little success has been reported (14) toward continuous monitoring. The present report describes a continuous automatic fluorine analyzer, developed to meet the requirements of a simple, low cost instrument having a short response time and a precision of about A 1% absolute. The instrument is capable of automatic control of processes. K THE
number of gases with fluorine and uranium hexafluoride and which has been applied to the static titration of some simple mixtures. Gases A and B are transferred from chambers called burets into the reactor, where combination is induced either on or after mising. The volumes of all chambers, or a t least the relative volumes of the vessels, have been predetermined. The titration is followed by plotting the pressure changes in the reactor after each incremental addition of the titrant gas. The method is applicable to most gaseous reactions which bring about a change in molar volumes. Sometimes the absorption of a product is desirable to permit a suitable analytical response-for example : CH,
+ 4Fz
+
CF,
+ 4HF (adsorbed on NaF)
The adaptation of the gas titration principles to a continuous analyzer, shown by Figure 2, involved the continuous introduction of the reactants and removal of the products, the selection of proper reaction conditions, and selection of the appropriate means of detecting the molar flow changes which are indicative of the concentration of the unknown. SULFUR DIOXIDE TITRANT FOR FLUORINE
PRINCIPLES OF GAS TITRATION
The chemical reaction selected for the analysis of fluorine results in the addition compound, sulfuryl fluoride, was originally prepared by Moisan (7) in 1901. Little practical use has been made of this reaction,
Figure 1 represents a basic system with which Barr ( I ) , Katz and Barr ( 5 ) , and the author in recent years have examined the molar reaction ratios of a
but during this study a number of advantages in the use of sulfur dioxide as
a selective titrant for fluorine became apparent. The reaction is essentially complete, absorption of product gases is not necessary, there is one mole change for each mole of fluorine, the reaction can be controlled to rule out interferences, the sulfuryl fluoride product gas is inert, sulfur dioxide is readily available and inespensive, and methods other than pressure measurement could be applied for delicate detection-dielectric constant (16) or infrared (8)-both of which would be specific and sensitive for sulfuryl fluoride detection. The change in molar flow is apparent in the reaction above; the greater the fluorine content of the sample gas, the greater the flow change. FLOW CONTROL SYSTEM
i3asic to the development of the fluorine analyzer was the design of a suitable flow control system, capable of control to a t least + 1% and free from corrosion by the highly reactive gases encountered. The complete system applied t o the continuous determination of elemental fluorine is illustrated in Figure 3. A constant standard flow was provided by controlling the pressure drop across a metering capillary and controlling the absolute pressure in the metering zone. The flow system for sulfur dioxide was identical to that for the fluorine-containing sample gas. The pressure control valves (Hoke, Inc., Part 1148) and the pressure transmitters (18) (Taylor, Models 206-RA and 206-RD for absolute and differential pressures, respectively) were the building blocks selected for automatic control. I n combination, as seen in the sample flow control system, they form simple pneumatic feedback systems; the response is transmitted (usually amplified) to a control valve which in turn affects the response. By converting the sample or reagent gas pressure signals to air pressure signals, the transmitters served also to isolate VOt. 32, NO. 3, MARCH 1960
387
- II
-0
REACTOR
PRESS"RE OAGL
Figure 1. Basic gas titrator for static samples
GAS 'A"
PRESSURE INDICATOR RECORDER
GAS "E"
OR
EVACUATION VALVE
P V ' nRT APaAn
U
Figure 2. Basic scheme of continuous gas titration type analyzer
REACTOR
Fp SAMPLE
3
0-100 mm Hg
*-METERING CAPILLARY PCV- PRESSURE CONTROL VALVE APT- ABSOLUTE PRESSURE TRANSMITTER D P T - D I F F E R E N T I A L PR,ESSURE TRANSMITTER CONTROL-AIR PRESSURE PRESSURE GAGE
A I R FROM TRANSMITTER CONTROLLED A I R PRESSURE
8
N E C T I N G SYSTEM V A L V E BODY
Figure 4.
Hoke pressure regulating control valve
(PCV)
ti S O z FLOW CONTROL S Y S T E M
Figure 3.
VACUUM PUMP&
Automatic continuous fluorine analyzer
the corrosive gases from the diaphragm control systems. As indicated in Figure 3, the controlled streams of sample and sulfur dioxide are mixed and fed to a reactor in which the analytical reaction occurs. The resultant product gas flow, which varies with fluorine concentration, is reflected in the fore pressure signal changes a t the exit constriction. I n the case illustrated in Figure 3, the pressure probe supplied a choice of transmitters to provide a recorded signal. The pressure control valves, manufactured by Hoke, Inc., are basically illustrated in Figure 4. One side of the diaphragm is supplied with a selected air pressure while the transmitter signal, applied to the opposite side, brings about varying degrees of unbalance which open and close the valve seat. The resulting flow changes are sensed by the transmitters so that the mutually dependent components provide a fixed condition, controllable by selection of the transmitter range and selection of applied air pressure. The entire system, with the exception of the reactor, was thermally insulated and heated a t 71" f 2" C. to prevent condensation of uranium hexafluoride, one of the principal impurities encountered. The reactor (see Reactor Conditions) was individually insulated and heated a t 200" f 1" C. To obtain reproducible test data it was necessary to treat the systems with sample gas diluted with nitrogen prior to full exposure. FLOW EQUATIONS AND CONSTRICTIONS
The following gas flow equations illus388
ANALYTICAL CHEMISTRY
trate the variables which required consideration in the selection of appropriate constrictions. For the metering capillary:
*'
'=
I'(
4- ");
KPT
Hagen-Poiseuille law (1O), simplified.
For the exit constriction: Q =
Q
L; orifice (critical) ( 1 7 ) ~
KWMT
=
simplified flow equation.
K 7-P f 2 . capillary, Hagen-Poiseuille
law further simplified, assuming PZ = 0 where
Q
=
PI Pz P,
= upstream pressure = downstream pressure = fore pressure on exit constric-
standard volumetric flow
tion AP = pressure differential q = viscosity M = molecular weight T = absolute temperature K , K', and K U = constriction stants
con-
The exit capillary equation was derived from the Hagen-Poiseuille law, assuming a negligible downstream pressure in the applied design. The constriction equations were applied assuming constant temperature conditions. I n the case of the metering capillary, if the pressure drop AI' is small compared with the absolute pressure which is maintained constant, the flow is es-
sentially linear with AP. The resultant flow is affected by viscosity, but for mixtures of interest this was not a serious problem, as will be shown later. The exit constriction fore pressure, P f , which is the analytical output of the instrument, was of special importance. With mixtures containing uranium hexafluoride with a molecular weight of 352, a capillary free of molecular weight effects was the better choice for minimizing errors in fluorine analysis. A finely tapered needle valve was first selected to permit adjustment in the capillary-type constriction. With wide valve openings, however, its characteristics were more like those of an orifice and it was replaced with a fixed capillary as the exit constriction, consisting of 140 inches of 0.051-inch (nominal inner diameter) nickel tubing installed in the form of a coil. The greatest concern, from the standpoint of interference by physical properties, was uranium hexafluoride. Although the molecular weight of uranium hexafluoride (and its square root which enters into the orifice equation) is considerably higher than the other gases, its viscosity is very close to those of the other constituents (Table I). Thus, a capillary exit constriction, affected by viscosity, was more desirable than an orifice for monitoring the molar flow changes affected by the fluorine content of the sample stream. The viscosity of sulfuryl fluoride was not established, but it appeared to be sufficiently close to that of sulfur dioxide, so that the exit gas viscosity was affected to a negligible extent. REACTOR CONDITIONS
The addition reaction producing sulfuryl fluoride occurs to a negligible ex-
90
50 S T D C C / M I N N 2
v)
L
g701
1
Y
t x
-
Y
6
0
"
Figure 5.
' " 50
100 I50 200 R E A C T O R TEMPERATURE I'C.1
Reactor temperature
VJ.
tent a t room temperature, but it can be induced under controlled conditions to eliminate chemical interference from the other gases. A number of induction methods were examined, including glowing filaments, electric spark, spectral radiation, and surface catalysts. The best system tested for inducing the reaction was a heated metal tube and a 3/8inch (outer diameter) Monel reactor, 28 inches long, was selected. The reactor was installed in the form of a double coil wrapped with appropriate Nichrome heaters and thermally insulated. Figure 5 illustrates the manner in which the most favorable reaction temperature was selected. The minimum in the curve defines the temperature a t which the reaction is substantially complete. The curve after this point parallels that of an inert gas combination and is rising because of temperature effects on the monitoring system. These data were collected with sample and reagent flows of about 50 standard cc. per minute. With 25 standard cc. per minute each and operating the reactor a t 200" j= 1O C., the residence time in the reactor was 2.5 to 5.0 seconds depending on the fluorine concentration. With samples covering the concentration range from 0.1 to 91% fluorine, mass spectrometer analysis of the exit gases indicated that the addition reaction with sulfur dioxide is essentially complete. The calibration curve in Figure 6 shows the exit capillary fore pressure responses obtained over the full range of fluorine concentration in nitrogen. The calibration was dependent on the iodometric analysis (IS) of bulb samples of the fluorine supply and synthesized mixtures. This curve agrees fairly well with that calculable from the exit capillary equation. EFFECT OF FLOW RATIOS
Just as in solution titrations where sensitivity is increased by using a larger sample or by diluting the titrant, continuous gas titrations are capable of similar manipulations. For example, in certain field applications where the range of concentration was limited, the sensitivity of the pneumatic monitoring
250
response
55
20
0
40
60
80
I00
% FLUORINE IN NITROGEN
Figure 6. Fore pressure response vs. per cent FZ (reactor temperature 200' C.)
and Table I. Viscosity and Molecular Weight Data for Gases of Interest
,/MX Wt. 6.16 0 2 5.65 HF 4.47 N2 5.29 UFs 18.76
Gas Fz
Viscosity, 7 (Poise) x 108 Reference 197( 0" C.) (4, 16) lS9( 0" C.) (9) 123 (25" C.) (11) 166 ( 0°C.) (9) 170(17" C.)
(12)
could be improved by changing the sample to reagent flow ratio. Figure 7 illustrates the types of curves obtained with different ratios; these are calibration curves applied in the analysis of fluorination tower exit gases and their shapes are not in complete agreement with theoretical curves. A more reliable analysis was obtained by comparison with a calibrat,ion curve established for a particular gas system, as unknown factors in the matrix gases could not be treated adequately on a theoretical basis. Analytical errors due to fluctuations in the matrix constituents, however, were a t least partially eliminated in this instrument as explained below. INTERNAL COMPENSATION FOR CHANGES IN SAMPLE VISCOSITY AND SYSTEM TEMPERATURE
The balance of gas flows through the analyzer, assuming specific detection and complete reaction of fluorine, may be expressed as: &exit
=
&sample
+
&reagent
-
&fluorine
where Q = standard molar flow of gas. The pertinent capillary flow equations for the exit and metering constrictions are :
By substitution, the flow balance equation becomes :
or
-1
For the first term within the brackets, temperature changes and fluctuations in sample viscosity, due to matrix changes or temperature effects, essentially cancel out. Errors due to contribution by the second term within the brackets can be made small, as under conditions of optimum sensitivity the excess reagent flow would be reduced. Through the above consideration of temperature effects, it was not necessary to provide refined thermal control of the system. Small blowers adequately kept the constrictions a t approximately the same temperature. By the same reasoning as above, it can be shown t,hat the alternate use of orifices in both the metering system and the exit constriction would have partially compensated for fluctuations in average molecular weight of the sample gas. POSSIBLE CHEMICAL INTERFERENCES
Among the constituents in many of the gas streams on which the continuous analyzer was evaluated, uranium hexafluoride, hydrogen fluoride, and oxygen may be expected to be the most reactive. However, neither response tests, mass spectrometer analysis of the exit gases, VOL. 32, NO. 3, MARCH 1960
389
w
72 5
70 0
-
Fp SAMPLE
Ln
-
I/
67.5
E
a W
$ 65.0 W E
n
w
2
62.5
Figure 8. 60.0
57.5
0
IO
20
30
40
50
60
Y. CLV'SRINE
Figure 7. Effect of flow ratios tower exit gases
on response to fluorination
nor examination of the reactor and constriction after exposures indicated reaction between these gases and the sulfur dioxide titrant a t the reactor conditions chosen. These conditions permitted a residence time in the reactor of 2.5 to 5.0 seconds a t 200" C. and pressures below 150 mm. of mercury (absolute). The reaction: UFs (g)
+ SO2 ( 9 )
4
UF4 (s)
+ SOzFz (9)
would have given a direct error in fluorine analysis. Nevertheless, no reaction was noted in gases containing a t least 58 mole % uranium hexafluoride. Tevebaugh et al. (19) have reported no reaction at temperatures as high as 150" C . Infrared studies by Maybury (6) have shown no significant reaction between hydrogen fluoride and sulfur dioxide; no interference was found in the fluorine analyzer when the gas stream contained hydrogen fluoride a t least as high as 15 mole %. The possibility of oxygen interference was examined with gas systems containing up to 20% concentration; no reaction was noted with the sulfur dioxide titrant under the reactor conditions. The formation of sulfur trioxide (8,s) to any appreciable extent requires temperatures considerably above 200" C. and probably the use of a contact catalyst. DISPOSAL SYSTEM
The exit gases from the analyzer were removed and disposed of with chemical traps and a mechanical vacuum pump. Of the exit gases, the excess sulfur dioxide proved to be the most dificult to remove. For long trap life and pump pro390
ANALYTICAL CHEMISTRY
tection, 20 pounds of activated alumina were used in the discharge trap. This quantity provided a trap life of about one month under operating conditions where i t was possible t o reduce the sulfur dioxide excess to about 6 standard cc. per minute. The trap was also satisfactory for the system uranium hexafluoride and hydrogen fluoride contents. The vacuum pump (capacity 33 liters per minute) maintained a pressure downstream of the exit constriction of about 3 mm. of mercury absolute with total incoming flows to the analyzer of 48 standard cc. per minute. The vacuum ballast provided by the trap system, valved off from the pump, allon-ed sufficient time for changing the pump or the oil so that the analyzer response was negligibly affected during the change. However, the incident rate for pump failures became negligible when the sulfur dioxide excess was reduced to 6 standard cc. per minute. PRECISION AND LOWER LIMIT OF MEASUREMENT OF THE AUTOMATIC CONTINUOUS FLUORINE ANALYZER
Because no satisfactory analytical method exists for use in determining precision by comparative evaluation, only crude estimates of the precision could be obtained this way. The calibration curves had been determined by iodometric analysis of bulb samples of the complex gas mixtures. For the middle curve of Figure 7, about 70 representative samples were analyzed in the range from 5 to 6001, fluoride. The calibration points matched the curve within =t2.5% fluorine per point a t the 95% confidence level, Most of the scatter can be attributed to the limitations of
Automatic fluorine analyzer-controller
bulb sampling and the iodomctric method of fluorine determination. From the small deviation noted on recorded measurements over many hours of stable operation with gas streams known to be essentially constant in fluorine content, the precision of measurement with the analyzer is estimated to be about 5 1% fluorine. The practical lower limit of measurement is set by the precision a t 1%. As the chemical reaction with sulfur dioxide is essentially complete at even lower concentrations, it is expected that use of pneumatic components capable of more precise measurement and control. or use of other detector systems such as infrared. would reduce the lower limit. AUTOMATIC FLUORINE ANALYZERCONTROLLER
A simplified block diagram of the fluorine analyzer-controller, designed by the Engineering Division of Oak Ridge Gaseous Diffusion Plant, is presented in Figure 8. At each of the three gaseous diffusion plants, several of these instruments are controlling t'he fluorination towers used in the conversion of powdered uranium tetrafluoride to uranium hexafluoride. Representing a simplification over the previously described research instrument, in which standard Cow control KLS beneficial, this analyzercontroller has a floating pressure in the flow systems and the reactor. Although the reactor pressure tends to vary with fluorine concentration, the response is fed back to the process powder feed t o maintain the level of fluorine excess. Thus, the omission of standard flow control was not a problem. In one of the ahernate monitoring systems, a differential pressure transmitter was used to give an increased response as fluorine concentration increased. A constant air pressure &-as applied to the high pressure side of the transmitter, so that as fluorine increased, giving a lower exit fore pressure, the AP increased. The simplicity of the basic system u-hich used the absolute pressure monitor was more acceptable and it was adopt,ed for the fluorination tower con-
trolkrs. Instrument lag is about 15 aeconds or less. These analyzers have performed well over the past two years, providing onstream efficiencies of 95 to 100% under !l.ighly corrosive conditions. Accuracy snd precision are each =tz 170 absolute up 60 100% fluorine. The principles of continuous gas titrat,inn are widely appiicable to many -.naiytical systems v h r r e changes in uolar flow occur. ::,;curacy is depend.:it on t,he specificity of the reaction; %us the titrant and reactor conditions must be selected for the particular probi-m. Kven minor differences in chem.(:ai rcactiviiy might be exploited to ;:rovidc a suitable analysis of binary gas nixturcjs. Detect’ion systems, such as mal conductivitj., infrared absorp, and others are app!icab!e with con.~-inuousgas .iit’ra:icn onalyzers and ir.ouid not require-. changes in moiar 5ow. ACKNOWLEDGMENT
The author thanks R. C. Ritter of the Enginc.cring Division, Oak Ridge Gase-
oils Diffusion Plaiit. f2r r,tBrinission C L mention the fluorination tower analyzer-controller, not ypt described in the literature. LITERATURE CITED
T.,
“Gas Titrations. Halogenation ot Gaseous Hydrocarbons,” K-889,K-25 Plant, ‘:arbide and Carbon Chemical Co., April 14, 1952. ( 2 ) Ephraim, F., “Inorganic Chemistry,“ p. 577, Gurney & Jackson, London, 1948. ( 3 ) Hijdebrand, J., “Principles of Chemist,ry, pp. 168, 239, Macmillan, New York, 1947. (4) Kanda, E., Bull. Chem. SOC.Japan 12, 463 (1937). (5) Katz, S., Sarr, Z. T., ANAL.C m x . 25, 619 (1953). (6) Maybury, It. El., others, J. Chem. Phys. 23, 1277 (1955). (7) Moisan, H., Lebeau, P., Compt. rend. 132,371 (1901). (5) Perkins, W, D., Wilson, M. K., J. Chem. Phys. 20, 1791 (1952). (9) Perry, J. TI., Ed., “Chemical Engineer’s Handbook,” 3rd ed., p. 370, ?rlcGraw-Rill, New York, 1950. (10) Ibid., p. 387. (11) Posey, J. C., “Viscosity of Gaseous ..inhydrous Hydrogen Fluoride,” K-
(1) Barf, J.
!063. I
