V O L U M E 26, NO. 8, A U G U S T 1 9 5 4 is neasured. The precision of the results are shown in Table 11. Th:: standard deviation of the electrode potential a t the end point is 00033 volt, which is equivalent to 0.00018 ml. of 0.1N ceric solution. The average end-point potential is 0.068 volt higher than the theoretical end-point potential and represents an error of 0.0015 ml. Bimetallic Electrodes. It is easy to substitute various reference electrodes for the calomel electrode because the potential of the reference electrode does not have to be known for use with the automatic titrator. Five 10-ml. aliquots of 0 . l N ferrous solution are titrated with standard ceric solution. The reproducibility of the end-point potential with a platinum indicator electrode and a platinum-lO% rhodium reference electrode connected to the input of the automatic titrator is given in Table I1 The end-point potential of the platinum electrode is measured with reference to a saturated calomel electrode. The standard deviation of the electrode potential a t the end point is 0.0034 volt, which is equivalent to 0.00019 ml. of 0.1,V ceric solution. The average end-point potential is 0.112 volt higher than the potential a t the end point determined by the classical method and represents a difference of 0.004 ml. Flow Rate Studies. In order to test the effect of the delivery rate of titrant on the precision and accuracy of the resultk, standard ceric solution was added to ferrous solutions a t five different flow rates varying from 0 4 to 2.8 ml. per minute. The reproducibility of the end-point potential a t all of the rates is about the same. There is, however, a rise in end-point potential with increase of flow rate. At the fastest flow rate studied, 2.8 ml. per minute, the end point is overshot by less than 0.02 ml. of 0.111’ ceric solution. These results indicate that for certain potentiometric titrations
1351 delivery rates of about 3 ml. per minute are possible without introducing significant error. Procedure for Performance of Titrations. The stirrer, a small, motor-driven glass propeller, is placed in a 150-ml. beaker, slightly offset from center and approximately one half inch from the bottom. The right-angled delivery tip of the buret is placed immediately above the ropeller and about 0.1 inch from the shaft of the stirrer, so t f a t the delivery flow is directed at the shaft. The reference and indicator electrodes are placed on the opposite side of the titration vessel about 0 . 2 5 inch from the bottom and close but not touching the sides of the titration vessel. The complete assembly is mounted so that the bottom of the titration vessel is above the table surface, thus allowing the vessel to be replaced without disturbing the electrode, buret, and stirrer assembly. The solution to be titrated is diluted with distilled water to approximately 50 ml., which allows sufficient volume to completely cover the electrodes, thus eliminating appreciable electrical noise during the titration. The stirrer is rotated as rapidly aa possible without introducing too much air into the solution. It is advisable to ground the stirrer and buret motor to prevent pickup of electrical noise a t the electrodes. The start button of the automatic titrator is pushed; completion of the titration is automatically determined. LITER4TURE CITED
(1) Beckman Instruments, Inc.. Pasadena, Calif., Bull. 239A (1951). (2) Blaedel, IT. J., and llalmstadt, H. 1‘ , ; ~ N Z L . CHEU..24, 450 (1 Q.52) (8) Ihid., p. 455. \ - - - - I
(4) Kolthoff, I. XI., and Furman, N. H., “Potentiometric Titrations.” 2nd ed., Yew York, John Wiley & Sons, 1931. ( 5 ) Lingane, J. J.. ANAL.CHEM..20, 285, 797 (1948). ( 6 ) Lingane, J. J., “Electroanalytical Chemistry,” New York, Interscience Publishers, 1953. (7) Precision Scientific Co., Chicago, Ill., Bull. 640A (1949). (8) Robinson, H. A., Trans. Electrochem. Soc., 92, 445 (1947). RECEIVED for review March 1, 1054. .4ccepted May 1.5, 1954
Automatic Recorder for Continuous Determination of Oxygen in Gases Using the Dropping Mercury Electrode TRESCOTT B. LARCHAR, SR., and MICHAEL CZUHA, JR. Government Laboratories, University o f Akron, Akron,
A constant control is needed to show the presence of oxygen in the head gas of reactors used for synthetic rubber production. By means of a continuous automatic analyzing system, the oxygen content of the exit gas from a nitrogen purification unit as well as in the hydrocarbon feed used in preparing synthetic rubber was placed’on a more reliable basis. This eliminates the error found in periodic sampling procedures and fulfills the need for more accurate and additional analyses of gases used in commercial systems where the presence of oxygen is detrimental.
0
S Y G E S plays an important role in the polymerization reac-
tions of olefins and diolefins. Small traces of this gas have been found to act as both polymerization inhibitors and activators, depending on the polymerization reactions involved. It is the general practice a t the Government Laboratories to reduce this variable to a minimum by purging polymerization e q u i p ment with butadiene or oxygen-free nitrogen prior to initiation. Because it is believed that a polarographic method is superior t o the classical manometric and Winkler methods for determining small quantities of oxygen, a potentiometric method ( 5 ) for the determination of oxygen in gases was developed a t the Government Laboratories and wed extensively in the analysis of gases
Ohio for purging polymerization units and laboratory assemblies (1). An automatic recording instrument was desirable in recording instantaneous changes in the oxygen content of the head gas in a 500-gallon reactor during purging and charging procedures. It was also desirable to record trawient phenomena in the stream from a nitrogen purification unit (6). Such recordings were not obtainable with the use of the manual apparatus. The design and operation of an electronic unit which automatically records the low concentration of oxygen in gases are described. THEORY
Molecular oxygen is irreversibly reduced a t the dropping mercury electrode a t applied potentials near zero (2, 3 ) . The diffusion current is reached a t approximately -0.3 volt us. the saturated calomel electrode. In neutral or alkaline solutions, the reaction proceeds according to the following equation: 02
+ 2H20 + 2e-
+
+ 20H-
H202
(1)
At potentials more negative than -0.5 volt, the second reduction step occurs:
HS02
+ 2e- +.20H-
(2)
The polarographic behavior of dissolved oxygen has been the basis of several methods for the determination of oxygen in gases
ANALYTICAL CHEMISTRY
1352 and water solutions ( 7 , 9). Generally, the diffusion current a t a constant applied potential is used as a measure of the amount of oxygen. A dilute aqueous solution, saturated with air and with containing approximately 0.1N electrolyte, is 2.5 X respect to oxygen, corresponding to 8 p.p.m., and gives rise to a diffusion current on the order of 4 pa. with an electrode of usual characteristics (4). The limit of sensitivity of the polarographic technique is usually around pa.; but because a t a concentration of oxygen below lyo,by volume, the residual current is in the same order of magnitude, the ordinary polarographic method becomes inaccurate for concentrations of oxygen less than 1%. However, a null-current measurement of the potential of the dropping mercury electrode shows considerable response to small changes in oxygen concentration.
T
10 CM.
BR ISTOL
REC 0 R D I N G POTE NT I 0 METER
j l OXYGEN
BECKMAN MODEL RX pH AMPLIFIER
I &,
Figure 1.
Automatic Recording Oxygen Analyzer
Figure 2.
Oxygen Cell
potassium chloride), are assigned the final expression becomes
( E - E,,,
) = 1.i2 X 10‘ Ct”2
where the potential shift, AE = E - Emax, is measured in nlillivolts, the concentration, C, in millimoles per liter, and the drop time, t , in seconds. Equation 4 relates the potential of the system with the oxygen dissolved in the electrolyte. To determine the actual oxygen content of any nonreducible gas, i t is necessary only to bring the electrolyte into equilibrium I=,ith the gas before measuring the potential. The expression then takes the form: Percentage of oxygen in gas =
The theoretical aspects of the method have been reported by Laitinen, el al. ( 6 ) . The method is based on measurement of the equilibrium potential between mercury drops forming in an electrolyte medium containing molecular oxygen and a reference electrode of constant electromotive force. Consideration of the characteristics of the dropping mercury electrode and transfer of charge during the electrolytic reduction of oxygen has led to the following expression:
where E is the potential of the dropping mercury electrode at equilibrium, E,,,. is the potential corresponding to the electrocapillary maximum (zero oxygen), nF is the number of coulombs of electricity involved per mole, C is the concentration of oxygen in millimoles per liter, D is the diffusion coefficient in square centimeters per second, t is the time in seconds, and k is the capacity in microfarads per square centimeter of the double layer on the positive side of the electrocapillary maximum. If values of n = 2, D = 2.6 X and k = 42.2 (0.1N
(4)
AE
__ loa
22.4
x
1.72
x
104t1/2
(5)
ahere 01 is the adsorption coefficient of oxygen in. the electrolyte expredsed in cubic centimeters per liter, and the total gas pressure is 1 atmosphere. i l t 25” C., 01 = 28.3; using a drop time of t = 4 seconds, the equation reduces to: Percentage of oxygen
=
2.30 X 10-3AE
(6)
(The value of 28.3 applies only to oxygen dissolved in water. In 0.1N potassium chloride, a loner value would be expected.) .4n oxygen content of 0.1% corresponds to a potential shift of about 40 mv., reproducible within 2 mv. From 0 to 1% of oxygen the method is accurate to nithin 0.01% (absolute). In actual practice there is a slight deviation from linearity a t concentrations near 1%of oxygen. Therefore, a calibration curve of A E versus percentage of oxygen may be employed for routine analyses. The question may arise as to the possible interference due to the anodic current-voltage curve of mercaptans. I t is the belief of the authors that the vapor pressure of Sulfole B-8 is too low for traces of mercaptan to be detected in the reactor head gas.
V O L U M E 26, NO. 8, A U G U S T 1 9 5 4
1353
APP4R4TL S
Thp oxygen analyzer (Figure 1) consists of a Bristol recording potentiometer, operating on the output of a Beckman Model RX pH amplifier in which the glass-calomel electrode system is replaced by the oxygen cell. The polarographic oxygen cell hato fulfill the following requirements (I, 5 ) :
140 -
I20
-
IO0
-
80
-
The potential of the cell is recorded on a Bristol recording potentiometer. A 12-microfarad oil-filled condenser is connected in parallel with the cell terminals on the Beckman RX pH amplifier to reduce fluctuation of voltage awociated with the grou th of merrui y drops.
v)
z 0
z 2
E,,,, and is approximately -0.500 volt us. the silver-silver chloride electrode. As oxygen is introduced into the cell, the potential shifts to less negative values; the magnitude of the shift, E - E,,,, is proportional to the amount of oxygen present. .4 shift in the potential of the electrocapillary maximum may be expected in the presence of foreign electrolytes introduced by the gas stream, the water used to make up the supporting electrolyte, or the mercury for the dropping electrode. HoMever, attainment of the theoretical value is not a requisite in the method, since the determination constitutes a differential nieasurement of potentials, and the system is calibrated accordingly.
0
z-
2
I-
o W
TABLE
0 047 % s IO DIV 0206% 139 " 0.510 % :83 '' I.OE8 *A =I36 "
J LL D W
a
60 -
I00
90-
W 0 L
0 0 w E
v)
0 2
s 0
s 2-
2 I-
OO
-
02
04
06
08
10
W o
A
80
-
70-
60 60
LL
0 W
O X Y G E N CONTENT, % BY VOLUME,
a
Figure 3.
Calibration of Oxygen Analyzer
W D 0 a W 0
40
-
30-
K
Arrangements have to be made to bring the electrolyte rapidly in equilibrium, with respect to its oxygen content, with the sample gas. The dropping electrode has to be shielded from any turbulence caused by flow of gas through the solution. The reference electrode has to have constant reversible potential and possess relatively low resistance. It has to be placed EO that the reducible products, if any, of the electrode cannot affect the dropping electrode. The mercury from the dropping electrode has to be removed in a manner to minimize contact with the electrolyte. Any mercurous chloride formed through interaction of the mercury with the electrolyte might affect the dropping electrode potential. To permit continuous operation, i t is advisable to construct the cell in such a manner as to permit cautious partial replacement of the electrolyte. This prevents accumulation of contaminants over a long period of time. The electrolytic cell (Figure 2), designed to provide rapid equilibration of the supporting electrolyte with the gas being tested, consists of a fritted-glass disk, A , within a cylindrical tube, through which the gas bubbles into the electrolyte. The dropping mercury electrode, B, is suspended directly above the disk and is shielded from turbulent flow by a small funnel, C, which also directs the mercury drops into an exit tube and trap, D Fresh mercury from reservoir E is supplied to the electrode capillary through polyethylene tubing. The mercury height (approximately 60 em.) is adjusted to give a constant drop time of 4 seconds. The electrolyte ( 0 . 1 S potassium chloride) is automatically maintained a t a constant head of 12 cm., and is continuously admitted to the cell a t approximately 1 ml. per minute through the capillary a t F . A41-liter bulb, G, acts as a reservoir for the electrolyte, which is maintained air-free by nitrogen bubbling through the fritted-glass disk, H . A cotton plug, I , prevents air from diffusing into the system. A silver-silver chloride reference electrode, J , completes the cell. Electrical connections are made a t K and L, the dropping electrode serving as cathode. The electrolyte is alternately equilibrated with purified nitrogen and the sample gas by means of a three-way stopcock, M . The potential obtained u i t h pure nitrogen bubbling through the cell corresponds to the electrocapillary maximum potential,
EO10-
Figure 4.
Oxygen Content of Nitrogen
The amplifier is operated with the right-hand switch in the S o . 3 position (see Figure 1). Controls 1, 2, and 3 are used as coarse and fine adjustments of the zero position. The recorder has provisions for various ranges; generally the 0- to 25-mv. range is used and covers the range 0 to 0.5% oxygen. For greater sensitivity the 0 to 25-mv. range may be spanned in increments of 5 mv., corresponding to approximately 0.1% oxygen for full scale deflection a t maximum sensitivity. To record concentrations above 0.5y0the 0- to 50-mv. range position is used. Because of the limiting sensitivity of the oxygen cell, i t is not practical to record concentrations above 1% okygen by volume. PROCEDURE
K i t h prepurified nitrogen bubbling through the cell and the recorder set on the 0- to 50-mv. range, adjust controls 1, 2, and 3 on the amplifier until the recorder pen is on scale near the left side of the chart. Turn the selector switch on the recorder to the appropriate range for standardization and allow it to record for 5 minutes to establish the reference line. Standardize the instrument by bubbling a known mixture of nitrogen and oxygen through the cell. If the recorder deflection does not coincide with the calibration curve, recalibrate the cell, using additional known mixtures of nitrogen and oxygen. RESULTS A N D DISCUSSION
Figure 3 shows a calibration curve for the oxygen analyzer using four standard mixtures of nitrogen and oxygen-0.047,
1354
ANALYTICAL CHEMISTRY
0.206, 0.510, and 1.028% oxygen by volume. Airco prepurified nitrogen, containing 0.001% oxygen, was used as the zero reference. Figure 4 shows a sequence of gases of increasing oxygen contents. The instrument responds immediately to a change in oxygen concentration and attains equilibrium within approximately 10 minutes. Figure 5 shows the reproducibility of the zero setting by alternately admitting the reference gas and the nitrogen-oxygen mixtures. The 0- to 50-niv. range was used in recording the 1.02870 level. Figure 6 shows the changes in the oxygen level of the head gas in a 500-gallon reactor during purging and charging procedures.
lo0l
!r
column. The analysis was made under conditions of high consumption near the depletion of activity of the solution in the column. The highest rate of nitrogen consumption in the pilot plant occurs when pressure tests are being made on the 500gallon reactors. The instrument was zeroed a t A in Figure 7 and then switched to the column gas at B. At low consumptions
mt
FIGURE 6
0 510%
90
eo -
ln
z
I I
0 2 2
I
70-
0
60-
z-
0
I-
W o i W
U
a
so-
-
8 cc
40-
0 w
rr
a
a
0
W
a 0 W u a
40
c
E
rr
I
- 1
0-1
HOUR-DI
Figure 5.
I
e
I
3
I 4
Oxygen Content of Nitrogen 0-1
The instrument was zeroed (-2 in Figure 6 ) a t the second chart division with prepurified nitrogen bubbling through the cell and the selector switch on the 0- to 50-mv. range. The reactor was evacuated and then pressured with 15 pounds of butadiene. .it this point, R, the concentration of oxygen was approximately l.OY0 by volume. The reactor was again evacuated and another 15 pounds of butadiene added The drop in oxygen content is shown along BC in Figure 6. The selector switch was turned t o the 0- to 25-mv. range a t C , doubling the sensitivity The curve along D represents the lag in sampling and equilibration in the oxygen cell. The concentration of oxygen a t this point was 0.03%. At E the soap solution was charged. The oxygen content rose sharply t o an apparent concentration of 0.25%, F , but immediately dropped to a level slightly higher than 0.0370, G. Apparently, a portion of air from the soap charging line had reached the gas sampling line before i t was mixed with the head gas. Sulfole B-8 was then charged and an appreciable quantitv of air was admitted, raising the oxygen content to O . l % , H. Styrene was then charged and more air was admitted raising thp oxygen level t o O.lS%, I. The reactor was purged twice with purified nitrogen by pressuring to 30 pounds per square inch and venting to 2 t o 3 pounds per square inch. Point J (0.04% oxygen) represents the dilution of the oxygen in the reactor by the first nitrogen purge. The second nitrogen purge, K , reduced the oxygen content to 0.02%. The zero point of the instrument was checked at L using prepurified nitrogen. The butadiene was then charged, M , and resulted in further dilution of the oxygen to approximately 0.01% at N . The oxygen content remained a t this level during the completion of the charge. Figure 7 shows the continuous analysis of the nitrogen stream (8)from a plant purification unit designed to remove oxygen from
regular grade nitrogen containing approximately 0.1 to 0.275 oxygen. The oxygen is removed by countercurrent scrubbing of the gas with a solution of sodium hyposulfite in a 16-foot packed
H O l l R W I
2
3
Figure 6. Oxygen Content of Head Gas in 500Gallon Reactor Figure 7. Oxygen Content of Nitrogen from Purification Unit
of the nitrogen, the oxygen content is maintained a t 0.01 to 0.02%, BC. Points D and E (0.09 arnd 0.21% oxygen, respectively) represent surges in the oxygen level during pressure tmta on the 500-gallon reactors. The zero check at F showed no drift in the reference point of the instrument. LITERATURE CITED
(1) Czuha, M., Office of Synthetic Rubber, Reconstruction Finance Corp., private communication. (2) Heyrovsky, J., Trans. Faraday Soc., 19, 785 (1924). (3) Kolthoff, I. &I., and Lingane, J. J., “Polarography,” New York, Interscience Publishers, 1941. (4) Kolthoff, I. M., and 3Iiller, C. S.,J . Am. Chem. Soc., 63, 1013 (1941). (5) Laitinen, H. d.,Higuchi, T., and Czuha, M., Ibid., 70, 561-6 (1948). ( G ) Lang, W. C., Office of Synthetic Rubber, Reconstruction Finance Corp., private communication. (7) Moore, E. W.,Morris, J. C., arid Okun, D. A,, Sewage Works J . , 20, 1041-53 (1948). (8) Reich, M. H., and Kapenakas, H. J., Office of Synthetic Rubber, Reconstruction Finance Corp., private communication. (9) Wise, W. S.,Chemistry &. Industry, 1948, 37-8. RECEIVED for review February 23, 1961. Accepted h l a y 26, 1964. Work performed as a part of the research project sponsored by the Reconstruotion Finance Corp., Office of Synthetic Rubber, in connection with the government synthetic rubber program.
