air compressors, would deactivate the catalyst, some oil was introduced into the catalyst chamber. The expected formation of large concentrations of carbon dioxide occurred for a while, but there was no loss of catalytic activity. The entire system has been in continuous operation for over tvio years. Except for routine maintenance and periodic checks of calibration with known mixtures, no attention has been required and there has been no sign of
catalyst deactivation. Figure 3 is a typical record of the hydrocarbon content of feed air in terms of parts per million of methane. The rapid response to changes in concentration is clearly evident. ACKNOWLEDGMENT
Clyde AIcKinley and Joseph T. Bernstein, .4ir Products, Inc., were helpful in the early stages of this jvork.
LITERATURE CITED
(1) Heaton, 1 ~ B., . ~ ~ J. T., ~ ANAL.CHEJI.31, 349 (1959). (2) Lawrey, D. M. G., Cerato, C. C., Ibzd., 31, (1959). (3) Loveland, J. W.,Adams, R. IT., King, H. H., Koxak, F. rl., Cali, L. J., Ibid., 31, 1008 (1959). ( 4 ) Martin, A. E , Smart, J., Nature 175,
422 (1955).
RECEIVED for review June 12, 1958. Accepted March 2, 1959. Second Delaware valley ~ ~ bfeeting, ~ ACS,i Philadelphia, Pa., February 1958.
Spectrophotometric Titration of Parts Per Million of Carbon Dioxide in Gases J. WEST LOVELAND, ROBERT W. ADAMS, H. H. KING, Jr., FRANCES A. NOWAK, and LAWRENCE J. CALI Research and Engineering Department, Sun O i l Co., Marcus Hook, Pa.
b In the spectrophotometric titration of traces of carbon dioxide in a gas stream, the carbon dioxide is absorbed in dilute sodium hydroxide and the excess caustic i s back-titrated with dilute hydrochloric acid. The titration is followed at 555 mp on spectrophotometer using phenolphthalein as an indicator. A special absorption bulb gives a complete recovery of all carbon dioxide up to gas flow rates of 100 cc. per minute. Sensitivity i s to about 1 p.p.m. of carbon dioxide for 20 liters of gas. The accuracy i s to about 1 p.p.m. and the repeatability standard deviation i s to 0.4 p.p.m. in the 1 to 10 p.p.m. range. Other acidic gases interfere.
A
of carbon dioxide in the range of 1 to 50 p.p.m. in gas streams containing no other acidic constituent TTas needed in conjunction with a long-path infrared nondispersive analyzer during studies of the oxidation of traces of hydrocarbons. Chemical methods for concentrations of less than 1% generally involve absorption of the carbon dioxide by a n alkaline reagent with several finishing steps available. A gravimetric analysis using ilscarite absorption tubes was described by Kolthoff and Sandell ( 2 ) . Others have used either the p H of a sodium bicarbonate absorber solution ( 1 ) or a continuous photometric method using phenol red as an indicator in aqueous bicarbonate (3). The latter method covered the range of 0.06 to 12% carbon dioxide. Spector and Dodge (6) report that a photometric method using the color of phenolphthalein in dilute sodium hydroxide is senDETERMINATIOS
1008
ANALYTICAL CHEMISTRY
sitive in the parts per million range. Pieters (4) recommends absorption in barium hydroxide solution and backtitration with hydrochloric acid which is sensitive to 0.1 mg. of carbon dioxide or about 5 p.p.m. for a 10-liter sample. Attempts to use Spector and Dodge's (6) method rvere not successful in the very 1017 parts per million range because of fading of the phenolphthalein color with time. Too frequent calibrations were necessary. The direct titration method of Pieters (4) appeared the most promising, although detection of the end point n-as difficult in the parts per million concentration range. However, this \vas overcome by using a spectrophotometer for the end point detection. Since the completion of this work, Toren and Heinrich (?) have reported a method n-hich measures pH a t the equilibrium point of saturated solutions of a n alkaline earth and carbon dioxide at concentrations from 1 p.p.m. to 1007, carbon dioxide in the gas streams. The method appears to have sensitivity and accuracy comparable to the spectrophotometric titration procedure. The titration method should be of use to laboratories where special infrared equipment is not available. The analysis requires that all the carbon dioxide in a k n o w volume of gas sample be absorbed in a given quantity of alkaline solution. During this work, it was found that absorption of carbon dioxide by a single bubbler is quantitative only when extrenielp lorn rates are employed. A number of absorption bubblers ere tried to find a single efficient bubbler to reduce handling of sample.
Table I gives the data obtained with these bubblers as rrell as the approximate dimensions and type of dispersing element for each bubbler. About 100 ml. of 0.0001.1' sodium hydroxide n-as used in each. For the first bubbler, from 70 to 300 y of carbon dioxide, measured in a gas pipet, ITas sir-ept through the caustic solution with nitrogen, using a total volume of sweep gas of about 3 liters a t a rate of about 70 cc. per minute. The amount of carbon dioxide absorbed was determined by spectrophotometric titration of the caustic solutions. For the second and third bubblers, k n o m blends of carbon dioxide in nitrogen were used for testing the efficiency a t about the same flow rate. Using the titration cell as the absorber or the longer column with a smooth surface gave 1011- recoveries. Apparently, the contact time was insufficient for the caustic to react completely lvith the traces of carbon dioxide. The best results were obtained with a Vigreus-type column. The restrictions in the column provided a stirring effect and a much longer residence time for the tarbon dioxide to be absorbed in the solution. A series of determinations with the Wgreux-type scrubbing system used varying flow rates of sample gas (Table 11). The sample used was a certified blend of 8 p.p.ni. carbon dioxide (Matheson Co., Inc.). About 10 liters of gas was used in each test. The flow rate is critical and should be maintained a t a rate below 100 cc. per minute. DISCUSSION OF PROCEDURE
The spectrophotometric titration of
~
Al-
Table I.
Effectiveness of Various Absorbers
Type of Bubbler Rectangular shaped cell 2J/2 X 1 - 3 / 4 X 3-l/2 inches, fitted with a coarse gas dispersion bubbler Tube
1-1/8
Kno-m, 298
coz
Determined, * y 208
-f4
198 70 69
100
48 59
P.P.31.
O.D. X 12-l/? inches long, n-ith smooth
sides; fitted as above
5.1 5.1 5.1 5.1
3.6 0.9 4.8 5.1
Vigreus-type column 1 O.D. X 16 inches in length; fitted as above 298 equivalent to 50 p.p,m. CO, for 3 liters of sveep gas used. b .lnalyzed by spectrophotometric titration. Q
9 ,
,$ 10130
Table II. Effect of Flow Rate of Gas on Absorption of Carbon Dioxide CO, by Titration,
P.P.X.
Rate, Cc./llin.
8.5, 8 . 0 8.9 6.4 6.8
70
85
Figure l . Apparatus converted for photometric titration A.
B. C.
D.
E. F. G.
H. 1.
1.
115 140
Stopcock for nitrogen purge gas Stopcock for outlet for droining cell Stopcock for admitting absorber solution to cell Two-way stopcock for caustic solution Three-way T stopcock for somple gas Stopcock for exit of purge nitrogen and sample gases Two-way stopcock for water wash Titration cell Magnetic stirrer Beckman Model B spectrophotometer
0.000111' sodium hydroxide with 0.002N hydrochloric acid using phenolphthalein as indicator gives well defined end points. The wave length of 555 mp was the most sensitive to changes in the color intensity of phenolphthalein. Plotting absorbance a t 555 m p us. milliliters of titrant gives two straight lines which intersect to give the end point. In the vicinity of the end point, the points do not fall on the lines, but give a curvature effect which can be ignored. At the start of the titration, the absorbance is greater than 1 and becomes less than 1 about 3 ml. before the end point. Generally, the end point requires about 5 ml. APPARATUS
Beckman Model B Spectrophotometer. The apparatus, designed to eliminate any possibility of carbon dioxide contamination from the atmosphere, is converted for photometric titration use as shown in Figure 1. The original cell compartment, cell cover, and back-plate are replaced by a magnetic stirrer and an aluminum back-plate and cover. Titration Cell. The 250-ml. rectangular glass cell, 7 X 9 X 4.5 cm. with ground-glass neck (45/50 T Joint) as shown in Figure 2 also contains an outlet for draining, under nitrogen pressure, the titrated solution without removing the cell. Rlicroburet. Self-filling, of 10-ml.
F
-711 4.5-4 Figure 2. A.
B. C. D.
E. F.
,
Titration cell
Inlet for nitrogen gas Outlet for draining cell Ball joint (female) 1 2 / 5 to absorber Female joint to buret Titrotion cell Tygon tubing
capacity with 0.02-ml. graduated intervals with delivery tip modified to attach directly to the titration cell head. Scrubber Apparatus. The gas scrubber with delivery pipets and gas absorption column is shown in Figure 3. It is equipped mith self-filling pipets for both the alkali solution and the wash water. All terminal inlets and outlets are equipped n-ith tubes containing Ascarite so that the apparatus can be readily purged and maintained carbon dioxide-free. Ket-Test Rleter. American Meter Co., Inc. wet-test meter with 0.001cubic foot graduated intervals or an equivalent meter graduated in liters. Flowmeter. Glass tube and float to indicate a rate between 70 to 95 cc. per minute (0.15 to 0.20 cubic feet per hour). REAGENTS, SOLVENT, AND SOLUTIONS
Sodium Hydroxide, 0.0001N. Dilute 1 ml. of 0.1N sodium hydroxide (4 grams per liter) to 1 liter with neutral distilled water, and add 5 ml. of phenolphthalein
-
L
8-M &--N
Figure 3.
Gas scrubber
A.
100-ml. self-filling pipet for sodium hydroxide solution B. 30-ml. self-filling pipet for wash water C. Two-way stopcocks, 2 mm. D. To sodium hydroxide solution reservoir E. To wash water reservoir F. 1 0 / 3 0 joints G. Woter spray dispersion tube, eight 1 -mm. diameter holes H. Sample gas outlet 1. Sample gas inlet 1. Vigreux column K . Three-way T, 3-mm. stopcock I. Gas dispersion tube, coarse porosity M . 3-mm. stopcock N. 1 2 / 5 male ball joint
indicator. Store in a polyethylene bottle. Hydrochloric Acid Solution, 0.002.47. Dilute accurately 0.1N hydrochloric acid which has been standardized against sodium carbonate using methyl orange as indicator. Phenolphthalein Indicator. Dissolve VOL. 3 1 , NO. 6, JUNE 1 9 5 9
1009
Table 111. Precision and Accuracy of Spectrophotometric Titration Method COa, P.P.M. Infra- TitraSample red tion Difference 1 2.4 2.4 0.0 2.5 0.1 2 4.4 5.5 1.1 5.0 0.6 3 5.7 5.1 0.6 5.9 0.2 4 sa 8.5 0.5 0.0
8.0
8.9 7.8 5
13a
6
23
0.9 0.2 2.8 2.7
15.8
15.7 25 2.0 23 0.0 7 40 37 3.0 39 1.o Std. dev.: Repeatability 0 to 10 p.p.m. range = 0.4; 10 to 40 p,p.m. range = 1.1. Std. dev.: Between methods 0 to 10 p.p.m. range = 0.5; 10 t o 40 p.p.m. range = 2.2. a Matheson Co., Inc., certified blends. Table IV. lr
Prepared Blends of Carbon Dioxide in Nitrogen
CO*, P.P.?rl.
Calcd.
Difference,
Found Difference
12 33
5.3 31 29
36
-6.7 -2
-7
%
128 6 24
0.3 gram of phenolphthalein in 100 ml. of ethyl alcohol. Prepurified Nitrogen, purchased from Air Products Corp. PROCEDURE
Assemble the apparatus as shown in Figure 1. Completely close the apparat u s from the atmosphere, and attach Ascarite drying tubes to all inlets and outlets. Fill the buret and pipet by nitrogen pressure. m i t h stopcock C open, degas the entire apparatus, including buret, water and caustic pipets, and cell, b y passing nitrogen through stopcock E, opening stopcocks A , B , D, G, and F , alternately. Close all stopcocks, once the apparatus is degassed. It should not be necessary to degas again if air is excluded at all times. Flush the scrubber and cell twice with wash water from the water pipet to ensure complete removal of a n y contamination. Close all stopcocks except A and B , and empty the cell b y applying nitrogen pressure a t stopcock A. When the apparatus is degassed and flushed as described, run a blank titration on the caustic solution. Because this caustic is very dilute and can change easily, run at least two such rea1010
0
ANALYTICAL CHEMISTRY
gent blanks each day. These blanks should check within 0.1 ml. of acid. Zero the spectrophotometer with the cell empty and the wave length set a t 555 m,u. Fill the caustic pipet and drain through the scrubber into the cell. Fill the water pipet with carbon dioxidefree distilled vater and spray it into the cell under nitrogen pressure. Turn on the magnetic stirrer, titrate the caustic with small additions of 0.0025 hydrochloric acid, and record the absorbance of the sample after each addition. Add the acid in 0.2-nil. increments for readings between 0.000 and 0.800 absorbance. Khere above 0.800, increments may vary between 0.5 and 1.0 ml. Add increments of acid until no change in absorbance is observed for three additions. Plot titration volume 2’s. absorbance. Take the end point a t the intersection of two straight lines. Ignore the points near the end point that will not fall on the two straight lines. Using Figure 1, the procedure for gas samples is as follons: With stopcock C closed, fill the caustic pipet and drain the solution into the scrubber. Connect sample gas stream to the sample inlet of the scrubber 11-ith stopcock E open to the atmosphere and degas the lines up to the gas dispersion tube for 10 minutes. Then admit the sample gas to the scrubbing column a t a rate of 70 to 95 cc. per minute by turning stopcock E and opening stopcock F . hleasure the gas volume with the wet-test meter on the outlet. Bubble a sufficient amount of gas through the caustic to give a difference between sample and blank titrations of a t least 0.5 ml. of acid. The amount of sample required will depend upon the carbon dioxide concentration, but can be detected b y the gradual fading of the phenoIphthaIein color. Close stopcocks, E and F . Zero the instrument, drain the caustic solution into the cell, and wash the scrubber with water from the n-ater pipet. Titrate the sample as in the case of a blank and determine the end point from the acid volume 21s. absorbance curve. Calculate the carbon dioxide concentration by: Parts per million CO, = (B - A) X N x T C X P
x 2.2 x 108
with certified analysis from hfatheson Co., Inc. were tested in duplicate (Table 111). The prepared blends were analyzed by a sensitive nondispersive infrared analyzer ( 6 ) ,which was factory calibrated and checked with certified carbon dioxide blends (Matheson Co.. Inc.). Subsequent calibrations of the infrared instrument a t various sensitivities were made using only blends analyzed by the titration technique. I n all cases the determined carbon dioxide concentration is in good agreement with the infrared and certified values. The accuracy is to about 1 p.p.m. in the 2 to 10 p.p.m. range and the repeatability standard deviation is to 0.4 p.p.m. in the 10 to 40 p.p.m. range, the accuracy is to about 2 p.p.m. and the repeatability standard deviation is to 1.1 p.p.m. During this investigation in which 1-4 cylinders (holding 1.5 cubic feet a t standard temperature and pressure) were used to prepare carbon dioxide blends, the amount of carbon dioxide found Iyas usually lower than the amount added to the cylinder. A blend made by diluting another blend gave lower results than expected. Table I V indicates the discrepancies between the amount added and the amount found by the titration technique, lvhich are believed to be due mainly to adsorption of carbon dioxide on the walls of the cylinder and by absorption in the moisture present in the cylinders. It is therefore necessary to analyze each blend of carbon dioxide prepared in cylinders. The spectrophotometric titration method is very sensitive and accurate for determining parts per million of carbon dioxide in gases and requires no comparative standard gas. The method is applicable only to gases which contain no acid gases other than carbon dioxide, because these would also reduce the alkalinity of the scrubber solution. The elapsed time required for a single determination in the 10 p.p.m. range is 2 hours and 4 hours in the 2 p.p.m. range.
where
LITERATURE CITED
B
(1) Yrjo, Kauko, I V A 20, 44 (1949). ( 2 ) Kolthoff, I. If., Sandell, E. B.,
= milliliters of acid needed t o ti-
trate blank A = milliliters of acid needed to titrate sample N = normality of acid C = cubic feet of sample used P = barometric pressure (mm. of Hg) T = temperature (absolute) OK. 2.2 x 103 = factor which converts gas measurements to standard temperature and pressures RESULTS
T o determine the precision and accuracy of the method, prepared blends of carbon dioxide in nitrogen and two blends
[‘Textbook of Quantitative Inorganic Analysis,” p. 385, Macmillan, ru’ew York, 1947. (3) Maxon, R.D., Johnson, hf. J., AXAL. CHEM.24, 1541 (1952). (41 Pieters, H. A., Anal. Chim. Acta 2, ’ 263 (1948). ( 5 ) Rosenbaum, E. J., Adams, R . W., King, H. H., ANAL. CHEM.31, 1006 (1959). (6) Spector, K. A., Dodge, B. F., Ibid., 19, 05 (1947). (7) Toren, P. E., Heinrich, B. J., Ibid., 29, 1854 (1957). RECEIVEDfor review June 12, 1958. Accepted January 1, 1959. Second Delaware Valley Regional Meeting, ACS, Philadelphia, Pa., February 1958.
