Precise Automatic Apparatus for Continuous Determination of Carbon Dioxide in Air MOYERD. THOMAS Department of Agricultural Research, American Smelting & Refining Company, Salt Lake City, Utah N VIEW of the important role of atmospheric carbon
I
possible direct observations without mechanical interference of rates of photosynthesis and respiration in plants-processes which are fundamental to metabolism and growth-and would thus afford an immediate quantitative measure of the effect of many types of experimental treatment. The apparatus should also be particularly useful in animal respiration studies because measurements could readily be made under a wide range of conditions. The modification of the laboratory model of the sulfur dioxide autometer (9) was accordingly undertaken in the hope of producing a machine which would measure continuously and automatically the carbon dioxide concentration at intake and outlet of fumigation chambers, thus determining the amount of gas absorbed or evolved by the experimental plants in the chamber. A few preliminary observations made it clear that the principal problem to be solved was the development of an efficient absorber for carbon dioxide, which unlike sulfur dioxide, is very difficult to remove from a rapid air stream, unless one uses a solid absorbent. With a liquid absorbent in an ordinary bubbler, strong alkali and low rates of aspiration are necessary to absorb all the gas. For example, Spoehr and McGee (7) used 60 to 125 ml. of 0.1 N barium hydroxide in a 10-bulb Pettenkofer tube, to effect absorption from an air stream of about 6 cc. per minute.
dioxide in many phases of biology, such as animal and plant respiration, photosynthesis, and air pollution, it is surprising that no adequate method has thus far been proposed for determining it continuously and automatically. The best methods are somewhat laborious and time-consuming, and have an admitted uncertainty of 5 to 10 parts per million. This applies to the gasometric method of Sonden as employed by Benedict ( I ) , the Pettenkofer evacuatedbottle method of Johnston and Walker (4)’ and the electro-
Type2.
7jpe3.
FIGURE 1. TYPESOF ABSORBERS
metric method of Spoehr and McGee (8). Even though these methods may indicate the total concentration of the gas in the air with sufficient precision for many purposes, they leave much to be desired-except under special conditions of operation-when they are applied to the problem of measuring small differences of concentration at two points in an air stream in order to evaluate the amount of carbon dioxide absorbed or evolved in an experiment. While this paper was being prepared for publication, articles by Heinicke and Hoffman (3) and Martin and Green (6) appeared. These methods, which are more promising than the others on account of their high rate of aspiration, will be referred to again below. With the development of the author’s automatic apparatus for the determination of small concentrations of sulfur dioxide in air (9, 10, 111, it became clear that a similar machine applicable to the determination of atmospheric carbon dioxide would be useful in studies of the action of sulfur dioxide on vegetation. Such a carbon dioxide autometer would make
-a01
If
10
20
30
40
50
60
70
Recorder Reading.
80 90
CHARTOF ABSORBERS FIGURE 2. CALIBRATION
The spiral absorber of Martin and Green (6), recently described, absorbs all the carbon dioxide from an air stream of 333 cc. per minute (20 liters per hour) in 30 ml. of 0.07 N barium hydroxide. Unfortunately the efficiency of this absorber falls rapidly as the concentration of the alkali is reduced. Since the atmosphere contains only 0.3 cc. of carbon dioxide per liter, it was evident that an absorber was necessary which could handle moderate volumes of air (several hundred cubic centimeters per minute) with hy-
193
ANALYTICAL EDITION
194
droxide of strength preferably less than 0.01 N in order that appreciable changes in the solution might be produced in short time periods (2 minutes). ABSORPTION APPARATUS A consideration of the problem indicated that it would be necessary to produce very small bubbles in the absorbing liquid in order to attain high efficiency for the apparatus, and for this purpose the use of fritted glass disks seemed promising. The commercial bubblers made of Jena glass were too coarse, and the finer Jena disks were not mounted in apparatus suitable for the purpose in hand. It was suggested to the writer in private conversation by R. A. Fulton of the U. S. Bureau of Entomology, Twin Falls, Idaho, that fritted glass disks could readily be made of Pyrex glass; and later the note of Bruce and Bent (a) on this subject was found. Accordingly a number of different types of absorbers were constructed of Pyrex glass, utilizing disks of different porosities. Figure 1 shows three of these absorbers. Types 1 and 2, with porous plates made of 150- t o 200-mesh glass, had an efficiency of about 60 per cent with air streams of 200 to 300 cc. per minute and alkali of about 0.005 to 0.01 N . Type 3 was constructed with 3 disks made from glass powder of 100 to 150 mesh. This absorber had an efficiency of 80 t o 85 per cent under similar conditions of operation. With 0.1 N alkali, the efficiency was raised t o about 90 per cent in types 1and 2, and 95 per cent in type 3. These gas distributors produced bubbles of an order of about 0.5 mm. in diameter, but offered considerable resistance to the gas stream, suction of 5 to 10 cm. of mercury being necessary to draw the gas sample through them. It was evident, therefore, that increased efficiency could not be sought with disks of finer pores on account of the mechanical difficulty of drawing the gas through the septum. It is well known, as pointed out by Maier ( 5 ) , that the size of the bubble is a function not only of the diameter of the orifice from which the gas emerges, but also of the surface tension of the liquid. It was therefore decided to add a surface tension depressant to the absorbing liquid. For this purpose the higher alcohols suggested themselves as being particularly suitable because they produce large depressions with low concentrations and because they do not affect the conductance of the system. Normal butyl alcohol was used in amounts ranging from 0.1 to 2 per cent, depending on the porosity of the fritted glass disks and the rate a t which the gas was to be aspirated. The addition of the surface tension depressant not only reduced the size of the bubbles, but also produced a froth which increased the time of contact of the
Vol. 5 , No. 3
gas with theliquid. I n fact, the apparent volume of the liquid phase was increased two or three times, and the tiny bubbles circulated around in the system, many of them rising to the top and returning to the bottom before finally emerging from the top of the liquid-air mixture. This frothing action, together with the smaller bubbles, raised the efficiency of all the absorbers to 100 per cent with 0.005 N alkali. Further, the resistance of the septum to the passage of gas was reduced somewhat by the alcohol. The absorber type 2, Figure 1, the form finally chosen, was designed to hold 30 to 60 ml. of solution. It was rovided with an intake tube, A , and a very small chamber, B, gelow the 22mm. fritted glass septum, C. Chamber B was made as small as possible, about 0.5 to 1.0 cc. capacity, in order t o minimize the trapping of liquid below the septum when the vessel was emptied through D. The absorber was provided with a tube for adding the liquid, E, and the outlet, F. All tubes extended above the absorber so that the vessel could be completely immersed in a water thermostat. The conductivity electrodes, H , were made by welding electrically in an inert atmosphere a piece of platinum foil around the end of a tungsten wire, then welding a platinum plate to the platinum tip. A pair of plates, suitably spaced, were sealed in a small glass tube, G, and mounted in the absorber. If the electrodeswere of considerable size, supporting glass bead was inserted between them. The measurement of the carbon dioxide absorbed was based on the fact that the conductance of a carbonate solution is only about half that of the hydroxide solution of the same concentration.
EFFICIENCY OF ABSORPTION The efficiency of the absorbers was determined as follows: Two vessels were mounted in an accurately controlled water thermostat, and air, measured in a gas meter, was drawn through them in series. At 2-minute intervals the aspiration was stop ed and the conductance of the two solutions measured wit: a recording Wheatstone bridge. The absorbing liquid was 0.005 N sodium hydroxide and the resistance of the bridge had a range from 230 to 550 ohms, so that it was possible to read the resistance of the solution to within about 0.3 ohm. This preoision was sufficient to detect with certainty losses from the first absorber as small as 1 per cent of the carbon dioxide in each 2minute sample, The results of this study are summarized in Table I, which gives the efficiency of the first absorber in percentage, with different rates of aspiration and different concentrations of carbon dioxide in the air and also with different amounts of normal butyl alcohol in the absorbing liquid. The efficiencies are shown a t different stages in the conversion of hydroxide to carbonate and bicarbonate. An accumulated efficiency is the
TABLEI. EFFICIENCY OF ABSORBERAB INFLUENCED BY RATEOF AIR FLOW, CONCENTRATION OF CARBON DIOXIDE IN AIR, AND AMOTJNTOF BUTYL ALCOHOL,WITH 0.0052 N NaOH, AT DIFFERENT STAGESIN CONVERSION OB HYDROXID~ TO CARBONATE OR BICARBONATE RAT~OP coz VOLWMEOF AIR
FLOW Cc./min.
ABBORBINQ
IN
AIR P . P . m.
I
SOLUTION 0.0045 MI. %
0.00400
%
EFFICIENCY OF ABSORPTION AT AVERAQE HYDROXYL NORMALITIES 0.0036 0.0030 0.0025 0.0020 0.0010 0.0000b -0.0010
%
%
%
%
%
100 97
96 96 96
96 9s 98 98
93 96 98 98 97 97
%
0.1 PER CBNT BUTYL ALCOHOL
170 280 280 330 340 316 340 355 440 460
820 920 1100 900 900 340 320 330 380 630
36 37 35 31 37 34 36 50 50 50
100
100 100 100
...
100 100 100 100 100
100 100
iib
98 ... 100
100 100
...
100
...
100 100 98 99 98 100 100 100
100 98
.. 98
96 98 9s 100 99 98
100 9s 96 98 96 96 9s 98 99 97
.. 97
*.
97
..
..
?
-0.0020C
%
%
93 91 86 87
90 88 77 87 88
93 91 92 91 94 94 96 96 97
91 90 94 92 93
100 100 95 92 93 80
99 98 95 87 89 63
..
..
85
..
83 a.
0 . 4 PER CENT BUTYL ALCOHOL
100 810 35 204 330 320 35 100 100 330 520 37 100 410 31 370 40 100 320 440 740 40 500 a 0.0040 NaOH 0.0012 NaaCOs. b 0.0052 NarCOa. C 0.0032 NszCOs 0.0020 NaHCOa.
+ +
...
io0
... 100 ...
96
100 100 100 100 100
...
100 100
166
.. 96
100 100
... 99
... 98
100 100 100 98 98 93
100 98 99 94 97
..
'
95 94
....
..
81
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
May 15, 1933
average of the values to a chosen stage of the process. The data indicate that if sufficient alcohol was present, the absorber did not begin to allow carbon dioxide to pass until practically all the hydroxide was converted into carbonate, when the rate of aspiration was less than about 350 cc. per minute. The efficiency was still about 80 per cent when 40 per cent of the alkali was converted to bicarbonate. The same results were obtained with air containing three times the normal amount of carbon dioxide. At 450 cc. per minute, however, the solution began to allow carbon dioxide to pass when about half of the hydroxide was changed to carbonate. I n other experiments, not recorded in Table I, an efficiency of 98 per cent was observed with velocities of 700 cc. per minute, using 25 ml. of 0.1 N alkali and 0.4 per cent butyl alcohol as the absorhent. Evidently the absorber can be used with 0.005 N alkali and velocities up to about 350 cc. per minute with practically 100 per cent efficiency of absorption until nearly all the hydroxide is changed to carbonate. It should be noted, however, that the efficiency decreased when the velocity was too low to produce froth in the solution. For example, in one experiment, at 30 cc. per minute without any foaming, 10 per cent of the carbon dioxide escaped, whereas it was all absorbed a t 300 cc. per minute.
TABLE11. EFFICIENCY OF ABSORPTIONOF CARBON DIOXIDE BY 250 cc. 0.1 Ba(OH)* IN ABSORBERSIMILAR TO THAT DESCRIBED BY HEINICKE AND HOFFMAN RATEOF AIR
FLOW
Cc./min.
320 850
2200 2200
195
f
Bleeder,
SURFACE EFFICIENCY TBNOF ABHEIGHT OF COLUMN Froth SION SORPTION Cm. Cm. Dzlnedcm. % . . 30 0.6 72 loo 72 97 30 5 72 93 30 30 63 96 30 60
Liquid
Heinicke and Hoffman (3) claim that all the carbon dioxide can be removed from an air stream of 1670 cc. per minute (100 liters per hour) in a large absorber tube containing 200 ml. of 0.1 N barium or potassium hydroxide, and provided a t the bottom with a 30-mm. fritted glass septum of 100- to 120-micron pore diameter. Table I1 shows that while this claim could not be strictly confirmed, the method is very promising. The absorber was similar to that of Heinicke and Hoffman, except that it had a 30-mm. Jena G 2 filter of 40- to 50-micron rated pore diameter and contained 250 ml. of 0.1 N barium hydroxide. The high efficiency at the maximum velocity employed, which could be further improved by the addition of a surface tension depressant, indicates that a considerable range of velocities of aspiration is possible if desired. Evidently the success of Heinicke and Hoffman’s absorber (3) is due in part to the formation of considerable foam even without a surface tension depressant.
HYDROXIDE SOLUTION From a consideration of the transport numbers of lithium and sodium hydroxide it was assumed that lithium hydroxide would show a greater change of conductance on being converted into carbonate than would sodium hydroxide, and much of the earlier work with the method was carried out with lithium hydroxide. However, as the calibration curves in Figure 2 show, the difference in behavior between these two akkalies is so small that one has no appreciable advantage over the other. Accordingly sodium hydroxide is now being used. Barium hydroxide might be used as the absorbent, in which case a much larger change of conductance would be observed on the addition of carbon dioxide to the solution because of the precipitation of barium carbonate. Experiments with barium hydroxide, however, indicated that while the presence of the precipitate did not affect the measurement of the conductance appreciably, especially if the absorbing vessel was cleaned occasionally with dilute hydrochloric acid, B serious
Iu I
FIGURE3. DIAGRAM OF AIR-FLOW CONTROL SYSTEM Showing method of alternating aoutoe of air aam le (top). steel gas meter and absorbers (aenter); and surge Eottle and control valves (bottom).
disadvantage was incurred because of the fact that the conductance of the solution decreased slowly for about half an hour after precipitation occurred. Apparently the solubility of barium carbonate was decreasing during this time, presumably because of growth in the size of the particles of the precipitate. The fact that equilibrium measurements could not be made quickly with barium hydroxide rendered that absorbent practically worthless for the purpose at hand.
CALIBRATION OF ELECTRODES The problem of calibrating the absorbers was finally solved by using the titration method of Walker, Bray, and Johnston ( l a ) to determine the stage of conversion of the hydroxide to carbonate. The ordinary method of double titration with phenolphthalein and methyl orange as indicators did not give the desired precision for this analysis on account of the high dilution of the alkali. The following procedure gave excellent satisfaction: A titration vessel was provided with a 3-hole stopper, one hole having a glass plug and the other two having short lengths of glass and rubber tubing. Two drops of 0.5 per cent thymol blue indicator were added to the bottle, the stopper was inserted and the vessel was flushed out with carbon dioxide-free air. definite amount of 10 per cent barium chloride solution, 0.01 N with respect to barium hydroxide, was pipetted into the titration bottle, which was then weighed before and after introducing the sample, and the excess hydroxide was titrated with 0.01 N hydrochloric acid after inserting the tip of the buret through one hole of the rubber stopper. The titration was carried to a definite
A
196
ANALYTICAL EDITION
end point at about pH 8.8, as compared with a well-buffered borax-boric acid solution. The method was entirely satisfactory, provided a large excess of soluble barium was present to insure the complete precipitation of carbonate and provided the end point was approached slowly. A titer greater than that of the barium hydroxide added represents hydroxide, while a smaller titer represents bicarbonate. A carefully prepared solution of
FIGURE4. SECTIONOF RECORDERPAPER,ILLUSTRATING TYPEOF RECORDOBTAINED Width of p&per,26 om, (IO in.); paper speed, 20 am. (8 in.) per hour.
sodium carbonate in carbon dioxide-free water does not change the blank titer. I n carrying out the calibration, about 50 ml. of alkali were introduced into the thermostated absorber and air was aspirated through the solution until the desired amount of hydroxide was converted to carbonate, as indicated by the conductance of the solution. Then carbon dioxide-free air was drawn through the solution until the recorder showed a constant resistance. The solution was withdrawn and titrated as indicated. Typical calibration curves are shown in Figure 2. Because of the fact that the temperature coefficients of conductance of hydroxide and carbonate differ appreciably, calibration must be made for each temperature at which it is desired to operate. The temperature compensation coil of the recorder employed was designed for dilute sulfuric acid. Dilute sodium hydroxide has nearly the same temperature coefficient as the acid and therefore the calibration curves at diflerent temperatures converge at the hydroxide end. AS the temperature coefficient of the carbonate is considerably greater than that of the hydroxide, the calibration curves become steeper at higher temperatures. Accordingly, the instrument may be expected to show a greater change of conductance for a given amount of absorption the lower the temperature of the solution. It will be noted that the response of
Vol. 5, No. 3
the recorder to the conversion of hydroxide to carbonate is linear over a wide range.
CARBONDIOXIDE AUTOMETER The conversion of the laboratory model of the suIfur dioxide autometer (9) into a carbon dioxide autometer was completed when a method was devised for drawing a definite small volume of air (for example, 300 cc. per minute) through absorbers of type 2 (Figure 1), which could readily be mounted in place of the sulfur dioxide absorbers. Since the air stream was aspirated through one absorber for 2 minutes before changing to the other, it was necessary to supply a sample of 600 cc. for each aspiration. The flowmeters on the sulfur dioxide machine were discarded and a steel gas meter was constructed, by means of which an exact volume of air could be delivered for each aspiration. A section of this meter is shown in Figure 3, which is a diagram of the air-flow control system. The meter consists of 2 flat chambers connected by a U-tube and mounted on a pivot. Each chamber communicated through glass and rubber tubing to an absorber and also through a valve (Hor I ) to the source of the sample. Enough mercury was added to 611 one chamber and the U-tube. Air volumes smaller than 600 cc. could be obtained by using more mercury. The two valves leading to the gas meter were operated by the same cam lifters which operated the valves used for shifting the air stream from one absorber to the other, so that air was permitted to enter one chamber of the gas meter from the outside source, while the air in the other chamber was being aspirated through the absorber. I n Figure 3, valves G and H were open, while valves I and K were closed, and vice versa. Electrical contacts were provided on each side of the gas meter so that when a chamber was completely N e d with mercury, the fact was recorded on the chart with the conductance record. The gas chambers were made as flat as possible, and also allowed to move on the pivot, in order to minimize the effect of the changing hydrostatic head of the mercury on the rate of flow of the gas through the absorber. The suction was adjusted by means of the needle valves and the 2.5-liter equalizing bottle, so that the full sample from the gas meter was drawn through one absorber a few seconds before the air stream was shifted to the other absorber. The bleeder served to maintain the pressure in the equalizing bottle at a constant reduced value and set a definite limit to the height to which the mercury could be drawn in the tubes above the gas meter. The valves A to F were used to alternate the source of the sample in order that any slight differences between the two absorbers could be compensated for, as in the case of the sulfur dioxide autometer. I n operation, whenever fresh portions of solution were placed in the absorbers, carbon dioxide-free air was drawn through valves A and F, to each of which was attached a large tube of soda lime. This aspiration adjusted the temperature of the solution and established an accurate zero for the determination. After each absorber had drawn one sample of carbon dioxide-free air, the valves A and F were closed and valves B and E opened, until seven aspirations had been completed in each absorber. Then fresh solution was placed in the absorbers, one aspiration each of carbon dioxide-free air given, and valves C and D opened for seven more aspirations. A complete cycle was accomplished in 64 minutes, and each solution was aspirated for 16 minutes out of the 32 minutes, during which it remained in the absorber. This arrangement proved to be suitable for long-continued studies of photosynthesis and respiration. For other purposes it might be desirable to employ a cycle of different type or length. On account of the frothing of the solution during aspiration, it was impossible to measure the conductance of the solution a t
May 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
197
that time, and the recorder was, therefore, connected while the solution was quiescent. Figure 4 is a photograph of a section of the recorder paper, illustrating the type of record obtained.
next half-hour period. Similarly, the analysis from absorber 2 in the first period was averaged with that from 1 in the second period. This method of calculation was necessitated by the fact that the machine alternated the source of the sample with each absorption period. Table I11 gives the average and At point A , the conductance of the solution in absorber 1 was maximum differences between the two absorbers from 135 measured and the solution was discharged, fresh solution being added immediately, which is indicated by point B. Points C and hourly determinations taken at random. The average differD represent a similar condition for absorber 2. During the latter ence was less than 0.5 per cent in each group of analyses, and time period, absorber 1 was being aspirated with carbon dioxide- exceeded 1.0 per cent in only 10 per cent of the cases. It may free air, the result of which is recorded at E and the correspond- be possible in the future to attain higher precision than this, ing value for absorber 2 is shown at F. G and H represent the results of the first aspiration in the two absorbers, respectively, because, while the uncertainty of reading the recorder was with air containing carbon dioxide. This process was continued about 0.1 scale division, corresponding to about 1 p. p. m. of with six more 2-minute aspirations in each absorber until the absorbing liquid was drained out after the conductivity was finally measured at I and K . The track M-N mas drawn by & pen mounted on a relay connected with the electrical contacts on the mercury gas meter. The pen was pulled to the right when each chamber was filled with mercury. The track M-N proves that the full gas sample was delivered at each aspiration. The average value of the carbon dioxide concentration during each half-hour period is given on the chart. Fifty milliliters of solution were used in the analyses illustrated in Figure 4. The volume now employed is 40 ml., which causes a correspondingly greater response of the recorder. The sulfur dioxide autometer was modified slightly in two other respects. The 500-cc. constant-level bottle was eliminated and capillary tubes of less than 1-mm. bore were mounted on the pipets to extend along and above the large supply bottle. The upper ends of these tubes were connected to the top of the bottle and provided with an outlet through a soda lime guard tube. The pipets delivered only 0.3 ml. more solution when the supply bottle was full than when it was nearly empty. The 20-liter bottle held enough solution for 125 hours of operation. Finally, it was found that the temperature of the autometer chamber could not be controlled closely enough for the carbon dioxide determination, and the absorbers were mounted in a small water bath thermostatically controlled to about 0.01" C. The thermostat was 10 X 25 X 25 em. and provided with a glass window in front and glass tubes soldered through the bottom for draining the absorbers. TABLE111. HOURLY DIFFERENCES BETWEEN Two ABSORBERSSAMPLING FROM SAME AIR STREAM (Average hourly concentrations calculated by same method as t h a t used for calculating absorption data) NUMBERO F AVERAGE HOURLY DIFFERENCE DATE HOUR@ CONCENTRATION BETWEEN ABSORBERS 1933 AVERAGED COz IN AIR Maximum Average P . p . m. % % Jan. 19, 20 22 322f 3 . 5 a 0.45 1.8 Jan. 25, 26 1.1 23 325f 9 . 4 0.42 Jan. 27 28 1.7 23 323i. 3 . 3 0.44 Feb. 2 '3 0.8 17 337f 7 . 8 0.32 Feb. l b 1.2 0.48 12 336 f l 6 . 5 Feb. 11-13 1.2 301 f 1 0 . 9 38 0.44 a Standard deviation.
PRECISION OF ABSORPTIONDATA During the autumn of 1932 an extended observation was made of the carbon dioxide exchange of alfalfa growing under natural conditions. This experiment will be described in detail in another paper. I n order to obtain a definite idea as to the error involved in the gas-exchange measurement, the autometer was operated during the following winter while the ground was covered with snow. Both absorbers drew air through the same intake tube. The machine was located near an open field 5 miles from Salt Lake City and there was little possibility of its being influenced by smoke from a few widely scattered farm homes in the neighborhood. The data were treated in a manner similar to that used in calculating the absorption data. The value of the carbon dioxide concentration indicated by absorber 1 during one half-hour period was averaged with the analysis from absorber 2 during the
FIGURE5 . COMPLETED ASSEMBLY 1, Supply bottle. 2, Pipets. 3, Absorbers and thermostat. 4 Mercury ga3 meter. 5 , Equalizing bottlk. 6 and 7, C a m systems. 8, Automatio valves.
carbon dioxide or about 0.3 per cent of 300 p. p. m., in practice the concentrations were calculated from both the six and the seven carbon dioxide aspirations in each series, and these values were then averaged, thus reducing the error of calculation. It is of course probable that at least some of the differences were due to occasional peaks of concentration of short duration which affected one absorber more than the other. It is interesting to note from Table I11 that the carbon dioxide of the atmosphere varied appreciably from day to day as well as during the day. SIMPLKFIED
APPARATUS
For long-continued observations of carbon dioxide absorption or evolution, automatic apparatus is practically indispensable on account of the time and labor which would be necessary to obtain the information without the autometer. However, if only a limited number of observations are required, a simplified hand-operated apparatus might be desir-
ANALYTICAL EDITION
198
able. Several assemblies of such apparatus have been constructed. Absorbers of type 2, Figure 1, were employed. The simplest method of drawing and measuring the air sample was by means of a large aspirator bottle, but the sample was most conveniently drawn through an extra valve on the equalizing bottle of the autometer, thus making use of the electrically operated suction pump. A water pump or a
carbon dioxide concentrations noted in Tables I and I V were observed in the air of the laboratory. The normal values represent for the most part samples taken from outside. TABLEIV. CARBON DIOXIDE IN SAME AIR STREAM AS DETERMINED BY TITRATION AND BY AUTOMETER
5
L
I
Vol. 5, No. 3
AV~RAQE DURATION OF CARBON DIOXIDE SOURCIO SAMPLINQTitration Autometer DIFFERENCE Min. P.p.m. P.p.m. % Outside 90 333 332 0.3 Outside 90 , 326 326 0.0 Outside 90 332 333 -0.3 Outside 90 330 388 0.6 Room 60 705 710" -0.7 COEvaried from 600 t o 850 p. p. m.
A hand-operated apparatus of the type described would doubtless give very satisfactory absorption data if it were provided with two absorbers sampling simultaneously. The concentration of the alkali solution could be measured either by titration, as indicated above, or by conductivity. If the latter method were chosen it would probably be advantageous to have at least two pairs of absorbers with different cell constants so that different concentrations of alkali could be employed. The analyses could thus be completed in a few minutes or extended for several hours, as desired. It would also be necessary to thermostat the apparatus accurately for conductivity measurements.
h Wurce 1
IAutomatii
//
SUMMARY
FIGURE 6. DIAGRAM OF HAND-OPERATED APPARATUS FOR DETERMINING CARBON DIOXIDE IN AIR
connection to the intake manifold of an automobile engine gave good results also. The gas was then measured in a wettest gas meter inserted between the absorber and the suction control apparatus. I n the early work, a capillary flowmeter was used instead of the gas meter, but it did not appear to offer the precision necessary for the purpose in hand. The flowmeter would be useful for less precise work. A diagram of a satisfactory assembly is shown in Figure 6. Difficulty was encountered in measuring the air volume accurately in the gas meter on account of the resistance of the absorber. This difficulty was overcome by attaching a three-way stopcock to the intake so that the air stream could be adjusted with carbon dioxide-free air before the sample to be analyoed was started. At the end of the analysis carbon dioxide-free air was aspirated again. The gas meter was read each time the three-way stopcock was turned. A number of analyses were made by the above procedure using 0.1 N sodium hydroxide. The excess alkali was titrated with 0.025 N hydrochloric acid by the method of Walker, Bray, and Johnston ( l a ) already discussed. The autometer was operated simultaneously, both sets of apparatus sampling from the same source. Each experiment was continued for 60 to 90 minutes. The average values of the carbon dioxide given by the two methods are summarized in Table IV. The maximum difference of 0.7 per cent between the two independent analyses indicates a satisfactory evaluation of concentration of the gas in air. The high
Automatic apparatus has been constructed for the continuous determination of atmospheric carbon dioxide from two sources of supply. For example, the machine can draw its air from intake and outlet of an experimental chamber, thus making possible the evaluation of the carbon dioxide exchange in an air stream passing through the chamber. The maximum error of the total concentration of the gas in the air is shown to be less than 1 per cent and the average error of the difference between the two sources of supply is less than 0.5 per cent. The apparatus employs an absorber capable of removing all the carbon dioxide from an air stream of 350 CC. per minute by means of 40 cc. of 0.005 N sodium hydroxide. The essential features of this absorber include a fritted Pyrex glass disk to divide the air stream into fine bubbles and a small amount of normal butyl alcohol to depress the surface tension of the liquid, thus increasing the persistency of the bubbles. The apparatus also utilizes a special mercury gas meter for dispeneing the air sample accurately. Finally a simplified hand-operated apparatus is described.
LITERATURE CITED Benedict, F. G., Carnegie I n s t . Wash. Pub., 166 (1912). Bruce, W. F., and Bent, H. E., J. Am. Chem. Soc., 53,990 (1931). Heinicke, A. J., and Hoffman, M. B., Science [N.S.], 77, 55 (1933).
Johnston, J., and Walker, A. C., J. Am. Chem. SOC.,47, 1807 (1925).
Maier, C. G., Bur. Mines, Bull. 260 (1927). Martin, W. MoK., and Green, 5. R., IND.ENQ. CHEM.,Anal. Ed., 5, 114 (1933). Spoehr, H. A., and McGee, J. M., Carnegie Inst. Wash. Pub. 325 (1923).
Spoehr, H. A., and McGee, J. M., IND.ENQ.CHEM.,16, 128 (1924).
Thomas, M. D., Ibid., Anal. Ed., 4, 253 (1932). Thomas, M. D., and Abersold, J. N., Ibid., Anal. Ed., 1, 14 (1929).
Thomas, M. D., and Cross, R. J., IND.ENG.Cnxx., 20,645 (1928). Walker, A. C., Bray, M. B., and Johnston, J., J. Am. Chem. SOC.,49, 1235 (1927). RECEIVED February 20, 1933
