Type 2 . Type 3 .
Fi g u r e 1 . Ty p e s o f Ab s o r b e r s
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 measur
ing small differences of concentration a t 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 (S') and M artin 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, 11), it became clear th a t a similar machine ap
plicable 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
Mo y e r D. Th o m a s
Department of Agricultural Research, American Smelting & Refining Company, Salt Lake City, Utah
Fi g u r e 2 . Ca l i b r a t i o n Ch a r t o f Ab s o r b e r s ' Resistance of 0.0052 JV alkali solutions as indicated by a re*
cording W heatstone bridge, a t different stages in conversion of hydroxide to carbonate. Positive values of hvdroxide concen
tration represent mixtures of hydroxide and carbonate; negative values represent mixtures of carbonate and bicarbonate.
The spiral absorber of M artin 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 th a t an absorber was necessary which could handle moderate volumes of air (several hundred cubic centimeters per minute) with
hy-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 treatm ent. The ap
paratus should also be particularly useful in animal respira
tion studies because measurements could readily be made under a wide range of conditions.
The modification of the laboratory model of the sulfur di
oxide autometer (9) was accordingly undertaken in the hope of producing a machine which would measure con
tinuously and automatically the carbon dioxide concentra
tion a t intake and outlet of fumigation chambers, thus deter
mining the amount of gas absorbed or evolved by the experi
mental plants in the chamber. A few preliminary observa
tions made it clear th at the principal problem to be solved was the development of an efficient absorber for carbon di
oxide, 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 G cc. per minute.
I
N VIEW of the im portant role of atmospheric carbon dioxide in many phases of biology, such as animal and plant respiration, photosynthesis, and air pollution, it is surprising th a t no adequate method has thus far been proposed for determining it continuously and automatically.The best methods are somewhat laborious and time-consum
ing, and have an adm itted uncertainty of 5 to 10 parts per million. This applies to the gasometric method of SoncUn as employed by Benedict (1), the Pettenkofer evacuated- bottle method of Johnston and Walker (4), and the
electro-193
194 A N A L Y T I C A L E D I T I O N Vol. 5, No. 3 and for this purpose the use of fritted glass disks seemed prom
ising. 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. I t was suggested to the writer in private conversation by R. A. Fulton of the U. S.
Bureau of Entomology, Twin Falls, Idaho, th a t fritted glass disks could readily be made of Pyrex glass; and later the note of Bruce and Bent (2) on this subject was found. Accordingly a number of different types of absorbers were constructed of
Type 3 was constructed with 3 disks made from glass powder of 100 to 150 mesh. This absorber had an efficiency of 80 to
ance to the gas stream, suction of 5 to 10 cm. of mercury being necessary to draw the gas sample through them. I t was evident, therefore, th at increased efficiency could not be sought with disks of finer pores on account of the mechanical difficulty of drawing the gas through the septum.
I t is well known, as pointed out by Maier (5), th a t the size of the bubble is a function not only of the diameter of the orifice from which the gas emerges, hut also of the surface tension of the liquid. I t was therefore decided to add a sur
face 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
gas with the liquid. In 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, to
gether with the smaller bubbles, raised the efficiency of all the absorbers to 100 per cent with 0.005 N alkali. Further, 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 plati
num foil around the end of a tungsten wire, then welding a plati
num 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 electrodes were of considerable size, a supporting glass bead was inserted between them. thermostat, and air, measured in a gas meter, was drawn through them in series. At 2-minute intervals the aspiration was stopped and the conductance of the two solutions measured with 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 resist
ance of the solution to within about 0.3 ohm. This precision was sufficient to detect with certainty losses from the first ab
sorber as small as 1 per cent of the carbon dioxide in each 2- minute 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
T a b l e I. E f f i c i e n c y o f A b s o r b e r a s I n f l u e n c e d b y R a t e o f A i r F l o w , C o n c e n t r a t i o n o f C a r b o n D i o x i d e i n A i r ,
I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 195 average of the values to a chosen stage of the process. The
data indicate th at 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.
In 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 absorbent. 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. I t should be noted, however, th a t the efficiency decreased when the velocity was too low to produce froth in the solution. For example, in one experiment, a t 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.
T a b l e II. E f f i c i e n c y o f A b s o r p t i o n o f C a r b o n D i o x i d e b y 2 5 0 c c . 0 .1 Ba(OH)s i n A b s o r b e r S i m i l a r t o T h a t
D e s c r i b e d b y H e i n i c k e a n d H o f f m a n Su r f a c e Ef f i c i e n c y
from | | tro m
SourceN0.1. A Source NoZFrom
Ra t eo f Ai r He i g h to f Co l u m n Te n o f Ab
Fl o w L iq u id F r o th s i o n s o r p t i o n
C c ./m in . C m . C m . D y n e s /c m . %
320 30 0 . 5 72 100
8 5 0 30 5 72 97
2 2 0 0 30 30 72 93
2200 30 60 63 96
¡¿fi3ödä.'üm e-:f i r Automatic telvM
Heinicke and Hoffman (3) claim th a t 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 II shows th a t while this claim could not be strictly confirmed, the method is very promising. The absorber was similar to th a t of Heinicke and Hoffman, except th a t 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 a t the maxi
mum 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.
Hy d r o x i d e So l t j t i o n
From a consideration of the transport numbers of lithium and sodium hydroxide it w’as assumed th a t lithium hydroxide would show a greater change of conductance on being con
verted 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 alkalies is so small th at 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 th a t while the presence of the precipitate did not affect the measurement of the con
ductance appreciably, especially if the absorbing vessel was cleaned occasionally with dilute hydrochloric acid, a serious
F i g u r e 3. D i a g r a m o f A i r - F l o w C o n t r o l S y s t e m S h o w in g m eth o d o f a lte rn a tin g source o f air s a m p le (to p );
s te e l g a s m eter a n d ab sorbers (cen ter ); an d surge b o ttle and co n tro l v a lv e s (b o tto m ).
disadvantage was incurred because of the fact th a t the con
ductance of the solution decreased slowly for about half an hour after precipitation occurred. Apparently the solubility of barium carbonate was decreasing during this time, presum
ably because of growth in the size of the particles of the pre
cipitate. The fact th at equilibrium measurements could not be made quickly with barium hydroxide rendered th a t ab
sorbent practically worthless for the purpose a t hand.
Ca l i b r a t i o n o f El e c t r o d e s
The problem of calibrating the absorbers was finally solved by using the titration method of Walker, Bray, and Johnston (12) 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. A definite amount of 1 0 per cent barium chloride solution, 0 .0 1 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
196 A N A L Y T I C A L E D I T I O N Vol. 5, No. 3 end point at about pH 8.S, 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 th a t of the barium hydroxide added represents hydroxide, while a smaller titer represents bicarbonate. A carefully prepared solution of
Fi g u r e 4 . Se c t i o n o p Re c o r d e r Pa p e r, Il l u s t r a t i n g Ty p e o f Re c o r d Ob t a i n e d
W id th o f paper, 25 cm . (1 0 in .); p aper s p e ed , 2 0 cm . (8 in .) per hour.
sodium carbonate in carbon dioxide-free water does not change the blank titer.
In carrying out the calibration, about 50 ml. of alkali were introduced into the thermostated absorber and air was as
pirated 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 th a t the temperature coefficients of conductance of hydroxide and carbonate differ appreciably, calibration must he made for each temperature a t 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 arid therefore the calibration curves at different temperatures converge a t the hydroxide end. As the temperature coefficient of the carbonate is considerably greater than th a t of the hydroxide, the calibration curves be
come steeper a t higher temperatures. Accordingly, the in
strum ent may be expected to show a greater change of con
ductance for a given amount of absorption the lower the tem
perature of the solution. I t will be noted th a t the response of
the recorder to the conversion of hydroxide to carbonate is linear over a wide range.
Ca r b o n Di o x i d e Au t o m e t e r
The conversion of the laboratory model of the sulfur dioxide autometer (5) into a carbon dioxide autometer was com
pleted 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 wras 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 fiat chambers connected by a U-tube and mounted on a pivot. Each chamber communi
cated through glass and rubber tubing to an absorber and also through a valve (H or I) to the source of the sample. Enough mercury was added to fill 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 th a t 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. In Figure 3, valves G and II were open, while valves I and K were closed, and vice versa. Electrical contacts were provided on each side of the gas meter so th a t when a chamber was com
pletely filled 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 th a t 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 a t 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 th a t any slight differences be
tween the two absorbers could be compensated for, as in the case of the sulfur dioxide autometer. In 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 aspira
tion adjusted the temperature of the solution and established an accurate zero for the determination. After each ab
sorber 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 w'hich it remained in the absorber. This arrangement proved to be suitable for long-continued studies of photo
synthesis 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
th at 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.
At point A, the conductance of the solution in absorber 1 was measured and the solution was discharged, fresh solution being added immediately, which is indicated by point B. Points C and
At point A, the conductance of the solution in absorber 1 was measured and the solution was discharged, fresh solution being added immediately, which is indicated by point B. Points C and