5 INDUSZ'RIAL A N D ENGINEERING: CHEMISTRY
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Investigations in Photosynthesis' An Electrometric Method of Determining Carbon Dioxide By H.A. Spoehr and J. M , McGee COASTALLABORATORY, CARNEGIEINSTITUTION OF WASHINGTON,CARMEL.CALIF
T
HE methods which A method has been devised for determining relatively large quantithe periods of determinahave been generally ties of carbon dioxide in an air stream with a high degree of accuracy, tion of the photosynthetic employed for deterSO that changes in a stream containing ouer twenty times the amount rates be as short as possible, mining the rate of Photoof carbon dioxide normally in the atmosphere can be ascertained in order that changes due to synthetic activity have been internal or external condito 0.002 per cent. The method is based upon the absorption of carbon dioxide in barium hydroxide solution and the determination of the tions can be detected readbased upon three Principles: (1)the liberation of oxygen! change .in concentration of the latter through electrical resistance ily. But the amount of car(2) the absorption of carbon measurements. The necessary precautions and apparatus are bon dioxide given off or fixed dioxide, and (3) the f ~ m a - described a s well a s the mode of constructing a resistance-concentraby a single leaf in as short a tion of organic ~ u b ~ t a n c e . fion curve by using, instead of sfandard hydrochloric acid, the Period 8%say, 15 minutes, While the use of each of is exceedingly small. The carbon dioxide in the atmosphere. The latter thus seroed U S a very dilute solution of carbon dioxide of known concentration of which problem then resolves itself these Principles Possesses certain definite advantages carefully measured volumes were used to standardize the barium into absorbing SO comhydroxide solution. pletely the relatively large in interpreting the phenomenon of photosynthesis, amounts of carbon dioxide they are not all of them in the air stream that the amenable to the same degree of accuracy and ease of very slight differences in the carbon dioxide exchange application. On account of the fact that in determining of the leaf can be determined with sufficient accuracy. It is photosynkhetic rates it is essential that consideration be also therefore necessary to adjust several factors in the experigiven to the rates of respiration, it has been found that a mental procedure so that the differences in the rate of photomethod based upon the differential determination of the con- synthesis and respiration fall within the experimental error centration of carbon dioxide in the medium surrounding the of the method. plant affordsthe easiest, as well as the most accurate, method It is necessary to have the volume of the absorbing solution of determining the rate of carbon dioxide fixation by the plant. and the measurements of the resistance commensurate with This does not imply that it is the only method which should the accuracy required. When a relatively large amount of be employed in the study of the photosynthetic Process; carbon dioxide is to be absorbed, as is the case when the in fact, data obtained by means of the other principles are periods of determination are long and several leaves are used Qf equal significance in gaining a clearer conception of the in the respiration chamber, 125 cc. of 0.12 N barium hydroxide mechanism of the process. However, the determination of solution and a simple cell consisting of a wide-mouthed carbon dioxide can be accomplished with greater accuracy glass vessel with dip electrodes can be used. Such a cell, and ease than of either oxygen or the synthesized products. described in a former report14had a cell constant of 1.284 a t 25" C. and had an observed resistance of 96.6 ohms when PRINCIPLE OF METHOD filled with 0.1 N potassium chIoride solution. The general principle of the electrometric detennbtion of When the carbon dioxide emitted or absorbed by a single carbon dioxide, as used in this laboratory, has already been leaf during a period of 2 or more hours was to be measured, described;2 it has since been amplified and improved to Yield a pipet-shaped ce11,6 the observed resistance of which was more accurate results, SO that 0.002 per cent of carbon dioxide 1940 ohms a t 25' C. and which had a cell constant of 25.026, by volume can be determined with certainty. The method was employed. is based upon the absorption of the carbon dioxide in an air Where the amount of carbon dioxide given out by a single stream by means of barium hydroxide, and the determination leaf in a period of 1 hour or less was to be measured, it was of the strength of the barium hydroxide solution from its found by calculation that neither of these cells would be electrical conductivity. AS first devised, the method was accurate enough, since a difference of 1mg. of carbon dioxide used for determining the carbon dioxide emission from an absorbed by the solution would make a change of but 0.21 entire plant or a larger number Of leaves; it has since been ohm in the observed resistance of the solution when measperfected so that the carbon dioxide exchange of a single ured in the simple cell and of 8.26 ohms in the observed reexcised leaf can be determined with a high degree of accuracy. sistance when in the second cell. Calculation showed that a cell having a sensitivity at least three times greater than PRECAUTIONS NECESSARY that of the second cell was required to measure the small In employing the method of differential determinations of changes of resistancethat would be found. carbon dioxide for determining photosynthetic activity, APPARATUS a number of factors must be b v i n careful consideration. For reasons which have already been discussed,s when workThe accuracy of this method is limited by the accuracy ing with land plants it is advantageous to use a single ex- with which the resistance of the solution can be determined, cised leaf. Moreover, it is desirable that the carbon dioxide The most convenient source of current is the commercial of the air be increased five to twenty times 110-volt, GO-cycle alternating current, and a very convenient that of normal air, On the other hand, it is essential that combination set-up of bridge and dial resistance boxes with Received September 12, 1923. Spoehr and McGee, Crrrnegde Inst. Pub., 925, 28, 89 (1923). a l b i d . , 925, 22 (1923). 1 2
4 Carnegie I n s t . P u b . , 885, 28 (1923). E l b i d . , 895, 89 (1023).
February, 1924
INDUSTRIAL A N D ENGINEERING CHEMISTRY
galvanometer can be obtained for such a current, The sensitivity of an electrolytic cell is proportional to its resistance limited by the accuracy of the resistance boxes and the degree of polarization and self-induction set up in the system used. With these considerations in mind, the resistance required to give the desired accuracy for the work with a single leaf during short periods was found, when calculated, to be 6000 ohms or more. A cell having this resistance when filled with 0.1 N potassium chloride was constructed and tested to see if the errors due to polarization or selfinduction were too large to be negligible. Since these errors were found to be very small, a cell was devised that would have this resistance and yet would not require a large volume of solution and that could be filled without exposing the solution to the carbon dioxide of the air. A pipet-shaped cell of 30-cc. capacity having two bulbs was found to meet all these requirements better than any other form in common use. The bulbs of such a cell served as the electrode chambers, The electrodes (0.25 sq. cm. in area) must be small in order to give the desired resistance. These were placed with the flat side vertical so that air bubbles which might form in the solution did not collect on the electrodes and change the resistance. Errors due to this last cause may be minimized by warming the solution t o be tested to the temperature of the thermostat before filling the cell. The pipet cell was fitted with a glass stopcock in the inlet tube and with a soda lime tube fitted into the upper bulb to protect the solution from the carbon dioxide of the air.
129
of BenedictG has shown that the n dioxide content of dry air is remarkably constarit. alysis of dry air from various open air sources differed than 0.002 per cent. Hence air was used as a constant source of carbon dioxide and his average value of 0.031 per cent by volume, the average of 212 analyses, was employed in all calculations. When a measured voldme of air at known temperature and pressure was drawn through the barium hydroxide solution, the weight of carbon ,dioxide that had been absorbed could be calculated. The atmosphere was thus used as a very dilute solution of carbon dioxide of known concentration, each liter of which, when dry, 076 mg. of carbon dioxide. TABLE I-SPECIFICRESISTANCE OF VARIOUS CONCENTRATIONS OF BARIUM HYDROXIDE SQLUTIONS
Volume of Water Displaced Liters
c.
AT 25' WITH THE CARBON O P 75 C C . O F THE SOLUTION
Volume of Dr$ Air at Oo C . and 760 Mm. Liters 0.0000 1.7534 2,5602 2.5273 4.4085 8.8300 13.2940 13.2550 17.9030 22.3910 25.8440 31.2960 35.844 40.428 44.843
Weight of COa in Dry Air Grams
DIOXIDE EQUIVALENT
COz Equivalent of 75 Cc. of Ba (0H)a Solution Grams
Specific Resistance Ohms
The apparatus by which a measured volume of air could be drawn through barium hydroxide solution consisted of tenbulb absorption tubes, the air entering the solution through narrow tubes inserted in the lower end of the absorption tubes. The latter were filled with 75 cc. of the barium hydroxide solution and air was drawn from out of doors by means of a Mariotte bottle aspirator. The rate a t which the air was drawn through the apparatus was regulated by means of a needle valve. The water from the outlet tube of the aspirator was collected and its volume measured, in order that the volume of air that had entered the aspirator might be known. A water-filled pressure gage attached to the aspirator permitted the air pressure inside to be measured, and the temperature was determined from a thermometer placed beside the bottle. The determinations with volumes of air greater than 15 liters were made in an underground constant-temperature room in which the temperature varied but 0.9 degree during the entire time the determinations were being carried out. The contents of the absorption tube, RESISTANCE-CONCENTRATION when the desired volume of air had been drawn through, CURVE
FIQ. ELECTROLYTIC CELL
I n order to translate the resisDu~lVIfY OF THE tance valuesof the barium hydroxide HYDROXIDE SOLUTION solution into concentration values, a resistance concentration curve, the accuracy of which would be equal to that of the data desired, had to be constructed. It was necessary to determine the concentrations of the different solutions of barium hydroxide used in constructing this curve more accurately than could be done by titration with dilute hydrochloric acid. This was accomplished by taking successively a number of 75-cc. portions of 0.1 N barium hydroxide solution and allowing each to absorb a known quantity of carbon dioxide,,permitting the barium carbonate to settle out, and determining the resistance of the clear supernatant solution a t 25" C. When attempting to measure a quantity of carbon dioxide of less than 10 mg. it was found imperative that all containers for the barium hydroxide solution be free of carbon dioxide. I n all this work the absorption tubes, sample bottles, pipet, etc.? were swept free of carbon dioxide, before being used, by a stream of air passed through soda lime. The work FOR
e Benedict, Carnegic Inst. Pub., 166, 114 (1912).
DWERWNING THE CON-
FIQ.2-sPECIFIC
RESISTANCE C U R V E O F BARIUM HYDROXIDE USED CARBONDIOXIDE DETERMINATIONS
IN
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
were immediately transferred to a sample bottle free of carbon dioxide and sealed with paraffin until the resistance could be determined. Thus the temperature, pressure, and weight of carbon dioxide absorbed from each volume of air were calculated and the specific resistance determined, as shown in Table I. As examples of the accuracy of this method, two determinations of the carbon dioxide in 3 liters of air showed a difference of 0.05 mg. carbon dioxide and two determinations of the amount in 5 liters of air a difference of 0.11 mg. carbon dioxide. These differences are equivalent to a difference of 0.001 per cent by volume of carbon dioxide in the determinations, an order of accuracy equal to that obtainable with the Sonden air analysis apparatus, hitherto the most accurate devised. Using the cell with a cell constant of 85.204, a difference of 1 ohm in the observed resistance of the barium hydroxide solution, when calculated as specific resistance, was equal to 0.45 mg. carbon dioxide on the middle portion of the curve. Hence a milligram of carbon dioxide when absorbed would change the observed resistance 'of the solution 21.7 ohms. Since the change of resistance with concentration is not linear, an ohm will have a slightly
Vol. 16, No. 2
different equivalent value in milligrams of carbon dioxide on different portions of the curve. From the data so obtained, knowing the original concentration of the solution, the carbon dioxide gram equivalent of each 75-cc. portion, after the desired volume of air had been drawn through, was calculated. This was plotted against the specific resistance of the solution, which was calculated from the observed resistance so that one chart could be used to translate the data obtained when using different cells. Since the curve was to be used in work where the amount of carbon dioxide absorbed by 75 cc. of solution was to be determined, the concentration of the barium hydroxide solution was expressed in gram equivalents of carbon dioxide instead of equivalents of barium hydroxide, thus saving much labor in calculating results. The data were plotted on a scale such that the readings from the chart would be equivalent in accuracy to the resistance measurements. The range of the curve drawn through the points so plotted is limited to a chart of convenient size. A curve allowing for the absorption of 30 mg. of carbon dioxide was found to be the maximum that could be drawn with accuracy.
T h e Simultaneous Production of Pentosan Adhesives and Furfural from Corncobs and O a t Hulls' By Frederick B. LaForge BUREAUOF
CHEMISTRY, WASHINGTON,
D. C.
P
This article describes in detail the process worked out at the to about 2 Per cent without ENTOSAN adhesives Bureau of Chemisfry experimental plant for the production of penserious adverse effecton the are thick solutions of gums obtained by extosan adhesives and furfural from corncobs and oat hulls. The adhesive. With further yields of the two products from both of the raw materials were deheating the adhesive is r a p tracting various materials, termined, a+ well a s the steam consumption, time of ,fhe reaction idly decomposed, with the such as corncobs and oat period, and the proper proportion of water to raw material. formation of furfural. With hulls, with superheated Special reference is made to the use of pentosan adhesives for a reaction period of 2 hours water and concentrating a maximum yield of furthe extract. Previous rebriquetfing anthracite coal. fural is obtained. ports on this subject by the Bureau of Chemistry ADHESIVES FROM CORNCOBS have not described the process in detaiL2 Owing to the simDlicity - - of the method used and the easy availability of SEMICOMMERCIAL PLANT~-FO~ the digestion of the cobs the raw materials, these adhesives can be manufactured at a a cylindrical steel digester, heated by the injection of live comparatively low cost. steam, was employed. The average charge consisted of about In the process as originally planned several by-products, one of which is furfural, are obtained. The main object of 200 pounds of cobs and 850 pounds of water a t room temperaor 950 pounds when hot water was used. At the end the first experiments, however, was to obtain the maximum ture, of the heating period the pressure was relieved by blowing yield of adhesive material. Later developments having off through a condenser, after which the contents of the brought out the importance of furfural as a commercial article, it seemed desirable to attempt to increase the yield digester weye discharged by means of a valve into a drain box. The solid residue was then further freed from the of this product. absorbed liquor by means of a hydraulic press. As a rule The former method, worked out by the Bureau of Chemthe press cake was moistened with water and the pressing istry for the preparation of corncob adhesive, consists in heating corncobs with water in a pressure digester and con- operation was repeated. The discharge and wash liquors were evaporated to the proper consistency in an open steamcentrating the resulting solutions of gums to about 28" BB. jacketed kettle. (about 65 per cent solids). With a temperature of about Most of the furfural formed in this process was found in the 180" C. and a heating period of about 28 minutes, the yield condensed blow-off vapors, the furfural content of the disof adhesive is 40 to 45 per cent and the yield of furfural is tillates varying from 0.75 to 1-00 per cent. A large part of 1 to 1.5 per cent, based on the air-dry weight of the cobs. total quantity produced, however, was found in the It was found that by prolonging the heating period for 7 the discharge liquor in about 0.1 per cent concentration. This to 15 minutes the same yield of adhesive could be obtained fraction was by analysis, but was not generally and a t the same time the yield of furfural could be increased isolated. In determined commercial practice, the portion contained in 1 Received September 26, 1923. the discharge liquor would be recovered as a weak solution
2 THIS JOWRNAL, 10,025 (1918); U. S. Patent 1,285,247 (November 19, lgl8); Chem. Age (N.Y.),a8.332 (1920).
THISJOWRNAL, 15, 828 (1923).
