1346 experimental - ACS Publications

1346

C. BURKE MILES AND HERSCHEL HUNT

HEATS OF COMBUSTION. I

THEHEATOF COMBUSTION OF ACETONE‘ C. BURKE MILES? AND HERSCHEL HUNT

Department of Chemistry, P u r d w University, West Lafayette, Zndiana Received May 16, 1941

INTRODUCTION Rossini (11) and others have recently demonstrated the inadequacy of much of the existing thermal data. I n particular, the existing values for heats of formation of many organic compounds are in disagreement by as much as several kilogram-calories per mole. Errors in the standard free energies of formation (linear functions of the standard heats of formation) affect the calculation of equilibrium constants very adversely, owing to the exponential nature of the relationship between the two. The present paper describes a modification of existing apparatus and technique for precision determinations of the heats of combustion of readily volatile organic compounds. The heat of combustion of acetone has been redetermined with an absolute accuracy of & 0.05 per cent.

EXPERIMENTAL I. METHOD

The method employed is, in its essentials, the so-called “ordinary” method of constant-pressure calorimetry. The procedure involved is non-adiabatic, appropriate corrections being made for heat exchange between the jacket and the calorimeter proper (figure 1). The apparatus is very similar to that employed a t the National Bureau of Standards by Rossini et al. (12),and is a modification of the assembly described by Dickinson (2). The recommendations of this latter publication are closely adhered to in the present investigation. The principal differences between Rossini’s method and the present one are indicated in what follows. (1) A nine-junction copper-constantan thermel of known thermoelectric power is employed to measure the temperature rises accompanying the combustions. The potential changes are read with a White double potentiometer of 10,000 pv. (microvolts) range (Leeds and Northrup No. 7622). Slight variations in the jacket temperature are followed with a similar thermel. 1 This article is baged upon a thesis submitted by C. Burke Miles to the Faculty of Purdue University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, June, 1941. 2 Present address: The Pennsylvania S< Manufacturing Co., Philadelphia, Pennsylvania.

HEAT OF COMBUSTION OF ACETONE

1347

A platinum resistance thermometer is used a t the Bureau of Standards. (2) The heat of combustion of hydrogen is the standard with which comparisons are made. The temperature rise obtaining during the combustion of a known amount of compound X is compared with the temperature rise accompanying the combustion of pure hydrogen gas to form a measured quantity of liquid water, both reactions being conducted in the same calorimeter and under very similar experimental conditions. Electrical calibration is employed a t the Bureau of Standards. The present method has the advantage of simplicity of operation, but is of

FIG.1. The calorimeter

course dependent upon the initial accurate determinn ion of he heat of combustion of hydrogen by electrical calibration. The apparatus to be described is readily available or easily constructed in laboratories where the requirements of the electrical method as to high precision in timing and measurement of heating current and voltage are not easily fulfilled. The heat of combustion of hydrogen, H2(g)

+ 1/2 Oz(g) = HzO(1)

to form liquid water under a constant total pressure of 1 atmosphere and a t 25OC. is -68,318 f 10 g.-cal.,,. per mole of water formed (13). This value has been accepted as a thermometric standard by the International Committee on Thermochemistry (16).

1348

C. BURKE MILES AND HERSCHEL HUNT 11. APPARATUS

A . Calorimeter system A semi-schematic diagram of the apparatus is shown in figure 1. Figure 2 shows the combustion chamber in more detail. The special features of the ice bath (to be discussed) are illustrated in figure 3. The combustion chamber, entirely of Pyrex glass except for a quartz flame tip and the platinum spark leads, rests upon a bakelite support. Its position in the calorimeter is precisely the same during all combustion experiments. The chamber is of one piece to the ground-glass joints just above the jacket lid. A spiral coil (6-mm. Pyrex tubing, 4 ft. long) surrounding the chamber insures that the carbon dioxide and water vapor formed during combustion leave a t the temperature of the calorimeter can. The calorimeter can itself rests upon three small ivory cones attached to the can. These cones rest in three small depressions on the floor of the jacket. The calorimeter can is thus given a constant and reproducible position with respect to the jacket. The jacket-can separation is very nearly 1 cm. a t all points. Adequate stirring for the can and jacket water is provided by a single constanbspeed motor mounted upon the air thermostat surrounding the calorimeter assembly. The section of the thermostat illustrated in figure 1 is not to scale. The bearings for the brass propeller shafts are all above the calorimeter. This arrangement eliminates all vibration in the calorimeter system. The stirrers for the jacket are entirely of brass. The single shaft for the calorimeter cap is of brass down to the coupling (not shown) just above the jacket lid. Through the jacket lid, the shaft is of Q-in. bakelite rod. Between the jacket and the lid of the calorimeter is a permanent coupling (F, figure 1); below, the propeller shaft is of +-in. brass rod. The calorimeter can and jacket are of copper; all metal parts are nickelplated and polished. The three gas leads to the combustion chamber are made of 4-mm. Pyrex tubing; the platinum spark leads pass through the jacket lid enclosed in Pyrex capillary tubing 6 mm. in diameter. The hydrogen and oxygen leads turn upon greased ground-glass joints (I1 and It, figure 1) and are connected to the combustion chamber by additional ground joints at G (figure 1). The thermel for measuring the temperature rise is indicated at E. The jacket is provided with a similar thermel centrally located (Q, figure 1). The jacket water and metal surfaces are maintained a t a constant temperature by means of an electric heater shown at D. A rheostat in series with this heater is adjusted as needed during a combustion experiment. The lid of the jacket is removable as a single unit, a water-tight fit being


1349

accomplished by thin rubber gaskets a t either end. Another gasket on the under side of the jacket lid effectively seals the inner calorimeter against any effects due to the air circulating in the air thermostat. The air thermostat serves to hold operating temperatures near 25'C. Mean combustion temperatures are limited to approximately this maximum by reason of the low range of the potentiometer and the high thermoelectric power of the thermels. B. Combustion chamber (figure 2 )

The combustion chamber is constructed entirely of Pyrex glass except for a quartz combustion tip and the platinum spark leads. The quartz

FIG.2. The combustion chamber

flame tip is a separate piece, tapered and ground to insure a close fit to the capillary tube through which hydrogen gas or acetone vapor enters the chamber. The quartz tip extends about 1 em. beyond the capillary lead; the poor thermal conductivity of quartz and the cooling effect of the gas streams thus combine to prevent fracture due to unequal thermal coefficients of expansion a t the union. Ignition of the gas issuing from the flame tip is accomplished by sparking between the platinum spark points for a measured period of time. Except a t the tips the spark leads are enclosed in glass tubing. Air gaps of approximately 1 mm. are provided a t the point where the leads enter the inner

1350


calorimeter can. Comparable thermal leakages are thus insured for both hydrogen and acetone combustions. In the acetone experiments the flame is not in contact with the spark tips. Preliminary combustion trials showed that considerable catalytic oxidation and/or pyrolysis occurs a t the surface of bright platinum. The higher rate of diffusion of hydrogen, however, causes the flame to assume a “mushroom” shape, so that the platinum tips are in the combustion zone. The capillary lead for the combustible vapors, together with about 3 cm. of glass wool above the capillary, was found to be necessary to prevent the flame from flashing back during ignition of the acetone mixture. The acetone vapor is carried into the chamber by a mixture of purified helium and oxygen. The main portion of the required oxygen enters the chamber through a separate lead at the top. Oxygen for the combustions is usually supplied in threefold excess of the stoichiometric requirements. The water formed during combustions is largely condensed in the chamber below tho flame tip. A small portion (about 0.1 g.) escapes as vapor. The heat effect accompanying this vapor loss is included as a correction in the final calculations. C. The ice point

The requirement of a constant reference temperature for the two thermels is provided for by the apparatus indicated in figure 3. A modification of the ice bath described by White (17) is employed. The points of difference between White’s method and the present one are indicated below: (1) So attempt is made to obtain a temperature of exactly 0°C. A temperature which is precisely constant during a combustion experiment is sufficient for the present experiments. Finely crushed commercial ice and once-distilled water are used in the two Dewar vessels. KO effort is made to free the water of dissolved gases. (2) The ice in the Dewar flasks is washed after installation (as recommended by White). The precooled wash water is contained in a vessel located in the same outer ice bath that surrounds the Dewar flasks and is connected to these flasks by a $-in. length of clean rubber tubing a t the T-seal (B, figure 3). The insulated outer ice bath is maintained with a temperature gradient of 3-4°C. from top to bottom by replenishing the ice (rather large lumps) once a day. The temperature is 0°C. at the top, where most of the small heat exchange with the inner ice bath occurs. The Dewar flasks are rubber-stoppered. (3) In preliminary trials with the relatively warm outer bath it was discovered that, over a period of several days, sufficient leakage of heat occurred a t the mouths of the Dewar flasks to cause a serious melting of the ice in contact with the glass tubes (CI and Cs, figure 3) into which the ther-

1351


mels are lowered. Examination showed that these tubes were no longer in actual contact with the ice in the Dewar flasks, melting having created an ice-free gap of several millimeters. This difficulty was eliminated by attaching closely fitting, finned copper sleeves (D1 and DO,figure 3) to the tubes. The slight melting is thus distributed over the ice bath and good contact with the ice is maintained. The leads a t A (figure 3) are of glass tubing and serve to accomplish washing of the ice by complete draining (as described by White). These tubes are stoppered when not in use. J

I I

k- I O C M . 4

FIG. 3. The ice bath

The flasks are fitted with metal sleeves a t the bottoms and are held rigidly in place by wedging upon large rubber stoppers cemented to the floor of the outer ice bath. With the arrangements described, it was possible to maintain stable ice points, ready for use, for periods as long as 7 to 8 days (with the temperatures in the outer ice bath as previously indicated). The operator is thus freed from the tedium of carefully preparing new ice baths for each combustion experiment. Checks with a differential thermel before and after washings showed that %he absolute temperatures of our ice points drifted a maximum of & 0.0003"C. in a period of 4 to 5 hr. The relative constancy of the two was generally much better. The day-to-day fluctuations amounted to

1352


approximately 0.002-0.003°C. The time consumed by a combustion experiment is 1 hr.

D. Precombustion purification of oxygen, hydrogen, and helium and absorption of products of the combustion Cylinders of commercial gases are used. I n the cases of the oxygen and the helium, purification is accomplished by (a) passing through quartz furnaces packed with copper oxide and maintained a t 800-900°C. and (b) absorbing all traces of acidic oxides and water vapor by passing through three tubes containing, in the order named, ascarite (a granulated sodium hydroxide-asbestos mixture), dehydrite (the trihydrate of magnesium perchlorate), and anhydrous phosphorus pentoxide. Oxidation of any traces of hydrogen or carbonaceous material in the oxygen or helium, respectively, is thus accomplished and the carbon dioxide or water vapor formed or present originally is eliminated. The hydrogen gas is merely passed through ascarite, dehydrite, and phosphorus pentoxide before use. The presence of a small amount of oxygen in the hydrogen is obviously not objectionable. Blank trials indicated the complete absence of acidic oxides or water vapor in the gases thus purified. The rate of flow of each of-the gases is measured by flow meters, using redistilled mercury as the indicating fluid. For an acetone combustion, helium and oxygen, after passing through their individual purification tubes and flow meters, are led into a common tube in approximately equal proportions by volume. This artificial “air” is then passed through acetone “absorber” tubes thermostated a t a temperature 1to 2” lower than that a t which the acetone vapor is thus delivered to the control stopcocks indicated in figure 1. The entire precombustion system is constructed of acid-washed Pyrex glass, and all necessary stopcocks are lubricated with a special grease of very low vapor pressure. The amount of chemical reaction during a combustion is determined by absorbing the carbon dioxide and/or water formed in two U-tubes connected to the exit line from the combustion chamber. The first tube contains dehydrite, followed by a small amount of phosphorus pentoxide. The second tube contains ascarite. Loss of water of crystallization in the ascarite is prevented by means of a short plug of phosphorus pentoxide a t the exit end of the U-tube. 111. EXPERIMENTAL PROCEDURE

The actual procedure employed during combustion experiments very closely follows that described by Rossini (12). The calorimeter electromotive force is read a t zero time and on evennumbered minutes thereafter. The jacket electromotive force is taken every fourth minute, as is the galvanometer deflection due to slow drift of


1353

the true zero point. Stray potentials in the galvanometer circuit are thus eliminated. The true zero point is plotted against time for each experiment. Precise values of the zero point are read for any instant from the curves obtained.

METHOD OF CALCULATION The calculations required by the present technique are part of the “prior art” and have been adequately described by Eckman and Rossini (3). Figure 4 shows a time-voltage plot of the data obtained in a characteristic combustion experiment. E, is the nearly constant jacket electromotive force. E, actually fluctuates slowly in a maximum range of 0.005”C. and is known a t each instant to the nearest thousandth of a degree,-or to approximately 0.4 pv. (microvolt). Calculations are actually made in

TIME

FIG.4. A time-voltage plot

terms of microvolts in order to avoid the unnecessary tedium of conversion into temperature units. Corrections for “sources” and “sinks” of energy follow those given by Rossini (12) with one exception: The “sink” provided by the water vapor remaining in the combustion chamber and leads after the gas flows are halted is neglected. The energy involved here is negligibly small, however, and is absorbed as part of the systematic errors which are eliminated by the similarity of experimental procedure in the several experiments. The energy of vaporization of this small amount of water ( < 0.002 g., assuming saturation in the chamber and leads) is of the order of 1 calorie. The atomic weights employed in the calculations are those of the 1939 revision: H = 1.0081, 0 = 16.oo00, C = 12.010. A single exception is the use of the weight 1.0078 for hydrogen in the calibration experiments. This is the atomic weight used by Rossini in his determination of the heat

1354


of combustion of hydrogen (12). One mole of hydrogen is thus taken as the equivalent of 18.0156 g. of water (weight in vacuo). The necessary thermal data were taken as follows: (1) The unit of energy employed is the small calorie (g.-cal.15-),defined as the equivalent of exactly 4.185 absolute joules. This is in accordance with recent convention, following the recommendation made by the Division of Electricity at the National Bureau of Standards (9). (9) For correcting the heat of combustion of hydrogen to the mean temperature of a given experiment, AC, for the reaction is taken as +32 joules or approximately 7.6 g.-cal.llo per mole of water formed. The heat of combustion thus increases by 7.6 calories for a temperature increase of 1°C. (3) For correcting the data of the acetone experiments to 25"C., AC, for the reaction is taken as $32.2 g.-ca1.160per mole of acetone burned. The heat of combustion thus increases by 32.2 calories for a temperature increase of 1°C. It should be observed that the heat of combustion according to the present notation (8) is a negative quantity when heat energy is evolved. ACCURACY IN TEE EXPERIMENTAL DATA Absolute accuracy of the determinations is dependent upon the following factors: (1) An accurate method of weighing the water and carbon dioxide formed during a combustion is required. The weights employed were calibrated a t the National Bureau of Standards. Extreme care was exercised in the weighing operation. The lengths of the lever arms in the balance employed had a ratio of l.ooOo3. The weights obtained were corrected to yield the true mass (weight in vacuo), employing exactly the same method outlined by Rossini (12). The weights are recorded to 0.05 mg. and are believed to be accurate to 0.1 mg. (2) Knowledge of the absolute accuracy of temperature rises during a combustion is not necessary, since the method is one of comparison. The relative precision required here is insured by relative accuracy in the resistances of the potentiometer coils and by day-to-day constancy of the electromotive force supplied by the standardizing cell. The resistance coils were calibrated relatively as provided for in the circuits of the White double potentiometer. Observations of the constancy of the standard cell were made over a period of several weeks prior to the actu?l combustion experiments. With good thermal insulation and a fairly constant

1355


room temperature, the cell was found to give a maximum day-to-day variation of i 0.01 per cent. The maximum deviation from the mean over this period was 0.02 per cent. With the galvanometer and optical lever employed, 1 pv. is read as a deflection of approximately 2.5 mm. This allows an estimation of 0.1 pv. as 0.25 mm. (3) The purity of the acetone must be very high. The acetone used in the present investigations was prepared from a commercial “analytical” TABLE 1 Calibration data

I I ~

~

_

_

_

_

_

_

~

_

EXPERIMENT NO.

1

2

3

~

Combustion period, in minutes. . . . . . . . 8 12 AE2o,.o,* in microvolts.. . . . . . . . . . . . . . . 691.91 825.71 Total mass of water formed, in grams (in UQCUO). . . . . . . . . . . . . . . . . . . . . . . . 1.81430 2.17025 Reaction energy, in g.-cal.w.. . . . . . . . 6880.6 8229.1 Spark energy correction, in g.-cal.15-. +8.30 +8.30 Gas energy correction, in g.-cal.l,~.. . . +0.61 +2.88 Mass of water vaporized, in grams.. . . 0,0462 0.0805 Vaporization energy correction, in g.-cal.L5~, ........................ -26.9 -46.8 Ket effective energy, in g.-cal.ls*.. . . . . 6864.9 8191.2 Apparent heat capacity of calorimeter at 25”C.,with 3500 g. of calorimeter water, in g . - ~ a l .perpv.-l.. ~~........ 9.9217 9.9203 Mean of three experiments 9.9197 0.0020

I

10 806.25 2.11760 8029.2 +8.30 -0.93 0.0701 -40.8 7995.8

9.9172

* This is the temperature rise during the “middle period”-figure 4-(expressed in microvolts) after correction for temperature change due to stirring, conduction, etc. (3). The values have also been corrected to precisely 25°C. and to a common mass of calorimeter water (3500 g , ) . grade analyzing 99.5 per cent acetone. The bulk of the impurity present in this starting material is probably water. Purification was accomplished in two stages: (a) The impure acetone was separated from impurities other than ketones by fractional crystallization, employing the addition compound with sodium iodide, according to the recommendations of Shipsey and Werner (14). (b) The “pure” acetone thus obtained was dehydrated by distillation over phosphorus pentoxide. During this distillation a slow stream of dry inert gas was passed through the receiving vessel to exclude contamination by moisture in the air. The entire distillation apparatus was constructed of Pyrex glass. The operation involves considerable loss, owing to formation of some

1356


condensation products of acetone (induced by the phosphorus pentoxide), The product obtained was further fractionated, employing a distillation column having ten theoretical plates. This distillation was performed under anhydrous conditions. The middle third of the acetone thus distilled w m used in the combustion experiments. The ratio of the masses of carbon dioxide and water formed during a combustion is taken as an indication of the purity of the acetone. A combustion chamber very similar to the one employed in the later experiments was used here. TABLE 2 Data f o r combustion of acetone

I

EWEEIMBNT NO.

1

Combustion period, in minutes. . . . . . . A . E l o , , ~ , *in microvolts.. . . . . . . . . . . . . . . .

12 939.95

Total effective energy input, in 9324.0 g.-cal.16-.. . . . . . . . . . . . . . . . . . . . . . . . . . . -8.30 Spark energy correction, in g.-cal.w. . Gas energy correction, in g.-cal.lv. , , . -0.98 Acetone blank, in grams. . . . . . . . . . . . . . 0.0045 0.1786 Mass of water vaporized, in grams.. . . Vaporization energy correction, in g.-cal.l,v.. . . . . . . . . . . . . . . . . . . . . . . . . . . +104.0 Reaction energy a t mean temperature . of experiment, in g.-cal.ls-.. . . . . . . . . 9418.7 Mass of carbon dioxide formed in grams (in vacuo),. . . . . . . . . . . . . . . . . . . 2.85795 Heat of Combustion a t temperature of experiment, in kg.-cal.lv per mole of acetone., . . . . . . . . . . . . . . . . . . . . . . . . 435.12 Heat of combustion corrected t o 435.12 25.00T.. . . . . . . . . . . . . . . . . . . . . . . . . . . . Mean of three experiments (at 25.OOC.).. . . . . . . . .

. . . . . . . . . . . . ..I

* See footnote to table

2

3

716.01

10 630.68

7102.6 -8.30 +0.95 0.0048 0.1336

6256.6 -8.30 +1.20 0.0052 0.1297

4-77.7

+75.4

7172.9

6324.9

10

2.17580

1.91735

435.25

435.52

435.28

435.55

435.32 f 0 . 2

1.

During the sparking operation a small amount of acetone escapes the flame tip unburned. This is the acetone blank listed in tables 2 and 3. This acetone is absorbed and weighed as water, thus causing an apparently large error in the experimental carbon dioxide-water ratio. That the apparent excess of carbon dioxide is not due to impurities in the acetone is demonstrated by performing two sets of combustion analyses. The acetone blank is found to remain constant regardless of the amount of combustion, thus indicating 100 per cent purity for the acetone within the

1357


limits of the weighing precision. The results of the analyses are given in table 3. The combustion chamber employed here was accidentally broken before final combustion experiments were performed. The discrepancy in the acetone blanks encountered with the two chambers is accounted for by the slight difference in construction of the two flame tips. Chamber No. 2, used in the final experiments, had a slightly larger dead space inside the quartz flame tip. This dead space provided a region of relatively low gas velocity, thus causing the flame to flash back once or twice before forming a quiet cone a t the tip. Between these flashes, of a very short duration, an additional small amount of acetone escapes unburned. (4) The purity of the reaction is guaranteed by the constancy of the ratio of carbon dioxide to water (after correction for the acetone blank). ComTABLE 3 Combustion analysis o j acetono WEIIIYENT NO.

1

2

a

4

~~~

Apparent mass of water formed, in grams.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3333 1.1576 0,1111 Mass of carbon dioxide formed, in grams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2532 2.8246 0.2682 Apparent mass ratio COz/H*O.. . . . . . . . 2.4400 2.4401 2.4140 Theoretical mass ratio. . . . . . . . . . . . . . . . . . 2.4428 Acetone blank, in grams. . . . . . . . . . . . . . . 0.0015 0.0013 0.0012 Mean for four experiments.. . . . . . . . . . . . 0.0014 Deviation from mean, in grams. . . . . . . . . f0.0001 -0.O001 -0.0002

0.1084 0.2607 2.4050 0.0017

t o .0003

pleteness of combustion is insured by the large excess of oxygen supplied in all experiments.

DISCUSSION OF RESULTS The “best” value for the heat evolved by the reaction (CHs)zCO(g)

+ 4oz(g) = 3coz(g) + 3Hz0(1)

a t a temperature of 25OC. and under a constant total pressure of approximately 1 atmosphere is taken as the mean of the experimental figures: 435.32 f 0.2O,Kg.-~al.,~~ per mole of acetone burned. The odds are four to one that the true heat of combustion lies within this range. Existing data on the heat of vaporization of acetone allow a fairly accurate calculation of the heat of combustion of liquid acetone. A thermodynamic calculation employing vapor pressure data from the International Critical Tables ( 5 ) yields the figure 7497 g.-cal.,,. as the heat of vaporiza-

1358


tion of 1mole of acetone a t 25OC. A graphical interpolation of calorimetric data taken from the same source (6) gives the figure 7,605 g.-cal.,,.. Weighting these two values equally gives for the heat of vaporization the figure 7550 f 60 g.-cal.,b.. Thus, the heat evolved by the reaction

a t 25OC. and 1 atmosphere constant total pressure is 427.77 f 0.26 Kg.cal.,5nper mole of acetone burned. Later experimental data on the vapor pressure of acetone are few. A recent determination (10) gives the figure 115.4 mm. of mercury as the vapor pressure of acetone a t 10°C. This is in very good agreement with the corresponding value (1 15.6)listed in the International Critical Tables. The two sets of data do not overlap except a t this one temperature.

EEAT OF COMBUSTION OF ACETONE

REFERBNCP)

kg.-xl. par mole

428.9 431.0 427.0 427.77 f 0.26

(15) (4) (1) Authors

All previous values for the heat of combustion of acetone are very old. The results of previous investigators are listed in table 4. These data are from Kharasch’s compilation (7); the reference numbers indicate the original sources. All figures listed refer to the heat of combustion of 1 mole of liquid acetone to form liquid water and gaseous carbon dioxide a t 25°C. and under constant total pressure of 1 atmosphere. We have brought Kharasch’s listed values into accordance with the modern molecular weight of acetone (58.0786). The first listed figure (Thomsen’s) was obtained by subtracting 7550 g.-cal.,,. per mole from the experimental heat of combustion of gaseous acetone (after making the molecular weight correction). The present figure is included for comparison purposes. REFEREXCES (1) D E L ~ P I NM.: E , Compt. rend. 131,745 (1900). (2) DICKINSON, H . C.: Natl. Bur. Standards (U. S.) Bull. 11, 189 (1915); Natl. Bur. Standards (U.S.) Sci. Paper No. 230 (1914). (3) ECKMAN, J . R . , AND ROSSINI,F . D.: Bur. Standards J. Research 3, 597 (1929). (4) EMERY, A . G . , AND BENEDICT, F . G . : Am. J . Physiol. 28, 301 (1911). (5) International CriticaE Tables, Vol. 111, p. 218. McGraw-Hill Book Company, Inc., New York (1928).

PHOTOSYNTHESIS IN FLASHING LIGHT

1359

(6) International Critical Tables, Vol. V , p. 137. McGraw-Hill Book Company, Inc., New York (1929). (7) KHARASCH, M. S.: Bur. Standards J. Research 2.359 (1929). (8) LEWIS, G . N . , AND RANDALL,M.: Tkrmodynamics. McGraw-Hill Book Company, Inc., Kew York (1922). (9) National Bureau of Standards (U. S.), Technical News Bulletin No. 156 (1930). (10) RADULESCU, D., AND ALEXA,M.: Bull. soc. chim. Romania 2OA, 89 (1938). (11) ROSSINI,F . D.: Ind. Eng. Chem. 29, 1424 (1937). (12) ROSSINI,F . D.: Bur. Standards J . Research 6. 1 (1931). (13) ROSSINI, F. D.: Bur. Standards J. Research '22,407 (1939). K . , AND WERNER, E . A.: J. Chern. SOC.103, 1255 (1913). (14) SHIPSEY, (15) THOMSEN, J.: Z. physik. Chem. 62, 343 (1905). (16) Union Internationale de Chimie: Premier Rapport de la Commit6 Permanenle de Thermochimie. Paris (1934). (17) WHITE,W. P.: J. Am. Chem. SOC.66,20 (1934).

PHOTOSYNTHESIS I N FLASHING LIGHT S. WELLER

AND

J. FRANCK

Department of Chemistry, University of Chicago (Fels Fund), Chicago, Illinois Received June 8, lO4l I. INTRODUCTION

For years there existed in photosynthesis the difficulty that the shape of the continuous light saturation curve speaks for a dark reaction lasting about 1 min., whereas direct measurement of the dark reaction time by the flashing light method (3) gives around 0.01 sec. for the half-period of the dark, or Blackman, reaction. That problem has been explained by Franck and Herzfeld (6) by the assumption that the limiting reaction a t light saturation is one in which a catalyst works on a photochemically made substrate which is unstable; all the substrate which cannot be handled by the catalyst is eliminated by back-reactions. We can avoid, in this way, any accumulation of substrate made by a photochemical process and not removed by the catalyst. A further consequence of the instability of the substrate is that a dark pause between illuminations will not permit the catalyst to continue working on accumulated material; by the time catalyst molecules have recovered from working once, any original excess of substrate will have disappeared by back-reactions. With this picture, then, the dark period found by Emerson and Arnold becomes identical with the working, or recovery, time of the catalyst. The results of Emerson and Arnold also indicated that low temperature and cyanide showed the similar effects of merely prolonging the Blackman

1346 experimental - ACS Publications

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