IiVDUSTRIAL d,VD ENGINEERING CHEMISTRY
912
Vol. 17, No. 9
The Oxy-Cal~rirneter’~‘ Principle and Application to the Determination of Energy Values of Fuels, Foods, and Excretory Products By Francis G. Benedict and Edward L. Fox NUTRITION LABORATORY, CARNEGIR INSTITUTION OF WASHINGTOX, BOSTON,MASS.
An oxy-calorimeter is recommended for use in industrial and dietetic or nutrition laboratories for the determination of the energy value of various substances. I t consists of a simple combination of a combustion chamber, a motor-blower unit, a soda lime absorption vessel, and a simple spirometer or expansion chamber. One form of oxy-calorimeter has a high degree of accuracy and is recommended for those laboratories not already possessing respiration apparatus, and particularly for those industrial laboratories wishing to measure the heating value of fuels with considerable exactitude. The apparatus in this form
is described in detail. Another, less accurate, form may be used to determine the energy values of foods and excreta. A series of standardization tests with pure substances and with fuels, accompanied by actual heat determinations in a bomb calorimeter, resulted in the accumulation of numerous heat factors. These factors may be applied to the direct determination of oxygen by the oxy-calorimeter and thus estimations of the energy values of fuels, foods, and excreta may be secured with a high degree of accuracy.
T
ence to the oxygen consumption. The principle depends upon the combustion of a known weight of substance in a confined volume of nearly pure oxygen, with provision for the rapid absorption by soda lime of the carbon dioxide produced, and the return of the purified oxygen to the combustion chamber. The quantitative determination of the carbon dioxide is not ordinarily made, but by inserting a calcium chloride bottle before and after the soda lime bottle, as originally used in the portable respiration a p p a r a t ~ sone , ~ can from the weight of the soda lime bottle and its subsequent calcium chloride bottle determine the carbon dioxide quantitatively. The contraction in volume due to the absorption of oxygen in the process of combustjon is measured either on a spirometer of known capacity or by means of a small, rubber-covered expansion chamber, which is brought back to its original volume by introducing a metered or measured volume of air or oxygen after the combustion ceases. The composition, the heat of combustion, and the oxygen required for complete oxidation are known or can be calculated for many pure substances, and from the total energy per gram and the volume of oxygen required for complete combustion of the substance-in which the end products are carbon dioxide, water, and, in the case of protein of foods, pure nitrogen-the calorific value of a liter of oxygen is easily computed. Thus, the heat of combustion of dextrose has been repeatedly determined as 3.74 calories per gram. The equation for complete oxidation is
HUS far but one method for the direct determination
of the energy values of fuels, food, and excreta has been available-i. e., the use of some form of bomb or pressure chamber in which the substance to be burned is placed in contact with highly compressed oxygen or some highly oxygenated substance, such as sodium peroxide or potassium chlorate. The calorimeter with which such a bomb may be used for the most part involves the complete submersion of the bomb in water in a well-insulated vessel. If the insulation is only moderately good, as is usually the case, there is an inevitable, complicated “cooling correction” to be made in the calculation of results. With the better insulation of a vacuum vessel this “cooling correction” is reduced, but i t is entirely eliminated by use of the adiabatic form of calorimeter, in which the temperature of the outer water jacket is automatically increased as the temperature of the water in which the bomb is placed rises. All these methods, however, involve an expensive bomb, an apparatus for filling the bomb with high-pressure oxygen and tightly closing it, an elaborate calorimeter with attested thermometers, and, finally, time-consuming calculations, which usually involve more mathematics than the highest limit of accuracy in the preparation of the sample from unhomogeneous material warrants. I n the case of fuels the comparison of the actually determined heat of combustion in the bomb with ultimate and proximate analysis has proceeded to such a stage that many formulas are now available for computing the energy of the fuel from the analysis. This latter of itself is a matter of no small difficulty, and hence the end r e s u l t 4 e., the heat valuation of the fuel-is not perceptibly simplified. Proposed Method The method proposed is the direct determination of the volume of oxygen required to burn a known weight of the substance, and the use of this measurement in the estimation of the heat value of these substances by means of a series of directly determined factors giving the calorific value of a liter of oxygen. The apparatus has been styled an “oxycalorimeter,” since it measures the heat indirectly by refer-
* Received May
28, 1925 This apparatus was publicly demonstrated at the Carnegie Institution of Washington, in Washington, D C , on December 11, 1924, and at a mceting of the Harvard Medical Society at the Peter Bent Brigham Hospital in Boston M a s s , on February 24, 1925 2
+ 6 0 2 = 6C02 + 6H20
CeH120e
It may be readily computed, therefore, that 1 gram of pure dextrose requires 746.2 cc. of oxygen, and that the heat per liter of oxygen, or the calorific value of oxygen, is 5.01 calories. Essentially, this same value is found with the various carbohydrates. With animal fat the calorific value of oxygen falls t o 4.72 calories per liter, and with alcohol it is 4.85 calories. By reversing the procedure and determining first the oxygen required in the combustion of various pure substances, one can readily compute the heat of combustion from the calorific value of oxygen as already established for these pure substances. This method may be applied, in the first place, to those substances that are exclusively used for fuel in industry, the exact heating value of which should be known in order to make an economic assessment of their sale and purchase 3
Benedict, Boslon Med Surn. J , 178, 667 (1918).
September, 1923
INDUSTRI.IL I S D ESGISEERING CHEAWISTRY
value. The sampling of large masses of coal, coke, and fuel oil is presumably so well developed that extreme accuracy in the determination of the heat of combustion is justifiable. Any method substituted for that using a good bomb calorimeter must demonstrate an accuracy, if not so great as that of the bomb itself, a t least equal to the inherent error in sampling fuels. Another class of materials, however, is frequently studied with the object of securing the heat of combustion-that is, materials burned in the living organism, particularly food for man and feeding stuffs for beasts. I n studies of the efficiency of digestion it is necessary, furthermore, to determine the potential energy in the excreta of man and animals, chiefly in the feces. The inherent error in the sampling of these two classes of material is so great (sometimes as much as 5 per cent) as to make the use of the highly accurate bomb calorimeter almost inconsistent. Yet exactly this procedure is followed whenever one determines with a bomb calorimeter the heat of combustion of foods or feeding stuffs for animals, and particularly the potential energy in the excreta of animals and man. The oxy-calorimeter in its highest perfection as developed particularly for the study of fuels is described in this paper. Apparatus
!I13
COOLINQSYSTEN-The excessive heat resulting from the combustion, and also subsequently the heat resulting from the absorption of the carbon dioxide by the soda lime in vessel B, is removed by passing the products of combustion through the cooling pipe, a, immersed in ice water. This cooling pipe may likewise be used to control the temperature by discharging cool air into vessel B, thus offsetting the heat of absorption and finally reducing the temperature (indicated on thermometer b) to the same level as a t the start of the experiment. e I
-0
IO
20
.
The oxy-calorimeter shown in the figure consists of a combustion chamber, A ; a carbon dioxide absorbing vessel, B; a blower to circulate the air current, C; and an expansion or, perhaps better named, a contraction chamber, D (in this case a spirometer of simple form), in which is measured the contraction of the volume of oxygen used. A cooling pipe, a, and a thermometer, b, in the air current are two further accessories. COMBUSTION CHAMBER-This consists of a water-sealed glass vessel, a lamp chimney of standard size, preferably of a glass with a loa- coefficient of expansion. The lower end of the chimney is sealed in water in a brass cup, through which is passed a 3/4-inch standard brass pipe and 2 rods of nickel or nickel alloy, fitted with electric binding posts a t the bottom, one of which is insulated from the brass base by hardrubber washers. Into the upper part of the brass pipe wide slots have been sawed so as to make three prongs into which a nickel crucible is easily placed, thus providing sufficient space to allow the free passage of oxygen between the bottom of the crucible and the opening in the pipe. The upright 3/~-inch pipe in the base is screwed into a standard 3/4-in~hbrass cross. One opening of the cross is closed with a plug and may be used for the introduction of volatile liquids through a capillary tube in connection with the combustion of alcohol, kerosene, and similar liquids. The opening a t the bottom connects directly with the cooling tube, a, immersed in a pail of water and ice. The third opening connects directly with a 3/4-inch, three-way plug-cock valve, d. The oxygen coming from the blower passes down through the delivery tube into the combustion chamber. Then it may pass either through the cooling tube, a, the three-way valve, d, and the carbon dioxide absorbing vessel, B, to the blower and back to the chimney, or it may pass directly from the cross below the combustion chamber through the three-way valve and thus not enter the cooling vessel. In the top of the lamp chimney is placed a one-hole rubber stopper carrying a standard l/2inch brass pipe, 15 mm. in internal diameter. This is fitted to a tee a t the top provided with a rubber stopper, e, and a side connection of '/*-inch standard pipe, which is connected by a rubber hose to the blower, C. At no point in the circuit is there an increased or decreased pressure sufficient to cause the water to leave the water seal. This seal does away with all possibilitiec of leaks a t this point.
T h e Oxy-Calorimeter f o r Use i n D e t e r m i n i n g H e a t i n g Value of F u e l s , Foods, a n d Excretory P r o d u c t s
CARBON DIOXIDEABSORPTION VESsEL-For the absorption of carbon dioxide soda lime in a standard soda lime bottle has been found very satisfactory. Obviously, any other form of soda lime bottle or can may serve with equal satisfaction. It is of the utmost importance, however, that the bottle or container be absolutely tight. The bottle shown in the figure holds 4 liters of soda lime and is commonly provided with two standard 3/4-inch hose couplings, one connecting with the three-way valve, d, the other with the tee in which the thermometer, b, is placed. The oxygen, containing carbon dioxide, enters the top of the bottle, rapidly passes down through the soda lime, and escapes up through the central tube. ROTARY BLOWER-TO secure a circulation of not far from 30 liters of air per minute, a rotary blower of good construction is essential. In this combustion system a simple rotary air impeller is sufficient. A very satisfactory simple blower4 connects directly with a 110-volt motor, and the length of bearing through the housing of the blower and the lubrication are such as to insure absence of leaks. The rate of speed may be controlled by a simple resistance in the line. The actual discharge of oxygen to the combustion chamber may further be regulated by the valve, d, or one can easily place a large screw pinchcock on the rubber tube connecting the blower and the lamp chimney. EXPAKSION CHAMBER OR SPIROLIETER-i%th the ignition of a substance in the combustion chamber there is momentarily a large production of heat with an expansion of air which, unless provided for, would blow water out of the water seal. To provide for the expansion of air immediately after ignition, and more particularly for the large contraction in oxygen as the combustion proceeds, a spirometer, D, is connected with the system a t a point between the blower and the combustion chamber.5 The spirometer should be well counterpoised a t a medium position of the bell. The diameter of the bell should be about 150 mm., thus allowing a volume of approximately 21 cc. for each millimeter length of the 4 This blower can be obtained from 1 %' E Cnl!ins, 584 Huntington Ave., Boaton, Mass 6 See Benedict lor c11 for description of spirometer used in this laboratory
914
IiVD L S T R I A L 14ND ENGINEERIiYG CHEMISTRY
spirometer bell, and the bell should be large enough to provide for a contraction in volume of preferably not less than 2.5 to 3 liters. The position of the counterpoise a t the start and its difference in level a t the end are read on a millimeter scale to 0.5 mm. A small thermometer attached to the bell of the spirometer indicates the temperature, which rarely varies appreciably during a combustion. The gas inside the bell must be saturated with water vapor. For this purpose a piece of wet blotting paper, with a hole cut out to allow the pipe entering the spirometer free play, is placed in the top of the spirometer a t p . The bell plays up and down in the annular space containing water. OXYGENSuPPLY-Commercial compressed oxygen is admitted through a petcock, n, in the line, after removing the rubber stopper, e, in the tee in the top of the combustion chamber. A gas-washing bottle, preferably provided with a small safety tube in the middle, indicates the rate a t which the oxygen is admitted. The entire system is rapidly filled with a high percentage of oxygen, as the heavier density of the oxygen-rich air drives out the lighter air. The stopper is then replaced and the spirometer bell filled to a sufficient height to allow for a contraction of not less than 3 liters. The actual amount of oxygen that disappears as a result of the combustion is measured by the contraction of the spirometer. TESTOF VENTILATION RATE-This is readily tested by disconnecting the combustion chamber and putting a weight on the spirometer bell sufficient to hold it at its lowest level. The motor is set a t the normal speed, the weight is then taken off the bell, and the rubber hose leading to the combustion chamber is simultaneously closed with the hand. The air is now discharged into the bell, which rises rapidly, and by noting the number of seconds required to introduce 3 or 4 liters of air into the bell the ventilation rate per minute can be readily calculated. This should be not far from 30 liters per minute. CRUCIBLES-PUR nickel crucibles, spun or stamped out of sheet nickel 0.5 mm. thick, are used. These crucibles are 22 mm. deep, with a diameter a t the top of 32 mm., and have a total capacity of 9 cc. CONDENSATION OF WATERI N COOLING DEvIcE-The water resulting from the combustion, the water vaporized from the water seal in the combustion chamber, and the water given up by the soda lime, particularly by some of the more modern forms of high-moisture soda lime, may condense in the cooling pipe, a, in amount sufficient to seal off the bend in the pipe a t the bottom. To prevent this after every third combustion a glass tube is run down the brass pipe, through the base of the combustion chamber and the cross, to suck out the 3 to 10 cc. of water accumulated in the bottom of the pipe. IGNITIoN-The electrical method of ignition, commonly employed in the calorimetric bomb, is likewise used here. SAMPLES-Most materials can be laid loose in the crucible, without previously compressing into a pellet, but it might be desirable to pellet light-weight materials. I n the standardization tests, however, the pellet press was used only with benzoic and salicylic acids. Standardization
The first proof of the efficiency of such an apparatus must be a demonstration of its capacity to measure accurately the oxygen consumed in the combustion of a substance of known purity. Fortunately, the one substance which is deemed preeminently useful in the standardization of bomb calorimeters-namely, sucrose-may equally well be used for the standardization of this oxy-calorimeter. Consequently the efficiency and accuracy of this apparatus has been proved by repeated tests with known weights of pure, standardized sucrose obtained from the Bureau of Standards.
Vol. 17, s o . 9
COMICSTIONOF SucRosE-From 2 to 2.5 grams of the substance are placed in the nickel crucible, which is then placed inside the combustion chamber. The ignition wire is attached to the two upright nickel-alloy rods, and a small pinch of powdered pumice stone is sprinkled about the wire where it rests on the sucrose. After replacing the lamp chimney in the water seal, the whole system is filled with oxygen, and after inserting the rubber stopper in the top of the lamp chimney the spirometer is filled. As the spirometer bell rises out of the water during the introduction of oxygen, some water adheres to the exterior surface of the bell and the vaporization of this water chills the air inside of the spirometer. It is necessary to wait until this cooling effect has ceased or to be sure that the spirometer temperature has not fallen below the room temperature. If the spirometer bell has previously been very lightly wiped with an oiled rag or is constructed with a highly polished, nickel-plated surface, a minimum amount of moisture will adhere to it. When the temperature conditions are uniform, the position of the counterpoise on the spirometer is read, as well as the temperature of the spirometer and the temperature of the line at b. The motor is then started and the valve d turned so that the air will pass down through the cooling pipe. The substance is then ignited. The brilliancy of combustion is such that colored glasses should be worn by the operator. The small amount of powdered pumice stone on the surface of the sugar makes the ignition certain. The pipe conducting oxygen into the combustion chamber restricts the discharge so that the oxygen impinges directly upon the surface of the burning sugar. This tends to control the combustion, prevents the substance from frothing unduly, and prevents particles of charred sugar from being blown out of the crucible. Usually the combustion is complete, leaving no residue except the ash from the pumice stone, a t the end of 1 or 2 minutes. Meanwhile a great amount of heat has been liberated, but the cooling system is very efficient, so that the air entering the soda lime bottle is practically a t room temperature, if not below. The carbon dioxide is absorbed in the soda lime bottle and there a certain amount of heat is also liberated, which is imparted to the air in the line and causes the temperature (as recorded on b ) to rise somewhat. The ventilation of the system is continued until the cold air entering the soda lime bottle finally brings the thermometer down to the initial point of reading-usually about 10 minutes. During this time the valve may be turned to send the air directly to the soda lime for a moment, in case the temperature a t b becomes too low before the chimney becomes cool. Finally, the blower is stopped, after which the thermometers in the spirometer and in the line and the position of the spirometer counterpoise and the barometer are accurately read. The difference in the spirometer readings can be ascribed wholly to the contraction of oxygen required in burning the sucrose. There is no correction for temperature, as the experiment is continued until the temperature reaches its initial value. The contraction in volume is directly computed on the basis of wet air and is reduced to 0" C. and 760 mm. by standard formulas and tables.6 There is a slight correction due to the oxygen required to burn the iron wire; this has been computed to correspond to 5 cc. The following formula is used to calculate the contraction in volume:
D X factor of bell X factor for pressure and temperature
-5=
reduced volume of oxygen
where D is the difference in level of the spirometer expressed in millimeters. The bell factor is the volume of the bell per C
Carpenter, Carnegic Inst. Pub., 803 A nTables 7 and 8, pp. 39 and 70
(1924).
September, 1925
INDUSTRIAL AND ENGINEERING CHEMISTR I'
millimeter of its length, and is usually not far from 21.00 cc. The factors for temperature and pressure are those commonly secured from standard tables. The minus correction of 5 cc. represents the oxygen consumed by the iron wire. By dividing the value thus found by the weight of sugar, one knows the oxygen consumed per gram of sucrose, as measured by the oxy-calorimeter. In calculating the theoretical amount of oxygen consumed the chemical formula for sucrose is used, and it is considered that enough oxygen combines with the sucrose to convert all the carbon to carbon dioxide, the hydrogen in the molecule being already provided with enough molecular oxygen to convert it to water. From this theoretical calculation it has been found that each gram of pure sucrose requires for its complete oxidation 785.5 cc. of oxygen at standard conditions of temperature and pressure. The regularity of the oxy-calorimeter may best be shown by the individual values obtained in a series of combustions made on one typical day. I n practically all cases 2.5 grams of sucrose were burned. Oxygen Required to Oxidize 1 Gram of Sucrose cc. c-~ c. - .. 785 775 788 782 780 789 779 784 785 782 Average 782.9 cc. or 99.7 per cent of theoretical
COMBUSTION OF LAcTosE-of the other carbohydrates easily obtainable in the purest form, dextrose burns only with difficulty and does not lend itself for standardization purposes. Lactose can be obtained in a very pure form, and complete combustion can be readily accomplished by placing a small amount of powdered pumice stone on the surface of the sample prior to ignition. Three combustions of lactose resulted in values for the oxygen required to oxidize 1 gram of the substance of 740, 751, and 746 cc., respectively, the average being 745.7 cc. of oxygen, as compared with the theoretical value of 746.2 cc. COMBUSTION OF BENZOICAND SALICYLIC Acms-These acids are readily sublimed or volatilized, and the combustion proceeds with almost uncontrollable vigor. Hence it is necessary to mix the finely powdered substance thoroughly with exactly one-quarter of its weight of powdered pumice stone, and then to burn weighed portions of the mixture compressed in pellet form. Although the combustion is not so readily controlled as in the case of sucrose, the rather general use of benzoic acid for standardizing heat-of-combustion apparatus makes its combustion in the oxy-calorimeter desirable for control. The complete oxidation of the last traces of carbon is difficult, and a t the end of the combustion there are frequently from 2 to 15 mg. of unburned carbon in the crucible. Correction is readily made for this incomplete oxidation by weighing the crucible a t the end of the combustion, then subjecting it to the strong flame of a Bunsen burner, and finally again weighing it. The loss in weight of the crucible during this process is ascribable to unburned carbon. As each milligram of carbon corresponds to 1.9 cc. of oxygen, a slight additive correction can readily be made. The average of seven combustions of benzoic acid (in which from 1 to 2.2 grams were burned) carried out in the oxy-calorimeter showed that the oxygen required to burn 1 gram of the substance was 1382 cc., compared with the theoretical amount of 1376 cc. Two combustions of salicylic acid showed an average oxygen consumption per gram of 1130 cc., whereas the theoretical amount is computed to be 1136 cc. COMBUSTION O F PURE NITROGENOUS SUBSTANCES-In the oxidation of a substance containing nitrogen, free nitrogen
915
is liberated. Accordingly, when such a combustion takes place in a confined volume of oxygen, the contraction, as measured, will always be materially less than that represented by the true volume of oxygen absorbed in the oxidation. By the use of pure organic substances containing a known amount of nitrogen, however, it is possible to employ a nitrogenous substance in checking the accuracy of the combustion apparatus. One such substance, hippuric acid, burned readily but gave a residue of unburned carbon considerably larger than that noted with either benzoic or salicylic acids. Indeed, the unburned carbon resulting from the combustion of 2 grams of hippuric acid ranged in a series of six combustions from 14 to 69 mg. After the usual additive correction for unburned carbon and a further additive correction for the nitrogen of the molecule, the average of six determinations showed an oxygen consumption per gram of 1222 cc., whereas the theoretical amount is 1220 cc. With uric acid the very high percentage (33.3 per cent) of nitrogen in the molecule makes the correction for nitrogen amount to about 580 cc. for each 2 grams of substance. The final values for the true amount of oxygen required to oxidize each gram of uric acid were, in the two determinations made, 603 and 610 cc., as compared with the theoretical 600 cc. Under ordinary conditions a substance with a percentage of nitrogen as high as that found in uric acid would rarely, if ever, be burned. I n unknown nitrogenous substances obviously a direct determination of the nitrogen (ordinarily by the Kjeldahl process) is a prerequisite. CONCLUSIONS ON STANDARDIZATION-From the standardizations and the confirmatory tests described, it is clear that this apparatus measures the oxygen required to oxidize completely 1 gram of any of these substances to within 1 per cent of the theoretical amount. It therefore makes available a general method for the rapid and accurate determination of the oxygen required for the combustion of unknown organic substances. It does not give directly the heat value of the substance; this must be computed by applying to the oxygen measurement a factor representing the calorific value of oxygen. Experiments on the Combustion of Fuels
As typical fuels, samples of anthracite and bituminous coal, coke, and heavy fuel oil were burned. The combustion of each of these products involved slight changes in technic, and specific instructions for the best method of burning these products are therefore given. ANTHRACITE COAL-The anthracite coal is pulverized and passed through a U. S. standard sieve with 80 meshes per inch (30 per em.). A 10-cc. nickel crucible is half-filled, loosely, with very finely powdered pumice stone, and the bottom of a duplicate crucible is then momentarily pressed into it to line the inside of the crucible completely with an even layer of powdered pumice stone about 2 mm. thick, thus making the crucible a suitable receptacle in which the powdered coals can be completely burned. Care should be taken to scrape away with a spatula the loose pumice left on the top and edges of the crucible. This procedure is much simpler and fully as satisfactory as the method of lining the crucible with ignited asbestos, recommended by the American Society for Testing Materials.' After the crucible containing the powdered pumice lining is weighed, 2 or 3 grams of the coal are very carefully added by delivering from a folded paper. The crucible is then weighed again, the difference in weight representing the weight of the sample to be burned. The crucible is placed in the 7
Standard Methods of Laboratory Sampling and Analysis of Coal, 33.
~ a a - a r p. ,
916
ISDCSTRIAL AND ENGI,VEERISG CHEMISTRY
combustion chamber and the iron ignition wire adjusted to dip into the coal. Oxygen is allowed to flow into the entire apparatus for about 2 minutes, to insure a high percentage of oxygen. The apparatus is then closed, and the spirometer fills with oxygen. When the walls of the spirometer bell appear to be dry-i. e., when temperature equilibrium has been secured-the readings of the spirometer level, the temperature of the spirometer, and the temperature of the line b are made. The ventilation is directed through a and simultaneously the coal is ignited. The ventilation a t the start must not be so strong as to blow away the finely powdered coal dust, although some fine sparks will necessarily be blown out of the crucible. These are almost wholly and instantly consumed in the oxygen. Sparks should not be allowed to blow out of the crucible for a distance of more than 2 to 3 em. Particles that are incandescent long enough either to strike the side of the chimney, or to be blown onto the surface of the water, or to pass down the exit tube, are liable to carry with them unburned material. Ten or eleven minutes is ample time for the combustion of a 3-gram sample of coal. If the combustion takes a longer time, owing to restriction of the oxygen ventilation, there is danger of unburned coal remaining in the crucible. The ventilation is continued until the temperature, as indicated on b, returns to the initial point. If this is reached before the lamp chimney is cold to the hand, valve d is turned so that the air will pass directly from the lamp to the soda lime bottle. When the entire system is a t the initial temperature, the ventilation is stopped and the position of the spirometer bell, the temperature of the spirometer, and the barometric pressure are noted. The lamp chimney can then be removed. Approximately 1.5 liters of oxygen will be required for each gram of coal burned, with variations due to the ash content. Thus, depending upon the size of the spirometer, one can use larger or smaller samples. Ten successive combustions of a sample of ordinary anthracite coal were made in the oxy-calorimeter, and the oxygen values (not corrected for any nitrogen in the coal) were as follows : Oxygen Required to Oxidize 1 G r a m of Anthracite Coal cc. cc. 1703 1710 1700 1712 1730 Average 1i 1T cc
Table I-Combustions
a
Ash content
%
39.0 21.6
19.6 6.7 33.4 16.0
7.4 21.2 17.6
... ...
8.6
4.3 4.5
... ...
burned. The true oxygen consumption per gram of coal is therefore 1724 cc. Samples of this same coal were likewise burned in the calorimetric bomb, which had been standardized so as to give the heat of combustion of pure sucrose as 3.949 calories per gram. The average of two well-agreeing determinations of the heat of combustion was 7.831 calories per gram. T h e calorific value of oxygen, obtained by dividing this figure by the volume of oxygen absorbed in the combustion of 1 gram-namely, 1.724 liters-was 4.54 calories per liter. The application of this factor to anthracite coals in general would obviously not be justified without further control. Fortunately, through the courtesy of Herman C. Lythgoe, of the Department of Health, State House, Boston, nine specimens of coal were secured that had been submitted to his office for analysis with particular reference to the ash content and heating value. No history of these coals was available, but from the great variation in ash content they were probably from different localities. The averages of the results on these coals compared with the average values for the laboratory samples are given in Table I. As is readily seen, the coals are characterized by a large variation in ash content. The percentage of nitrogen was essentially that of the laboratory sample, although actual determinations were made only upon Samples 320 and 318. It was assumed that the other samples contained 0.9 per cent of nitrogen. The most striking feature of this table is the uniform calorific value of the coals per liter of oxygen. The average value for all the anthracite coals, including the laboratory sample, is 4.49 calories, with a minimum of 4.42 and a maximum of 4.54 calories. No special correlation is noted between the calorific value of oxygen and the ash content of the coals. The variability in the calorific values is within h 1 . 5 per cent and it is believed that for anthracite coals the average factor of 4.49 calories per liter of oxygen may well be employed, as this is well within the ordinary limits of the methods of sampling. BITUMIKOCS Coa~--An ordinary sample of bituminous coal was studied by a series of eight combustions in the oxycalorimeter and four determinations with the calorimetric bomb. The eight combustions with the oxy-calorimeter gave the following values: Oxuaen Required to Oxidize 1 G r a m of B i t u m i n o u s Coal cc. cc 1794 1789 1783 1783 1798 17S0 1796 1788 Average 1780 cc.
Subsequent determination of the nitrogen in this sample, by the Kjeldahl process, showed that it contained 0.9 per cent of nitrogen. Hence to the oxygen value found by the combustion 7 cc. of oxygen must be added for each gram
COMBUSTIONS IN Weight Oxy- Bomb of SAMPLE calorim- calorim- sample Anthracite eter eter Grams 3 2 2.8665 320 2.7268 326 3 3 4 3 2.8155 327 2.0918 318 3 2 4 2 2.3526 321 2 3.2983 335 331 3 3 -9 3.6208 328 3 2 3.2772 319 4 3 2.8731 Lab. 10 2 2.0396 Bituminous 2.1063 Lab. 2.2910 8947 2.4905 9017 2.5506 9092 2 2.7471 4 Coke 1.0867 Fuel oil 2 Corrected for 0.0055 gram unburned carbon.
Vol. 17, S o . 9
d determination of the nitrogen in this coal showed that there was 1.3 per cent. Therefore the combustion figure should be increased by 10 cc. per gram of coal, and the true oxygen consumption per gram is accordingly 1799 cc.
of Various S p e c i m e n s of Fuels
NITROGEN COSTENT Oxygen 70 equivalent wzht Cc. 0.6 14 0.9 20 0.9 20 18 1.1 0.9 17 0.9 24 0.9 26 0.9 23 0.9 21 0.9 15 1.3 1.3 1.3 1.3 1.0
...
22 24 26 26 22
..
CC OXYOEN CONSUMPTION CALORIES HEATPRODUCED Per Per As Corrected Total gram liter measfor per of of ured nitrogen gram coal oxygen 1062 4779 4.50 3029 3043 3830 3850 1412 6343 4.49 4200 1492 6677 4.48 4180 3713 3731 1784 8055 4.52 2785 1184 5242 4.43 2768 5025 5049 3531 6767 4.42 6287 6313 1744 7724 4.43 4687 4710 1437 6483 4.51 4413 4434 1543 7004 4.54 3503 3518 1724 7831 4.54 3768 3902 4468 4531 4534 2360
3790 3926 4494 4557 4556 2370a
1799 1714 1804 1787
1658
2181
8356 7874 8337 z214 ,339 10191
4.65 4.59 4.62 4.60 4.43 4.67
Sept,ember, 1925
I N D U S T R I A L A S D ESGISEERI.1'G CHEJIISTRE'
The coal burns very freely, in fact too freely, during the first 2 or 3 minutes, causing the mass to puff up and have a tendency to be blown over. This puffing can be stopped by reducing the ventilation, either by slightly pinching the rubber tubing leading t o the chimney or by decreasing the rate of ventilation by adjusting the valve d. Four well-agreeing combustions in the bomb Calorimeter showed a calorific value of 8.356 calories per gram of coal. A comparison of the average consumption of oxygen, as determined by the oxy-calorimeter (1799 cc.), with the heat of combustion as determined in the bomb gives an average calorific value of 4.65 calories per liter of oxygen, a value about 3.5 per cent higher than the average for the ten samples of anthracite coal. As with the anthracite coal, however, it was necessary to control this factor by making similar determinations with other samples of bituminous coal, again secured through the courtesy of Mr. Lythgoe. Three such samples were studied and the average values obtained with them are also recorded in Table I. Assuming the same percentage of nitrogen (1.3 per cent) in all the samples, the calorific values of oxygen for these three samples were 4.59, 4.62, and 4.60 calories, respectively; the average for all four samples was 4.61 calories. The deviation from this average of any one of the saniples is measurably less than 1 per cent. It is thus clear that a general factor of 4.61 applied to the determinations of oxygen, as made in the oxy-calorimeter, will give with a high degree of accuracy the heat of combustion of bituminous coal. Com-One sample of gas coke was burned, and the average of five closely agreeing determinations of the oxygen absorption (after corrections for the 1 per cent of nitrogen found in the coke by actual determination) was 1658 CC. of oxygen for each gram of coke burned. The data are given in Table I. The average of four well-agreeing determinations of the heat of combustion in the bomb calorimeter gave 7.339 calories per gram. Thus the calorific value of oxygen, when burning coke, is 4.43 calories per liter of oxygen. FUELOIL-TO secure the best controlled combustion it is necessary to mix the oil with sufficient powdered pumice stone into a thick putty or paste, so that it will stand up in the center of the crucible. Under these conditions the crucible is not lined with the pumice stone. Inasmuch as even in these heavy fuel oils there are light volatile constituents, the oil should be burned as soon as possible after it is weighed and thoroughly mixed. A piece of nichrome wire or a short bit of glass rod, which is left in the crucible, is usually employed in the mixing process. Even by retarding the combustion with pumice stone there is danger of too vigorous combustion, and all the precautions cited in the standardization of the apparatus by burning benzoic acid, particularly with regard to the control of the ventilation, should be observed. The results of these combustions will also be found in Table I. No determinations of nitrogen were made on the oil. As was found in burning benzoic acid, salicylic acid, and also other fats and oils, there was a tendency for a small amount of unburned carbon to remain in the crucible. This amounted on the average to 0.0055 gram, corresponding to 10 cc. of oxygen per gram of oil, and the average oxygen value given in Table I includes this correction. Determinations of other fuel oils were not made, but it seems clear that the calorific value of oxygen will not materially alter, and it is believed that the factor of 4.67 calories niay be taken as reasonably standard. GENERALCoNcLvsIoNs-Although innumerable experiments have been made to secure most representative samples of laage shipments of fuel, it is doubtful if any sample, when prepared ready for combustion in the bomb calorimeter, has
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anywhere near the inherent accuracy represented by the combustions in the bomb itself, and it is highly probable that with the oxy-calorimeter it is possible to determine the heating value of coal well within the limits of accuracy of random sampling. The small initial cost of the oxy-calorimeter, the slight expense for oxygen and soda lime, the rapidity with which the experiments can be made, and the simplicity of calculation make the apparatus especially adaptable for the determination of the heating value of fuels. Trial tests of the heating value of wood and other commonly used combustibles have not been made, but the oxy-calorimeter should be much more accurate than any possible existing method of sampling. Simplified Oxy-Calorimeter for Determining Energy Values of Foods and Excreta
Although the oxy-calorimeter is not complicated and calls for little special apparatus other than the blower and the spirometer, a simpler form, with but slightly less precision, may be used for all determinations involving foods, feeding stuffb, and excreta, where the error of sampling is presumably very much greater than in the case of fuels. The fundamental principle remains unaltered-namely, the combustion of a known weight of substance in a current of nearly pure oxygen, and the accurate measurement of the contraction in volume of the gas due to the absorption of oxygen in the combustion and the attendant absorption of carbon dioxide by soda lime. I n the combustion of materials in which the error of sampling may be as high as 5 per cent or more, special provisions for the control of the temperature, the pressure, and particularly uniform humidity are entirely unnecessary and, indeed, not justified. The simplified form permits the use of any standard respiration apparatus of the closed-circuit type and determines energy values with greater accuracy than that involved in the present methods of sampling foods and excreta. This form will be described in detail elsewhere. Calorific Value of Oxygen when Burning Various Organic Substances
As a result of the standardization tests of the oxy-calorinieter and the tests demonstrating the practicability of using standard respiration apparatus as oxy-calorimeters, it has been possible to determine for a large number and variety of organic substances the amount of oxygen required to oxidize 1 gram of the substance. Since in all instances the actual heat value of the substances was also directly measured in the bomb calorimeter, the data are available for establishing a series of average factors indicating the energy values of these substances per liter of oxygen required in the oxidation. For the pure substances used in the standardization tests the energy values obtained by the combined use of the oxycalorimeter and the bomb calorimeter agree very well with the theoretical factors computed from the chemical formulas. Thus, the pure sucrose furnished by the Bureau of Standards had a tested heat of combustion of 3.949 calories per gram and required, according to the oxy-calorimeter, 783 cc. of oxygen in the combustion of 1 gram. The heat of combustion, 3.949 calories, divided by the oxygen consumption per gram, 783 cc., gives 5.04 calories per liter of oxygen, a factor exactly the same as that computed from the chemical formula for oxidation. By the same methods of combustion and computation the calorific values per liter of oxygen were found for various other substances (Table 11). I n the computation of the factors for hippuric and uric acids, and indeed all nitrogenous substances, a correction for nitrogen (content of 7.8 and 33.3 per cent, respectively) is necessary. From these results it is seen that the carbohydrates, sucrose, lactose, starch, and dextrose all have a calorific value
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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close to 5.04, that of sucrose. Lactic acid, acetone, p-oxybutyric acid, and ethyl alcohol have values very close to 4.85. Human and animal fats have an average energy value of 4.75, whereas that of the four acids used to standardize the oxycalorimeter and the calculated value for protein are all about 4.60. Table 11-Calorific
Vol. 17, NO. 9
Spectrophotometric Identification of Dyes‘*2 111-Basic Violets of the Triphenylmethane Group
Value of Various Substances
SV8STANCE Pure Substances: Sucrose Lactose Benzoic acida Salicylic acid’ Hippuric acid‘ Uric acid Commonly Metabolized Compounds: Starch Dextrose Lactic acid Animal fat Human fat Protein Acetone &Oxybutyric acid Ethyl alcohol Fuels: Anthracite coal Bituminous coal Coke Fuel oil Foods: High carbohydrate substances: Dried skimmed milk Oyster crackers Corn meal N u t bread Cheese sandwich Chicken sandwich Salmon salad sandwich Club sandwich Doughnut Highly nitrogenous substances: Glidine (vegetable protein) Ossein Collagen Plasmon Fats: Olive oil Corn oil Cottonseed oil Cod-liver oil Goose fat Butter Mixed foods: Beef stew Mince pie Animal foods: Hay, Specimen I Hay, Specimen I1 Cottonseed meal Linseed meal Gluten meal Excreta Human feces Steer feces a Correction for unburnt carbon necessary.
By Walter C. Holmes
Calories per liter oxygen 5.08 5.00 4.58 4.65 4.65 4 55 5.06 5.01 4.85 4.72 4.79 4.60 4.82 4.85 4.85 4.49 4.61 4.43 4.67 4.89 4.90 4.88 4.88 4.95 4.85 4.98 4.93 4.90 4.67 4.69 4.70 4.65 4.74 4.71 4.70 4.70 4.75 4.62 4.84 1.97 4.80 4.86 4.66 4.76 4.85 4.97 4.84
The fact that much of the food of man and practically all the food of domestic animals has a high carbohydrate content suggests that this value of 5.04 calories will play a large role in establishing the heat factors for other food materials and food mixtures. Although some apparent irregularities are seen in the values, in general the factor is not far from 4.6s for the rich nitrogenous substances, 4.70 for fats, and nearly 5.0 for those of a high carbohydrate nature. If one considers the average food of man and realizes that in determining the metabolism of humans from the oxygen measurements the calorific value of a liter of oxygen is commonly taken as 4.825, one can see that this average value would not be far from correct for all food mixtures, particularly if a composite sample of the total daily meals were taken. No doubt subsequent research will slightly refine some of these figures, but it does not seem justifiable a t the present time to refine a figure that is far inside the limit of accuracy possible in preparing the sample for combustion. According to advice from Paris the Manufactures des Glaces et Produits Chimiques de St. Gobain, important French manufacturers of superphosphates and other chemical products, plan t o increase their capital from 120,000,000 francs to 161,000,000.
COLOR
LABORATORY, BUREAU OF CHSMISTRY, WASHINGTON, D . C.
B
ASIC violets of the triphenylmethane class find important application as biological stains and therapeutic agents. An investigation has been made of all samples of these dyes which were available, in order to obtain such data as would afford a reliable means of differentiating between them. Samples of methyl violet (C. I. 6SO), crystal violet (C. I. 681), ethyl violet (C. I. 682), benzyl violet (C. I. 683), and gentian violet were examined. It is generally held that the gentian violets which have been supplied for biological staining have been mixtures of crystal and methyl violet, and the term will here be employed with that significance. It may be noted, however, that a sample of pre-war Griibler “Gentianaviolett” was found to be a typical methyl violet (of very low dye content) rather than a mixture of the type mentioned. Absorption in Dilute Alcoholic Solution
With dilute solutions of the dyes in question the absorption spectra in aqueous and alcoholic solutions are very similar. The decided tendency of aqueous solutions to dye the containing cells, however, makes it more convenient to carry out spectrophotometric measurements in alcoholic solutions. The data recorded in the first three columns of the table of constants, accordingly, were obtained with solutions containing 90 per cent alcohol. The measurements were made with a Hilger wave length spectrometer provided with a Nutting photometer. The approximate maxima of the bands (Column 1) were located with the usual technic, and the values were supplemented with determinations of the ratio of extinction coefficients a t wave lengths selected on each slope of the bands (Column 2). This type of ratio will frequently serve, as in the present instance, to establish a differentiation between similar spectra and to define the spectral locations of absorption bands more definitely than will the mere determination of maxima, particularly when the bands in question are somewhat broad and indefinite. Influence of Acidity
The dyes under investigation exhibit differences in stability to both hydrogen and hydroxyl ions. Ethyl violet is decidedly more stable to hydroxyl ions and less stable to hydrogen ions than the remaining dyes, which vary among themselves in only a relatively minor degree. The effect of acidity is more conveniently measured than that of alkalinity. The curves in Figure 1 illustrate the transition between the normal dye and its di-acid salt with increasing acidity in aqueous solutions of crystal violet. With aqueous solutions of the dyes the immediate modification of color and absorption which is brought about by the change in hydrogen-ion concentration is followed by a gradual decolorization, due to a process of increasing aggregation of dye molecules. I n alcoholic solutions more acid is required to produce an equivalent effect and the resulting 1 Received
May 8, 1925. Contribution No. 106 from the Color Laboratory, Bureau of Chemistry, Washington, D . C. . 2
