Correlation of Methods for Measuring Heat of Hydration of Cement R. W. CARLSON
AND
L. R. FORBRICH, Massachusetts Institute of Technology, Cambridge, Mass.
M
EASUREMESTS of heat of hydration of cement have been made extensively only during the past few years, beginning a t a time when little was known about either the characteristics of heat liberation or the thermal properties of the materials involved. It is not surprising, then, that discrepancies between results from different calorimeters were encountered. The present paper is an attempt to correlate the results from different types of calorimeters and to point out the limitations and advantages of each. A brief description of each type of calorimeter is given and the sources of errors that may be involved in each are discussed. Supporting data are the result of researches conducted a t the Massachusetts Institute of Technology during the past three years. Heat of hydration of cement is important to engineers mainly because of the temperature rise that it produces in thick concrete structures. In slabs less than about 30 cm. (1 foot) thick, the temperature rise is small and heat of hydration is usually unimportant. If it is desired to compute the temperature changes that are likely to occur in a body of concrete from which some heat escapes (f), the heat of hydration is not used directly, but is first expressed in terms of the equivalent temperature rise that it would cause in the concrete if no heat were lost. A plot of the equivalent temperature rise against time is called an adiabatic timetemperature curve, which is of interest to engineers. For the purpose of comparing cements, on the other hand, heat of hydration is best expressed directly as calories per gram of cement. The heat expressed in this way does not involve either the aggregate or the proportions employed in making the concrete. Adiabatic temperature rise of concrete can be computed from heat of hydration of cement by multiplying the latter by the weight of cement per unit weight of concrete, and then dividing the resulting product by the specific heat of the concrete. The specific heat varies with the type of aggregate and with temperature. Four factors that affect the conversion of heat of hydration of cement into adiabatic temperature rise of concrete are (1) the immediate heat liberation, (2) the variation with temperature of the specific heat of concrete, (3) the watercement ratio, and (4) the curing temperature. Some of these factors were overlooked when heat of hydration studies first began to be made only a few years ago. Because they are of general interest, they will be discussed before the various calorimeters are described.
heat of hydration (not shown) for the cements listed in Table I. The immediate heat liberation is included in the determination of heat of hydration by the heat-of-solution method, but is apt to be missed in determinations by other methods. This is one of the factors to explain the discrepancies that were encountered when results from different types of calorimeters were compared.
Specific Heat of Concrete The specific heat of dry cement is almost unaffected by temperature changes within the range commonly encountered by concrete structures. Likewise, the specific heat of water is almost constant. But the specific heat of cement paste increases with temperature to an extraordinary extent. At ordinary temperatures, the specific heat of the hydrated paste is lower than the sum of the heat capacities of the amounts of water and cement contained in a gram of the paste, as would be expected if the water and cement were chemically combined. I n other words, hydration reduces the specific heat of cement paste a t ordinary temperatures. Because of the increase with temperature, however, the specific heat of hydrated paste may be greater than that of corresponding unhydrated paste at temperatures of about 65.56" C. (150" F.) and above. TABLEI. Cement Symbol AH BH DH EH
FH
GH HH AS CS
FLLP
MM
IMMEDIATE
HEATLIBERATION O F CEMENTS
Total Heat Liberated 3 min. 15 min. 30 min. --Calories per gramHigh early strength 3.5 5.2 5 6 High early strength 3,5 5.3 5.9 High early strength 1.1 3.3 3.7 High early strength 3.1 5.2 6.3 High early strength 2.7 4.1 5.0 High early strength 0.6 3.0 4.0 High early strength 2,5 4.2 5.1 Standard 3.7 4.0 4.3 Standard 1.5 2.2 2.8 Standard 3.6 4.1 4.4 Portland pozzuolana 0.9 1.2 1.5 Low heal 3.0 3.8 4 0 hfodified 2.4 4.1 4.3 Type of Cement
Accurate data on the variation with temperature of the specific heat of both neat cement and concrete were first obtained in the laboratories of the Bureau of Reclamation in Denver. Recently, further tests have been made a t the Massachusetts Institute of Technology with the aid of special adiabatic calorimeters designed and constructed for this express purpose. The specific heats of well-hydrated cement paste in two water-cement ratios are shown in Table 11. Note that the increase from 21.11' to 65.56" C. (70" to 150" F.) is about 50 per cent for either water-cement ratio. The results shown are for a particular, standard cement; there appears to be some variation among cements. The large change in apparent specific heat with temperature is believed to be due to heat absorption accompanying the decreasing affinity of hydrated cement for water as the temperature is raised. Extended research on this subject is now being conducted at the Massachusetts Institute of Technology in an attempt to obtain a more adequate explanation. It now seems clear that the variation in specific heat of concrete with temperature is almost entirely that of the
Immediate Heat Liberation of Cement When cement is mixed (in a Dewar jar) with water of equal temperature, an appreciable temperature rise occurs almost immediately and continues a t diminishing rate for a half hour or more. This temperature rise is believed to be due mainly to the solution of free oxides and impurities, and only a small amount is believed to be due either to the hydration of primary compounds or to the wetting of the cement grains. The amount of immediate heat liberation, computed from temperature rise of cement paste, is shown in Table I for a number of different cements. The amount of heat liberated up to 30 minutes varies, for the cements tested, from 1.5 to 6.3 calories per gram. The average 30-minute heat liberation is about 5 per cent of the average potential 382
JULY 15, 1938
ANALYTICAL EDITION
hydrated cement and that the aggregate is not responsible; in other words, the variation is in the cement paste. I n Table 11, the observed specific heats of a particular concrete a t various temperatures are compared with corresponding computed values on the assumption that the concrete is composed of cement paste having the measured variation in specific heat, and of aggregate having a constant specific heat. The agreement is as good as the test data, and the discrepancy that exists is in the right direction to be explained by a small increase in specific heat of the aggregate. No measurements were made on the aggregate, which was siliceous, but aggregates in general exhibit a slight increase in specific heat with rising temperature. The specific heat of limestone, for example, shows an increase of 5 per cent due to a change in temperature from 0' to 100' C. TABLE
11.
Ternperature F. 70 90 110 130 150
383
appreciably with increasing water-cement ratio, however, even in the range of ratios encountered in concrete. I n Table I11 are given the measured heats of hydration a t various ages for a standard cement in three water-cement ratios, from 0.30 to 0.50 by weight. While the effect of increasing water cantent is not great a t the age of 3 days, it is relatively important at the later ages. Specimens of still higher water-cement ratios were prepared by slowly rotating vials of the paste during setting to prevent separation of water. The results were not consistent with those for lower water-cement ratios, where test methods were straightforward, so they were not included in the table. Auxiliary tests indicated that the rotation during setting had an effect on the hydration. Until further studies are made of the higher water-cement ratios, it is believed to be safe to extrapolate the results in Table I11 up t o a ratio of about 0.60.
SPECIFIC H E a T OF COiXCRETE AND CEMENT PASTE Neat-Cement Paste Water-cement Water-cement ratio, 0 . 2 5 ratio, 0.60 r
0.265 0.277 0.303 0.340 0.400
Concrete Observed Computeda Calories per ovum per 1 C . 0.380 0.226 0.225 0.408 0.232 0.230 0.455 0.240 0.237 0.244 0.505 0.249 0.580 0.26C 0.255
a Computed on basis of 14.4 per cent of paste by weight ( 1 bbl. of cement per cu. yd. of concrete) and 85.6 per cent aggregate (specific heat assumed constant = 0.20).
With such variations in specific heat of cement pastes and concretes as are shown in Table I1 it is clear that adiabatic temperature rise of concrete cannot be translated accurately into heat of hydration of cement, or vice versa, without taking the variations into account. Before leaving the subject of specific heat, mention should be made of one phase that remains indefinite. The specific heat of fresh concrete is the weighted-average specific heat of the ingredients. After hydration, a considerably different value prevails. Considering only the cement paste, which alone is affected by hydration, the specific heat a t 21.11' C. (70' F.) is decreased by hydration to the extent of 30 to 40 per cent. A certain amount of error must be involved in assuming that the specific heat of well-hydrated paste or concrete applies a t all ages. Unfortunately, it is difficult to measure the specific heat of cement paste a t the early ages, when heat is still being generated at an appreciable rate. TABLE111. EFFECTOF WATER-CEMENT RATIOo x HEAT OF HYDRATIOP Water-Cement Ratio
Heat of Hydration 7 days 28 days Calories per orom 0.30 45.7 58.3 74.3 0.40 49.2 61.8 82.9 0.50 52.3 69.8 91.4 a Tests made by heat-of-solution method. Cement contained (potential) 7 per cent CIA and 51 per cent CIS. 3 days
Interpolation between the known hydrated and unhydrated values, in accordance with the degree of hydration, seems to offer the safest means of ensuring a reasonable degree of accuracy where specific heat is involved. Water-Cement Ratio Heat of hydration of cement is usually determined on a neat-cement paste of relatively low water-cement ratio. Because only a part of the water in concrete was believed to combine with the cement, it was first thought that the difference in water-cement ratio between neat paste and concrete was not important. The heat of hydration increases
Curing Temperature
A change in curing temperature appears to affect mainly the rate of heat liberation of cement. The heat liberation a t higher temperature is generally greater a t early ages but about the same at later ages as at lower temperature. Heatof-hydration results are shown in Table IV for a standard cement cured a t three different temperatures. The 3-day values for curing a t 4.44' and 39.44' C. (40' and 104' F.), are seen to be 29.5 and 72.3 calories per gram, respectively, a difference of about 150 per cent. But a t 28 days the difference is only 8 calories, and at 90 days it is only 4 calories. Many tests have indicated greater heat liberation for lower curing temperature a t later ages. This is in agreement with tests reported by Hornibrook and associates ( 3 ) . TABLE
Iv. EFFECTOF
Temperature O
F
40 74
104
TEMPERATLTRE O N HEATOF HYDRATION
CKJRISG
Heat of Hydration for Water-Cement Ratio 0.40 3 days 7 days 28 days 90 days Calorzes p e r o r a m 88 8 43 5 78 4 29 5 83 6 90 8 52 4 72 4 86 8 93 1 72 3 80 3
-
It is unfortunate that mass-concrete temperatures cannot be duplicated readily in the laboratory. Thus, it is necessary usually to convert heats of hydration obtained under constant temperature conditions to temperature rise of concrete curing a t variable temperature. There is need for further tests aimed toward the establishing of conversion factors for variations in curing temperature. Calorimeters THE CALORIMETRIC PROBLEM. The measurement of heat of hydration of cement is difficult because of the great variation in the rate a t which the heat is liberated When the cement is f i s t mixed with water, an appreciable amount of heat is liberated immediately, as described above. After the immediate heat liberation has subsided, the rate is low for a time, but as the major compounds begin to hydrate, the rate increases, first slowly and then more rapidly, until a t about 8 hours for an average cement a maximum rate is reached. The maximum rate does not correspond to final set, but usually occurs later; final set is an arbitrary hardness that does not require as much hydration and heat liberation as generally occurs up to the time of the maximum rate of heat liberation. After the maximum rate is passed, there is a rapid decline in rate followed by a slower, continued decline that may bring the rate at the end of a week to only about 1 per cent of the maximum. And yet, this slow but continued heat liberation accumulates to an appreciable amount over a period of days or weeks.
INDUSTRIAL AND ENGINEERING CHEMISTRY
384
The most obvious method of determining the heat of hydration of cement is to measure the temperature rise of an insulated specimen in a room of constant temperature. But even after carefully sealing the specimen against moisture loss and making correction for heat losses, reliable results can be obtained by this method only up to about 3 days. Later results are usually lacking in accuracy because the corrections are larger than the quantity being measured. Under typical conditions, one might find that a specimen of 181.44 kg. (400 pounds) weight, insulated all around with 15 cm. (6 inches) of kapok, would reach a maximum temperature a t about 3 days and thereafter would decline in temperature despite its heat generation. This example illustrates the difficulty of measuring heat of hydration over a long period of time and shows that the early method, consisting of measuring temperature rise of insulated concrete, could not give satisfactory long-time results.
amount of water, sealed in metal, and thus the thermal properties of concrete are simulated. A few inches from the specimen and its water jacket is a copper container that is maintained at the same temperature as the water jacket, so that no heat can escape from the specimen. Advantages of adiabatic calorimeters in general are: (1) the adiabatic time-temperature curve of concrete is obtained directly, ( 2 ) any type of cement can be tested, (3) the hydration temperature simulates what would develop in a heavy mass, and (4) they provide the mass-curing condition for simultaneously testing other specimens for other properties. TABLE V. HE.4T OF
FIGURE 1. ADIAB.4TIC CALORIMETER I N WHICH CONCRETE Is SIMULATED BY NEAT CEMENT AND WATER
ADIABATIC CALORIMETER. The adiabatic calorimeter avoids heat losses from a specimen of concrete by keeping the room exactly as warm as the specimen. I n the most satisfactory type of adiabatic calorimeter, an automatic controller actuates heaters that maintain zero temperature difference between a thermometer in the specimen and another in the room. Temperatures are either registered by a separate recorder or are observed from time to time to provide the time-temperature curve. An economical type of adiabatic calorimeter has recently been developed a t the Massachusetts Institute of Technology. It takes advantage of the fact that the variation in specific heat of concrete is almost solely in the cement paste, and employs a concentrated neat-cement specimen to replace the bulky concrete specimen usually employed. A cross-sectional drawing of such a calorimeter is shown in Figure 1. The neat-cement specimen, weighing about 1.36 kg. (3 pounds), is surrounded by a jacket containing a measured
EFFECT OF CARBONATION OF TESTSAMPLES ox HYDRATION DETERMINED BY HEAT-OF-SOLUTION METHOD
CeTVaterment Cement Ratio No.
c To confroller
VOL. 10, NO. 7
Age Days
0.40
60
0.60
75
0.40
90
Condition Protected r u t protected Protected S o t protected Protected N o t protected
CO? 0.9 2.2 2.0 4.5 0.8 3.9
H e a t of Hvdration Corrected
for COX Caloraes p e r g r a m
Observed 65.1 75.1 106.9 120.2 107.1 126 3
59 9
62 3
95 94 102 103
3 1
5 7
Disadyantages of adiabatic calorimeters are: (1) they are expensive and require close temperature control, (2) large specimens are generally required, and (3) they are not accurate for the first hour after mixing the concrete for the specimen. HEAT-OF-SOLUTION CALORIMETER. Roods and his coworkers ( 6 ) , realizing early that continuous observations on specimens for determining the heat of hydration would be exacting and expensive, applied the heat-of-solution method to the problem. In this method, it is necessary only to determine the difference in heat of solution of corresponding samples of cement a t two ages of hydration to have the amount of heat liberated between those ages. If one of the ages is zero-in other words. if one sample is dry cement-and the age of the corresponding sample is 28 days, the difference in heat of solution of the two samples represents the total amount of heat evolved up to 28 days. A description of the heat-ofsolution method in simplified form has been given by Lerch (b), and no detailed account need be given here. Heats of hydration determined by the heat-of-solution method in most American laboratories have been too high to indicate the correct temperature rise of concrete. Also in England, tests reported by Lea (4)show the heat-of-solution method to give considerably higher values than the adiabatic method. Sources of error not commonly considered in the heat-ofsolution method are carbonation and drying of hydrated samples during preparation for testing. Of these two sources of error, carbonation is the one that makes heat-of-hydration results too high. Carbonation of a cement sample before it is dissolved in acid reduces its heat of solution, because the heat of solution of calcium carbonate is less than that of calcium hydroxide. In the particular acid solution employed, the heat of solution of calcium carbonate (ignited basis) was found to be only 102 calories per gram as compared with 557 for calcium hydroxide. As each per cent of carbon dioxide corresponds to 1.27 per cent of transformed calcium hydroxide (ignited basis), each per cent of carbon dioxide would be expected to cause an error of (5.57-1.02) X 1.27 or 5.8 calories per gram. Each per cent of absorbed carbon dioxide would then be expected to reduce the heat of solution of the hydrated sample by 5.8 calories per gram. The difference between values for dry and hydrated samples would then be greater, and the heat of hydration as determined by the heat-of-solution
JULY 15, 1938
.4NALYTICAL EDITION
method would be too great by 5.8 calories per gram for each per cent of carbon dioxide absorbed by a hydrated sample. An investigation revealed that it was common for hydrated samples to absorb more than 0.5 per cent of carbon dioxide during grinding. Dry samples were relatively unaffected by carbonation. The computed effect of carbonation was checked by testing samples carbonated to different extents. In one series of tests, one sample was protected so as to minimize carbonation and the corresponding sam’ple was purposely carbonated more than usual during grinding. Results of such tests on three cements are shown in Table T‘. After applying the correction of 5.8 calories per gram for each per cent of carbon dioxide, the results are in fair agreement. The accuracy of the calorimeter was about 2 calories per gram. It should not be concluded that the effect of carbonation is as simple as merely changing free calcium hydroxide to the carbonate. Actually, the carbonation seems to affect mainly the lime contained in the gel of the hydrated cement, and to affect but little the crystals of calcium hydroxide. Therefore, the carbonation involves another step, that of separating the lime from the gel, and this was not considered in deducing the correction value of 5.8 calories per gram for each per cent of carbon dioxide. It is indicated that the separation of a small amount of lime from the gel requires little energy and that the correction value of 5.8 calories is therefore approximately correct where small amounts of carbon dioxide are involved. Turning to the effect of drying of hydrated samples during preparation for test, an error in the opposite direction is encountered. Any extensive drying of a hydrated sample would be expected to increase its heat of solution and hence the heat of hydration obtained by this method would be too small. If this fact is realized, samples can readily be prepared without drying to the point of introducing appreciable error. Drying a t 50” C., for example, was found to increase the heat of solution by 3 calories per gram, while drying a t 110’ C. caused an increase of about 16 calories per gram. Ordinarily, drying to the equivalent of 50” C. is not encountered, although this amount of drying can be obtained a t room temperature when the humidity is low. The factors t o be borne in mind in determining equivalent temperature rise of concrete from heat-of-solution results are as follows: 1. Heat of hydration values are often reported on ignited basis (making values too high). 2. Carbonation of hydrated samples may have occurred (making values too high). 3. Drying of hydrated samples may have occurred (making values too low). 4. Loner water-cement ratios are generally used for heat-ofsol2tion specimens than for concrete (making values too loa). a. Immediate heat of hydration is included in heat of solut ion. 6. Specific heat of concrete varies with temperature. 7. Temperature of curing test specimens is usually loiver than that of concrete.
The advantages of the heat-of-solution calorimeter are that small neat-cement specimens may be used and that the specimens require no attention other than temperature control between tests. Disadvantages are that a high degree of accuracy is necessary in the measurement of heats of solution to get fair accuracy in heat of hydration, and that some cements, particularly Portland pozzuolana cements, do not dissolve quickly enough in acids to be tested by the heat-ofsolution method. The seven factors listed above are not considered to be disadvantages of the method, because they can be eliminated or corrected. VANE AND COXDUCTION CALORIMETERS.Reliable results cannot be obtained on highly insulated neat-cement speci-
385
F I G ~ R2.E COSDUCTIOX CALORIMETER
mens because abnormally high temperatures develop that not only affect the heat of hydration but make even high insulation inadequate. The vane calorimeter ( 2 ) employs neatcement specimens but avoids high temperatures by conducting the heat away through metal vanes almost as fast as it is liberated The rate a t which the heat is conducted away is determined by measuring accurately the small temperature difference that develops between the specimen and the outer edge of the vanes. When a continuous record of the rate of heat removal from the specimen is obtained, the total amount of heat removed up to any age can be computed. Because the specimen varies so little in temperature that only a small amount of heat is stored in the specimen, the total amount of heat removed is the heat of hydration. A modification of the vane calorimeter is the “conduction” calorimeter, exactly the same in principle but with a metal tube replacing the vanes. Figure 2 presents a cross section of the conduction calorimeter, showing how heat is removed from the specimen by a tapered copper rod and how substantially all heat is caused to flow in the direction of the metal tube by reason of a surrounding Dewar jar. Thermometers, not shown in the figure, are at either end of the conducting tube. The advantages of the conduction over the vane calorimeter are (1) greater accuracy, ( 2 ) use of smaller specimens, and (3) a more faithful response to changes in rate of heat liberation. The advantages of vane and conduction calorimeters as a type are: (1) early heat liberation can be studied in detail, (2) any cement can be tested, and (3) continuous results up to about 7 days can be obtained a t low cost. Disadvantages are that results are lacking in accuracy after about 7 days and that curing conditions are practically limited to a substantially constant temperature.
IXDUSTRIAL AIVD ENGIKEERING CHEMISTRY
386
Comparison of Results from Different Calorimeters Comparable tests, employing the three types of calorimeters described above, indicated that almost identical results could be obtained from all three calorimeters when due regard was paid to possible sources of error. Results of heat-of-solution tests made on neat specimens cured on the time-temperature curve of corresponding concrete, when converted into temperature rise of concrete, closely represented the time-temperature curve obtained from an adiabatic calorimeter. Likewise, results of heat-of-solution tests on neat specimens cured a t 21.11" C. (70" F.) checked very well with results on similar neat specimens tested in vane Ealorimeters a t 21.11 " C. (70" F.), Results were not in agreement, however, until proper account was taken of (1) immediate heat of hydration, (2) carbonation of heat-of-solution specimens, (3) water-
VOL. 10, KO. i
cement ratio, and (4)variations in specific heat of concrete with temperature. The effects of the other possible sources of error discussed above either were not involved in the comto be subject to proof Of parisons Or were too in the tests that were made.
Literature Cited (1) Carlson, R. W., Proc. Am. Concrete Inst., 34, 89 ( 1 9 3 7 ) . (2) Carlson, R. IT., Proc. Am. SOC.Testing Materials, 34, Part 11, 322 (1934). (3) Hornibrook F.
B., Kalousek, G. L., and Jumper, C. H., J . Research Na'atl. Bur. Standards, 16, 4 8 7 (1936). (4) Lea, F. M., "Comparison of Methods for Measuring the Heat of Hydration of Cements," London Congress Intern. .4ssoc. Testing Materials, 1937. Lerrh. Wm.. Eno. News-Record. 113. 523 ( 1 9 3 4 ) . (5) (6) Woods, Hubert,-and Steinour, 'H. H., Proc. Am. Concrete Inst., 3 (31, 195 (1931). Bpril 12, 1938
Separation of the Chrysanthemum Carboxvlic Acids J
Destructive Effect of Steam Distillation on Chrysanthemum Monocarboxylic Acid ATHAN A. PANTSIOS, University of Chicago, Chicago, Ill.
T
HE acid methods of estimating the pyrethrin I and I1
content of pyrethrum flowers and their extracts depend upon the separation of the chrysanthemum acids by steam distillation of the steam-volatile monocarboxylic acid (6, 6, 7). Estimations of pyrethrin I by these methods are usually lower than those obtained by difference after estimation of total pyrethrins by the Gnadinger-Cor1 (1) reduction method and of pyrethrin I1 by the Haller-Acree (4) methoxyl method. TABLEI. Loss OF CHRYSANTHEMLT~ MONOCARBOXYLIC ACID ON REPEATED STEAMD~STILLATION OF THE SAMESAMPLE Operation 1 2 3 4 5 6
7
8 9 10 11
12
I
(Acid expressed as cc. equivalent of 0.0200 N base) Total Acid in Distillate --LossLoss 52.0 4i.3 42.5 38.0 33.5 29.6 25.8 22.2 20.1 18.8 17.4 16.1
4.7
4.8 4.5 4.5 3.9 3.8 3.6 2.1 1.4 1.3 1.3
%
%
9: 1 10.2 10.6 11.8 11.6 12.8 13.9 9.5 7.0 6.9 7.5
9:1 18.3 26.8 35.2 43.0 50.4 56.9 61.4 64.0 66.5 69.0
Acid in Residue 0:i, 0.1 0.05 0.1 0.1 0.1 0.05 0 1 0.1 0 1 0.1
Graham (2) has found that steam distillation of perfumed oil extracts to remove the essential oil (as directed in the Seil method, 6 ) results in a 25 per cent loss of pyrethrin I, and ( 3 ) that there is a lack of uniformity among the results of different analysts. This study was undertaken t o show the effect of steam distillation on chrysanthemum monocarboxylic acid and to develop a different method of separating the acids.
Procedure A sample of pure chrysanthemum monocarboxylic acid (b. p., 140-2" a t 9 mm.) was subjected to repeated steam distillationsthe acid after extraction from the distillate and
titration with standardized base was reacidified and stearndistilled again. All steam distillations were carried out as directed by Seil. After eleven steam distillations (Table I) 69 per cent of the original acid present was destroyed. The average loss for each distillation was over 10 per cent. A series of samples containing various amounts of the chrysanthemum monocarboxylic acid was prepared both by direct weighing of the pure acid and by measuring off aliquots from a standardized alkaline solution of the acid. Each sample was steam-distilled and the acid reestimated as in the Seil method. The results of a series of determinations are condensed in Table 11. It is evident that steam distillation destroys a n average of over 10 per cent of the chrysanthemum monocarboxylic acid; consequently, all methods of estimation of the pyrethrins involving steam distillation give low values for pyrethrin I. TABLE11. ON
Sample
-4 B C
D E F
G H I J K
L
LOSS O F CHRYSANTHEMUM MONOCARBOXYLIC ACID STEAMDISTILLATION OF VARIOUSSAMPLES
(bcid expressed as 00. equivalent of 0.0200 N base) Acid Acid in Present Distillate --Loss% 31 . B 37.6 36.0 33.3 36.6 38.3 36.9 43.8 43.9 48.2 33.4 41.0
27.5 34.5 31.8 28.6 32.0 32.8 31.5 39.4 37.6 43.8 28.5 35.0
4.4 3.1 4.2 4.7
;:!
5.4 4.4 6.3 4.4 4.9 6.0
13.8 8.2 11.6 14.1 12.6 14.4 14.6 10.0 14.6 9.1 14.6 14.6
Acid in Reeidue 0 1 0.1 0 0 0 1 0 1 0 1 0 1 0 0 0 1 0 1 0 1 0 1
That any degree of precision is possible in the acid methods of pyrethrin analysis can be explained by the fact that under comparable conditions the loss of the monoacid on steam distillation is approximately proportional to the amount of acid present. The lack of agreement in the results of different analysts is also understandable.
