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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Vol. 17, No. 1
Heterogeneous Catalysis! 11-Hydrogenation of Marine Oils By A. S. Richardson, C. A. Knuth, and C. H. Milligan THEPROCTER & GAMBLECo., IVORYDALE, OHIO
N AN earlier paper: it has been shown that the hydro-
less well-defined breaking point, similar to that observed for genation of cottonseed, peanut, and soy bean oils in the other oils. It was believed that the initially constant rate presence of catalytic nickel is characterized by the prefer- of reaction signified the selective hydrogenation of glycerides entia1 conversion of linoleic acid to oleic acid and its isomers. derived from acids more unsaturated than oleic and that the I n other words, relatively little saturated acid is formed dur- break in rate of reaction occurred when only 10 to 20 per cent ing the hydrogenation of these vegetable oils until practically of such glycerides were present, but no direct evidence reall the more highly unsaturated fatty acid radicals are re- garding the composition of hydrogenated whale oil was given by Armstrong and Hilditch. duced to the oleic acid stage. The present paper deals Experimental with the selective action of Hydrogenation of whale oil and of menhaden oil in the catalytic nickel in the hyThe fractional distillapresence of nickel catalyst results initially in the preferentions mentioned below were drogenation of whale oil and tial conversion of the more highly unsaturated fatty acids carried out according to the menhaden oil. According to to less unsaturated acids without formation of substantial method described in a rethe best available evidence, quantities of completely saturated acids. A n abrupt cent paper by the authors the unsaturated acids of change occurs at an iodine value of approximately 84, at on the composition of whale vegetable oils are chiefly or which value nearly all the acids of more than two double In other respects the entirely of eighteen carbon bonds have disappeared. Below this critical point hydroworking methods used in c o n t e n t . On t h e other genation results both in the formation of saturated acids t h e p r e s en t investigation hand, as has been recently and in the conversion of Cao and C ~ acids Z containing two were essentially similar to pointed out by the a u t h o r ~ , ~ double bonds to corresponding acids of one double bond. those used in the study of the fatty acids of marine oils The hydrogenation of these marine oils is markedly differhydrogenated v e g e t a b l e appear to be complicated ent from that of typical vegetable oils, in which substanoils.2 Consequently, no atmixtures of acids of various tial increase in the quantity of saturated acids present is tempt will be made in the molecular weights , the coincident with the almost complete disappearance of present paper to explain in highly unsaturated acids acids containing more than one double bond. detail the preparation and being chieflv of more than examination of samples. eighreen carbon content. A critical point in the hydrogenation of any fat is that at Consequently, studies involving the composition of marine oils are likely to present difficulties considerably greater than which the solid saturated fatty acids begin to increase in substantial amount. This point may or may not be well defined. those encountered in similar studies of the vegetable oils. If hydrogenation were perfectly selective, the point would Previous Work always be sharply defined and would be identical with the Ubbelohde and Svanoe* followed the hydrogenation of point at which all acids more unsaturated than oleic disapwhale oil by preparing, from the fatty acids of whale oil pear. In order to determine this critical point and in order hydrogenated to varying degrees, the solid bromides insoluble to make a general study of the change in composition with in ether. It was concluded that clupanodonic acid absorbs progress of hydrogenation, whale oil was hydrogenated to two mols of hydrogen and is converted into linoleic acid different iodine values under the same conditions of reaction. without the formation of substantial quantities of the inter- The whale oil used was obtained from the United States mediate linolenic acid. Ubbelohde and Svanoe were evi- Pacific coast and was from the same sample as that previously dently laboring under the mistaken impression that the more estimated3 to have the composition shown in Table I. highly unsaturated acids of whale oil are of eighteen carbon TABLEI-COMPOSITION OF FATTY ACIDS OF W H A L E OIL content, but this fact does not necessarily invalidate their Per cent evidence to the effect that hydrogenation of whale oil does c 1 4 Myristic 4.5 not lead to the accumulation of substantial amounts of unC l 6 Palmitic 11.5 Palmitoleic 17.0 saturated fatty acids containing three double bonds. In the CIS Stearic 2.5 present paper it is preferred to use the term “selective hydroUnsaturated (nearly all oleic) 36.5 genation” to denote preferential conversion of the more highly Cao Unsaturated 16 unsaturated fatty acids to less unsaturated acids of corre10 Czz Unsaturated Cad Unsaturated 1.5 sponding molecular weight, without the formation of substanUnsaponifiable 0.7 tial quantities of completely saturated fatty acids. I n this sense, the work of Ubbelohde and Svanoe gives little or no Data on the change of composition of whale oil with indication of the extent to which hydrogenation of whale progress of hydrogenation are shown in Table 11. oil is selective. A sample of menhaden oil from the Gulf of Mexico was Armstrong and Hilditch6 have shown that the hydrogen- likewise hydrogenated to varying iodine values under the ation of whale oil proceeds a t a constant rate up t o a more or same condition of reaction. Some idea of the composition of the menhaden oil used may be gathered from Table 111, 1 Received August 25. 1924 2 THIS JOURNAL, 16, 519 (1924). which shows the results of distilling the methyl esters of the 8 J. Am. Chem. SOC.,46, 157 (1924). mixed fatty acids of this oil a t a pressure of 7 to 9 mm. through 4 Z. angew. Chem., 32, 279 (1919). two‘vigreux distilling towers (90 cm. long and 25 mm. in 6 Puoc. Roy. SOC.(London), 96A, 137 (1919).
I
INDUSTRIAL A N D ENGINEERIhTG CHEiVlISTRY
January, 1925
cross section) arranged in series with an extra flask a t the bottom of the second tower. Data on the change in composition of menhaden oil are shown in Table IV. TABLE 11-WHALE
OIL HYDROGENATED WITH 0.3 PER CENT NICKEL AT
198O TO 204”
c.
,-----COMPOSITIONSatu- UnsatuSo!id rated rated ------IODINE VALUEMixed Solid Liquid acids acids acids Oil acids acids acidsa Per cent Per cent Per cent 150.5 5.6 15.5 84.5 121.6b 1 2 7 . 2 16.7 24.3 18.4 120.1 25.2 93.7 81.6 98.0 116.6 25.3 18.3 81.7 25.5 95.3 91.1 112.0 27.6 18.5 81.5 26.6 91.3 87.3 27.8 18.6 109.1 26.9 84.8 81.4 88.7 30.0 19.1 105.2 81.4 80.9 28.6 85.1 105.5 28.8 20.3 29.8 84.1 80.4 79.7 104.4 29.0 21.6 31.8 78.2 78.4 81.8 3 4 . 3 2 3 . 2 1 0 4 . 3 76.6 76.8 3 7 . 5 80.1 34.1 23.7 103.0 38.2 76.3 75.1 78.6 3 1 . 5 1 0 0 . 9 2 2 . 7 77.3 3 5 . 0 74.6 78.1 38.4 24.1 102.1 42.0 74.1 75.9 77.5 3 1 . 7 2 4 . 7 1 0 0 . 9 72.7 75.3 38.1 76.0 33.6 25.8 97.6 69.2 74.2 41.2 72.4 36.2 24.3 63.8 49.7 93.1 56.8 59.4 a The term “liquid iodine value” or “iodine value of liquid acids” is used to denote the iodine value of the total unsaturated acids, including those whose lead salts are precipitated along with the lead salts of the saturated acids. b Original oil.
In Tables I1 and IV the percentage of the saturated fatty acid is estimated from the observed weight and the iodine value of the solid fatty acids isolated by the Twitchell method. Strict accuracy would require also a knowledge of the iodine value and molecular weight of the unsaturated acids present in the solid fraction, the determination of which for each individual sample would require an enormous amount of labor even if a method for this determination were available. Fortunately, however, no great error can be introduced if we arbitrarily assume that the unsaturated acids in the solid fraction are isomeric with oleic acid, and this assumption has been made in the calculations. I n other words, the same formulas have been used for calculations in connection with Tables I1 and IV as were previously used for the cases of the cottonseed, soy bean, and peanut oils, except that no attempt has been made to divide the unsaturated acids into linoleic, oleic, and “iso-oleic.” In view of the complexity of the unsaturated acids in marine oils, it would he absurd to express their composition in terms of the acids of eighteen carbon content ~
TABLE111-DISTILLATIONOF METHYLESTERS OF MENHADEN OIL Iodine value of methyl esters, 166.6; average molecular weight of methyl esters, 293.0 Boiline Weight of Predominating limit) fraction Molecular Iodine series of O c. Grams weight value fattv acids 145 252.0 7.45 10.4 150 248.0 13.81 11.9 Cl4 155 249.0 11.07 20.1 160 255.0 24.30 44.1 165 265.2 72.9 32.03 170 271.6 75.8 87.73 Cl8 175 271.9 70.7 24.78 190 291.2 39.27 132.7 195 271.7 19.93 150.4 CIS 205 289.7 10.30 135.8 210 312.7 44.96 318.1 c20 215 310.1 17.46 308.5 225 326.3 14.51 319.6 230 332.9 19.90 333.6 331.2 240Q 5.76 229.9 398.2 Residue 19.90 199.4 c 2 2 SaDonifiable in iesidue 16.94 338.0 203.88 Unsaponifiable in residue 2.96 Total 393.16 a This fraction was distilled through the first of two columns and obtained from the flask a t bottom of the second column at end of distillation. b Iodine value of free fatty acid instead of ester.
g;:;
The results shown in Table I1 indicate that the hydrogenation of whale oil is highly selective down to an iodine value of about 84. Until this point is reached, hydrogenation proceeds with very little increase in the percentage of solid saturated acids present. Below this critical iodine value hydrogenation results in a steady and substantial increase in
81
solid fatty acids. Table IV shows a similar breaking point in the hydrogenation of menhaden oil and, by coincidence, the abrupt increase in solid saturated fatty acids occurs also in this oil a t an iodine value of approximately 84. This break in the rate of formation of saturated acids during the hydrogenation of whale and menhaden oils is shown graphically in Fig. 1.
T
/
I
c9c I
I
I
80 70 ‘ 60 Iodine V a l u e FIG. CHANGE IN COMPOSITION DURING HYDROGENATION OB ( A ) WHALE OIL AND ( B ) MENHADENOJZ
90
With cottonseed, peanut, and soy bean oils, the abrupt increase in solid saturated fatty acids was coincident with the virtual disappearance of fatty acids more unsaturated than oleic. I n this respect whale oil and menhaden oil are very different from the vegetable oils. High liquid iodine values, corresponding to considerable quantities of fatty acids more unsaturated than oleic, persist in samples of marine oil hydrogenated far beyond the point where solid saturated acids are formed in large amount. If the hydrogenation of whale oil were perfectly selective, an iodine value of about 71 instead of 84 would be reached without increase in the percentage of solid saturated acids present. TABLE IV-MENHADEN OIL HYDROGENATED WITH 0.5 PERCENT NICKEL AT 224’ t o 228O C. -COMPOSITION--Satu- Unsatur -IODINEVALUESo!id rated rated Mixed Solid Liquid acids acids acids Oil acids acids acids Per cent Per cent Per cent 169.8a 3.6 238.8 177.4 91.1 129.2 95.2 25.2 73:7 2 7 . 1 87.7 124.9 91.9 73.4 83.4 27.9 119.6 87.2 72.9 117.2 81.0 28.4 84.6 72.2 112.9 76.8 29.4 80.3 71.1 28.7 77.5 74.2 111.5 69.5 65.2 104.8 68.1 26.8 66.0 56.4 23.2 68.9 99.0 59.5 a Original oil.
-
The break in the hydrogenation curves a t about 84 iodine value appears to be closely related to the disappearance of fatty acids of more than two double bonds. The hexabromide test, according to the method of Eibner and Muggenthaler,G was made on samples of hydrogenated whale oil having iodine values of 93.7, 87.3, 84.6, 81.4, and 78.2, respectively. h’one of these samples gave a crystalline bromide, but the first four gave a liquid bromide insoluble in ether. The sample of 78.2 iodine value gave no bromide insoluble in ether. Ubbelohde and Svanoej4 using a slightly different hexabromide test on hydrogenated whale oil, report that the fatty acids giving solid bromides insoluble in ether disappear a t an iodine value of approximately 85. From a study of the liquid iodine values given in Table 11, it may be estimated that in hydrogenated whale oil of 84 iodine value about 19 per cent of the total fatty acids consists of acids of 20 and 22 carbon content containing two double bonds. On account of less accurate information as to 8
Farben-Zlg., 18, 235 (1912)
Vol. 17, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
82
'its composition, a similar estimate for menhaden oil is subject t o greater inaccuracies, but examination of the liquid iodine values reported in Table IV indicates that the percentage of acids of 20 and 22 carbon content containing two double bonds i s even greater in menhaden oil of 84 iodine value than in whale oil of the same iodine value. Considerable information regarding the character of hydrogenation of marine oils below the breaking point may be gained by comparing numerically the increase in solid saturated acids with the decrease in iodine value. Immediately below 80 iodine value, Fig. 1 shows that the increase in solid saturated acids is approximately 1 per cent for each change of two units in iodine value. Accordingly, in this region, somewhat less than half the total hydrogenation of whale oil and of menhaden oil can be accounted for by the formation of solid saturated acids from unsaturated acids containing one double bond. In order to gain a further insight into the progress of hydrogenation below the critical point, the methyl esters prepared from a sample of whale oil hydrogenated to an iodine value of 69.2 were fractionally distilled a t a pressure of 7 to 9 mm. through one of tke two distilling towers previously mentioned. The results are shown in Table V. TABLE V-DISTILLATION OF
THE METHYLESTERS OF WHALE OIL OF 69.2IODINE VALUE
Boiling point
%?; c.
Weight of fraction Grams 3.75 15.28 13.11 32.40 5.85 59.50 6.24 16.50 5.32 2.56 13.75 9.05
Molecular weight 251.6 261.2 274.2 276.5 289.5 299.9 308.6 324.7 335.6 338.2 360.1 414.1
Iodine value 25.8 31.0 44.6 43.3 S6.O 70.1 76.9 105.2 111.1
TABLEVIII-DISTILLATION O F METHYL ESTERSO F HYDROGBNATED
HYDROGENATED
Predominating series of fatty acids
165 170 175 180 190 195 215 220 230 111.1 235 116.1 255 101.2 Residue Saponifiable in residue 7.75 362.0 101.la Unsaponifiable in residue 1.30 Total 183.31 a Iodine value of free fatty acid instead of methyl ester.
I n fact, there is no justification for expressing the results of Table VI1 in terms of the mixed fatty acids of the whole oil, except for the purpose ol indicating roughly the order of magnitude of the percentage of a particular type of acid present. For instance, Table VI1 indicates that 10.6 per cent of the fraction of the oil in question consists of solid principal CZO saturated acids. Since the CZOacids constitute about 16 per cent of the mixed fatty acids of the whole oil, we might conclude that 1.7 per cent-i. e., 10.6 per cent of 16 per cent-of the fatty acids of the hydrogenated whale oil of 69.2 iodine value consists of arachidic acid. Actually, we are only justified in concluding that the oil in question contains very small amounts of arachidic acid, probably less than 2 per cent. Likewise it appears probable that this oil contains a t most only traces of behenic acid. Tables VIII, IX, and X show for partially hydrogenated menhaden oil what Tables V, VI, and VI1 show for partially hydrogenated whale oil. I n Tables IX and X there have been also included, for the sake of comparison, data concerning the composition of some of the principal fractions obtained by distillation of the methyl esters of the mixed fatty acids of menhaden oil before hydrogenation.
ClE
C1a CZO
czz
The principal fractions recorded in Table V were examined by the Twitchell method of separating solid fatty acids. The results are shown in Table VI.
Boiling point (Upper limit)
MENHADEN OIL OF 68.1 IODINB VALUE Weight
of Predominating fraction Molecular Iodine series of Grams weieht value fattv acids 29.84 146 246.9 6.6 Cl4 245.6 9.79 150 7.8 Clt 7.41 16.7 155 250.5 27.06 46.3 160 264.9 CIS 270.3 82.11 46.6 165 ClS 170 272.0 38.33 40.1 ClE 10.99 42.6 175 278.1 14.70 63.4 180 291.3 23.83 71.4 190 298.4 CIS 195 300.7 19.50 71.6 C1a 205 312.1 21.33 96.8 32.15 210 324.6 117.4 CZO 328.5 23.19 220 113.2 czo 8.50 225 336.5 113.8 345.9 10.44 230 120.6 352.4 20.74 118.4 240 c 2 2 Residue la 370.5 102.6 3.45 416.1 12.91 Residue 2 114.2 a This fraction was distilled through the first of two columns and obtained from the flask a t the bottom of the second column a t the end of distillation.
c.
TABLEIX-EXAMINATIONOF PRINCIPAL FRACTIONS FROM HYDROGENATED MENHADEN OIL OF 68.1 IODINEVALUEAND FROM ORIGINAL OIL Predominating -IODINE VALUE Solid series of Ester Mixed Solid Liquid acids fatty acids (or oil) acids acids acids Per cent ' Hardened oil 68.1 71.2 28.4 109.2 50.9 Original oil (169.8)a (177.4) (3.6) (238.8) (26.8) 160-5 ClE 46.6 49.2 13.3 100.0 58.6 (165-70) (CIS) (75.8) (79.8) (4.0) (138.3) (44.1) 180-90 ClI 71.4 74.9 41.4 103.9 51.7 (190-5) (CI$ (150.4) (158.0) (16.3) (182.2) (16.2) 205-10 117.4 122.7 68.7 131.1 39.6 (215-25) (Cd (319.6) (333.9) (164.6) (12.8) 230-40 c 1 1 118.4 123.3 68.8 l2+:8 44.0 a Figures in parentheses refer t o the original oil before hydrogenation.
.
Boiling HYDROGENATED point C.
TABLE VI-EXAMINATIONOF PRINCIPAL FRACTIONS FROM WHALE OIL O F 69.2 IODINE VALUE Boiling predominating -IODINE VALUESolid point series of Ester Mixed Solid Liquid acids O C. fatty acids (or oil) acids acids acids Per cent Wholeoil 69.2 72.4 33.6 97.6 41.2 170-5 Cl6 44.6 47.0 14.8 89.5 55.7 190-5 70.1 - 73.4 51.7 92.8 49.1 215-20 C1a c zo 105.2 109.9 58.0 122.9 36.3 Residue C22 97.2 101.1 74.2 120.9 42.9
It is not possible to estimate the composition of these fractions accurately in terms of the actual fatty acids present. However, in Table VI1 such an estimate has been attempted and rests on the following assumptions: (1)that each fraction contains only fatty acids of the carbon content indicated; (2) that the unsaturated acids present in the solid acids are of one double bond only; and (3) that no acids of more than two double bonds are present in any of the fractions.
TABLEX-APPROXIMATECOXPOSITION OF PRINCIPAL FRACTIONS PROM HYDXOGEXATED LIESHADES OIL O F 68.1 I O l l I N Z V A L U E A N D O F O R I G I X A L OIL
(2
a
TABLEVII-APPROXIMATECOMPOSITION OF PRINCIPAL F R ~ C T I OFROM N HYDROGENATED WHALE OIL OF 69.2 IODINE VALUE PER CENT LIQUIDACIDS 2 double Predominating PER CENT SOLIDACIDS 1 double series
ClE CIS
CZO C21
Satd. 47.5 20.9 10.6 0.0
Unsatd. 8.2 28.2 25.7 42.9
bond 44.3 48.5 19.4 22.2
bonds 0.0 2.4 44.3 34.9
It should be noted that the percentage values in Table VI1 refer strictly to the individual fractions and not to the mixed fatty acids of the hydrogenated whale oil as a whole.
PER CENT LIQUIDACIDS PER CENT SOLID ACIDS 1 double 2 double Satd. Cnsatd. bond bonds 60.8 7.8 41.4 (42.3)~ (34.1) (21:8) (1.8) 23.8 27.9 37.4 10.9 (13.3) (2.9) 0) 33.2 6.4 9 55.5 40.5 56.0 3.5 0.0 Figures in parentheses refer to the original oil before hydrogenation.
Predominating series
-
Effect of Varying Conditions upon Hydrogenation
Finally, it is of some interest to inquire into the effects of conditions of hydrogenation upon the extent to which the hydrogenation of marine oils is selective. In this connection, the effect of quantity of catalyst and the effect of temperature of hydrogenation have been investigated a t some length by the authors, with results which are, in the main, concordant with those obtained in the study of cottonseed, peanut, and
January, 1926
I N D U S T R I A L A N D ENGINEERING CHEMIXTRY
soy bean oils. A few typical experimental results are shown in Table XI. Increasing quantity of catalyst has been found rather consistently to favor selective hydrogenation. I n a large majority of experiments, although not invariably, increasing temperature has been found to favor selective hydrogenation of marine oils up to about 200" C., above which results are conflicting. T A B LXI-WHALE ~
OIL HYDROGENATED TO 75 IODINE VALUE UNDER VARYING CONDITIONS Temperature Nickel Solid acids C. Per cent Per cent 22.6 21.7 23.3 23.4 23.0 22.2 22.0 23.0 21.7
19.8 22.3 21.2
Conclusion
In view of all the experimental results set forth above, it appears that the hydrogenation of whale oil in the presence
83
of catalytic nickel results initially in the preferential conversion of the highly unsaturated Cza and CB acids to acids of either one or two double bonds without the formation of substantial quantities of saturated acids. At an iodine value of approximately 84, the character of the hydrogenation changes abruptly. Immediately below this critical point, hydrogenation results in the formation of substantial amounts of palmitic and stearic acids, while the Cm and CZZacids containing two double bonds are simultaneously converted to corresponding acids of one double bond, possibly with the formation also of very small amounts of saturated acids of 20 and 22 carbon content. Even a t an iodine value of 57, substantial quantities of unsaturated acids containing two double bonds are present in the hydrogenated oil. Menhaden oil behaves in a similar manner on hydrogenation, and on account of similarity in composition it is probable that the same will be found true of other marine oils. It must, however, be regarded as mere coincidence that the critical points in the hydrogenation of the particular samples of whale oils and of menhaden oil used in the present investigation were reached a t practically identical iodine values.
Perfection of Chromic Acid Method for Determining Organic Carbon' By J. W. White and F. J. Holben THE PENNSYLVANIA STATE COLLEGE, STATECOLLEGE, PA.
the procedure of Ames and H E need of a method Results of the present study show conclusively that Gaither, suggeststhe partial organic materials are capable of complete oxidation in a for organic combusboiling mixture of sulfuric and chromic acids. substitution of phosphoric tion adaptable to the The sulfur trioxide absorption tube, used for the first for sulfuric acid, which estimation of organic carbon, both in solution and in time in this study, greatly simplifies the usual analytical was observed to reduce procedure and eliminates the need of a combustion tube. the volume of acid fumes. dry substances, is emphaThe data reported as the sized by the many attempts The proposed method has the advantage over dry comresult of the present study to perfect the chromic acid . bustion in that it eliminates the possibility of leaving were secured by boiling the method proposed in 1848 by behind the residue of undecomposed carbonates; moresulfuric-chromic acid mixRogers and Rogers.2 The over it may be used for the estimation of carbon both in solution and in dry substances. ture for 30 minutes. The early studies of the method sulfuric acid fumes given off by Warrington and PeakeJ3 The Knorr apparatus is suggested for chromic acid digestion as carried out in this work. were intercepted by solution together with the investigations of Cameron and Breain a U tube containing sulfuric acid of constant boilzealeJ4furnished data which led to the conclusion that oxidation of carbon in soils by means ing point (98.33 per cent) in contact with coarse glass of a mixture of chromic and sulfuric acids gave results lower wool prepared and arranged as described below. The rethan those obtained by furnace combustion in a current of sults secured by this method will be seen to agree with the oxygen. dry combustion results. Various substances were tested, As suggested by Ames and Grtither,6 the failure of chromic including such resistant materials as peat, alfalfa meal, acid to bring about complete oxidation, resulting in the libera- charcoal, barnyard manure, soils, etc. The use of the protion of carbon compounds other than carbon dioxide, is due to posed sulfur trioxide absorption tube eliminates the necessity the fact that the mixture of acids has been kept considerably of secondary combustion and greatly reduces the time rebelow the boiling point. quired to complete a determination. Results are reported Those who have previously studied the chromic acid method including digestion in concentrated acid mixture and also have made no attempt to determine the effects of boiling the with 25-cc. dilutions of 4 per cent ammonium hydroxide acid mixture and of intercepting the sulfuric acid fumes evolved and 3 per cent sodium hydroxide. incident to boiling. Ames and Gaither5 report results sePreparation of Materials cured by boiling the acid mixture, but made no attempt to prevent the acid fumes from passing into the carbon dioxide SULFURIC ACID OF CONSTANT BOILINQ POINT(338 C.) absorption tower. Schollenberger,e in his modifications of 1 Received August 6. 1924. -The acid used in the sulfur trioxide absorption tube 2 A m . J . Sci., [21 5, 352 (1848). is prepared by boiling sulfuric acid (specific gravity 1.83 to 8 J . Chem. SOC.(London), 37, 617 (1880). 1.84) for 2 hours in a Kjeldahl flask. The flask is allowed to 4 J . A m . Chcm. SOC., 26, 29 (1904). 'cool for a few minutes and then closed with a rubber stopper 8 THIS JOURNAL, 6, 561 (1914). to which is attached a sulfuric acid-pumice drying tube. 8 I b i d . , 8, 1126 (1916).
T
