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Figure 1 contained 2 oleate to 1 stearate, while Figure 2 contained ester in the ratio of 1 oleate to 1 stearate. The backgrounds in Figures 1 and 2 appear black. They were actually white and clear as Figure 3, but the lighting was arranged to give this effect in order to show the crystals by contrast. When ethyl oleate is present in the proportion of 4 parts oleate t o 1 part stearate, this crystallization of the stearate from the film is not observed; the ratio of 3 oleate to 1stearate was not studied. The authors believe that this experiment throws light on the blooming phenomena (often observed in varnishes, in certain simple oil-nitrocellulose lacquer coatings, and in patent leather finishes) which appear after repeated elevation and dropping of the temperature incidental to climatic changes. The development of tacky, odorous, and darkcolored produets by the films composed of the ethyl esters of linseed mixed fatty acids also furnishes evidence for the source of the development of odors and color in varnished insulation fabrics as well as varnished papers used for bottle caps and for other purposes. I n comparison with the films of ethyl esters, one was prepared containing polymerized oil. This film remained flexible and dry and showed no crystallization phenomena. It yellowed on 10 months’ storage slightly more than the ethyl oleate films, but by no means t o the extent that the esters of unpolymerized linseed oil did.
Vol. 20, No. 12
When exposed outdoors on the roof for 1 month (September) polymerized linseed-oil films remained flexible, while those of all the esters mentioned first became sticky and then brittle, thus demonstrating the added advantage of lowered iodine number of the oleaginous component of the films brought about through polymerization. Conclusions
The esters of the less unsaturated fatty acids of the type of oleic are very stable in films. The esters of the more unsaturated acids of linseed oil when present in the unpolymerized form are not so stable, as evidenced by rapid tendency to become sticky, odorous, and dark-colored. It is therefore of advantage to compose films of lower iodine number fatty constituents which may be effected by use of oleic derivatives 01 substances or similar iodine number range obtained by polymerization of linseed oil, or, certain mixtures of oleic and stearic derivatives obtained by hydrogenation of linseed oil. This eliminates the tendency to become sticky and dark in color. The darkening in color of certain of the films as contrasted with the failure t o darken in others affords support to the current belief that yellowing of drying-oil films arises from oxidation of the highly unsaturated fatty acid groups and that it and drying of linseed films are not interdependent. It is also independent of the presence of glycerol and is a necessary consequence of such oxidation.
Action of Ultra-Violet Light upon Ferric Citrate Solutions’” H. Shipley Fry and Elmer G. Gerwe DEPARTMENT OF CHEMISTRY. UNIVERSITY OF CINCINNATI, CINCINNATI, OHIO
HIS quantitative study of the action of ultra-violet
T
light upon solutions of citric acid in the presence of ferric salts was suggested and fostered by John Uri Lloyd,3 a pioneer in the manufacture of pharmaceutical preparations containing compounds of iron. Such solutions, containing citrates in the presence of ferric salts, when unprotected from the action of sunlight, suffer decomposition to such an extent that stoppers may be blown from their glass containers by the accumulation of the evolved carbon dioxide. Accordingly, the purpose of this study was to determine whether or not carbon monoxide or any other deleterious decomposition products of citric acid are formed, and to ascertain not only the specific nature of the involved photochemical changes but particularly to what extent, quantitatively, these changes might be dependent upon the concentrations of the ferric salts present when standardized solutions containing citric acid and varying multiple molar quantities of ferric sulfate were exposed to the action of ultra-violet light. Previous Work Eder4 and Vries5 found that ordinary light affected the reduction of ferric salts to ferrous in the presence of oxalic, citric, tartaric, and malic acids. Vries stated that the ferric Received July 28, 1928. Synopsis of a section of a thesis presented by Elmer G. Gerwe t o the faculty of the Graduate School, University of Cincinnati, June, 1927. in partial fulfilment of the requirements for the M.A. degree. 8 Proc. Am. Pharm. Assocn., 27, 741, 743, 744 (1879). 4 Bn.,82, 606 (1880). 6 Chem. Zcnlr., 66, 219 (1885). 1 2
salts act as catalysts, “carriers” of oxygen, since a small amount of ferric chloride oxidized a great excess of acid. This was due to the fact that the reactions were conducted with access to air, which continuously re-oxidized the ferrous salt to ferric and thereby led to the complete oxidation of the carbon compound. Wisbare and Seekamp’ conducted photochemical reactions in the presence of uranium salts. Seekamp observed that uranic oxide, similarly to ferric salts, suffered reduction. Neuberg8 has made an extensive biochemical study of the action of both sunlight and ultra-violet light upon sixtyodd substances, some in the presence of ferric salts, others in the presence of uranic salts. He also exposed drugs containing iron to the action of sunlight and measured the extent of the reaction by quantitative determinations of the yields of certain products, but did not establish any stoichiometrical ratios for possible specific reactions. His work embodies some excellent ~ u m m a r i e s . ~ None of the earlier investigators attempted to establish exact stoichiometrical relationships between the quantities of the ferric salts employed and the yields of the various products of oxidation. However, Berthelot and Gaudechon,’” the first to employ ultra-violet light as the source of energy, determined quantitatively the proportions of carbon dioxide and hydrogen involved when solutions of oxalic acid were Ann., 262, 232 (1891). I b i d . , 278, 373 (1894). 3 Bioclrem. Z., 44, 495 (1912). 9 I b i d . , 13, 305 (1908); 17, 270 (1909); 27, 271, 279 (1910); 99, 158 (1912); 44, 495 (1912); 71, 219 (1915). 10 Compt. rend., 168,1791 (1914). 6
7
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1928
1393
The function of the ferric sulfate may be regarded as exposed and noted that the relative volumes of these gases varied with the wave lengths of the rays used. The continua- the oxidation of the hydrogen formed in equation (1) in tion of this work by Allmand and Reeve” was directed to a conformity with the following equation: study of the energetics invohed as the most accurate method Fe2(S04)3 HZ +2FeS04 HzSOi (4) of determining to what extent a secondary or tertiary reaction From these points of view the photochemical change, takes place. They made no attempt to isolate any decomposition products other than gaseous ones, and their results are equation (l),is promoted by the oxidizing action of the ferric given with the understanding that they hold for the “initial sulfate in removing the hydrogen, as noted in equation (4), and reactions (1) and (4) are followed by the concurrent stages” of the photolytic reactions only. As to the photochemical decomposition of citric acid in reactions (2) and (3), which are not, per se, oxidation-re. (2), presence of ferric chloride, Benrathlz noted that acetone and duction processes. The summation of equations (.1),, . ., (3), and (4)gives: c a r b o n dioxide were the final products of oxidation, (CHzCOOH)tC(OH)COOH Pharmaceutical solutions containing citric acid and d i c a r b o x y l i c acetone and + Fe2(S04)3--f CH3COCH2 3COn $- 2FeSOd ferric compounds are readily decomposed when exacetoacetic acid b e i n g t h e HzSO4 (5) posed to light. Carbon dioxide is evolved, acetone is intermediate products. formed, and the ferric compounds are reduced. Euler and Ryd,lJ and likeThe establishment of the This quantitative study of the action of ultra-violet wise Ciamician and Silber14 hitherto undetermined light upon solutions containing unit concentrations of recorded the formation of stoichiometrical ratio, Fezcitric acid in excess and varying concentrations of acetone and carbon dioxide. (SO&: 3C02, which is the ferric sulfate shows that three molecules of carbon It is of historical interest i m m e d i a t e object of this dioxide are liberated for every molecule of ferric sulfate to note that Liebig15in 1859 quantitative study, will not present. found that acetone and caronly confirm the proposed It was assumed that the photochemical change inbon dioxide were products s u m m a t i o n equation (5), volves the oxidation of citric acid, with the liberation of the oxidation of c i t r i c representing the final result of one molecule of carbon dioxide, to the unstable acid by m a n g a n e s e p e r of the action of ultra-violet acetone-dicarbonic acid. This also loses a molecule oxide. light upon solutions of citric of carbon dioxide, thereby yielding acetoacetic acid. acid in the presence of ferric Preliminary Experiments The latter readily decomposes forming acetone and a sulfate, but will also lend third molecule of carbon dioxide. The summation of direct support to the theory A review of the literature, the equations for these assumed intermediate reactions of the reaction mechanism as briefly noted, furnishes gives a complete equation as involving the several reno quantitative data to es(CHzCOOH)&(OH)COOH + F e ~ ( S 0 d r--f CHaCOCHa + 3COs actions represented b y + 2FeS01 + HzSOl tablish any stoichiometrical equations (I), (2), (3), and which calls for the stoichiometrical ratio Fe2(SO&: relationships between t h e (4)* 3C0,. The quantitative data obtained confirm this initial quantities of f e r r i c salts present and the correratio and the proposed reaction mechanism. Experimental sponding yields of carbon ULTRA-VIOLET LIGETd i o x i d e when citric acid To this end the experimental procedure involved the use is decomposed by the action of ultra-violet light. Preli&ary experiments, using first sunlight and then of a quariz mercury vapor lamp, spectra range 1850 A. ultra-violet light, confirmed the qualitative observations of to 14,0000 A., two-thirds of the total radiation being less than the previously noted investigators. As the reactions pro- 4500 A. No attempt was made to employ specific radiation ceeded to completion the green solutions became colorless by means of interposed filters. I n all experiments quartz and the ferric salt (ferric sulfate) was completely reduced tubes, 2 X 13 cm. of 20 cc. capacity, containing the standto the ferrous state. ardized reaction mixtures, were placed practically adjacent Since acetone dicarboxylic acid and acetoacetic acid are and parallel to each other, directly beneath and equidistant the products intermediately formed, the complete reaction from the source of light, and inclined a t an angleof 45 degrees. mechanism may be assumed to involve four distinct changes. PREPARATION O F REAcTIoN-MIxTUREs-Three sets Of The first, a photochemical change, is presumably the con- runs (A, B, and C) were conducted and in each two quartz version of a molecule of citric acid to acetope dicarboxylic test tubes were used-one designated as “sample,” the other acid with the elimination of one molecule each of hydrogen as “blank.” The contents of the standardized reaction and carbon dioxide according to the following equation: mixture in the “sample” tubes were made up according to this tabulation of concentrations of reacting components: (CHzCOOH)zC(OH)COOH+(CHZCO0H)zCO Hz COS
+
+
+
+
+ +
(1)
The instability of acetone dicarboxylic acid immediately leads to the formation of acetoacetic acid and a second molecule of carbon dioxide, thus: (CHtC0OH)zCO
--f
CHsCOCHzCOOH
+
COz
(2)
Lastly, the instability of acetoacetic acid yields acetone and a third molecule of carbon dioxide according to the equation: CHaCOCHzCOOH + CHaCOCHa + COS (3)
14
J. Chem SOC, 1926, 2834. Z.p h y s i k . Chem., 74, 118 (1910); Ann., 382,222(1911). Biochem. Z., 51, 97 (1913). Bcr., 46,1558 (1913).
15
A n n . , 113 (1859).
11 12 18
RUN A B C
CITRIC ACID FERRIC SULFATE (1 M soln.) (0.1725 M soln.)
cc.
cc.
9 9
5 7.5 10
9
WATER CC. 5
2.5 0
TOTAL VOLUME CC.
19 19 19
I n juxtaposition to each sample tube in each of the runs there was placed a blank tube containing 9 cc. of 1 M citric acid solution and 10 cc. of distilled water-likewise a total volume of 19 cc. Thus the concentration of citric acid was identical in each sample and in each blank tube; but in the sample tubes, in runs A, B, and C, the molar concentrations of the ferric sulfate stood, respectively, in the ratio 1:1.5:2, and the molar concentration of the citric acid was many times that required by the molar proportions of citric acid
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Vol. 20, No. 12
and ferric sulfate, as indicated ia the complete summation the citric acid to acetone and carbon dioxide, in conformity equation ( 5 ) . with the proposed summation equation (5), had been comThe blank tubes were used in determining the quantity pleted. Table I embodies all the data of the three runs. of carbon dioxide eliminated through the action of ultraDiscussion of Results violet light upon citric acid alone. Subtraction of the weight of carbon dioxide evolved from the blanks from the weight At the end of each 2-hour period during the runs the blank of that evolved from the samples gave the weight of carbon tubes, each containing the same concentration of citric acid dioxide eliminated by the action of the light upon the citric and no ferric sulfate, evolved fairly uniform quantities of acid in the presence of ferric sulfate. Accordingly, in order carbon dioxide-that is, the Ab values varied from 0.0025 to verify the stoichiometrical ratio, Fes (so&:3co2, required gram to 0.0051 gram; but from the sample tubes at the end by equation ( 5 ) ,the quantities of carbon dioxide so eliminated of each corresponding 2-hour period, the quantity of carbon in runs A, B, and C should conform to the respective molar dioxide (As) evolved in each C run was greater than that of concentrations of the ferric sulfate-namely, 1: 1.5: 2. I n each corresponding B run, and in like manner the B-run other words, run B should yield 1.5 times, and run C, 2 times quantities were greater than corresponding A-run quantities. as much carbon dioxide as is obtained in run A. In other words, with increasing molar quantities of ferric DETERMINATION OF CARBON DIoxrDE-Each sample and sulfate, increasing quantities of carbon dioxide were evolved. each blank tube, used concurrently in each run, practically These values are noted as As - Ab in the last column. The total quantities of carbon dioxide thus evolved during filled with their respective reaction mixtures (thus excluding any air and thereby preventing excessive oxidation through the total time required for the completion of the reaction the regeneration of ferric sulfate), was securely fitted with a according to the proposed summation equation ( 5 ) were two-holed rubber stopper bearing an inlet and an exit tube. found by adding all the increments, As - Ab, possessing a The inlet tube, of capillary dimension, extended to the bottom plus value, for when As - Ab attained a minus value no of the quartz tube and served for the introduction of a slow, carbon dioxide was being eliminated through the action of steady, continuous current of pure nitrogen, which kept the ferric sulfate which was entirely reduced to ferrous sulfate. reaction mixture well stirred and carried over all of the At this stage As - Ab would necessarily have a minus value, evolved carbon dioxide through the exit tube to a train of since the concentration of citric acid in the blank was greater apparatus comprising two U-shaped drying tubes filled with than that in the sample. Accordingly, the total quantity calcium chloride, a Liebig bulb containing concentrated of carbon dioxide evolved by the action of the light and sulfuric acid, and lastly, a weighed Stesson-Norton glass- ferric sulfate upon citric acid in run A is closely approximated stoppered carbon dioxide absorption bottle filled with As- and estimated as the sum of the increments As - Ab from carite (a proprietary sodium hydrate asbestos absorbent A-0 to A-10, inclusive. For run B the summation includes mixture), for the fixation and ultimate determination of the increments As - Ab from B-0 to B-10, and likewise run C evolved and dried carbon dioxide from each blank and from includes the increments from C-0 to C-12. Table I1 embodies the molar quantities of ferric sulfate each sample tube, respectively. used in runs A, B, and C; the corresponding yields of carbon Table I--Yields of Carbon Dioxide i n R u n s A. B. a n d C dioxide evolved according to the proposed summation equaWT. ASCARITE WT. WT. ASCARITE WT. tion ( 5 ); the theoretical yields of carbon dioxide calculated COZ BULB co2 BULB RUN TIME (Sample) (Sample) (Blank) (Blank) COa in conformity with the stoichiometrical ratio Fe2(SO& : 3C02 As Ab As-Ab Hours Grams Gram Grams Gram Gram of equation (5); and the per cent theoretical yield of carbon 0 0 0 93.0520 0 91.6342 A-0 dioxide based upon the same ratio. 0 0 0 90.4628 0 95.0324 B-0 c-0
0
A-2
R-2 c-2 A-4
B-4 c-4
A-6 B-6 C-6 A-8 B-8
C-8 A-IO B-10
C-10 ~ - 1 2
€3-12
(2-12 A-14 B-14
C-14
6
6 6 8 8 8 IO
10 10 12 12 12 14 14 14
92.7562 93.0869 90.5137 92.8213 93.1520 90.5718 92.8920 93.1567 90.6138 92.9503 93.1698 90,6320 92.9730 93.1778 90,6385 92.9816 93.1823 90.6422 92.9879
0 0.0349 0,0509 0.0651 0.0381 0.0581 0.0707 0.0317
0.0420
0.0583 0.0131 0.0182 0.0227 o.oo80 0.0065 0.0086 0.0045 0.0037 0.0063
90.8140 91.6378 95.0362 90.8165 91.6419 95.0405 90.8193 91.6464 95.0457 90.8231 91.6502 95.0506 90.8288 91.6555 95.0542 90.8334 91.6604 95.0587 90.8379
0 0 0.0036 0.0313 0.0471 0.0038 0.0025 0.0626 0.0041 0.0340 0.0043 0.0538 0,0028 0.0679 0.0272 0.0045 0.0368 0.0052 0.0038 0.0545 0.0038 0.0093 0.0133 0.0049 0.0057 0.0170 0.0053 0.0027 0.0036 0.0029 0.0046 0.0040 0.0049 -0.oon4 0.0045 -0.0008 0.0018 0.0045
Completed Completed Completed Completed Completed Completed Completed Completed Completed Completed
92.9926
0.0047
90.8430
0.0051 -0.0004
PRocEDuRE-In each run one sample and one blank tube containing its standardized contents of reaction mixture were simultaneously exposed to the same intensity of ultra-violet radiation for a total period of 12 to 14 hours-that is, until the difference between the yield of carbon dioxide evolved from the sample (As)-and that evolved from the blank (Ab) was practically constant. Thus when As - Ab reached its approximate minimum value-that is, just before this difference gave a minus reading-it was naturally assumed that the rates of the decomposition of the citric acid, both in the sample and the blank, were practically the same or, in other words, all the ferric sulfate in the sample had been reduced to ferrous sulfate and the concomitant oxidation of
Table 11-Summarized Data of R u n s A, B, a n d C COZFOUND 0.1725 M CARBON DIOXIDB RUN Fez(S0i)s Found Theory Per cent theory
cc. A B
C
5 7.5 10
0.1045 0.1539 0.2078
0.1138 0.1707 0.2277
91.8 90.1 90.3
The actual yields of evolved carbon dioxide in runs A, B, and C stand in the ratios of 1:1.5:2, which are the ratios of the quantities of ferric sulfate initially present in the respective sample tubes of these runs. This result, also expressed in Table I1 as the practically constant per cent theoretical yield of carbon dioxide, shows that, under the present experimental method of procedure, the effect and the extent of the action of ultra-violet light upon solutions of citric acid and ferric sulfate are directly proportioned to the molar concentrations of the ferric sulfate employed, in conformity with the stoichiometrical ratio Fe2(S04)3: 3C02 indicated in the proposed summation equation (5). The quantitative verification of this stoichiometrical ratio also lends support to the proposed reaction mechanism involving the intermediate reactions represented by the equations (l), (2), (3), and (4), the summation of which gives the established equation (5). Analysis of the gas evolved in separately conducted runs showed that carbon dioxide was the only gaseous product. No deleterious carbon monoxide was found. The authors gratefully acknowledge the valuable suggestions and assistance in fellowship grants of John Uri Lloyd which have made this study possible.
