May, 1932
INDUSTRIAL
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ENGINEERING
experimental specific heats as t,he conflicting data of this field will permit, and in most cases lead to calculated equilibrium constants in good agreement with the observed results. It is worthy of notice that the equations of the Planck-Einstein type cited’ are derived almost entirely from molecular spectra with only slight adjustment to experimental specific heat data, yet it is doubtful whether these formulas show deviations from the observed values in the literature which exceed in magnitude the experimental uncertainty of the latter. The molecular configurations assumed in the case of water and carbon dioxide appear to be in accord with Dennison’s new treatment of vibration in polyatomic molecules [Rev. Modern Phys., 3, 280 (1931) 1. W. M. D. BRYAST E.I. D U POSTD E N E M O U RBiS Co. WILMINGTON, DEL. February 29, 1932 1 It must be remembered t h a t Planck-Einstein functlon gives only a close approximation of the true vibrational heat because of t h e simplifying assumptions employed. There is a further heat increment d u e t o t h e increase of potential energy attending stretching of t h e molecule a t high temperatures, which is not considered in this formula. This increment is small below 1000° C. in the known cases of hydrogen a n d hydrogen chloride.
The Yellowing of Drying Films Editor of Industrial and Engineerang Chemistry: In a paper on ‘(The Drying and Yellowing of Trilinolenic Glyceride” [IND.ESG. CHEM.,23, 881-6 (1931)], A . C. Elm criticizes a theory propounded by us to explain the yellowing displayed by linseed oil and by similarly constituted substances when they are dried and left in the dark for some time. This theory was based on an extensive investigation into ( a ) the oxidation products of a pure crystalline glyceride--e. g., 8-eleostearin obtained from tung oil-and ( b ) a very large number of paint trials with white pigments in linseed oil, whereby it m s shown that the nature of the pigment was a factor in the yellowing [ J . 0 2 1 Colour Chem. Assoc., 10, 186 (1927)l. A published account of a more recent investigation [ J . Soc. Chem. Ind., 50, 27 (1931)l had apparently not come to Elm’s notice, as he makes reference only to earlier papers, and his comments are restricted to only one factor in the problem-i. e., the presence of hydioxy compounds in the “oxyn” or oxidized glyceride. The purpose of this communication is to point out that the figures which are given by Elm as hydroxyl values .and on which his criticism is based are not acceptable for the following reasons: (1) True hydroxyl estimations were not made, but “iodine substitution values” were determined. No experiments are recorded, however, to justify the assumption that the hydroxyl content of these oxidation products is given by iodine substitution value. In the paper cited by Elm [McIlheney, J . Am. Chem. Soc., 21, 1084 (1899)] in this connection, there is no reference to the estimation of hydroxyl groups. We experienced considerable difficulty during our investigations in estimating the hydroxyl contents of these oxidized glycerides because of the inapplicability of the usual methods to these substances. We would suggest that the hydroxyl contents be estimated by the process already described [Analyst, 56, 428 (1931)], as this has been carefully examined and found to be reliable for the type of compound under investigation. (2) The -C(OH) :C(0H)grouping which has acidic properties and is present in many of these substances has been shown to be labile. A measure of the hydroxyl group of these oxidation products can therefore be considered only side by side with their carbonyl content, due regard being paid to variations fn the conditions under which the two estimations are made. The methods of estimation the carbonyl groups present have also been investigated by us (Ibid., 508).
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Further experimental data concerning the yellowing, bleaching, and softening of oxidized glycerides and drying oil films have recently been obtained in support of the theory we put forward. I t may be noted that it is now possible by chemical methods to demonstrate the degree of polymerization of certain oxidized glycerides obtained from p-eleostearic glyceride. R. S.MORRELL 8. M . 4 R K S U X l V E R 3 I T Y O F BIRYINGHhhI I S D T H E CENTRAL TECHNICAL COLLEQE BIRWNGHIM,E N G L A N D Decemher 12. 1931
. . . . . . . . .. Edttor of Industrial and Engineering Chemistry: The criticism of my work on the yellowing of oils by Morrell and Marks forces me to call attention to a misconception prevailing among the investigators of this field. The yellowing of oil or paint films sealed in glass vessels is hardly identical with the yellowing observed in a poorly lighted room or behind pictures, furniture, etc. As Morrell and Marks point out, the presence or absence of air is of importance. Under ordinary room exposure conditions, aldehydic decomposition products are partly volatilized and partly oxidized to colorless acids. When enclosed in a glass vessel, however, those aldehydes which in themselves are colorless are retained and resinified to more or less colored compounds. This resinification is catalyzed by alkalies. This explanation of the effect of alkalies which are not absolutely necessary to produce color, although they may intensify it appreciably, appears to deserve preference over that suggested by Morrell and Marks. The color changes of oil films enclosed in glass tubes caused by the resinification of aldehydic decomposition products are apt to obscure the common type of yellowing, and great care should therefore be exercised in the interpretation of such laboratory results and their correlation with actual indoor exposure tests (see Morrell’s experiments 1, 2, 3, 4, 5 ) . In my work on yellowing, I was interested in and concentrated on only the normal type of yellowing of oils and white oil paints. In this case the yellow color is probably due to underpolymerized polyketones formed from oxidized drying oils by a simple intramolecular rearrangement. This type of yellowing seems to be the one Morrell and Marks mention under (b). However, they claim that the color is caused by the tautomeric change of the keto-hydroxy compound to its corresponding dihydroxy form. It is true that this or similar groupings of atoms cause selective absorption of light. It has also been observed frequently that alkalies intensify this type of selective absorption. But, the absorption bands being located in the invisible ultra-violet region, the color produced is invisible. This grouping can, therefore, never create yellow color. A detailed discussion of this question a t this place and time would lead too far and is therefore reserved for a future publication when additional experimental evidence recently collected will be presented. A . C. ELM THENEW JERSEY ZIXC Co. PILMERTOK, Pa January 27, 1932
Tenderness of Meat-I.
Correction
Line 7, column 2, page 243 of the article, “Tenderness of Meat, I. Determination of Relative Tenderness of Chilled and Quick-Frozen Beef,” ISD. ENG. CHEM.,24, 242-5 (1932), should read: “A shallow cylindrical container about 1.5 inches in diameter. . . . .” instead of “15 inches in diameter.” D. K. TRESSLER ET AL.
