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INDUSTRIAL AND ENGINEERING CHEMISTRY
amount of reactive solute close a t hand to maintain the concentration at t h e surface. But t h e relative motion of the liquid to the bubble is viscous, and replacement of converted carbonate can occur only by diffusion through liquid strata concentric with the bubble. In the fraction of a second during which the bubble was exposed t o t h e liquid, there was time for the rapid chemical reaction to coat the surface with bicarbonate but insufficient time for the two-way diffusion of dissolved salts to take place appreciably. The explanation of the much slower rates found by Ledig and Weaver for 2.5 N sodium carbonate solution as compared with pure water apparently depends upon these two factors: (1) Their observations were completed within one second after gas and liquid came in contact; ( 2 ) The diffusion of reactive solute into t h e liquid film] and of reaction product out of the film, was relatively slow. As compared with water, therefore, the technic of Ledig and Weaver in effect led to observing the life of a gas bubble surrounded by a solution of
sodium bicarbonate. Sothing discordant remains in connection with Table I. The variation of eightfold in the initial rate of absorption by water between Becker’s value and that of Davis and Crandall is clearly attributable to speed of stirring. The very high rates reported by Ledig and Weal-er and Whitinan, Long, and \Tang (10) are probably due t o the fact t h a t true initial rates were measured in these cases, rather than the practical initial rates under more general discussion. I n one case a bubble of fresh gaq was quickly exposed to a large volume of pure iyater; in the other case a drop of fresh water rva9 suddenly exposed t o a large volume of pure gas. In both, the total time during which absorption was followed v a s one second or less.
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Somenclature A V a b
area of interface] sq. cm. gas absorbed, cc. activity; exponent in equationf(z) = eo2 exponent in equation YO = k(u c.)b/z concn., gram equivalents/liter (normallty) base of natural logarithms, 2.718.. . . any function k = constant in given equation yo = dV/Ade, cc./sq. cm. min., a t zero time z = viscosity, centipoises A = an increment 0 = time SubscriDts: i= interface (e. g., as used here, ci = interfacial concn. of CO, gas, dissolved) = solute (e. g., Na2CO8,KzC03, etc.)
= = = = c = e = f() =
+
Literature Cited Becker, IIVD. ENQ.CHEM.,16, 1223 (1924). Davis and Crandall, J. Am. Chem. Soc., 52,3757 (1930); Davis, TND. ENG.CHEM.,25,1023 (1933). Hatta, Tech. Repts. Tohokulmp. Uniu., 10,630 (1932). Hatta andBaba, Ibid., 11, 365 (1934). Hitchcock, IND. ENG.CHEM.,26, 1158 (1934). Hitchcock and McIlhenny,Ibid., 27,461 (1935). Ledig and Weaver, J . Am. Chem. Soc., 46, 650 (1924); Ledig, IND.EKG.CHEM.,16, 1251 (1924). Payne and Dodge,Ibid., 24,630(1932). Walker, Bray, and Johnston, J . Am. Chem. Soc., 49,1235 (1927). Vhitman, Long, and Wang, IND. ENG.CHEM.,18,363 (1926). Williamson and Mathews, Ibid., 16, 1157 (1924). RECEIVED March 2 5 , 1935. Presented before the Division of Industrial and Engineering Chemistry at the 89th Meeting of the American Chemical Society, New York, N. Y., April 22 t o 26, 1935. This paper is a contribution from the Department of Engineering and the Cobb Chemical Laboratory, University of Virginia.
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Calcium Availability in Foods Containing Oxalates A Preliminary Report E. F. KOHJI.IN ASD N. H. S INBORN, Research Laboratories, National Canners Association, Washington, D. C. H E role of calcium in the diet has long been recognized as of primary importance. That the discovery of the vitamins has not overshadowed interest in this phase of nutrition is indicated in the following statement by Sherman ( 1 ) : “Probably a larger proportion of the ordinary dietary both of adults and of children can be improved by enrichment in caIcium than in any other one chemical element.” In previously reported experiments ( 2 ) involving a comparison of canned, ordinarily cooked, and raw market foods through a number of generations and with a large number of animals, it has been found consistently that a higher and more nearly optimum bone ash results from canned foods. This a pears to be is not ina matter of calcium availability in which vitamin volved. A logical explanation is the effect of the more intense cooking employed in canning in lowering the absorption effect of the vegetable fibers. Data u-ere presented at the same time indicating that ordinary foods have definite antirachitic properties. S o study of calcium availability would be comprehensive, however, if it ignored the effect of oxalates. The confusion of ideas and the amount of misinformation regarding the occurrence of oxalates and their behavior in the diet TTas recently pointed out (3). Sherman states in the last edition (1932) of his “Chemistry of Foods and Nutrition:” “Because of lack of knowledge as to handling of oxalic acid and oxalates in the body, foods reported as containing these cannot be discussed satisfactorily in this connection.” It appears from data we have that oxalates may be a normal constituent of all plant tissue, but that thc amounts in most foods are biologically negligible. While the availability of calcium in all of its salts other than calcium oxalate that might occur in foods has received attention, there appear to be no data regarding the behavior of calcium oxalate or the oxalate in excess of calcium in certain foods. State-
8
ments that oxalates interfere with calcium availability seem to be based on the known insolubility of calcium oxalate under conditions used in the laboratory for analytical purposes. The pH under such circumstances is higher than normally encountered in the stomach. If calcium availability is interfered with by oxalates, this question is one worthy of a comprehensive study. Preliminary feeding experiments with newly weaned rats 21 days of age, fed for three weeks on diets containing calcium oxalate and soluble oxalates in varying proportions and amounts, throw some light on the subject in so far as the rat is concerned. Calcium oxalate does supply some calcium for bone formation, but the availability of calcium in calcium oxalate is of a low order. If soluble oxalates are present a t the same time, calcium otherwise available is rendered unavailable by the soluble oxalates. This results in part because the oxalates prevent calcium being absorbed from t,he digestive tract and in part because oxalates absorbed from the digestive tract carry calcium out of the systeni. This is evidenced by the fact that soluble oxalates in the diet cause increased excretion of calcium in the urine. These preliminary results suggest the necessity of a thorough investigation. We ”are therefore undertaking a comprehensive survev of the occurrence of oxalates in foods and their effect in the diet o? the rat.
Literature Cited (1) Sherman, H.C.,J . Am. M e d . Assoc.,97,1425 (1931). (2) Kohman, E. F., and Eddy, W. H., and Associates, IND.ENQ CHEX., 23,1064 (1931); 26,758 (1934).
(3) Kohman, E. F.,J . Am. Diet.Assoc., 10, 100 (1934); Editorial J . Am. M e d . Assoc., 103. 1152 (1934). RECEIVED May 20, 1935.
