July 15, 1934
INDTJSTRIAL AND E N G I N E E R I N G CHEMISTRY
in less than 14 days by the sample cured a t 212" F., whereas those cured a t 70" and 100' F. require 6 months or more to reach a maximum. It is clear that when it is desirable to know the ultimate heat developed by the cement, a curing temperature of 212" F. would be advantageous in greatly accelerating the reaction. The results here shown are of course to be considered as preliminary and merely indicative of the possibility of using elevated temperatures for accelerated tests. It is likely that valuab1,e information on the thermochemistry of the cement reactions may be obtained by further studies along these lines.
ZINC OXIDEAS
A
SECONDARY STANDARD IN HEATCAPACITY ATIONS DETERMIX
The electrical calibration of the calorimeter to determine its heat capacity is probably the most reliable method of obtaining an accurate absolute value. However, it is somewhat tedious and gives a temperature-time curve which is unlike that obtained with cement. There is also the possibility that the heater may be attacked by the acid and rendered useless for calibration purposes. Therefore it was desirable 80 find a chemical substance which might be used as a secondary standard. Calcium oxide has been proposed, but its high reactivity with moisture and carbon dioxide introduces manipulation difficulties, in addition to the precipitation of calcium fluoride which it produces in the acid charge. After some investigation it was found that zinc oxide was suitable for the purpose, being only slightly hygroscopic and giving a highly reproducible heat of solution. The best results are obtained if the zinc oxide is first ignited a t about 950" C. for several hours. It goes into solution very rapidly and produces a time-temperature curve which approximates that of a cement, in contrast to the electrical heating which gives a slow temperature rise. Table IV shows a series of results obtained with the substance and indicates its suitability as a standard of this kind. In one set of comparative tests it was found that both Merck C. P. and Baker c. P. samples gave the same value.
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TABLEIv. REPRODUCIBILITY OF CALORIMETER TESTS HEATCAPACITY OF HEATOF SOLUTION Zinc oxide (c. P.,ignited) CALORIMETER Dry cement 6C30 Date Cal./g. Date Cal ./g . Date Cal./O C . Apr. 17 May 3 May 19 June 26 July 26 Sept. 20 Deo. 14
a
1165.3 1163.5 1162.6 1163.3 1164.8 1164.1 1162.5
Dec. 20 Dee. 30 Dee. 31 Feb. 1 Feb. 7 Feb. 17
Av.
572.8 572.5 573.0 573.8 572.6 573.1 573.0
May 10 May 10 May 10 June 27 July 26 Sept. 20 Dee. 5 Dec. 5 Deo. 14
256.65 2 5 6 . 6 9 Lot 1 256.70 256.66 256.75 Lot 2 256.55 256.10
i i
256.34 Av. 1163.7 Av. 256.48Q Using heat capacity 1163.7, average heat of solution is 256.1.
REPRODUCIBILITY OF RESULTS It was found that by exercise of the precautions given above, the errors could be reduced to those which may be called purely calorimetric-i. e., involving errors due to the calorimeter alone. Tables I1 and IV show the kind of reproducibility of which the calorimeter is capable and indicate that the probable error in the heat of solution tests is less than one calorie. Such accuracy is quite satisfactory when the number of operations involved is considered, but its price is great care and skill in conducting the operations. ACKNOWLEDGMENT The authors are indebted to R. G. Folsom, R. $. Brown, and J. L. Dickinson for their suggestions and skillful work which have made possible whatever advance in cement calorimetry has been made during these investigations. LITERATURE CITED (1) Biddle, S.B., and Kelly, J . W., Proc. Am. SOC.Testing Materials, 1933, preprint 47. (2) Davis, R. E., Carlson, R. W., Troxell, G . E., and Kelley, J. W., J . Am. Concrete Inst., 4, 413 (1933). (3) Dept. Interior, Bur. Reclamation, Specifications 590-D, 1932. (4) Morris, S. B., J . Am. Walerworks Assoc., 25, 1350 (1933). (5) White, W. P., "The Modern Calorimeter," A. C. S.Monograph 42, Chemical Catalog Co., N. Y., 1928. (6) Woods, H., Steinour, H. H., and Starke, H. R., IND. ENG.CHEM., 24, 1207 (1932) ; Eng. News Record, 109, 404 (1932). RECEIVED .March 19, 1934.
Dilution Method for Micro-Kjeldahl Determinations OLIVEHARTLEY,Bureau of Home Economics,
T
U. S. Department of Agriculture, Washington, D. C.
HE use of sulfuric acid as a dilution medium in microKjeldahl determinations, where no microbalance is available, is convenient for materials insoluble in water, such as wool and silk, and reduces the time of digestion as compared with that required for water solutions. So far as the writer is aware, this has not been previously suggested in the literature. Rapid solution occurs, without charring, when wool is heated with concentrated sulfuric acid in a thick-bottomed container, over a hot plate, with frequent shaking to avoid local overheating. A 0.2-gram wool sample will dissolve in 10 cc. of sulfuric acid in 3 to 5 minutes, giving a clear amber solution. Since special volumetric flasks with thick bottoms would be needed to apply a volumetric method, the solution was carried out in this laboratory in 30 cc. dropping bottles. Portions containing 25 drops were then transferred to microKjeldahl digestion flasks, the weights determined by difference, and the catalyst added. The amount of catalyst may also be regulated by dissolving it in sulfuric acid and using a definite number of drops. Selenium, which was found t o give much quicker results with wool than Pregl's CuS04-K2S04 catalyst, was prepared in 1 per cent solution by strongly heating reduced selenium in
concentrated sulfuric acid, until it dissolved to a pale yellow solution. A round-bottomed flask is required. A 1 to 1 ratio of selenium to wool is the optimum concentration. The digestion liquor is clear within 15 minutes, but 25 to 30 minutes should be allowed for complete nitrogen recovery. Hydrogen peroxide is seldom required. It was also found advantageous to use a volume rule instead of Pregl'sl time rule for the steam distillation in Parnas and Wagner's micro-Kjeldahl apparatus. Distillation is continued until 10 cc. of distillate have condensed in the receiver. Graduated centrifuge tubes of 50 cc. capacity are satisfactory for collection and subsequent titration and boiling. Aliquots of the same wool solution checked within 1 part in 160. Typical results were 16.23, 16.11, and 16.18 per cent of nitrogen in a wool blanket composed of two parts of three-eighths blood and one part of one-quarter blood wool, and 16.00, 16.01, and 15.93 per cent of nitrogen in a wool blanket containing one part of one-quarter blood wool and two parts of reworked wool. RECEIVED March 24, 1934. 1 Pregl, F., "Quantitative Organic Microanalysis," 2nd English ed., tr. by E. Fyleman, P. Blakiston's Son & Co., Philadelphia, Pa., 1930.
