V O L U M E 19, NO. 4, A P R I L 1 9 4 7 of tyrosine with this‘ procedure, possibly because of excess water introduced by the saturated solution, which allows the destruction of the catalyst during heating before the digest becomes concentrated enough for actual oxidation of the tyrosine to occur. By the use of solid potassium persulfate with only a little water this difficulty was avoided. Excess foaming was also much reduced. It was found t h a t complete clearing of the digest was not a criterion of complete oxidation. Tyrosine, for instance, required a t least 5 additional hours in the sand bath a t 375” C. before complete recovery of nitrogen. This agrees with the findings of Miller and Houghton (3) who used a different digestion mixture. A t least 5 hours’ heating after complete clearing is obtained has been made a standard procedure in all the analyses of biological material reported here. After several digestions have been carried out, the Kjeldahl flasks may become etched and scale may form on the bottom. This may interfere somewhat with the transfer of the flask contents to the still. The scale may be largely removed by vigorous scouring with a motor-driven rod tipped with a short length of rubber tubing, using fine sand or other abrasive. Distillation. I n early trials, i t was noticed that the first distillation of a series nearly always yielded low ammonia values. This could be prevented by “seasoning” the still by distilling over a sample or two of ammonia from ammonium sulfate before each series of analyses. The reason for this is not completely clear, but i t is possible that an equilibrium existed between the ammonia in’the water which always was present in the traps between the still and the condenser, and the ammonia in the still atmosphere. When this equilibrium was established by distilling over ammonia and allowing the water in the traps to absorb its full complement of ammonia, loss of ammonia during subsequent analyses was never observed. The use of 40% sodium hydroxide which is sometimes recommended for neutralizing digests prior to distillation was unsatis-
283 factory because of violent bumping when the strong alkali was added to the concentrated acid of the digest. When heavy bumping occurred low nitrogen values were always obtained, apparently because some volatile hydrogen-ion-yielding material (HIS?) was carried over into the receiver with the consequent liberation of too much iodine when potassium iodide was added. The presence of large amounts of chloride in the digests gave a similar result, but 20% sodium hydroxide was found t o be completely satisfactory for neutralizing digests and could be run In freely without fear of bumping. Best results were obtained when every step in the distillation procedure-amount of wash water, time of distillation, etc.-waa standardized and rigidly adhered to in all distillations. Undoubtedly the best conditions for distillation will vary considerably with the particular still, heating system, etc., and this should always be investigated before experiments are begun. The entire analytical procedure should be carefully checked against standard substances. It was not necessary to run blank determinations through the entire procedure, since in the experiments in which standard ammonium sulfate was carried through the whole procedure the expected values for nitrogen were obtained, ihdicating the freedom from nitrogen of the distilled water and other reagents used. LITERATURE CITED
(1) Gregg, J. R., and Ballentine, R., J . E z p t . Zool., 103, 143 (1946). (2) Kassner, J. L., and Kassner, E. E., IND.ENQ.CHEM.,ANAL.ED., 12, 655 (1940). (3) Miller, J., and Houghton, J. A,, J . Biol. Chem., 159, 373 (1946).
(4) Niederl, J. B., and Niederl, V., “Micromethods of Quantitative Organic Analysis”, 2nd ed., p. 54, New York, John Wiley 8: Sons. 1942. (5) Ibid.,-p. 69. (6) Peters, J. P., and Van Slyke, D. D., “Quantitative Clinical Chemistry”, Vol. 11, Methods, p. 355, Baltimore, Williams and Wilkins Co., 1932. (7) Ibid., p. 525. I
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CORRESPONDENCE Measuring the Distribution of Particle Size in Dispersed Systems SIR: Dotts (1) has recently described his use of a sedimentation technique for the determination of particle-size distribution in emulsion systems. He has apparently overlooked a previous paper (2) by the author, in which improvements such as reported by him are discussed. This note is to point out that the sensitivity of the method can be increased considerably by an added feature which has been successfully used in our laboratories. If a bulb, such as that shown in the accompanying figure, is added to the side arm near its union with the large tube, one can use a different liquid in the side arm from that of the suspension medium. The two liquids should be immiscible. The increase in sensitivity is inversely proportional to the ratio of the density of the side-arm liquid to that of the dispersion-e.g., the use of petroleum ether in100 . creases the sensitivity over the use of water by a factor Per0.66 haps a more important factor in the use of petroleum ether is its lower surface tension, as compared to that of water, which reduces the tendency of the side-arm liquid to stick during sedimentation. The large-diameter bulb, where the interface between the two liquids exists, is necessary, so that no appreciable movement of this Side Arm interface need occur if the bore of the side-arm tube is relatively small. RAYMOND H. LAMBERT Eastman Kodak Co. Rochester 4, N. Y .
SIR: The stated aim of the author’s recent paper (1) on particlesize distribution measurement was to establish a foolproof technique suitable for the occasional needs of the majority of investigatora. The apparatus discussed was stripped to its essentials, yet it provided sufficient accuracy and versatility for general use. The time required to set up the apparatus and obtain complete data was minimized by eliminating all calibration and by treating only those sources of error which affect an accuracy of 1%. Experimental considerations yielding greater significance in results were discussed. The author refers investigators of extremely fine dispersions and those desiring optimum sensitivity to a paper ( 2 ) by Lambcrt in which an automatic photographic recorder for sedimentation continuing several days and a method for correcting for lag in the movement of the side-arm liquid are discussed. Lambert has also pointed out that the use of a side-arm liquid specifically lighter than the dispersion medium and immiscible with it yields an increase in sensitivity for any system. Lambert‘s improvements are useful for investigators doing very accurate or specialized n-ork. LITERATURE CITED
(1) Dotts, W.M., IND. ENG.CHEM.,ASAL.ED.,18, 326-5 (1946). and Wightman, E. P., J . Optical SOC.A m . , 11, (2) Lambert, R. H., 393-402 (1925). WALTERM . DOTTS Quaker Hill, Pawling, N. Y.
