BIOCHEMICAL LABORATORY NOTES W. E. THRUN Valparaisa University, Valparaiso, Indiana
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N THE directions (1,2, 3,4)given for the frequently performed experimental determination of the isoelectric point of casein a solution of pure casein (according to Hammarsten) in exactly 0.1 N sodium acetate is required. This is prepared by dissolving the casein first in water to which has been added an exact amount of exactly 0.1 N sodium.hydroxide solution. The equivalent amount of exactly 0.1 N acetic acid is then added slowly with stirring, whereup6n the solution is made up to volume. In this laboratory it was found that this solution can he prepared more readily and more satisfactorily by students when the followingdirections are given. Dissolve 0.3 g. of sodium caseinate (obtainable from the S. M. A. Corporation, Cleveland) in about 30 ml. of cold distilled water contained in a 50-ml. volumetric flask. Dissolve 0.41 g. (weighed accurately) of C.P. sodium acetate crystals in about five ml. of distilled water. Transfer this quantitatively with rinsing into the flask, which should be rotated during the addition. When the flask and contents have attained room temperature make up to the mark with distilled water and-. In the Nesslerization of a blood filtrate or urine digest gum ghatti solution (5) is usually added as a protective colloid. This solution is usually prepared by suspending the gum just under the surface of the water contained in a tall cylinder overnight, and straining it
through cloth. Rapid methods for preparing this solution are as follows. Pick out colorless tears of the gum and break into small pieces or into a powder. Transfer 2 g. of the gum to bottle containing 100 ml. of water and about 25 glass shot. Violent shaking for about 2 minutes will give a solution which is ready for use. If colorless tears are not available the colored tears are broken into small pieces, but not Gowdered. About 2.5 g. of the gum is treated as given above; however, the shaking is not continued until the complete dissolution of pieces. The undissolved remainder retains the brown woody substance and prevents its dispersion. The colorless solution is then decanted for use. If a brownish powdered gum only is available the solution obtained as directed above will have to be centrifuged or filtered through coarse paper. The addition of a drop of 0.1 N hydrochloric acid facilitates the removal of the brown suspension. LITERATURE CITED
(1) BonANs~yA m FAY,"Laboratory manual of physiological chemistry," John Wiley & Sons, 2nd ed., 1931, p. 62. (2) COLE,"Practicd physiological chemistry," W. H&er and Sons Ltd., Landon, 7th ed., 1926, p. 78. (3) HAWK AND BERGEIM,"Practical physiological chemistry," Blakiston, 10th ed.. 1931, p..134. (4) MICHAELIS a m PECHSTEIN. Bwchem. Z., 4 4 270 (1912). (5) HAWK AND B E R G B Iibid., ~ ~ , 46, 417, 821 (1912).
