AVING had occasion to prepare sodium hydroxide solutions of low carbonate ... comprehensive study of all methods of preparing alkali free from carbonate ...
ANALYTICAL EDITION
192
purity of the plasticizer was determined by refractive index measurements, as well as by saponification values if the plasticizer belonged to the ester type. SUMMARY A rapid method for the separation of plasticizers from organic cellulosic plastics, described in this article, has been shown to be to cellulose, cellulose, and cellulose acetate plastics. Its accuracy, when- applied quantitatively to the determi-
Vol. 5 , No. 3
nation of plasticizer content of organic cellulosic plastics, was found satisfactory for practical purposes.
LITERATURE CITED (1) Bradley, IND. ENO.CHEM.,Anal. Ed., 3, 306 (1931). (2) Gardner, “Physical and Chemical Examination of Paints, Varnishes, and Lacquer,” 5th ed., pp. 491-550, 818-28, 850, Institute of Paint and Varnish Research, Washington, 1930. (3) Hickman and Sanford, J. Phys. Chem., 34, 637 (1980). RECEIYBD December 19, 1932.
Preparation of Sodium Hydroxide Solutions of Low Carbonate Content by Centrifugation NELSONALLEN AND GEORGEW. Low, JR. Frick Chemical Laboratory, Princeton University, Princeton, N. J.
H
AVING had occasion to prepare sodium hydroxide solutions of low carbonate content, the use of a centrifugein connection with the oily alkali method (6) suggested itself. This method has long been used for preparing sodium hydroxide solutions and possesses the advantages of simplicity and freedom from the introduction of contaminating ions. On the other hand it has the disadvantages of extreme slowness and incomplete removal of carbonate. Clark (1) speeded up the process by filtering the alkali through paper, and Kolthoff (4) filtered it through a Jena sintered-glass crucible. Han and Chao (g), who have recently made a comprehensive study of all methods of preparing alkali free from carbonate, clarified the solution by heating in a water bath (6) and followed this by filtration through glass. Apparently no one has made use of the centrifuge for the clarification.
EXPERIMENTAL Samples of the oily alkali were prepared by dissolving 50 grams of pellet sodium hydroxide in 50 ml. of carbon dioxidefree water. After cooling, each lot was transferred to a Pyrex centrifuge tube, made from a 125-ml. Erlenmeyer flask by rounding out the base and blowing out an opening to fit a No. 2 rubber stopper. The neck of the flask was drawn off and a tube 5 cm. long and 0.8 cm. inside diameter sealed on. Centrifuging for 30 minutes a t 2200 to 2300 r. p. m. gave absolutely clear solutions with the undissolved carbonate and alkali tightly packed in the bottom. TABLE I. CARBONATE CONTENT OF ALKALI SAMPLE
TOTAL VOL. ACID AS 0.0998 N HC1
VOL.HCl FOR
M1.
M1.
cos - -
PERCENTHCI FOR
cos--
BEFORE CENTRIFUQINQ
1 2 3 4
273.36 270.67 306.66 276.24
1
271.28 278.37 309.36 276.33 330.64
2.40 2.42 2.48 2.26
0.88 0.88 0.81 0.82
.
AFTBR CENTRIFUQINQ
2 3 4
0.314 0.284 0.610 0.280 0.450
0.12 0.10 0.16 0.10 0.14 Av. 0.12
Samples of alkali were analyzed for carbonate before and after centrifuging by titration in the absence of carbon dioxide with standard hydrochloric acid. On reaching a phenolphthalein end point the titration was continued with 0.0998 N hydrochloric acid, from a microburet, to a pH of
4.2 as determined by methyl orange indicator and a buffer solution. The natural offset between the two end points for pure water was found to be 0.140 ml. and this correction was applied. Some typical data are given in Table I. DISCUSSION OF RESULTS Nan and Chao (2) secured an average value of 0.15 per cent for the hydrochloric acid used for the carbonate in their solutions, whereas the above data show a n average of 0.12 per cent for the centrifuged alkali. This clearly indicates that centrifugation gives a lower content of carbonate in the alkali with a very much smaller expenditure of time and labor. Then, too, no elaborate heating or filtering devices are needed. Forty milliliters of a 0.1124 N sodium hydroxide solution prepared by diluting the centrifuged alkali with carbon dioxide-free water required an average of 0.190 ml. of 0.0998 N hydrochloric acid to go from the phenolphthalein end point to pH 4.2. This value less 0.140 ml. (the natural offset) equals 0.050 ml., which corresponds to the carbonate content. Solutions prepared in this manner are perfectly satisfactory for all ordinary titrations. MODIFIEDTITRATION HEAD In connection with the above work the titration head recommended by Hillebrand and Lundell (3) has been modified by leading the carbon dioxide-free air stream through a tube passing through the cap by a ring seal and extending down about 8 cm. below the top of the head. This change insures that the titration flask will be completely swept out by the air stream. The head was made of very heavy Pyrex tubing and of a size to fit over a 250-ml. Erlenmeyer flask. Such a titration head makes a very rugged and useful piece of equipment, LITERATURE CITED (1) Clark, “Determination of Hydrogen Ions,” Williams and Wilkins, 1928. (2) Han and Chao, IND. ENQ.CHEM.,Anal. Ed., 4, 229 (1932). (3) Hillebrand and Lundell, “Applied Inorganic Analysis,” p. 141, Wiley, 1929. (4) Kolthoff and Furman, “Volumetric Analysis,” Vol. 11, Wiley, 1929. (5) Pregl, 2. anal. Chem., 67, 23-7 (1925). (6) Sbrensen, Biochem. Z . , 21, 186 (1909). RECEIYED January 31, 1933.
