JULY 15, 1940
ANALYTICAL EDITIOS
tained 70 to 90 per cent of silica, 1 to 20 per cent of aluminum and other metallic oxides, and the remainder moisture. Removal of the silica was attempted by adding hydrofluoric acid to the sample and volatilizing fluosilicic acid and the excess hydrofluoric acid on a hot plate. The residue was dissolved in hydrochloric acid and the sodium determined without removal of the metals. The results obtained were very erratic; some duplicate results differed as much as 100 per cent. Experiments with pure aluminum fluoride, aluminum sulfate, sulfuric acid, and sodium fluoride (Table 111) led to the conclusion that a readily soluble fluoride such as sodium fluoride will not interfere with the sodium determinations but varying amounts of aluminum fluoride will be precipitated in the concentrated reagent. This difficulty was overcome by adding 1 ml. of concentrated sulfuric acid with the hydrofluoric acid and evaporating to dryness. This procedure removed all the silica and fluoride and eliminated the former erratic results.
417
Summary The magnesium uranyl acetate method for determining small amounts of sodium can be successfully used in the presence of beryllium, cerium, lanthanum, neodymium, thallium, thorium, vanadium, and zirconium. Silica, if present, can be removed with hydrofluoric acid and sulfuric acid. Columbium and tantalum n ill precipitate in the acid reagent, but no difficulty is encountered if these elements are removed previous to the sodium determination.
Literature Cited (1) Barber and Kolthoff, J . Am. Chem. Soc., 50, 1825 (1928). (2) I b i d , 51, 3233 (1929). (3) Bridges and Lee, IXD.ENG.C H E V . , Anal. Ed., 4, 264 (1932). (4) Caley, J . Am. Chem. S o c , 54, 432 (1932). (5) Caley and Faulk, Ibzd., 51, 1664 (1929). (6) Caley and Sickman, Ibid., 52, 4247 (1930). (7) Kolthoff, Z . a n d . Chem., 70, 397 (1927).
Weighing Flask for Precision Standardization of Strong Alkalies in the Absence of Carbon Dioxide PAUL NAHINSKY AND H. A. LAITINEN Institute of Technology, University of Minnesota, Minneapolis, RIinn.
D
URING a recent investigation involving the testing and use of several alkalimetric and acidimetric primary standards, it became necessary to carry out a large number of highly precise acid-base titrations in the absence of carbon dioxide. None of the weighing flasks described in the literature (1) was suitable for the purpose. This note describes a rapid and highly reproducible method for the standardization of strong alkalies under such conditions.
A set of consecutive results selected from the data for the standardization of nearly saturated solution of barium hydroxide is given in Table I. The conditions under which these titrations were carried out were extremely unfavorable, the humidity being close to, the saturation point and the laboratory temperature about 30" C.
A 125-ml. Pyrex Erlenmeyer flask is fitted with a standard interchangeable two-in-one glass stopper, A , A', and inflow, B , and outflow, B', tubes carrying small ungreased stopcocks. The gas inlet tube is somewhat constricted at the tip to permit a fine stream of bubbles to present a maximum of surface and stirring effect. The complete assembly should not weigh over 60 grams. METHODOF PERFORMING TITRATION.Suppose that it be required to standardize a stock solution of strong alkali, using either constant-boiling hydrochloric acid or an appropriately diluted solution of it. The flask, previously dried with a stream of dustless air and tared with a glass vessel of approximately equal surface, is weighed empty to the nearest hundredth gram (or milligram if desired). A roughly determined amount of the standard acid is introduced and the flask weighed a ain. Two or three drops of an indicator solution are added an% weighed (if an aqueous indicator solution is used it may be weighed with the empty flask, but with an alcoholic indicator solut'ion the above procedure is recommended to prevent loss by evaporation). OF BARIUM HYDROXIDE TABLE I. WEIGHTKORMALITY SOLUTION
I
I1
I11
0.17875 0,17877 0.17876 0 17877
0.17881 0,17881 0.17880 0.17881
0.178888 0.178883 0.178887 0,178886
The flask is then connected directly to the stock buret, which is provided with a long tip carrying a small rubber stopper t o fit stopper A ' , A stream of nitrogen or carbon dioxide-free air is bubbled through the acid for several minutes, the alkali is added with one drop in excess, and the vessel is weighed with stopper A in place. Stoppers A and A' are then removed, the sides are washed down Tvith freshly boiled distilled water, and the backtitration is performed with dilute acid from an ordinary buret. With practice, using two weighing flasks, four such determinations can be made per hour with high precision.
If a solid primary standard, such as benzoic acid or sulfamic acid, is employed, the procedure is similar, except that the standard is weighed into the flask from a weighing bottle and dissolved in an appropriate amount of water. S o danger of overloading the analytical balance exists, since the maximum total weight of the flask and solution never exceeds 150 grams. Literature Cited (1) Bessenberger, F. K., J. Am. Chem. Soc., 39, 1321 (1917).
