VOL. 3,NO. 11
LECTURE TABLEDE%ONSTRATIONS
1321
LECTURE TABLE DEMONSTRATIONS OF THE NATURE OF CONCENTRATED SULFURIC ACID* FRANCIS F. HZYROTH, UNIVERSITY OQ CINCINNATI, CINCINNATI, Orno The progressive dissociation of a salt upon the gradual addition of water to its concentrated solution is rendered visible in the case of cupric bromide by the difference in color of the undissociated molecules and that of one of its ions. The conversion of undissociated 96% sulfuric acid, inactive toward metals, into the active dilute acid may be similarly rendered visible by the aid of an indicator. Dimethylaminoazobenzene is red in aqueous solutions in which the hydrogen-ion concentration is greater than 10-8 to lo-', and yellow in solutions in which the hydrogen-ion concentration is less. The addition of a few drops of either a red or a yellow aqueous solution of this indicator to 10-50 cc. (depending upon the size of the lecture room) each of normal hydrochloric acid, normal sodium hydroxide, and 9.5-96% sulfuric acid solutions causes the appearance of red, yellow, and yellow colors, respectively. The similarity in the color of the alkaline and concentrated acid solutions indicates the probability that the concentration (It is true that the of hydrogen ions in the 96% acid is less than lo-'. effect of changing the medium about the indicator from water to sulfuric acid may alter in some unknown manner the inversion point of the indicator. However, this does not d e c t the validity of the conclusion drawn from this demonstration, since, in that, no attempt is made to determine the actual hydrogen-ion concentration of the sulfuric acid.) The addition of an equal volume of water to the yellow solution in concentrated sulfuric acid changes the color to red, showing that the water has induced the dissociation of enough molecules of HzSOato provide a hydrogen-ion concentration greater than 10FS. On the further addition of sodium hydroxide solution, the color again becomes yellow. The indicator has not been destroyed by the strong hot acid. This behavior is not peculiar t o this indicator; methyl orange, methyl red, and many others may be substituted. Congo red is, however, unsuitable. A further experiment may serve to show that the water required for the completion of the ionization process may be derived from varied sources. To a solution of a few drops of methyl orange solution in 25 cc. of sulfuric acid is added 10 cc. of water; it is readily apparent that the further addition of a small amount of water will produce a red color, although yellow still predominates. This solution is then divided into four separate beakers, the first of which is reserved for comparison. To the second is gradually 'This paper is based upon one presented by the author bdm the Cincinnati Section of the American Chemical Sodety in March, 1922.
added from three to five grams of anhydrous sodium carbonate. The color becomes red because of the dilution of the solution by the water formed in the reaction. The careful addition to the third beaker of a stick of potassium hydroxide about 4 cm.long results in a vigorous reaction during which the solution becomes red. This experiment, in which the usual color change in a neutralization reaction is reversed by carrying it out at a concentration different from that ordinarily employed, and in which the acidity of a strong acid is made manifest only upon the addition of an alkali, suggests an analogy with the biological process of immunization, which, like this, occurs in a system containing relatively little free water. To the fourth beaker is added 4 grams of fused sodium acetate. For a time there is no reaction; if the beaker is heated, acetic acid is slowly evolved and the color of the solution becomes red. In this case it is not the diluting action of water, but the actual reaction of the acetic acid produced, which is demonstrated. That this is the case may be shown by adding a few drops of methyl orange t o glacial acetic acid; an intense red color develops. The addition of a very small amount of water to the "weak" 100 per cent acetic acid suffices to produce an acid reaction toward methyl orange, while, because of its tendency to b i d water as hydrates, a very much greater quantity must be added to the "stronger" sulfuric acid t o produce the same reaction. The quantities employed in the above experiment may be increased indefinitely, provided that approximately the same relative amounts of reagents are used.
summarg Experiments are given in which the dissociation of sulfuric acid is followed by means of indicators. A lagging of the dissociation behimd the dilution serves t o demonstrate the power of sulfuric acid to bind water as hydrates. The following references may serve t o throw light upon the theory involved. F. Liebert. C h . Weekb~,12,1088(1915). F. Heyroth, I.Am. Cbenz. Soc., 38, 57 (1916). A. Hantzsch, Z. Ekklrochem., 24,207 (1918);Ber., 58,941 (1915). H. C. Jones, Z. physik. Chnn.,13,419 (1894). Miles and Fenton, J. C k m . Soc., 117,59 (1920). V. &PI. Rec. hav. chim.. 44,476 (1925).
The task of an investigator requires for its success the toughness of a soldier, the temper of a saint, and the training of a scholar.-Hmmy DAVY
