JOHN P. PHILLIPS University of Louisville, Louisville, Kentucky
ONEof the diiculties in teaching instmmental aualysis is the emphasis necessarily placed on operational details. Generally only the simplest processes are investigated instrumentally in order to avoid complicating the task of making the instrnment work. The result may he that the student knows how the instmment works, but has only a vague idea of the kinds of chemical investigation that are best performed with it. This is particularly tme of ultraviolet and visible spectrophotometry where the determination of the absorption spectrum of some colored compound or an indicator constant appears to be a favorite experiment. The student is left with the limited notion that one can determine absorption spectra and do simple colorimetric analyses with a spectrophotometer; the more interesting determinations of molecular weights, formulas of soluble colored complexes, ionization constants of colorless acids and bases (in the ultraviolet), other equilibrium constants of many varieties, steric effects, and similar topics of stmctural organic chemistry' may be overlooked. To demonstrate the versatility of a spectrophotometer more suitably the following experiments, requiring not more than three hours working time, have been in use here for several years. Although the procedures are written for the ionization constant of benzoic acid and the formula of the complex between chromate and hydrogen ions, other aromatic acids, phenols and polyfunctional acids may be substituted m the first experiment and other complex-fonning pairs in the second. The Beckman DU Spectrophotometer, for which separate operating instructions are provided, is used. Ionization Constants of Weak Acids. The ionization constant of a weak acid can be obtained if there is a significant difference a t any wave length between the absorbancies of the free acid and the acid anion. If the absorbancies of the compound in acid solution, basic solution, and a buffer such that the acid is partially in the anion form are determined a t a suitable wave length, the ionization constaat, Kc,is given by PK.
= pH
-e + log eb-a
where the pH is that of the buffer solution of the acid having absorbancy e, and a and b are the absorbancies of the same concentration of acid in 0.1 N hydrochloric acid and 0.1 N sodium hydroxide respectively. Procedure. Ten-ml. portions of a saturated solution of henzoic acid are pipetted into three 25-ml, volumetric flasks. One solution is diluted to the mark with 0.2
' FERGUSON, L. N.,
"Electron Structures of Organic Mole-
odes," Prentice-Hall, Inc., New York. 1952, p. 292.
N hydrochloric acid, one with 0.2 N sodium hydroxide, and the third with 0.2 N acetate buffer (pH 4.7, composed of equimolar amounts of acetic acid and sodium acetate). Measure the absorbancies of the solutions a t 286, 288, and 290 mp. Record the temperature of the solutions. Calculate values a t each wave length for the ionization constant and compare with values a t this temperature in the literature. Better results may be obtained if additional buffer solutions in the pH range 4 to 6 are measured, and the calculation made graphically from a plot of absorbancy against pH. The thermodynamic constant, K,, may be calculated with the following equation log K. = log Kc -
I%
where I is the ionic strength of the solutions. Complex Formula Determination by the Method of Continuous variation^.^ If two equimolar solutions of substances that form a soluble colored complex are mixed in varying ratios, that ratio which corresponds to the mole ratio of the components in the complex will have a maximum (sometimes a minimum) absorbancy a t a suitable wave length. If either component is colored, the absorbancy must be corrected for the contribution to the color from this component. By plotting the difference between the measured absorbancy and the absorbancy calculated for no interaction against mole per cent (or its equivalent) of either component the complex formula is readily obtained. Procedure for Chromate-Acid Reaction. Prepare 0.1 M solutions of potassium chromate and hydrochloric acid. Using graduated 10-ml. pipets and 25-ml. volnmetric flasks make up the following nine solutions: 10 ml. chromate; 9 ml. chromate, 1 ml. acid; 8 ml, chromate, 2 ml. acid; etc. It will not be necessary to prepare the solution containing 10 ml. acid since it may safely be assumed that its absorbancy in the visible region is negligible. Dilute all solutions to the mark and mix. Measure the absorbancies of all solutions a t 470, 505, and 525 mp. Correct the absorbancies of the mixtures by subtracting from each the contribution expected from the amount of chromate present. Plot these corrected absorbancies ( Y values) against volume of chromate a t each wave length. What is the formula of the complex between chromate and hydrogen ion as indicated by these data? Consult the literature2for an explanation. Vossun~n,W. C., 63, 437 (1941).
AND
G. R. COOPER, J. Am. Chem. Soc.,
