A Different View of Using ~itrimetricData Acid-base titration is oneof thp mmst r m p o m t trchniquesoinnalyrical chemlsrry. Thegeneral p d u r c is todetermine the amount of. let u.say, an acid by adding an rquirnlrnt measur~damount of Imr. o r \.ice versa'l'he clearest reprermntatim of what happens during the titration is given bv rhe ritratim curve, a plot oipH as a function of the volume of added ritrant. I n practie&situationsi many titrations involve the reaction of a weak acid with a strong base or vice versa, and Stephens and Jonciehl have proposed a method to obtain the dissociation constant of the acid being titrated from the titration curve without further exoerimental work. The DUrDOSe . . of this note is to oresent a sliahtlv . . different ~ o i noft view of using. e x.~ e r i mental titrimetric data in order to obtain the d ~ s s o c ~ a t ~ nmstmt on of the acid under considrrntion. The ralculatim is lwed on eqn. r l ) . the familiar approxlmatp expression for the hydnwn ion concentration in a buffer [HA1 [Hf] = K " [A-I > K . > 10V and the concentrations of the acid and base Equation (1) applies as long as the acid has a K . such that are greater than 0.05. I t can be shown3 that
VbCb [A-1 = -
va+ v b
..
where C. is the initial malyrical or iormal conwntration of the mid, C, is the formal rrmentration ofthe bape uspd 8s reagent, v i. t he rnrtlal ". volume of acid titrated, and \'b is the wlume 01 ba%eadded to i t at any point. Substitution of eqns. (2) and (31 Into (11c i w i [H+] = K .
V.C.
- VbCb VbCb
Equation (4) can he rearranged to
so that a plot of l/Vb versus [H+] should be linear and have a slope of CdV.C.K. and an intercept of Cb/VaCw Thus K . = interceptlslope. Students need only to collect several pairs of Vb and pH values, calculate 1/Vb and [Hf], and make the appropriate plot to find both V.(V. = V.C& is the volume of titrant added a t the theoretical equivalence point) and K.. The method is oractical for the measurement of dissociation constants with K . values between 10V and
[email protected]~itethe ao~roximations oAhe method, student results agree well uith literature valuesof p ~ . .For example. ~n a determinatim id &, ior acrtlc acid, an average value of 4.T4 at ? S T was h a i n r d . Results were similarly with other weak rnonoprutir acids uith values generally agreeing with literature values withm.0'9 Stephens, S. J., and Joncich, M., J. CHEM. EouC., 54, 711 (1977). See, for exam~le.Laitinen and Hanis "Chemical Analysis," 2nd ed., McGraw-Hill. New York, 1975, p. 37. Skoog, D. A,, and West, D. M., "Fundamentals of Analytical Chemistry," Holt, Rinehan and Winston, New York, 1963, pp. 293-294. Carlos Castillo S. Universldad del Valle Apartado a h 0 25360 Cali. Colombia
Volume 62
Number 5
May 1985
443
