0. T. Zajicek University of Massachusetts Arnherst
Automatic Titrimetry in Introductory College Chemistry
The experiment here described employs an automatic pH titrator as a tool for the teaching of a detailed treatment of simultaneous equilibria. It has been used in an accelerated course for selected freshmen covering the principles of general inorganic, qualitative, and quantitative chemistry in one year. The experiment is quite feasible for any chemistry course for majors, according to our experience. The instrument used consisted of a Sargent constant rate buret, Sargent pH recording adapter, and a Sargent Model SR recorder; a Beckman combination electrode is used. The instrument provides results reproducible to within 0.02 pH units. The device recently described by Malmstadt1 should make it possible for schools with modest equipment budgets to perform this and similar experiments in introductory college courses. The principles of the iustrumentatiou are covered in a lecture prior to the laboratory session. Each student is issued an "unknown" which consists of a mixture of a strong and a weak acid. After bringing the volume to 100 ml (a convenient volume for our apparatus) each student titrates his sample with standard sodium hydroxide using the titrator. The titratiou time per student usually does not exceed 3 min; counting turnover time, it is quite possible to carry out twelve titrations per hour. The student is required to locate the equivalence points (from derivative plots) and from these calculate the total acid, wealc acid, and strong acid concentrations in the original sample in a conventional manner. I n addition, he must determine the acid dissociation constant of the weak acid, both graphically and mathe matically. I n the discussion that follows, the following symbols will be used.
At the half titration point, [A+] = [HA] and equation (1) may be reduced to PK,
=
pHus
(2)
Mathematical Calculation. The following relationships are established: Equilibria [H+l[OH-] = K ,
(4)
Material Balance C, = IHAI
+ IA-I
C = [Cl-] (if HCI is used)
(5)
(6)
Charge Balance [H+l
+ [Na+l
=
[A-I
+ [Cl-] + [OH-]
(8)
I
