I II
Robert L. S m i t h and H. R. Pinnick, Jr. Southeast Missouri State university Cape Girardeau. MO 63701
I
The Thermodynamics of Ionization of the Bisulfate Ion A physical chemistry laboratory experiment
One of t h e authors (RLS) has made use of the titration calorimetry method in the undergraduate physical chemistry laboratory for t h e past several years. T h e advantage of t h e titration calorimeter over the traditional ampule-smash or vial-opening calorimeter is t h a t the calorimeter can be used in t h e single addition mode (analoeous t o amoule-smash) or in the runtinunus m d e . In the lotter,a number of data points van he vollected durin:: R single titration run. T h e disndvantnye, n i course, is that all heat messuremenrs muat be corrected for heat d stirring-heat leak effects and for the heat effect due t o any difference in temperature between titrant a n d calorimeter solutions when t h e titration is initiated ( I ,
2). A manual detailing several different types of titration calorimetry experiments has been published (Z), complete with a FORTRAN computer program for data processing. An application of continuous titration is the entropy titration technique. I n this case, for reactions having equilibrium constants, generally, i n t h e range 1.0 < logK < 4.0
(1) it is possihle t o e\,aluute buth t h e equilibrtum constant and the v n t h a l ~ vchange from ralorimerric data 13).l'his isdune by a n iterative caieulation t o find t h e equilibrium constant value t h a t gives a minimum in t h e error-square sum,
u = X(Qexot- Qca1J2
(2)
where = the experimentally
determined, corrected heat term at each data point
Q,,,,
and Q,I, = the calculated heat term for a given equilibrium constant value at each data point It has been found in this laboratory t h a t the determination of the thermodynamics of ionization of the hisulfate ion agrees well with t h e literature values and does n o t involve tedious purification procedures. For the titration of perchloric acid into a sulfate solution, ammonium sulfate was selected as the source of sulfate ion in order t o avoid the competing equilibria ( 4 ) of Nat
+S O F
2
NaSOa-
tinnal methods. The calorimeter was a Tronac 450 isaperibol titratian calorimeter. The calorimeter bridge response was monitored w ~ t ha digital multimeter and the thermogram trace was recorded on a milliwlt recorder. Buret delivery time and calorimeter energy equivalent measurements were measured wlth the aid of a clock. Lead and trail
including thermogram analysis and data input to the computer, can he done in 1.5-2.0 hr. Results The results of five trials are shown in the table. Each titration involved addineabout .. . 3 ml of 0.5115 M HClOa into 50 ml of O.0492OM tI\H,,.SU,. The ~.cmrmrr;,tww01rpec1t.r at the md $01 s titr.ltwn \:&lrd ;I" iknir qrrcngth o l d I1 .
