A Timed Constant Current Source for Calorimeter Calibration Richard Fuchs University of Houston. Houston, TX 77004 A critical step in the calorimetric determination of heats uf solution (1-3), heats of reaction in solution ( 4 ) , heat capacities of liquids ( 5 ) , and heats of vaporization (61 is the calibration of the system, in which, in effect, the heat capacity of the system must he determined ( 7 ) . We describe here an accurate, inexpensive, and convenient timed constant current source which is particularly suited to this purpose. When a heat of solution produces a certain recorder deflection resulting from a temperature change of the system, it is necessarv to calculate the amount of heat involved. The is done electrically. In determining heats of solution, for example, the heat produced hv an electric current in the calorimeter heater is-used to cal&late the molar heat of solution (in Joules per mole): -defl(S)
A H , = ---X drll(H)
molar mass s o m o k mass
X IZHt
(1)
Drfl(S) is the recorder deflectionproduced by the dissolving sample, and dcfl(H) is the deflection produced by passage of current I (Amperes) through the heater of resistance R (Ohms) for a ~ e r i o do f t seconds. I t is not necessarv to know . accurately the temperature changes (AT) producing the recorder deflections, hut one must establish that the recorder deflection is linearly related to AT. The latter condition is met adeauatelv for small temoerature changes .. bv . a thermistor temperature sensor which is the unknown resistance in a Wheatstone bridge circuit. H w t s 01 w l u t ~ ~nlmsurrd m in or~linar?r;ilc,rimerers, with ION ~ . m u ) l~c~nc,~ntratio!>~of r 10- t o i V 2 .A1 t o a s ~ ~in1'1nitv re dilutinn;esults, are likely to have uncertainties of 1-3 per cent. Small liauid samples are usuallv added from a microliter svringe, with a volume error of as much as one per cent. (If the sample is measured by volume, the quantity "sample mass" in eqn. (1) is replaced by "density X volume".) The overall experimental uncertainty is ~
on = [ n p 2
+ oRZ+ r r r 2 +
..
livP
(2)
where a,, a ~a,,, and rr, represent the uncertainties in I, R, T, and sample volume. If a, = 1 per cent, improving a,, U R , and at from 0.1 per cent to 0.01 per cent will change uo from 1.030 to 1.0003 per cent. The overall uncertainty in the measurement of AH, is limited by the large uncertainty of a,. It is, therefore, reasonable to measure the other experimental quantities with uncertainties no hetter than 0.1-0.25 per cent. The source of heater current has usuallv been a lead-acid battery (1,2), and the current is calculated from the voltage drou across a standard resistor in series with the heater. The current flow from this "constant" voltage source can he affected appreciably by small changes in the resistance of the switch and connections, and by the condition of discharge of the batt,ery. We have constructed several integrated circuit constant current sources operating from 120 VAC power, which avoid many of the prohiems of hattery voltage sources. These have been in regular service for as long as six years without attention otherthan occasional confirmation the current setting
of
594
Journal of Chemical Education
using a digital multimeter in series with the heater circuit. A digital panel milliammeter may. he permanenth inct~roorated . in;
