Last Update: August 25, 2008
The Parr model 1451 solution calorimeter is shown in Fig. 1. The instrument consists of an insulated sample compartment (shown in Fig. 2), which is constructed to minimize heat loss during an experiment. The temperature change in the solution is accurately and precisely measured by a thermistor which sits inside a Dewar (see Fig. 2) within the sample compartment. Also integrated into the body of the calorimeter is a Parr 1661 calorimetry thermometer, but this is not functional on our instrument. Instead, a Parr 6772 calorimetry thermometer is used to record the temperature.
Figure 1. Parr model 1451 solution calorimeter.1
Figure 2. Cutaway schematic of the sample chamber of the Parr 1451 solution calorimeter.1
Figure 3. Sample bell, left, and Teflon dish, right.1
In preparation for a run, either a pure solvent (when a ΔsolutionH is to be measured) or solution containing one of the reactants (as when we attempt to measure ΔrxnH for a reaction taking place in solution) is placed in the Dewar. This is most conveniently done volumetrically, but it may also be done by weight. Be careful not to get solution or solvent on the outside of the Dewar and not splash the Dewar's contents on the inside of the calorimeter. The Dewar can now be left to equilibrate with the calorimeter while the sample is prepared.
The sample is contained in a bell-shaped tube attached to a removable Teflon dish, which makes a leak-tight seal with the bell tube. The separated sample bell and Teflon dish are shown in Fig. 3. The sample bell is very fragile, so handle it with care. When attaching or detaching the Teflon dish, handle the bell only by the large end. Be very careful not to damage the contact the edges of the Teflon dish.
To load a solid sample, remove the Teflon dish and weigh it on an analytical balance. Add the powdered sample to the dish and obtain its mass by difference. The sample must be powdered using a mortar and pestle to assure a sharp ΔT. Be sure that all of the sample is in the dish, and not on the contact edges or in the push rod socket in the center of the Teflon dish. Move the dish containing the sample to a clean, flat surface and carefully press the bell tube onto the dish.
To load a liquid sample, first connect the Teflon dish to the bell tube. Add the liquid to the cell through the top by pipet. Measure the amount of material added either volumetrically or by weight.
Once the sample is in the cell, attach the cell to the stirring shaft on the calorimeter cover by carefully sliding the plastic coupling onto the shaft as far as it will go. Turning the thumb screw finger tight (see Fig. 2) to finish the connection. Do not over-tighten this screw! Holding the cover, with the attached cell, in a horizontal position, lower it carefully until it rests on a clean, flat surface. Insert the glass push rod through the central pulley hub and lower it until it just rests on the Teflon dish's socket. With a gentle twisting motion, press the end of the rod into the socket in the Teflon dish (see Fig. 2). It is not necessary to push the rod more than a millimeter, or so, into the socket. The push rod is very fragile, exercise extreme care when performing this procedure.
Transfer the the entire assembly into the calorimeter containing the previously-filled Dewar. Carefully place the thermistor through one of the holes in the cover (it will snap in place). It is a good idea to position the thermistor opposite the stirring motor. After the samples have been loaded into the calorimeter, and the thermistor probe inserted, the drive belt is placed over the pulleys and the motor is switched on.
Record the temperature readings every 30 sec, or so, until a slight but steady temperature change is observed. If the reactants are below room temperature, you will observe a slight increase in the temperature, but if the reactants are above room temperature a slight decrease in the temperature will be observed. For best results, the reactants should be at essentially the same temperature when loaded into the calorimeter, and this temperature should be close to room temperature. Continue these pre-mixing temperature measurements for ~5 min. When a sufficient number of pre-mixing data points have been measured, quickly press down on the push rod to release the dish and hence the reagent in the rotating cell. It is advised that you hold the lid in place with one hand while depressing the rod. This operation should be done swiftly, but not violently, and it is a bit nerve-wracking at first because the belt will slip during the one, or two, seconds when pressure is applied to the push rod (as it is designed to do).
Note the time at which the addition was made and continue readings at 30 sec for about 5 min, which is usually sufficient to observe the post-reaction behavior. After the reaction is complete, turn off the motor and remove the pulley. Remove the thermistor, rinse with distilled water and carefully dry with a Kimwipe. Place it out of harm's way. Lift the calorimeter lid carefully. If the Teflon dish did not fall off the push rod during the run, gently push both the rod and the dish back into the bell, as you did to load a sample. Holding the bell firmly on a cushioned flat surface, gently remove the rod by pulling and twisting it out of the socket. Do not break the push rod. Remove the push rod and carefully remove the dish. Detach the bell tube. Rinse the bell tube and dish with distilled water, and dry with a Kimwipe or air dry. Remove the Dewar, empty the liquid, rinse with distilled water and wipe dry. Do not get water on the outside of the Dewar as it can become trapped between the Dewar and the inside of the calorimeter.
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