Online Photographic Pre-lab Demonstration for Solution Calorimetry
Note: the complete text of the experimental procedure from the "Physical Chemistry Lab Manual" is given here (in this type). If additional comments or directions are needed, they are added (in italics). So that each photo has its own block of text, extra numbers have been added as needed (i.e. 2a), 2b), etc.). The text is above the picture illustrating it in each case. For simple or known operations, no photo is included.
Set-up and Calibration of the Calorimeter
1) Plug in the chart recorder and calorimeter and turn each on. Make sure the chart recorder is properly attached to the calorimeter (use the color of each recorder wire to hook it to the proper calorimeter terminal – red to red, black to black, green to green). The Solution Calorimeter. The red, green and black wires and terminals are center top.
2) Allow time for the chart recorder and calorimeter to warm up (at least ten minutes and preferably half an hour after being plugged in). Make sure the calorimeter reads 20.000 °C before calibrating (black knob on 20 °C, dial on “000”). Calibrate the chart recorder as follows. The chart recorder (off).
3) Turn the chart recorder on to a speed of two or three centimeters per minute. Make sure to take the cap off the pen (typically the upper pen is used). Zero the pen by adjusting the chart recorder knob so that the pen trace is on the zero line (the chart’s baseline, closest to the recorder controls). The chart recorder is on (green light), the pen cap is off and the pen is leaving a mark on the paper, the chart speed is set at 3 cm/min, and the upper pen voltage is set at V and 1(V).
4) Set the chart recorder range at 1.0 V (1000 millivolts or mV). The width of the chart then represents 10.0 °C, so each large square on the chart is 1.0 °C. Please note that many other settings are possible, for example 0.5 V (500 mV) is 5.0 °C (each large square is now 0.5 °C), 0.1 V (100 mV) is 1.0 °C (each large square is now 0.1 °C), 0.01 V (10 mV) is a full scale range of 0.100 °C, and so on. See previous picture.
5) Turn the black thermometer selector switch to the “ZERO” position, and use the “ZERO” knob on the calorimeter to bring the recorder pen back to the baseline. In this picture all the positions for the thermometer selector switch are shown. The switch is set to "READ" and the thermometer is set at 22.175 °C as the baseline temperature.
6) Now turn the black thermometer selector switch to the “NULL” position, and use the “NULL” knob on the calorimeter to bring the recorder pen back to the baseline. Please see the photo for step 5 for the thermometer selector switch positions.
7) Now turn the black thermometer selector switch to the “CAL” position, and use the “CAL” knob on the calorimeter to bring the recorder pen to the full scale position (this is the “baseline” at the opposite side of the chart). Please see the photo for step 5 for the thermometer selector switch positions.
8) Finally, turn the black thermometer selector switch on the calorimeter to the “READ” position, and the recorder pen will read the temperature measured by the thermistor. Since the voltage is set at 1.0 V and the calorimeter has a base temperature of 20.000 °C, the actual temperature can be read by the number of squares above the baseline. If it is 25.40 °C, the pen will be 5.4 large squares above the baseline, if it is 23.75 °C, the pen will be three fourths of the way between the third and fourth large squares, etc. This calibration process only has to be done at the start of each lab. However, if results seem odd, it is always good to check the calibration again. Also recalibrate if the calorimeter has not been used for more than half an hour during a lab. Setting the knob to "READ".
Measurements with the Calorimeter
9) The baseline temperature must be set before each and every trial. The default is 20.000 °C (also the minimum), the maximum is 29.999 °C. It is good to set the temperature so that the pen is near the bottom of the chart but above the baseline (for exothermic reactions, where the temperature will increase) or near the top of the chart but below full scale (for endothermic reactions, where the temperature will decrease). This allows most of the chart to be used to record the temperature change of the reaction or process. For example, if the solutions are at 22.354 °C before an exothermic reaction, set the baseline at a temperature close to (but below) this temperature. Turn the temperature selection knob to 22 °C, and then “dial in” the desired decimals (i.e. 22.300 °C or even 23.350 °C). Be careful with the decimal knob as it can not go past 0.999 and can be locked (so it must be unlocked before changing the temperature). Never force this or any instrument knob or dial. It is a good idea to choose an initial temperature which allows easy reading of the chart (so 22.354 °C would be a bad starting baseline both because it makes the chart units weird (you want the big squares to be 22.3, 22.4, 22.5, etc. or 22.35, 22.40, 22.45, etc.). It is also good to pick a setting that allows the temperature to fluctuate, because if the temperature goes off the chart for any reason, the trace is worthless (as there are no grids off the chart to measure the temperature). ALWAYS record the baseline temperature directly on the chart before starting the experiment. If you do not, the data is worthless (as the chart records ΔT relative to the baseline temperature). Setting the baseline temperature to 22 °C.
Dialing in the decimals on the baseline temperature (the current baseline shown is 22.082 °C.
10) The following limits must be observed when using the solution calorimeter. The cell will not hold more than 20 mL (liquid) or 1 gram (solid) of sample. Similarly, the dewar will not hold more than 120 mL, but must have at least 90 mL of solution.
TRIS and THAM are two names for the standard used. Between the bottles a glass reaction cell and Teflon dish bottom are shown.
11) Calculation of the amounts and concentrations of solutions and reagents and their preparation must be done before starting the run. Make all solutions in volumetric glassware for best results. Generally solutions used in the dewar will be about 0.1 M and approximately 100 mL. Solutions in the sample cell will be more concentrated (as the volume is 20 mL or less). So for a 1:1 reaction, if the solution in the dewar is 100 mL and 0.1 M, the sample cell solution will be 20 mL and 0.5 M (or if a solid, 0.01 mol). Solids may also be placed into the sample cell (the capacity is 1 g or less). If 1 g of solid is less than 0.01 mole, adjust the concentration of the solution.
12) Generally, the dewar is filled first with solution (or pure liquid) and then placed into the calorimeter housing. The amount of solution added can be measured either by mass (place the dewar on the balance) or volume (use volumetric glassware). Make sure the ring is around the dewar to center it properly in the housing. Try not to spill or slosh any of the liquid from the dewar into the housing. If any is spilled, clean it up immediately and redo that trial. Do not use that data, if any.
13) Next the cell is filled and carefully inserted into the dewar. The cell may be filled with either liquid or solid. If liquid is used, make sure the Teflon dish is firmly in place on the bottom of the cell before putting any liquid into the cell (otherwise it will run out the open bottom). While inserting liquid into the cell can be difficult, a dispo pipette works well (add the liquid dropwise slowly through the hole where the push rod goes). The amount of solution added can be measured either by mass or volume. If a solid is used, please note that solids are generally easier to place inside the cell than are liquids (solids are simply placed on the teflon dish, which is then carefully inserted into the bottom of the cell bell). Solids dissolve better and faster if ground to a powder first (use a mortar and pestle). The cell assembly (left), filling the cell with a dispo pipet (right).
14) As the cell is inserted into the dewar, the calorimeter lid is attached to the top of the cell, so it is placed on top of the well for the dewar.
The thermistor probe is carefully inserted into the hole in the lid (it is glass so treat it very gently).
15) Make sure the chart recorder is properly set. The voltage gives the temperature range on the chart (see number 4, above), and the speed (2 or 3 cm/min) must also be chosen. Make sure the pen cap is off so the pen marks the chart. For each trial you must write the voltage and speed on the chart itself, as well as the date, group name, and reaction identification (i.e. TRIS trial 4). Inserting the glass push rod into the cell.
16) It is prudent to calculate the approximate change in temperature expected for each measurement beforehand to be certain that the data recorded does not go off the chart (for example if the chart recorder is set to measure a maximum of 1 °C temperature change and the reaction instead produces a 1.8 °C change). This expected value may be calculated from the literature value for the process involved (or estimated from data for an analogous process). If the data goes off-scale on the chart recorder, the experiment must be repeated. That trial’s data can not be used in any report.
17) Similarly, if the data occupies only a small portion of the chart recorder range (the change in temperature occupies less than 40% or four large squares), you will not get good results and should repeat the trial. These trials go relatively quickly, so a first test run can be made to determine if the recorder scale is satisfactory (if not, make adjustments). Data from such a trial can only be used after consultation with the instructor.
18) When everything is set, run the drive belt from the stirrer drive motor shaft to the cell. Then turn on the stirrer drive motor to start stirring. Make sure to allow stirring to continue for several minutes before reaction to get a good baseline.
19) To begin the reaction or process, carefully push the glass push rod down. Since the cell is rotating, some pressure is needed to push the Teflon dish off the cell bottom, which initiates the reaction or process. As the cell opens, the contents mix with the solution in the dewar, and the rotating cell stirs everything. Pushing the glass rod down while the cell is rotating. Left before pushing down, right after.
20) Allow the reaction to proceed until the temperature change is done and there is enough of a trace for a good baseline after reaction to be established.
21) Turn off the stirrer motor and carefully remove the thermistor from the lid. Remove the cell and carefully wash and dry it. Dispose of the solution in the dewar according to the pre-lab directions and carefully wash and dry it. Stop the chart recorder motion (do not turn the recorder off, just the 2 or 3 cm/min motion of the chart) and cap the pen.