MANUAL No. 154

INSTRUCTIONS FOR THE

1451

SOLUTION CALORIMETER

The unit of heat used in this Manual is the thermochemical calorie, which is equal to 4.1840 absolute joules.

ASSEMBLY INSTRUCTIONS

Unpack the calorimeter carefully and check the individual parts against the packing list. If shipping damage is discovered, report it immediately to the delivering carrier. Handle the glass sample cell and the thermistor probe with care as these parts are fragile and easily broken. Set the calorimeter on a bench or table in a location that is free from drafts and protected from sources of radiant heat. Temperature changes in the room should be minimal.

Attach the motor to the rear of the calorimeter case using the two mountings screws which are provided. Set the cover on the calorimeter air can; drop the geared drive belt over the motor and stirrer pulleys; connect the motor to a 115/ 120 volt, 50/60 Hz grounded outlet and start the motor. The drive system should run freely. Although the belt may appear to be unusually loose, it is intended to operate under very light tension to minimize friction in the stirrer bearing. The gearing on the belt and pulleys will prevent slippage.

Turn the selector switch on the thermometer panel to the OFF position; then connect the power cord to a 11S/120 Volt, 50/60 Hz grounded outlet. Plug the lead wire from the thermistor probe into the jack on the thermometer panel and lay the probe on the table near the calorimeter. Handle the probe very carefully as the glass tip will break easily and, if broken, it cannot be repaired.

The unit of heat used in this Manual is the thermochemical calorie, which is equal to 4.1840 absolute joules

Recorder Requirements. Instead of reading temperatures directly from the thermometer bridge with the aid of a null indicator, it will be best in most applications to operate this calorimeter with a potentiometric strip chart recorder to produce a thermogram showing the temperature changes which occurred during a run. The recorder to be used with this calorimeter should have an internal impedance of 100 ohms or more, and it should have plotting ranges of 10, 100 and 1000 millivolts full scale. Most laboratory recorders meet these requirements.

Connect the recorder to the polarized terminals at the top of the thermometer panel and plug the power cord from the recorder into a 115/120 volt, 50/60 Hz grounded outlet. Run a ground wire from the grounding terminal on the panel to a similiar terminal on the recorder. If there is no grounding terminal on the recorder it may be necessary to connect the ground wire to the instrument case but, in doing so, do not introduce a ground loop circuit. Good grounding is very important, not only as a safey measure, but also to ensure satisfactory thermometer and recorder performance. If there is any question about the reliability of the ground connection through the power cord, run a separate wire from the grounding terminal on the calorimeter to a good earth ground.

Before starting to use the calorimeter for the first time it will be well to make a dry run with the calorimeter completely assembled but with no liquid in the Dewar and no sample in the rotating cell. This will give the user an opportunity to become familiar with the individual parts of the calorimeter and the manner in which they must be handled. Detailed instructions are given in the operating sections which follow.

CALORIMETER OPERATIONS

Sample Size. The rotating sample cell will hold up to 20 ml of liquid sample or a solid sample weighing up to one gram, More than one gram of solid may be used in some cases, but smaller samples are preferred so that the heat capacity and ionic strength of the system will not change significantly when the reactants are mixed. The Dewar must be filled with not less than 90 ml and not more than 120 ml of liquid to properly cover the rotating cell.

Filling the Dewar. It is best to lift the Dewar out of the air can during the filling operation. The liquid to be placed in it can be measured volumetrically, or the Dewar can be placed on a solution or trip balance and filled by weight. After filling the Dewar set it in the air can and gently push the spacer ring down as far as it will go.

Loading a Solid Sample. Solid samples should be suitably ground so that they will dissolve quickly or mix uniformly with the liquid in the Dewar. Place the 126C Teflon dish on an analytical balance and weigh the sample directly into the dish. Be careful not to drop any of the sample into the push rod socket. After the final weighing, set the dish on a flat surface and carefully press the glass bell over the dish to assemble the cell. Do not grasp or press the thin-walled glass stem during this operation; it is fragile and will break easily. Instead, grasp the bell and press it firmly onto the dish. Then lift the cover from the calorimeter and attach the cell to the stirring shaft by sliding the plastic coupling onto the shaft as far as it will go and turning the thumb screw finger tight.

Hold the cover in a horizontal position and lower it carefully until the bottom of the rotating cell rests on a firm, flat surface; then insert the push rod through the pulley hub and press the end of the rod into the socket in the 126C sample dish.

Figure 1.

Loading a Liquid Sample. Liquid samples can be measured into the rotating cell either by volume or by weight. Best precision is obtained by weighing, but filling from a volumetric pipet may be adequate in come cases. Set the 126C Teflon dish on a flat surface and press the glass bell over the dish, handling the glass carefully as described above. If the sample is to be weighed, tare the empty cell on a laboratory balance; insert a pipet through the glass stem and add the liquid; then reweigh the cell. Attach the cell to the stirring shaft and insert the push rod.

Install the thermistor probe in the cover opening and press the bushing firmly into place to anchor the probe in its proper position. Handle the cover carefully after installing the probe since the glass stem will break easily.

Lower the cover assembly with the cell and thermistor probe into the Dewar and set the cover in place on the air can; then drop the drive belt over the pulleys and start the major.

Combining the Reactants. Each test in a solution calorimeter can be divided into three distinct time periods: (1) a preperiod during which the reactants are allowed to come to thermal equilibrium; (2) a reaction period during which the reactants are combined and an enthalpy change occurs in the system; and (3) a postperiod during which the calorimeter again comes to equilibrium. At the end of the preperiod, start the reaction by pressing the push rod downward to drop the sample out of the rotating cell. This should be done quickly without interrupting the rotation of the rod without undue friction from the finger. Push the rod down as far as it will go; after which it should continue to rotate with the pulley. Let the stirrer continue to run during the postperiod until a uniform slope is established, as explained later in these instructions.

THERMOMETER AND RECORDER OPERATIONS

The temperature measuring system in this calorimeter consists of a thermistor probe and a special bridge designed for use within the ten degree span from 20° to 30° C. Within this range the response of the thermometer is linear, with each 100 microvolt change in output representing a temperature change of exactly . 001° C. Thus, when the out-put signal is fed to a recorder and plotted on a 10, 100 or 1000 millivolt chart, temperatures can be read directly from the chart without applying a conversion factor. The following values will apply when equating changes in thermometer output to temperature changes:

100 microvolts (0. 0001 V) = 0. 001° C

10 millivolts (0. 010 V) = 0. 100°

100 millivolts (0. l00 V) = 1. 000°

1000 millivolts (1. 000 V) = 10.00°

Once these basic relationships are understood the bridge can be balanced to a zero output at any baseline temperature from 20° to 30° C and a recording range can be selected to produce a full-scale trace corresponding to a temperature change of 0. 1°, 1. 0° or 10° C. The true temperature at any point on the chart can then be determined by adding the chart reading to the baseline setting shown by the unit temperature switch and digital potentiometer located within the marked boxc on the thermometer panel.

There are five switch positions on the selector switch in the center of

the thermometer panel, the middle three of which have adjusting knobs.

OFF ZERO NULL CAL READ

In the OFF position no power is supplied to the thermometer and the system is dead. The switch should remain in this position whenever the calorimeter is not in use.

The ZERO control adjusts the output of the bridge to zero voltage.

The NULL control adjusts the bridge to indicate a temperature of

exactly 20° C at zero voltage output.

The CAL adjustment sets the full scale output of the bridge at exactly 1000 millivolts ( 1.000 V), corresponding to a temperature exactly 10° C above the bridge null.

The READ position is used to feed the thermometer signal to the recorder.

Calibrating the recorder. Most strip chart recorders can be set to produce a full scale trace with inputs of 10, 100 or 1000 millivolts, which cover the ranges generally used for the 1451 Calorimeter. The recorder should have a chart speed selector and an adjustment for setting the pen to a zero baseline. Any specific instructions furnished by the recorder manufacturer should be observed when using this equipment.

After connecting the recorder to the thermometer bridge, balance the bridge and calibrate the recorder using the procedure described below. These steps should be taken in sequence when using the recorder for the first time. It is not necessary to repeat these adjustments in each subsequent run, but the settings should be checked from time to time to be sure that they have not changed.

1. Turn the recorder on and turn the thermometer selector switch to ZERO for a warm up period before making any adjustments. Although the thermometer will usually warm up in ten minutes, a longer period up to thirty minutes may be required to reach maximum stability.

2. Start the chart drive at a speed of one inch per minute.

3. Move the recording pen to the zero baseline on the chart as instructed by the recorder manufacturer.

4. Set the range switch on the recorder to 1000 millivolts (1. 000 volt) full scale. In this position the full scale of the chart represents a span of 10° C.

5. With the thermometer selector switch in the ZERO position, use the ZERO adjustment on the thermometer bridge to bring the pen back to its zero baseline.

6. Turn the selector switch to NULL and bring the pen to its zero baseline with the NULL adjustment.

7. Turn the selector switch to CAL and use the CAL adjustment to move the pen to its full scale position.

8. Now set the unit temperature switch and the digital potentiometer to read exactly 20. 000°C and turn the selector switch to the READ position. The pen will then move to a position on the chart indicating the temperature sensed by the thermistor.

Example:

If the chart paper has 10 major units in its ruling and the recorder is set at 1000 millivolts (10. 00°) full scale, each major unit on the chart represents 1°C. Therefore a reading of 4. 52 units on the chart indicates a temperature of 24. 52° C in the calorimeter (20° baseline + 4. 52° on the chart scale).

Better precision can now be obtained by changing the baseline setting and increasing the sensitivity of the recorder after it is known that the temperature being measured is near 24. 52°. Move the temperature setting on the bridge to exactly 24. 000°, then change the range selector on the recorder to 100 millivolts (1. 000°) full scale. If the pen then moves to 5. 23 major divisions on the chart, the temperature in the calorimeter is 24. 523° (24° baseline + . 523° on the chart).

Or, for best precision, set the temperature dials to exactly 24. 520°

and change the range selector on the recorder to 10 millivolts (0.100°)

full scale. Now use the recorder as a null indicator and adjust the digital

potentiometer to bring the pen back to the zero baseline. If the digital

meter then reads .525, the temperature in the calorimeter is 24. 525° C.

PRODUCING THE THERMOGRAM

Choosing the Range. Before starting a run, try to estimate the total

energy change involved in the experiment so that the recorder can be set

in a range which will produce the best thermogram. The following set-

tings are recommended:

Up to 10 calories, set at 0.01 V (0.100°) full scale.

10 to 100 calories, set at 0.10 V (1. 000°) full scale.

100 to 1000 calories, set at l. 00 V (10. 00°) full scale.

If the sign and magnitude of the enthalpy change calmot be estimated before starting an experiment, set the recorder at 0.1 V (1. 000°) full scale and make a trial run starting with a baseline in the middle of the chart. The temperature change observed in this experiment can then be used as a guide for setting the recorder to a different range and baseline to produce a better thermogram in a subsequent run.

Resetting the Pen. If the reaction to be examined is expected to be endothermic, the pen must be raised to a higher position in order to record a temperature drop during the reaction. To raise the pen, simply turn the unit temperature switch or the digital potentiometer to a lower setting. This change can be made at any time without affecting the range and calibration of the recorder.

In exothermic experiments, a baseline adjustment may be desirable at this time if the temperature in the calorimeter has changed significantly during the initial equilibration period.

Beginning the Trace. The liquid in the Dewar and the sample in the rotating cell must reach thermal equilibrium; and the recorder must trace a straight line for at least 3 or 4 minutes before starting the reaction. To minimize this equilibration period, the reactants should be at approximately the same temperature when they are placed in the calorimeter. This is particularly important when working with reactions which produce low enthalpy changes. In such cases any temperature difference between the two solutions in a liquid-liquid system should not exceed 0. 2° when the calorimeter is loaded. The calorimeter should then be allowed to run for about 15 minutes before starting the trace. Solid-liquid systems will usually come to equilibrium within a shorter period.

Completing the Trace. Having established the initial drift, start the reaction by depressing the push rod to open the rotating cell. This will produce a distinct temperature change which will soon taper off to a uniform drift as the calorimeter again comes to equilibrium. Continue the trace until the drift line becomes straight and remains straight for at least three minutes. Usually this condition will be reached within ten minutes or less after starting the reaction.

At the conclusion of the test, stop the recorder; lift the pen and turn the thermometer selector switch to ZERO. Remove the chart from the recorder and mark it to identify the run and the reactants involved. Also, write in the baseline temperature and show the recorder range setting for this run.

Emptying the Calorimeter. Stop the calorimeter motor; raise the cover carefully and wipe any excess liquid from the parts that were immersed in the Dewar. Remove the thermistor probe from the cover and remove the sample dish from the end of the push rod; then remove the rod and release the glass cell from the drive shaft. Lift the Dewar out of the air can and empty it; then wash and dry all wetted parts carefully.

READING THE THERMOGRAM

In order to determine the net temperature change produced by the reaction it is necessary to locate a point on the thermogram at which the temperture reached 63 per cent of its total rise (or fall). This can be done easily using the graphic procedure which is described below, although other variations of this method can be used as well.

1. Place a straight edge over the preperiod drift line and extend this line well past the point at which the cell was opened to start the reaction.

2. Moore the straight edge to the postperiod drift line and extrapolate this line backward to the time when the cell was opened. If there are fluctuations in the drift lines due to noise or other variations in the signal, use the best average when drawing these extrapolations.

3. Using a centimeter scale, measure the vertical distance, R, between the two extrapolated lines at a point near the middle of the reaction period.

4. Multiply the distance, R, by 0. 63, then

5. Set the zero end of the centimeter scale on the extrapolated preperiod drift line and move the scale along this line to locate a vertical intercept with the thermogram which is exactly 0. 63R above the preperiod drift line. Draw a vertical line through this point to intercept both drift lines.

6. Read the initial temperature, T_i s and the final temperature, T_f, at the points of intersection with the drift lines and subtract to determine the corrected temperature rise, DT

DC = T_f - T_i

Although the thermogram shown on page 13 to illustrate this graphic method is taken from an exothermic reaction, the same steps can be used to establish the corrected temperature change for an endothermic reaction.

If it was necessary to reset the pen during the test, the graphic procedure

can still be used by taking into account the two different baseline tempera-

tures when reading the intercepts. However, in such cases it usually

will be desirable to re-run the experiment using a different baseline or

a higher span on the recorder to produce an unbroken thermogram.

CALCULATING THE ENERGY CHANGE

The energy change, Q. measured in this calorimeter is calculated by multiplying the net corrected temperature change, DT_c by the energy equivalent, e, of the calorimeter and its contents.

Q = ( D T_c) (e)

If DT_c is measured in degrees C and e is expressed in calories per degree C, Q will be reported in calories.

The energy equivalent, e, is determined by a calibration procedure which is described below under Standardization.

The change in enthalpy, DH, at the mean reaction temperature is equal to -Q divided by the amount of sample used in the experiment, expressed either in moles or grams.

DH_T = -Q/m

where T is the temperature at the 0. 63R point on the thermogram.

Enthalpy values are usually expressed in kilocalories per mole. Procedures for converting enthalpy changes, DH. to thermodynamic standard conditions and for using DH in other computations can be obtained from thermodynamics or thermochemistry textbooks, or from literature references.

STANDARDIZING THE CALORIMETER

As explained above, in order to calculate the energy change, Q. involved in a reaction, the energy equivalent, e, of the calorimeter and its contents must be known. Values of e are determined by running several calibration experiments in which the calorimeter is operated in the usual manner but with reactants which release (or absorb) a known amount of energy. The energy equivalent is then calculated by dividing the known energy input, Q_E, by the corrected temperature rise, DT_c .

e = Q_E/DT_c

Standardization with TRIS. A sample of tris(hydroxymethyl)aminomethane, commonly called TRIS, is furnished with the 1451 calorimeter to provide a reliable standardizing reagent. TRIS is furnished as a dry powder which can be used directly from the bottle as supplied without further preparation, but undue exposure to air and moisture should be avoided in order to preserve the integrity of the standard.

For standardizing the 1451 calorimeter, solid TRIS can be dissolved in dilute hydrochloric acid in a controlled reaction for which the amount of heat evolved is well established. In the recommended standardization procedure described below, O. 5 gram of TRIS is dissolved in 100 ml of 0.1 N HC1 to evolve 58. 738 calories per gram of TRIS at 25° C. This is the procedure:

1. Tare the Dewar on a solution or trip balance and add exactly 100. 00 ± 0.05 grams of 0.100 N HC1.

2. Weigh 0. 50 + .01 gram of TRIS into the 126C Teflon dish on an analytical balance to an accuracy of ± . 0001 g.

3. Assemble the rotating cell; place it in the calorimeter and start the motor.

4. Let the calorimeter come to equilibrium; then set the recorder

at 0. l0V(1. 000° C) full scale; set the baseline at the bottom of the chart

for an exothermic reaction and trace a thermogram as previously described.

5. Analyze the thermogram to determine the net corrected temperature rise, DT_c.

6. Calculate the known energy input by substituting in the equation:

Q_E = m[58. 738 + 0. 3433(25 - T _.63R)]

where, Q_E = the energy input in calories

m = the weight of TRIS in grams

T _.63R = the temperature at the 0. 63R point on the thermogram

The term: 0. 3433(25 - T_.63R) adjusts the heat of reaction to any temperature above or below the 25° C reference temperature.

7. Calculate the energy equivalent of the calorimeter and its contents by substituting in the equation:

e = Q_E/ DT_c

where e will be expressed in calories per °C.

8. Determine the energy equivalent of the empty calorimeter by subtracting the heat capacity of the 100 g of 0.1 N HCl from e, as follows:

e’ = e - (100. 00)(0.99894)

where, e’ = the energy equivalent of the empty calorimeter in calories per °C.

100. 00 = the weight of 0.100 N HCl in grams

0.99894 = the specific heat of 0.1 N HCl at 25° C

Example:

A standardization reaction involving:

0.5017 grains of TRIS, and

100.00 grams of 0.100 N HCl

produces a net corrected temperature rise of:

DT_c = 0.244° C

with 0.63 rise, T_.63R , at 24.301 °C

In this reaction the known energy input is:

Q_E = 0.5017 [58.738 + 0.3433(25 - 24.301)]

= 29. 589 calories

The energy equivalent, e, of the calorimeter and its contents is then computed:

e = 29.589/0.244

= 121.27 calories/°C

The energy equivalent, e’, of the empty calorimeter is then computed:

e’ = 121.27- (100.00)(.99894)

= 21. 38 calories/ ° C

EXAMPLE A - An Exothermic Reaction

Problem: Determine the change in enthalpy for solid sodium sulfate, Na₂SO₄, when dissolved in a 5 gram/liter aqueous solution of barium chloride, BaCl₂·2H2O .

Na₂SO₄ m = 0.1458 grams

Ba⁺⁺ solution = 100. 00 grams

Corr. temp. rise DT_c = .042 C

(from chart, p. 20)

T_.63R = 24.885 C

Energy equivalent e = 121.46 cal/ C

Energy evolved Q = (DT_c)(e)

= (.042)(121.46)

= 5.1013 calories

Enthalpy change DH_T = -Q/m

= -5.1013/0.1458

= -34. 99 cal/g @ 24. 885° C

Or, multiplying by 142.04 (the molecular weight of Na₂SO₄)

DH_T = (- 34. 99)(142.04)

= -4. 970 Kcal/mole @ 24.885 C

EXAMPLE B - An Endothermic Reaction

Problem: Determine the heat of solution of solid potassium nitrate,

KNO₃, when dissolved in water.

KNO₃ m = 0. 7180 gram

Distilled water = 100. 00 grams

Corr. temp. rise DTC = -. 508° C

(from chart, p. 2 2)

T_.63R = 25.400 C

Energy equivalent e = 121. 46 cal/°C

Energy evolved Q = (DT_c )(e)

= (-. 508)(121. 46)

= -61 . 70 calories

Enthalpy change DH_T = -Q/ m

= -(-61. 70)/. 7180

= +85. 94 cal/g @ 25. 400° C

Or, multiplying by the molecular weight of KNO₃

DH_T = (85. 94)(101. 10)

= 8. 6 9 Kcal / mole @ 25.400 ° C

GENERAL INSTRUCTIONS

• For best results, always operate the calorimeter as close to room temperature as possible .

• When working with two liquid reactants, both liquids should be adjusted to nearly the same temperature before they are loaded into the calorimeter. This can be done in various ways, such as: storing the two flasks in a constant temperature cabinet or on a heavy alum-inum plate, or by immersing the two flasks in the same water bath.

• Be consistent in technique. For example, if the Dewar is filled by weight during standardization runs, weigh the liquid sample into the Dewar during experimental runs.

• Do not obstruct the breather hole in the air can or the passage to it through the surrounding insulation.

• Turn on the recorder and the thermometer and let them warm up for at least ten minutes (and preferably up to thirty minutes) before starting the first run in a series.

• Clean the Dewar and the sample cell thoroughly after each run to prevent the accumulation of contaminants.

REFERENCES

Since this is primarily an operating manual, the user may want to consult other references for additional information on calorimetric and thermochemical theory. Suggested references include:

1. Dickinson, H. C., Combustion Bomb Calorimetry, Bulletin of the U.S. Bureau of Standards, 11, 189 (1915).

2. Eatough, Christensen and Izatt, Experiments in Thermometric Titrimetry and Titration Calorimetry, Brigham Young Univer-sity Press, Provo, Utah (1974).

3. Lewis and Randall, revised by Pitzer Brewer, Thermodynamics, 2nd Edition, McGraw-Hill Book Co., New York (1961).

4. Rossini, F. D., (Ed. ), Experimental Thermochemistry, (IUPAC), Interscience Publishers, Inc., New York (1956).

5. White, Walter P., The Modern Calorimeter, The Chemical Catalog Co., New York (1928).

MAINTENANCE INSTRUCTIONS

• Examine the rotating cell periodically for leaks by closing the cell and submerging the empty bell in a beaker of water for a period of time comparable to a complete run in the calorimeter. If water migrates into the sealing area between the Teflon dish and the glass bell, the seal is deteriorating and the 126C Teflon dish should be replaced. 0

• If the plastic coupling becomes detached from the glass stem of the sample cell, the two parts may be rejoined by applying a small amount of a slowcure epoxy adhesive to the top of the glass stem.

• After one hundred hours of operation, or once each year, apply one drop of instrument oil or household oil to the motor bearing, and one drop of oil to the bronze stirrer shaft bearing. Be sure to remove any excess oil that may appear at the bottom of the shaft.