Parr Instrument 6755 - Manual (Page 12)

3
Operation
P a r r I n s t r u m e n t C o m p a n y 10
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 sys-
tem will not change signifi cantly when the reactants
are mixed. The Dewar must be fi lled 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 fi lling 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 fi lled by
weight. After fi lling 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 PTFE Dish on
an analytical balance and weigh the sample directly
into the dish. Be careful not to drop any of the sam-
ple into the push rod socket. After the fi nal weigh-
ing, set the dish on a fl at surface and carefully press
the glass bell over the dish to assemble the cell. Do
not grasp or press the thin-walled glass stem dur-
ing this operation; it is fragile and will break eas-
ily. Instead, grasp the bell and press it fi rmly 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 thumbscrew fi nger tight. If the
thumbscrew is not tight against the shaft, the con-
tents will not be released. If necessary, use a 9/64
Allen wrench to tighten further. Hold the cover in
a horizontal position and lower it carefully until
the bottom of the rotating cell rests on a fi rm, fl at
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.
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 fi lling from a volumetric
pipette may be adequate in some cases. Set the
126C PTFE Dish on a fl at 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
pipette through the glass stem and add the liquid,
then reweigh the cell. Attach the cell to the stirring
shaft and insert the push rod.
Installing the Loaded Cover Assembly
Install the thermistor probe in the cover opening
and press the bushing fi rmly into place to anchor
the probe in its proper position. 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
motor as required.
Combining the Reactants
Each test in a solution calorimeter can be divided
into three distinct time periods:
1. A pre-period during which the reactants are al-
lowed to come to an initial thermal equilibrium.
The thermometer will beep to inform the opera-
tor that it has established the initial equilibrium
and that it is now time to initiate the reaction.
2. A reaction period during which the reactants are
combined and an enthalpy change occurs in the
system.
3. A post-period during which the calorimeter
again comes to equilibrium. The thermometer
will produce a report when the fi nal equilibrium
has been achieved and that the test is complete.
At the end of the pre-period, 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 fi nger. Push the rod down
as far as it will go; after which it should continue
to rotate the pulley. Let the stirrer continue to run
during the reaction and the calorimeter reports its
results.