5
7. There are three different ways to handle the reporting of volume.
•
Do not calibrate drop volume
In this case, you can skip to Step 8, and begin
collecting data.
•
Manually enter a drops/mL ratio value
If you have done a previous
calibration with a particular apparatus, you can enter the drops/mL ratio.
•
Perform an Automatic drops/mL calibration
You can use a 10 mL graduated
cylinder and perform this calibration (see “To Calibrate Drop Volume” section
below for specific directions on this procedure).
8. Start data collection.
Important:
No data will be collected until the first drop
falls through the Drop Counter’s slot. Completely open the lower 2-way valve
(the upper valve should still be set to a slow drop rate). You will now see pH vs.
volume data being plotted on a graph. When you are satisfied that the titration has
proceeded past the equivalence point, stop the data collection, and shut off the
lower 2-way valve of the reagent reservoir. A plot of pH
vs
. volume will be
displayed for your data collection.
9. After collecting data, the program will allow you to view graphs of the first or
second derivative
vs.
volume—second derivative
vs.
volume is shown here. Data
from calculators and Palm OS handhelds can be imported into Logger
Pro
3 for
further analysis or enhanced display or printing capability.
To Calibrate Drop Volume (in mL)
To calibrate drops so that the volume of titrant is recorded in units of milliliters,
choose Calibrate Drops in your program. The Automatic menu choice helps you to
measure the volume of an individual drop by counting the number of drops that pass
through the Vernier Drop Counter and dividing by the total volume of the drops. To
calibrate drops:
a. Place a 10 mL graduated cylinder below the slot of the Drop Counter.
b. Fill the reagent reservoir with titrant.
c. Choose to start the calibration routine in your program.
d. Open the bottom 2-way (on-off) valve to begin releasing drops from the
vessel through the Drop Counter (the top valve finely adjusts the drop rate).
e. Continue to release drops until there are between 9 and 10 mL of liquid in
the graduated cylinder.
f.
Close the bottom valve on the vessel to stop the drops.
g. Type the precise volume of liquid in the graduated cylinder in the Volume
(mL) box of the Calibrate drops dialog box.
h. Choose to stop the calibration routine of your program.
i.
The number of drops/mL will be displayed. It may be helpful to record the
value for that particular reagent reservoir for future titrations.
j.
You can now continue with the titration (return to Step 8 above).
Alternately, the Manual choice allows you to input a previously determined value for
drops/mL. Simply enter the drops/mL value (e.g., 28.0), then continue with the
titration.
6
To Calibrate the pH Sensor (or a different sensor)
For many titrations, it will not be necessary to calibrate the pH Sensor. Without
calibration, you can expect pH readings to be accurate to ±0.25 pH units. Having pH
offset by this much will not affect the location of the equivalence-point volume. In
those cases where a very accurate calibration is required, however, our programs
allow you to do a two-point calibration, using two pH buffer solutions. The general
procedure, using a pH-4 buffer and a pH-7 buffer as an example, is:
First Calibration Point
a. In your program, choose to do a new calibration.
b. Rinse the pH Sensor with distilled water and place it into a buffer of pH 4.0.
c. Type
4
in the edit box as the pH value. Swirl the sensor, wait until the
voltage stabilizes, then choose to keep the calibration point.
Second Calibration Point
d. Rinse the pH Sensor with distilled water, and place it into a buffer of pH
7.0.
e. Type
7
in the edit box as the pH value for the second calibration point.
Swirl the sensor and wait until the voltage stabilizes. Click Keep, then click
OK. This completes the calibration.
f.
You are now ready to collect data using the new calibration.
Note:
If you
want to use these new calibration values at a later time, you can simply
record the slope and intercept values found on the Manual Entry tab and
type them in when needed.
The Manual Entry tab allows you to enter the Slope and Intercept values determined
during a previous two-point calibration.
