7. Remove the internal glass electrode from the pH 7.0 buffer.
Rinse the electrode with DI water and blot dry.
8. Immerse the internal glass electrode in a beaker of pH 4.0
buffer.
9. Begin data recording again. Once the reading has
stabilized, record the potential in mV.
The potential reading in pH 7.0 buffer should be 0±25 mV. The
potential difference between the two readings should be greater
than 168 mV. If the difference is less than 160 mV, consider
replacing the sensor. Contact PASCO if the sensor is still within
the warranty (1 year from purchase), or purchase a new sensor.
If the internal glass electrode functions properly but the
completely assembled electrode does not, replace the
membrane module with a new one filled with Internal Fill
Solution. A spare membrane is included with the electrode.
Follow the steps under Check electrode operation (slope)
again after replacing the membrane.
NOTE: All solutions should be prepared fresh.
For best performance, use ISA in all solutions.
Reading samples with the electrode
Various procedures may be used to determine the concentration
of a sample. The most common is the Direct Calibration method,
which is described in the following sections. Contact PASCO’s
Technical Support department for details of other methods.
In Direct Calibration, a series of standard solutions of differing
concentration is used to calibrate the electrode. From there,
each sample requires only a single reading, which is compared
with the calibration readings to obtain the sample concentration.
ISA is added to all solutions to ensure that the samples and
standards have very similar ionic strength and pH.
Measurement set-up
1. Prepare the electrode as described in Electrode
assembly.
2. Connect the electrode to PASCO Capstone or SPARKvue
and configure it as a Carbon Dioxide ISE, as described in
Set up the electrode.
3. Prepare two standards that differ in concentration by a
factor of ten and bracket the expected sample
concentration range. Both standard solutions can be
obtained by diluting the 1000 ppm Standard Solution
(provided with the electrode) in DI water. For example, if
your expected sample concentration is 50 ppm carbon
dioxide, dilute 10 mL of the 1000 ppm standard solution in
a 100 mL volumetric flask to obtain a 100 ppm high
standard, then dilute 10 mL of the high standard in an
identical flask to obtain a 10 ppm low standard. All samples
and standard solutions should be at the same temperature.
Calibrating with direct concentration reading
Follow the procedures below for your program of choice if using
a pH sensor with direct concentration reading capabilities, such
as the Wireless pH Sensor (PS-3204).
SPARKvue:
1. From the Sensor Data screen, check the box next to
Carbon Dioxide (aqueous) ISE in the list of available
measurements, then select a template to open the
Experiment Screen.
2. From the Live Data Bar on the bottom left of the
Experiment Screen, click on Carbon Dioxide (aqueous)
ISE and select Calibrate measurement from the list.
3. Make sure that the following settings are selected in the
Calibrate Sensor window:
- Sensor: {Name of the sensor you are using}
- Measurement: Carbon Dioxide (aqueous) ISE (mg/L)
- Calibration Type: 2 point (Adjust Slope and Offset)
4. Click Continue.
5. Remove approximately 10 mL of your low standard solution
from its volumetric flask and set it aside until Step 7. Add
1 mL of ISA to the flask and invert repeatedly to mix.
6. Use lab pH paper to check the pH of the standard. The
value must be between 4.0 and 4.5; if it is not, add ISA in
1 mL increments and continue mixing until the value is
within this range.
7. Fill your flask to the 100 mL mark with the low standard
solution that you removed in Step 5. (If you do not have
enough solution to reach the 100 mL mark, simply add all
of the removed solution.) Invert repeatedly to mix, then
pour the entire solution into a 150 mL beaker.
8. Rinse the electrode with DI water, blot dry, and place it into
the beaker. Wait for a stable reading in the Current Value
boxes. (This may take several minutes.)
9. Enter the known concentration of the low standard solution,
in mg/L, into the Standard Value box under Calibration
Point 1, then click Set Calibration.
TIP: For solutions dissolved in DI water, 1 ppm
and 1 mg/L are approximately equivalent.
10. Remove the electrode from the low standard solution
beaker. Rinse the electrode with DI water and blot dry.
11. Remove approximately 10 mL of your high concentration
standard from its volumetric flask and set aside until the
next step. Add 1 mL of ISA to the flask and invert to mix.
12. Repeat Step 6 for the high standard solution, then fill the
flask to the 100 mL mark with the high standard solution
removed in the previous step. Invert to mix, then pour the
solution into a new 150 mL beaker.
13. Place the electrode into the high standard solution beaker
and wait for a stable reading in the Current Values boxes.
14. Enter the known concentration of the high standard
solution, in mg/L into the Standard Value box under
Calibration Point 2, then click Set Calibration.
15. Review your new calibration, then click OK.
Product Guide | 012-15719B
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