Unified pH measurement v2
Agnes Heering, Ivo Leito, Markus Lahe, Martin Vilbaste, John Paulo Samin
Abstract
The purpose of this document is to present technical guidance of measuring pHabsH2O difference (ΔpHabsH2O value) between two solutions by differential potentiometry in a cell with two compartments connected by a salt bridge filled with an ionic liquid.
Before start
Turn on the thermostat and electrometer at least half an hour before starting the work.
Steps
Software
Start Quick IV Measurement Software.
On the left hand pick Function “Source & Sampling”.
Right hand side click on tab “Channel Communication”. Click on “Search Channels”, which opens a new window. Select USB interface and search for the channel. After the search click on the channel name to choose the channel and close the window.
Settings can be saved as QIVM file, which can be used for measurements. The saved file can be opened under tab “File”. The settings will remain the same between the measurements if the program is not closed.
Filling the cell
25.0°C
Equipment
| Value | Label |
|---|---|
| VistaShield | NAME |
| Faraday cage | TYPE |
| Gamry | BRAND |
| VistaShield | SKU |
| https://www.gamry.com/ | LINK |
Equipment
| Value | Label |
|---|---|
| UT cell | NAME |
| Special water-jacketed glass cell | TYPE |
| Rettberg | BRAND |
| Glass cell | SKU |
| https://www.rettberg.biz/en/ | LINK |
Equipment
| Value | Label |
|---|---|
| CORIO CD-200F | NAME |
| Refrigerated / heating circulator | TYPE |
| Corio | BRAND |
| 9012701.03 | SKU |
Fill capillary with a syringe or pipette with the ionic liquid, so that the level of ionic liquid is 1 mm to 2 mm below the bottoms of the measurement compartments.0.1mL
The test solutions are added simultaneously to both compartments, raising solution levels in the compartments synchronously.20mL
After adding the test solutions, the air bubbles are removed from the capillary ends with a syringe or a pipette.
Insert electrodes one-by-one.
Equipment
| Value | Label |
|---|---|
| EST-0601 | NAME |
| Glass electrode half-cell | TYPE |
| Izmeritelnaya tekhnika | BRAND |
| EST-0601 | SKU |
Electrode is rinsed with water, gently dried, and then rinsed with solution to be measured.
Connect electrodes to the instrument.
Equipment
| Value | Label |
|---|---|
| B2987A Electrometer / High Resistance Meter | NAME |
| Electrometer | TYPE |
| Keysight | BRAND |
| B2987A | SKU |
Crocodile clips are as following:
red is signal and connects to ISE1,
black is shield and is connected to ISE2 and
green is guard. Green stays unused.
Equipment
| Value | Label |
|---|---|
| N1415A Triax to Alligator Cable | NAME |
| Cable | TYPE |
| Keysight Technologies | BRAND |
| N1415A | SKU |
Measurement
Start the measurement by clicking on the "Measure" button on the upper panel.
Data collection. Point is taken at 10 s interval.
Save data. Files are named as Date_ISE1_Solution1_vs_ISE2_solution2.
Go to table view, right click on the table, choose “Export as CSV…” and save the results. The data can be automatically imported into excel file while measuring or after (both under “Excel Navigation”), but this causes constant shift in the comma position.
Washing
Remove the electrodes, rinse them and put them in storage solution.
Remove the solutions.
Rinse the cell with small amount of ethanol5mL
Rinse three times with plenty of water20mL
Rinse once with acetone to ease the drying. 5mL
Dry cell with compressed air.
Data analysis
Points from most stable 15 min is used. The standard deviation of the points must be ≤ 1 mV and the drift ≤ 4 mV/h. Otherwise the measurement is left out from minimization.
Take the average of the chosen data points.
ΔpH = Δ E / average slope of the glass electrodes
Insert the ΔpH into the ladder.
Mesurement
Repeat with a new pair of solutions.
Repeat the data analysis with all the measured potentials.
Insert the data into the ladder.
Do the minimization process.
Obtain the pH values for all the measured solutions from the minimization process.
Calculate uncertainty.
