Sep 24, 2021
Version 1
Unified pH measurement V.1
- Agnes Heering1,
- Ivo Leito1,
- Markus Lahe1,
- Martin Vilbaste1
- 1University of Tartu
External link: https://analytical.chem.ut.ee/, uniphied.eu
Protocol Citation: Agnes Heering, Ivo Leito, Markus Lahe, Martin Vilbaste 2021. Unified pH measurement . protocols.io https://dx.doi.org/10.17504/protocols.io.bybxpspn
License: This is an open access protocol distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Protocol status: Working
We use this protocol and it’s working
Created: September 17, 2021
Last Modified: September 24, 2021
Protocol Integer ID: 53335
Keywords: unified acidity, pHabs, unified pH, pH, unified ph measurement, measuring phabsh2o difference, phabsh2o difference, δphabsh2o value, differential potentiometry, solutions by differential potentiometry, ionic liquid, measuring
Funders Acknowledgements:
EMPIR
Grant ID: 17FUN09
Estonian Research Council
Grant ID: PRG690
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.
Materials
The ionic liquid triethylpentylammonium bis(trifluoromethylsulfonyl)imide [N2225][NTf2] salt bridge has to be used.
Before start
Turn on the thermostat and electrometer at least half an hour before starting the work.
Software
Start Quick IV Measurement Software.
Note
Computer cannot go to sleep during measurements or the communication between computer and instruments is lost and data collection stops.
On the left hand pick Function “Source & Sampling”.
Position of the Function choice “Source & Sampling”.
Note
Alternatively, open previously saved QIVM file with measurement settings.
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.
Position of the tabs “Channel Communication” and “Search Channels”.
Note
This must be done every time the program is started.
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.
Program settings.
Filling the cell
8m
Cell from Gebr. Rettberg GmbH.
25.0 °C
Equipment
VistaShield
NAME
Faraday cage
TYPE
Gamry
BRAND
VistaShield
SKU
Equipment
UT cell
NAME
Special water-jacketed glass cell
TYPE
Rettberg
BRAND
Glass cell
SKU
Equipment
U2c
NAME
Thermostat
TYPE
MLW
BRAND
U2c
SKU
Note
The side ports must be open.
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.0.1 mL
Note
Ionic liquid has high viscosity, therefore flows very slowly, and can leave a film on the wall of the pipette tip or syringe during dispense.
2m
The test solutions are added simultaneously to both compartments, raising solution levels in the compartments synchronously.20 mL both test solutions
Note
Equal amount by weight (not volume!) of solutions must be used. The volume of the lighter solution is calculated from the weight of the heavier solution using the densities of the two solutions in hand. This is especially important for solutions containing acetonitrile.
Note
Minimum volume that can be used is 13 ml.
One option how to fill the cell. This configuration gives positive ΔpH values. Figure shows connections to a pH meter, a potentiostat and an electrometer.
2m
After adding the test solutions, the air bubbles are removed from the capillary ends with a syringe or a pipette.
2m
Insert electrodes one-by-one.
Equipment
EST-0601
NAME
Glass electrode half-cell
TYPE
Izmeritelnaya tekhnika
BRAND
EST-0601
SKU
1m
Electrode is rinsed with water, gently dried, and then rinsed with solution to be measured.
1m
Connect electrodes to the instrument.
Equipment
B2987A Electrometer / High Resistance Meter
NAME
Electrometer
TYPE
Keysight
BRAND
B2987A
SKU
LINK
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.
Connections to electrodes.
Note
Cable goes through the port in Faraday cage. Electrometer is not in the Faraday cage.
Equipment
N1415A Triax to Alligator Cable
NAME
Cable
TYPE
Keysight Technologies
BRAND
N1415A
SKU
LINK
Measurement
1h
Start the measurement by clicking on the "Measure" button on the upper panel.
Data collection. Point is taken at 10 s interval.
1h
Save data. Files are named as Date_ISE1_Solution1_vs_ISE2_solution2.
Note
SAVE THE FILE BEFORE NEW MEASUREMENT! Otherwise, data is lost. Data is given in volts and seconds.
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.
1m
Remove the solutions.
3m
Rinse the cell with small amount of ethanol5 mL
2m
Rinse three times with plenty of water20 mL each
10m
Rinse once with acetone to ease the drying. 5 mL
2m
Dry cell with compressed air.
2m
Data analysis
20m
Points from 30 min to 60 min are used for analysis.Points from 30 min to 60 min are used for analysis.Points from 30 min to 60 min are used for analysis.
Example of a good graph with non-aqueous system.
Take the average of the chosen data points.
ΔpH = ΔE / average slope of the glass electrodes
Start Quick IV Measurement Software.
Note
Computer cannot go to sleep during measurements or the communication between computer and instruments is lost and data collection stops.
On the left hand pick Function “Source & Sampling”.
Position of the Function choice “Source & Sampling”.
Note
Alternatively, open previously saved QIVM file with measurement settings.
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.
Position of the tabs “Channel Communication” and “Search Channels”.
Note
This must be done every time the program is started.
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.
Program settings.
Note
Range is "Fixed 20V" in case of EST-0601 calibration, otherwise it is "Fixed 2V".
Special water-jacketed glass cell from Gebr. Rettberg.
25 °C
Equipment
UT cell
NAME
Special water-jacketed glass cell
TYPE
Rettberg
BRAND
Glass cell
SKU
Equipment
VistaShield
NAME
Faraday cage
TYPE
Gamry
BRAND
VistaShield
SKU
Equipment
U2c
NAME
Thermostat
TYPE
MLW
BRAND
U2c
SKU
Note
The side ports must be open.
Note
A beaker can also be used if temperature is controlled.
Fill capillary with standard aqueous buffer with pH 7.00.50 mL
2m
Insert a glass electrode and a reference electrode.
Equipment
K401
NAME
Calomel reference electrode
TYPE
Radiometer
BRAND
K401
SKU
2m
Electrode is rinsed with water, gently dried, and then rinsed with solution to be measured.
2m
Connect electrodes to the instrument.
Equipment
B2987A Electrometer / High Resistance Meter
NAME
Electrometer
TYPE
Keysight
BRAND
B2987A
SKU
LINK
Crocodile clips are as following:
red is signal and connects to the glass electrode,
black is shield and is connected to reference electrode and
green is guard. Green stays unused.
Note
Cable goes through the port in Faraday cage. Electrometer is not in the Faraday cage.
Equipment
N1415A Triax to Alligator Cable
NAME
Cable
TYPE
Keysight Technologies
BRAND
N1415A
SKU
LINK
Start the measurement by clicking on the "Measure" button on the upper panel.
Data collection. Point is taken at 10 s interval.
Note
Calibration duration must be the same as later used in pH measurements.
1h
Save data. Files are named as Date_GE_vs_ref_pHx.
Note
SAVE THE FILE BEFORE NEW MEASUREMENT! Otherwise, data is lost. Data is given in volts and seconds.
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.
Remove the electrodes.
Remove the solution.
Rinse three times with plenty of water20 mL each
Rinse once with acetone to ease the drying. 5 mL
Dry cell with compressed air.
Repeat measurements with standard buffers pH 4 and pH 10.
Points from 30 min to 60 min are used for analysis.
Take the average of the chosen data points.
Plot ΔE vs pH.
Calibration graph of a glass electrode half-cell.
Obtain the slope and intercept of the glass electrode-half cell.
Mesurement
Repeat with a new pair of solutions.
Data analysis
20m
Repeat the data analysis with all the measured potentials.
Insert the data into the ladder.
Obtain the pH values for all the measured solutions from the minimization process.
Calculate uncertainty.
Note