TITLE: Unified pH measurement 
AUTHORS: Agnes Heering, Ivo Leito, Markus Lahe, Martin Vilbaste

[DESCRIPTION]
<div class = "text-blocks"><div class = "text-block"><span>The purpose of this document is to present technical guidance of measuring pH</span><span style = "vertical-align:sub;">abs</span><span style = "vertical-align:super;">H2O</span><span> difference (ΔpH</span><span style = "vertical-align:sub;">abs</span><span style = "vertical-align:super;">H2O</span><span> value) between two solutions by differential potentiometry in a cell with two compartments connected by a salt bridge filled with an ionic liquid.</span></div></div>

[STEPS]
?. [Filling the cell]
25 °C
The side ports must be open.
?. [Filling the cell]
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
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.
?. [Filling the cell]
The test solutions are added simultaneously to both compartments, raising solution levels in the compartments synchronously.
[both test solutions]
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.
Minimum volume that can be used is 13 ml.
?. [Filling the cell]
After adding the test solutions, the air bubbles are removed from the capillary ends with a syringe or a pipette.
?. [Filling the cell]
Insert electrodes one-by-one.
?. [Software]
Start Quick IV Measurement Software.
Computer cannot go to sleep during measurements or the communication between computer and instruments is lost and data collection stops.
?. [Software]
On the left hand pick Function “Source & Sampling”.
Alternatively, open previously saved QIVM file with measurement settings.
?. [Measurement]
Start the measurement by clicking on the "Measure" button on the upper panel.
?. [Measurement]
Data collection. Point is taken at 10 s interval.
?. [Measurement]
Save data. Files are named as Date_ISE1_Solution1_vs_ISE2_solution2.
SAVE THE FILE BEFORE NEW MEASUREMENT! Otherwise, data is lost. Data is given in volts and seconds.
?. [Washing]
Remove the solutions.
?. [Measurement]
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.
?. [Filling the cell]
Electrode is rinsed with water, gently dried, and then rinsed with solution to be measured.
?. [Filling the cell]
Connect electrodes to the instrument.
?. [Filling the cell]
Crocodile clips are as following:red is signal and connects to ISE1,black is shield and is connected to ISE2 andgreen is guard. Green stays unused.
Cable goes through the port in Faraday cage. Electrometer is not in the Faraday cage.
?. [Software]
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.
This must be done every time the program is started.
?. [Software]
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.
?. [Data analysis]
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.
?. [Data analysis]
Take the average of the chosen data points.
?. [Washing]
Rinse the cell with small amount of ethanol
5 mL
?. [Washing]
Rinse three times with plenty of water
[each]
?. [Washing]
Rinse once with acetone to ease the drying.
5 mL
?. [Washing]
Dry cell with compressed air.
?. [Data analysis]
ΔpH = ΔE / average slope of the glass electrodes
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?. [Data analysis]
Insert the ΔpH into the ladder.
Symmetry 2020, 12 (7), 1150; https://doi.org/10.3390/sym12071150
?. [Data analysis]
Calculate uncertainty.
Analytica Chimica Acta, 2021, 1182, 338923, doi.org/10.1016/j.aca.2021.338923
?. [Washing]
Remove the electrodes, rinse them and put them in storage solution.
?. [Mesurement]
Repeat with a new pair of solutions.
?. [Data analysis]
Repeat the data analysis with all the measured potentials.
?. [Data analysis]
Insert the data into the ladder.
?. [Data analysis]
Obtain the pH values for all the measured solutions from the minimization process.
?. [Data analysis]
Do the minimization process.
Symmetry 2020, 12 (7), 1150; https://doi.org/10.3390/sym12071150