TITLE: Unified pH measurement v2
AUTHORS: Agnes Heering, Ivo Leito, Markus Lahe, Martin Vilbaste, John Paulo Samin

[DESCRIPTION]
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]
SECTION: Software
1. Start Quick IV Measurement Software.

 

SECTION: Software
2. On the left hand pick Function “Source & Sampling”.

 
  
 


SECTION: Software
3. 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.

  
 


SECTION: Software
4. 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.

   

SECTION: Filling the cell
5.   25.0 °C
 

 


 


 


SECTION: Filling the cell
6. 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 

 

SECTION: Filling the cell
7. The test solutions are added simultaneously to both compartments, raising solution levels in the compartments synchronously.20 mL 

 

 
  
SECTION: Filling the cell
8. After adding the test solutions, the air bubbles are removed from the capillary ends with a syringe or a pipette.
SECTION: Filling the cell
9. Insert electrodes one-by-one.
 

SECTION: Filling the cell
9.1. Electrode is rinsed with water, gently dried, and then rinsed with solution to be measured.
SECTION: Filling the cell
10. Connect electrodes to the instrument.

 

SECTION: Filling the cell
10.1. 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.

  
 

 

SECTION: Measurement
11. Start the measurement by clicking on the "Measure" button on the upper panel.
SECTION: Measurement
12. Data collection. Point is taken at 10 s interval.

SECTION: Measurement
13. Save data. Files are named as Date_ISE1_Solution1_vs_ISE2_solution2.

 

SECTION: Measurement
13.1. 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.

SECTION: Washing
14. Remove the electrodes, rinse them and put them in storage solution.
SECTION: Washing
15. Remove the solutions.
SECTION: Washing
16. Rinse the cell with small amount of ethanol5 mL 

SECTION: Washing
17. Rinse three times with plenty of water20 mL 

SECTION: Washing
18. Rinse once with acetone to ease the drying. 5 mL 

SECTION: Washing
19. Dry cell with compressed air.
SECTION: Data analysis
20. 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. 
SECTION: Data analysis
21. Take the average of the chosen data points.
SECTION: Data analysis
22. ΔpH = ΔE / average slope of the glass electrodes


 

SECTION: Data analysis
23. Insert the ΔpH into the ladder.

 

SECTION: Mesurement
24. Repeat with a new pair of solutions.
SECTION: Data analysis
25. Repeat the data analysis with all the measured potentials.
SECTION: Data analysis
26. Insert the data into the ladder.
SECTION: Data analysis
27. Do the minimization process.
 

SECTION: Data analysis
28. Obtain the pH values for all the measured solutions from the minimization process.
SECTION: Data analysis
29. Calculate uncertainty.