1. Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering pH measurements The probe, from chemistry to voltage to a number + + V

2. pH probe + + + + + + + + + + + + ++ + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + + + + + + + + + + Silver/silver chloride electrode Porous reference junction 4 M KCl solution Glass bulb (insulator) with anionic sites + + V Sensing electrode Reference electrode Buffered KCl solution 

3. + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + Glass membrane (Insulator) ä pH 4 solution (high H + ) ä Voltage across glass membrane ä Solution voltage is 180 mV higher than reference! ä We need a way to measure the solution voltage ä pH 4 solution (high H + ) ä Voltage across glass membrane ä Solution voltage is 180 mV higher than reference! ä We need a way to measure the solution voltage + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Reference Voltage Call it zero! + ++ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

4. pH Measurements ä The porous frit provides electrical contact between the solution and the electrolyte ä Must be in contact with the sample solution ä Probe won’t work well if frit is clogged (fouled) ä The voltage measurement requires a very high __________ circuit (high resistance) because a pH probe can’t produce much current ä Gentle stirring keeps the solution next to the glass bulb from being depleted of protons ä The porous frit provides electrical contact between the solution and the electrolyte ä Must be in contact with the sample solution ä Probe won’t work well if frit is clogged (fouled) ä The voltage measurement requires a very high __________ circuit (high resistance) because a pH probe can’t produce much current ä Gentle stirring keeps the solution next to the glass bulb from being depleted of protons + + impedance

5. Difficult Measurements? ä pH is difficult to measure in poorly buffered solutions ä Distilled water ä Rain ä Between pKs of dilute buffers ä pH is difficult to measure in poorly buffered solutions ä Distilled water ä Rain ä Between pKs of dilute buffers

6. Nernst Equation: Voltage = f(pH) Reference (known) [H+] Voltage at Faraday constant Gas constant

7. Nernst Equation

8. Slope vs. Temperature ä Temperature compensation is important!

9. pH Calibration ä It would be possible to make a pH measurement without any calibration ä Based on theoretical values ä This is how the software recognizes buffers! ä Calibration accounts for non-ideal probe behavior (fouling) as well as electronic measurement errors ä It is important that buffers be used covering the range of pH measurements ä It would be possible to make a pH measurement without any calibration ä Based on theoretical values ä This is how the software recognizes buffers! ä Calibration accounts for non-ideal probe behavior (fouling) as well as electronic measurement errors ä It is important that buffers be used covering the range of pH measurements

10. The Challenge ETBuffer pH Current measurement (E, T) pH 0 to 3 known buffers used as standards Temperature compensation optional

11. Calibration Cases 0 or 1 standards w/ Temperature No standards (assume ideal slope and intercept) Single standard (assume ideal slope)

12. Multiple Point Calibrations w/ Temperature Compensation ä How would you use this information to calculate pH? E (mV)T (C) Buffer pH 8237 185244 -1702010 Current measurement (E, T) pH Piecewise linear fit Calculate E/T