1. 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 

2. + + + + + + + ++ + + + + + + + + + + + + + + + + + + + + + + + + ++ + + + + + + + Glass membrane (Insulator) ä pH 4 solution (high H + ) ä Voltage across glass membrane ä Solution voltage is 180 mV higher than reference! ä pH 4 solution (high H + ) ä Voltage across glass membrane ä Solution voltage is 180 mV higher than reference! + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + Reference Voltage Call it zero!

3. Nernst Equation: Voltage = f(pH) Reference (known) [H+] Voltage at Faraday constant Gas constant An easy reference condition!

4. Nernst Equation For T in Celsius

5. Voltage vs pH

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

7. pH Measurements ä The voltage measurement requires a very high impedance circuit (high resistance) because a pH probe can’t produce much current 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 Gentle stirring keeps the solution next to the glass bulb from being depleted of protons ä The voltage measurement requires a very high impedance circuit (high resistance) because a pH probe can’t produce much current 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 Gentle stirring keeps the solution next to the glass bulb from being depleted of protons

8. 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

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 Accumet meter recognizes buffers! ä Calibration accounts for non-ideal probe behavior (fouling) ä It is important that buffers be used covering the range of pH measurements ä Typically a piecewise linear fit is used ä It would be possible to make a pH measurement without any calibration ä Based on theoretical values ä This is how the Accumet meter recognizes buffers! ä Calibration accounts for non-ideal probe behavior (fouling) ä It is important that buffers be used covering the range of pH measurements ä Typically a piecewise linear fit is used

10. Buffer pH vs Temperature ä Buffers change pH slightly as the temperature changes. ä Accumet meters automatically compensate for this when calibrating ä Buffers change pH slightly as the temperature changes. ä Accumet meters automatically compensate for this when calibrating