pH is a measure of hydrogen ion concentration in an aqueous solution. It is measured using electrochemical methods with a glass electrode that responds to hydrogen ions and a reference electrode like silver/silver chloride. The glass electrode potential depends on the pH through the Nernst equation. Proper calibration and accounting for temperature effects are important for accurate pH measurement. Other methods like indicator papers and ISFET electrodes also exist.

1. pH Measurement
By- Praful Hanmante

2. Need of pH measurement
• To produce products with defined properties
• To protect equipment
• For research and development

3. Basics about pH
• pH is the abbreviation of pondus hydrogenii
• pH = -log [H+]
• [H+] is in molar concentration

4. Basics about pH

5. Basics about pH
 In any aqueous solution
Kw = [H+] [OH-] = Dissociation constant
It depends on temperature
 Acids
[H+] increases and [OH-] decreases
 Bases
[OH-] increases and [H+] decreases

6. Electrochemical method
• Depends on hydrogen
activity in the solution
• It consists of two
electrodes
• 1. Measuring electrode
• 2. Reference electrode

7. Nernst Equation
• E=E0 + 2.3RT*log aH+
F
E0 = constant
R = the gas constant
T = temperature(k)
At 25˚C,slope=59.18mV/pH
• Comparing with
standard formula for
straight line
• Y= a+bx
a= offset
b= slope of the line

8. Glass electrode
• Dependent on hydrogen
activity
• Glass membrane
sensitive to H+ ions

9. Glass membrane

10. Reference electrode
• insensitive to the H+
ions in the solution
• most widely used is the
silver/silver chloride
electrode
• another commonly
used reference
electrode is the calomel
electrode

11. Combined electrode

12. 12 Metrology
Course –Lecture 1.ppt /
15.02.2006 / Hermersdorf
Calibration

13. Calibration
• Offset adjustment
• Slope adjustment
• Two point calibration

14. Temperature effect
Temp Slope mV/pH
15 -57.2
20 -58.2
25 -59.2
30 -60.1
35 -61.1

15. Offset adjustment

16. Sodium Error
• Occurs at high pH
[H+] less than [Na+]
• Electrode respond to [Na+]
• Measured pH is lower than the actual pH

17. Liquid Junction Potential
• Equitransferent KCL is
used as fill solution
• Different mobility of
ions result in liquid
junction potential

18. Diagnostics
• Glass pH Electrode Impedance
- normally in the range 0f 10-100 MΩ
- crack in glass electrode will result in short
• Reference Electrode Impedance
- Coating or blockage of the liquid junction
will increase the result

19. Other methods
• Optical Methods
-Indicator papers
• ISFET electrode
-CMOS based electrode

20. ISFET
• reference electrode in
the solution replaces
the metal gate
• silicon nitride, Si3N4,
overlying the SiO2 is
used to improve pH
response