pH Measurement

By- Praful Hanmante

Need of pH measurement

• To produce products with defined properties

• To protect equipment

• For research and development

Basics about pH

• pH is the abbreviation of pondus hydrogenii

• pH = -log [H+]

• [H+] is in molar concentration

Basics about pH

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

Electrochemical method

• Depends on hydrogen

activity in the solution

• It consists of two

electrodes

• 1. Measuring electrode

• 2. Reference electrode

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

Glass electrode

• Dependent on hydrogen

activity

• Glass membrane

sensitive to H+ ions

Glass membrane

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

Combined electrode

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

Calibration

Calibration

• Offset adjustment

• Slope adjustment

• Two point calibration

Temperature effect

Temp Slope mV/pH

15 -57.2

20 -58.2

25 -59.2

30 -60.1

35 -61.1

Offset adjustment

Sodium Error

• Occurs at high pH

[H+] less than [Na+]

• Electrode respond to [Na+]

• Measured pH is lower than the actual pH

Liquid Junction Potential

• Equitransferent KCL is used as fill solution

• Different mobility of ions result in liquid junction potential

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

Other methods

• Optical Methods

-Indicator papers

• ISFET electrode

-CMOS based electrode

ISFET

• reference electrode in the solution replaces the metal gate

• silicon nitride, Si3N4, overlying the SiO2 is used to improve pH response