pH Meter and Measurement

pH is a qualitatitve measurement the acid and basic value of a solution

Basically it is a measure of the hydrogen ion concentration [H+] in a solution. The concentration of [H+] and [OH-] ion ranges from 1M to 10-14M

The pH of a solution is related to hydrogen ion conc. as follows:

pH = -log10 [H+]

Conversely pOH = -log10 [OH- ]

In a neutral solution at 25 C [H+] = [OH-]= 1.0 x 10-7 (pH + pOH = 14)

Therefore the pH of a neutral solution is pH = -log10 [1.0 x 10-7] = 7

The higher the pH number, the lower the hydrogen ion concentration and vice versa. The pH scale is logarithmic: If 2 solutions differ by 1 unit, it means that one solution has 10 times the hydrogen ion concentration of the other

Measurement of pH Measure pH using a glass electrode

and a reference electrode that contains saturated calomel (mercurous chloride, HgCl2)

Measure the e.m.f. in the cell using a pH meter

The pH meter measures the electrical potential between the HgCl2 of the reference electrode

and glass electrode

The scale on the pH meter has been calibrated to read the pH

directly.

Glass Electrode Most commonly used electrode to measure

pH

The electrode potential exists when a metal is brought in contact with a cation solution of the metal. Eg Ag in a solution of AgNO3 (Ag/Ag+)

A potentiometer or a voltmeter measures the potential

To measure the potential of a complete sel we need two half cells

In measuring pH 1st half cell: test solution and the electrode known as the indicator electrode 2nd half cell: the potential of this cell does not depend on the potential of the test solution. This electrode is called the reference electrode

There are many types of electrodes but the glass electrode is most suitable as it is sensitive to changes in ion concentration.

This electrode is not affected by the presence of oxidising or reducing agents

Operative over a wide pH range

Responds fast and functions well in physiological systems

Indicator electrode:Glass electrode

Internal Reference

electrode & Ag/AgCl electrolyte

Electrode potential is constant & determined by HCl concentration

Components of a Glass Electrode

Internal solution (dilute HCl)

Glass Membrane: pH sensitive; 0.03 – 0.1 mm thick

Internal Reference Electrode: Ag/AgCl

Reference Electrode: Calomel

Saturated calomel electrode

Comprises of a) mercury (Hg) b) calomel (mercury (I)

chloride c) saturated potassium

chloride solution At 25°C: electrode

potential is 0.242V

Complete Cell

Reference electrode (internal)

Reference electrode (external)

Calomel

H+

(Test solution)

Glass Membrane

H+

(internal)

The pH meter measures the electrical potential between the mercuric chloride of the reference electrode and its potassium chloride liquid, the unknown liquid, the solution inside the glass electrode and the potential between the solution and the silver electrode

Complete Cell Reference electrode (internal)

Reference electrode (external)

Calomel

H+

(Test solution)

Glass Membrane

H+

(internal)

Voltage of the cell is given by: Ecell = k + (2.303RT/F). pH (unknown)

k* = potential of the two reference electrodes + liquid junction potential + asymmetry potential

liquid junction potential = potential at the glass membrane due to

H+ asymmetry potential = potential across the membrane that is

present even when both sides of the membrane are identical. Due to:

a) non uniform composition of the membrane b) stains within the membrane c) mechanical and chemical effects on the

external surface d) degree of hydration on the glass surface

1. Asymmetry potential can be corrected by calibrating the pH electrode daily.

2. This can be achieved using a standard pH solution

E sel = k + (2.303RT/F). pH (test solution)

Therefore, pH (test) = pH (std) + [E (std cell) – E (unk. cell)]/2.303RT/F

Theory of the Glass Membrane Potential 1. The glass electrode functions as a result of ion exchange on the

surface of a hydrated layer

2. The membrane of a pH glass electrode consists of chemically bound Na2O and SiO2

3. The surface of a glass electrode contains fixed silicate groups associated with sodium ions, SiO– Na+

4. The outer surface of the electrode must remain hydrated

-SiO-Na+ + H+ -SiO-H+ + Na +

5. Glass has a high affinity for H+ ions: The equilibrium constant for the above exchange is high therefore although other ions can be exchanged with Na , the surface of the glass will be entirely silicic acid (except in very alkaline solutions)

Solid Solid Solution Solution

2 Types of Errors in the Measurement of pH Alkaline Error: • Due to the capability of the

membrane to respond to other cations besides hydrogen.

• In alkaline solutions, hydrogen ion conc. is low and other ions can compete successfully.

• Error negligible at pH less than 9

• Although the pH of the solution is above 9, the pH reading may be lower

Acid Error

• Also known as water activity error

• Occurs because the membran depends on the activity of the water

• Occurs in very acidic solutions when the activity of water is less than 1

• We get a positive error

• A similar result is obtained when with a high salt concentration in a non aqueous solvent eg ethanol

Factors that Affect pH measurements

Hydrogen ion activity Standardizing solution used Temperature