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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
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