4. Cont…
• The concept of pH was first introduced by the danish
chemist sorenson in 1909.
• P means “puissance d” and H mean “Hydrogen”.it is the
french word,which means strength/power of hydrogen.
• Defination :-The formal defination of pH is the
negative logarithm of the hydrogen
ion activity.
pH = ¯log [Hᶧ]
• pH is a unit of measure which describes the degree of
acidity or alkalinity(basic) of a solution.
• It is measured on a scale of 0 to14.
5. pH value
• The pH value of a substance is directly related to the ratio of
the hydrogen ion and hydroxyl ion concentrations.
• Low pH values correspond to high concentrations of Hᶧ and
high pH values correspond to low concentrations of Hᶧ.
• If the Hᶧ concentration is higher then OH¯ the material is
acidic and the pH is less than 7
• If the OH¯ concentration is higher than Hᶧ the material is
basic and the pH is more than 7
• Number of Hᶧ equals the number of OH¯ 7 is neutral < is
acidic > 7 is basic.
6. The pH scale
• The ion product of water, Kw is the basic for the pH
scale.
• The pH scale corresponds to the concentration of
hydrogen ions.
• It is a covenient means of designating the actual
concentration of Hᶧ in any aqueous solution in the range
between 1.0MHᶧ and 1.0M OH¯.
• Sorensen, used a logarithmic scale for expressing the Hᶧ
concentration this scale was called pH.
7.
8. Measurement of pH
• The pH can be measured by :
(1) pH strips
(2) pH indicators
(3) pH meter
pH strips :-
-pH strips are pieces of paper that change color depending
on the pH - the acidity or alkalinity –of a liquid.
- pH strips are Brownish yellow in colour (whatman filter);
by this method approx value is obtained,which are less
accurate.
9. Cont…
Ph indicators:-
-indicators are the organic compounds of natural or synthetic
origin whose colour is dependent on pH.
-The pH indicators are either weak acids or weak baseshaving
different colours in undissocisted and dissociated forms :
Hln ↔ Hᶧ + ln¯
(colour-A) (colour-B)
10. Cont…
• pH meter :-
-The pH meter is a laboratory equipment which used to
measure acidity or alkalinity of a solution.
-The pH meter measures
The concentration of
Hydrogen ions [Hᶧ] using
An ion-sensitive electrode .
-It is the most reliable and
Convenient method for
Measuring ph.
12. Types of buffers
• Buffers are 2 types :
(1)ACIDIC BUFFERS –
solution of a mixture of a weak acids with
their salt with a strong base.
E.g CH₃COOH + CH₃COONa
(Weak acid) (Salt)
(2)BASIC BUFFERS -
solution of a mixture of a weak bases with their
salt with a strong acid.
E.g NH₄OH + NH₄CL
(Weak base) (salt)
13. How buffers work :-
• Equilibrium between acid and base.
• Example : ACETATE BUFFER
CH₃COOH ↔ CH₃COO¯ + Hᶧ
• If more Hᶧ is added to this solution,it simply shifts
the equilibrium to the left,absorbing Hᶧ,so the [Hᶧ]
remains unchanged.
• If Hᶧ is removed (e.g. by adding OH¯)then the
equilibrium shifts to the right,releasing Hᶧ to keep
the pH constant.
14. Buffer Capacity
• It is defined as the number of moles of an acid or a base
required to be added to one litre of the buffer solution so
as to change its pH by one.
• Buffer capacity = no.of moles of the acid or base added
to 1 litre of buffer
change in pH
• The greater the buffer capacity the less the pH changes
upon addition of Hᶧ or OH¯.
• Choose a buffer whose pKₐ is closest to the desired pH.
• pH should be within pKₐ ± 1
17. Biological buffer systems
Phosphate buffer system :
• It is mainly intracellular buffer.its concentrationin plasma
is very low.
• The phosphate buffer system is found to be effective at a
wide pH range,because it has more than one ionizable
group.
• In the body,Na₂HPO₄/NaH₂PO₄ is an effective buffer
system.
18. Cont…
Protein buffer system :-
• The protein buffers are very important in the plasma and
the intracellular fluids but their concentration is very low
in cerebrospinal fluid,lymph and interstitial fluids.
• The proteins exist as anions serving as conjugate
bases(Pr) at the blood pH 7.4 and from conjugate
acids(HPr) accepting Hᶧ.
• They have the capacity to buffer some H₂co₃ in the blood.
H₂CO₃ + Pr¯ ⇌ HCO₃¯ + HPr
