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NSC1951 Lecture 18
Acids & Bases
Objectives
• Properties of acids and bases
• The pH scale
• Distinguish between strong and weak acids
and list the clinical uses of these acids
• Distinguish between strong and weak bases
and list the clinical uses of these acids
• Understand neutralisation and the clinical
applications of neutralisation
1. PROPERTIES OF ACIDS & BASES
 Acids
 Produce hydrogen ions (H+) in H2O
 Taste sour
Marieb, Fig 15.1
Turn blue litmus (vegetable dye)  red
Act as electrolytes in solution
Neutralise solutions containing hydroxide ions
(OH -)
React with several metals releasing H2(g) 
corrosion
React with carbonates releasing CO2(g)
Destroy body tissue
 Bases
 Produce or cause an increase in hydroxide ions
(OH-) in H2O
 Taste bitter
Marieb, Fig 15.1
Turn red litmus  blue
Act as electrolytes in solution
Neutralise solutions containing hydrogen ions
(H +)
Have a slippery, ‘soapy’ feel
Destroy body tissue/ dissolve fatty (lipid)
material
2. THE pH SCALE
 Ion Product of Water
Pure H2O at 25°C
Some molecules ionise
H2O  H+ + OH[H+ ] = 1 x 10-7 M = [OH- ]
 Ion Product of H2O:
[H+ ]
x [OH- ] = [1 x 10-7 ] x [1 x 10-7 ]
* Add exponents
= 1 x 10-14
• Acidic solution
[H+ ] > [OH- ]
• Neutral solution
[H+ ]
= [OH- ]
• Basic solution
[H+ ]
< [OH- ]
Timberlake, Fig 9.3
 Using the pH scale
Timberlake, Figs 9.4, 9.5
Marieb, Fig 2.12
Exponential values for [H+ ] & [OH- ]
inconvenient in a clinical workplace
Simplify  pH scale
 acid-base concentration
p  potential or Power
H  Hydrogen
• pH describes [H+ ] & [OH- ]
 Indicates if a fluid is :
0
Acidic
[H+ ] = 100
[OH- ] =10-14
7
Neutral
[H+ ] = 10-7 [OH- ] =10-7
14
Basic
[H+ ] = 10-14
[OH- ] = 100
On the pH scale, values below 7 are acidic, a value of 7 is
neutral, and values above 7 are basic.
Marieb, 2.12
3. STRENGTHS OF ACIDS
Strong Acids (very few)
Eg
HCl Hydrochloric Acid
~ Stomach acid
Marieb, Fig 26.11
HNO3 Nitric Acid
~ May be used to cauterise warts
~Drugs, explosives, fertilisers, dyes
H2SO4 Sulphuric Acid
~  conc. to treat stomach hypoacidity
~ Fertilisers, dyes, glues
Strong acids are:
• Strong electrolytes
• ~ 100% ionisation  good conductors
• Severe burns to body tissue
*** Stomach lining protected against HCl by
mucus
Dissociation in Water : Strong acids
Polar covalent molecules  ions
Eg.
HCl(l)
HNO3(l)
H2O
H+(aq) + Cl-(aq)
H2O
H+ (aq) + NO3- (aq)
H2O
H2SO4(l)
2H+ (aq) + SO42- (aq)
Weak Acids (most acids in nature)
See lecture 19; Marieb, Fig 26.11
CH3COOH Acetic Acid
~ Antimicrobial solution  ears, plastics, dyes,
insecticides
H2CO3 Carbonic Acid
~Bicarbonate buffer system, carbonated drinks
H3PO4 Phosphoric Acid
~ Drugs, fertilisers, soaps, detergents, animal feed
• Weak acids are:
– Weak electrolytes
– Small % ionisation  weak conductors
 Dissociation in Water : Weak acids
Polar covalent molecules
 Mainly stay as molecules
• Dissociation in water : Weak acids (cont)
H2O
H+ (aq) +
CH3COOH (l)
CH3COO- (aq)
H2O
H+ (aq) + HCO3-(aq)
H2CO3 (l)
H2O
H3PO4 (l)
H+ (aq) + H2PO4- (aq)
Marieb, Fig 26.11
4. STRENGTHS OF BASES
Strong Bases
Eg
NaOH Sodium Hydroxide
~ Removes grease – drains, ovens
Mg(OH)2 Magnesium hydroxide
~ Antacid
~ Laxative
Al(OH)3 Aluminium hydroxide
~ Antacid
~ Absorbs toxins, gases,
~ Causes constipation
Strong bases are:
• Strong electrolytes
• ~ 100% dissociation in water  good
conductors
• Severe damage to skin & eyes
(Group 1A elements)
Dissociation in Water : Strong bases
Metal hydroxides  ions
Eg.
NaOH(s)
Mg(OH)2(s)
Al(OH)3(s)
H2O
Na+(aq) +
H2O
H2O
OH-(aq)
Mg 2 + (aq) + OH- (aq)
Al 3+(aq) +
OH- (aq)
Weak Bases
Eg
NH3 Ammonia
~ Waste product of protein break down in body.
CO3 2-
In antacids
HCO3 –
In antacids, buffers
HPO4 2-
In buffers
• Weak bases are:
– Weak electrolytes
– Do not contain OH – but react with H2O  small
numbers of OH –
 Reaction with Water : Weak bases
NH3(g) +
H 2O
HCO3 – (aq) + H2O
NH4 + (aq) + OH – (aq)
H2CO3
(aq)
+
OH-(aq)
5. ACID-BASE NEUTRALISATION
 Neutralisation Reaction
Acid +
HCl +
H+ +
Base 
NaOH 
OH – 
Salt +
NaCl +
Neutralise each other
Must be equal concentrations
Water
H2O
H2O
 Antacids – clinical applications
for side effects!!)
(Check
~ Neutralise excess stomach acid
~ Raise stomach pH > 4
Pepsin inactive
~ Assist with ulcer treatment
~  solubility in H2O but still produce high %
of ions
Eg
Mg(OH) 2
2HCl +
Al(OH) 3
3HCl +
Milk of Magnesia & in
Mylanta
Mg(OH) 2 
MgCl 2 + 2H2O
In Mylanta
Al(OH) 3 
AlCl 3 + 3H2O
CaCO 3
2HCl +
CaCO 3  CaCl 2 + H2O + CO 2 (g)
~Also a Ca 2 + supplement
Long term overuse  Ca 2 + levels
 risk kidney stones (renal calculi)
NaHCO 3
Baking Soda
Not recommended!!
HCl + NaHCO 3  NaCl + H2O + CO2 (g)
~ Elderly tend to OD
 Stomach can ‘explode’
Acids and Bases
Key Concepts:
Acid
Base
produce
H + ions
Ionization in water
produce
gives
Small %
100%
Small %
Weak
acid
Strong
base
Weak
base
to form
OHproduct
[H+] x [OH-]
Strong
acid
Neutralization
OH- ions
H+
100%
undergo
is
pH
Salt
&
Water
REFERENCES
Cree & Rischmiller, 2001
Ch. 8 pp 205-216
Marieb, 2004
Ch. 2 pp 41-43,
Ch. 16 p556
Ch. 27 p1049
Review questions
1. List the properties of acids & bases.
2. Discuss the pH scale:
*Define the ion product of water & indicate how
this is determines the pH scale.
* Use the pH scale to determine if a given
solution is acidic, neutral or basic.
3. Distinguish between strong acids & weak acids:
List clinical uses of these acids & write
equations for their dissociation in water
Review questions (cont)
4. Distinguish between strong & weak bases;
List clinical uses of these bases & write
equations for their dissociation in water.
5. Complete simple equations for the
neutralisation reaction of an acid & a base;
Discuss clinical applications of acid-base
neutralisation.