Acids, bases and buffers 03/10

Question 1

Acid definition

Answer 1

A molecule or ion that tends to donate protons (H+) in solution
HX <–> H+ + X-

Question 2

How do protons exist in water?

Answer 2

hydronium ion, H3O+ (written as H+ for ease)

Question 3

Base definition

Answer 3

A molecule or ion that tends to acquire (mop up) protons in solution
X- + H+ <–> HX
e.g. NH3 + H+ <–> NH4+

Question 4

Equation for the dissociation constant

Answer 4

Ka = [H+] [X-] / [HX]

Question 5

pKa equation

Answer 5

pKa= -log Ka

Question 6

Why is water pH7?

Answer 6

Ka = [H+][OH-] / [H2O]
Ka = 1.8 x 10^-16
Conc. of pure water is 55M so [H+][OH-] = 55 x 1.8 x 10^-16 = 10^-14
so [H+] = 10^-7
pH = -log 10^-7 = 7
so higher [H+] are acidic while lower are basic (gives a larger pH)

Question 7

Importance of carbonic acid

Answer 7

Formed when CO2 dissolves in blood tissue fluids. CO2 + H2O –> H2CO3 <–> H+ + HCO3- . Acts as a buffer at physiological pH

Question 8

Lactic acid equation, formation and removal

Answer 8

CH3CH(OH)COOH <–> H+ + CH3CH(OH)COO-
Formed in human cells from glucose during anaerobic respiration, especially in muscle, which causes cramp. Converted to glucose by gluconeogenesis in the liver (Cori cycle)

Question 9

Role of hydrochloric acid in the body

Answer 9

HCl <–> H+ + Cl-
Secreted into the stomach in the digestive process

Question 10

Under what conditions do oral bacteria produce acids?

Answer 10

Anaerobic

Question 11

Examples of acids produced by oral bacteria

Answer 11

Lactic acid, formic acid, acetic acid, propionic acid, butyric acid.

Question 12

Equilibria for dissolution of calcium phosphate (hydroxyapatite) by acids

Answer 12

Ca10(PO4)6(OH)2 + 8H+ <–> 10Ca2+ + 6HPO4 2- + 2H2O
Ca10(PO4)6(OH)2 + 14H+ <–> 10Ca2+ + 6H2PO4 - + 2H2O
An increase in H+ shifts the eqm to the right, increasing the solubilisation of calcium phosphate.

Question 13

How does saliva/certain foods aid remineralisation?

Answer 13

Saliva contains calcium and phosphate which shift the eqm to the left, inhibiting solubilisation and promoting remineralisation.
Calcium containing foods (e.g. milk) protect teeth from decay.

Question 14

Tooth erosion cause

Answer 14

Caused by the direct effect of acids in the diet on teeth e.g. many soft drinks with low pHs. NOT due to acids produced by metabolism of bacteria.

Question 15

Chelating agents

Answer 15

Agents that bind strongly to divalent and trivalent cations (e.g. Ca2+). Many fruit juices contain organic acids (e.g. citric acid) that act as chelating agents.

Question 16

Effect of chelating agents

Answer 16

Bind to Ca2+ which are no longer available to counteract calcium solubilisation. Contribute to tooth erosion

Question 17

Two ways of making a buffer

Answer 17

1. neutralising excess acid with OH-
   e.g. CH3COOH + NaOH –> CH3COONa + H2O
2. mixing a solution of acid and a salt

Question 18

When does maximum buffering capacity occur?

Answer 18

When the acid has dissociated 50%.
So [HX] = [X-]
Ka = [H+]
pKa = pH

Question 19

Henderson-Hasselbalch equation

Answer 19

pH = pKa + log10[X-]/[HX]

Question 20

2 buffer solution mixtures

Answer 20

Either a weak acid and one of its salts, or a weak base and one of its salts e.g. NH3 and NH4Cl

Question 21

Range of effective buffering capacity

Answer 21

Within +- 1pH unit of the buffer’s pKa

Question 22

Main buffer in saliva

Answer 22

HCO3 - (bicarbonate)

Question 23

2 other buffers in saliva

Answer 23

Phosphate and protein

Question 24

Where does bicarbonate in saliva come from?

Answer 24

• blood
• carbonic anhydrase in the salivary glands that catalyses formation of carbonic acid

Question 25

Equation for formation of carbonic acid

Answer 25

H2O + CO2 –> H2CO3

Question 26

Carbonic acid buffer equilibrium

Answer 26

H2CO3 <–> H+ + HCO3- (pKa 6.37)

Question 27

pH of saliva

Answer 27

around 6.7

Question 28

How does phosphate act as a buffer?

Answer 28

H3PO4 can donate H+ ions to counteract an increase in alkalinity and it can regain the H+ ions to counteract an increase in acidity

Question 29

How many pH ranges can phosphate act as a buffer? What are the equilibria?

Answer 29

3 because 3 equilibria are involved:
H3PO4 <–> H+ + H2PO4- pKa 2.16
H2PO4- <–> H+ + HPO4 2- pKa 7.21
HPO4 2- <–> H+ + PO4 3- pKa 12.32

Question 30

Location on a titration curve where buffering is occurring

Answer 30

Point of inflexion - smallest gradient

Question 31

Feature of proteins that enable buffering

Answer 31

acidic or basic side chains in many amino acids e.g. aspartic and glutamic acids contain carboxyl group in side chains.

Question 32

Feature of histidine that gives buffering capacity

Answer 32

Imidazole side chain with a pKa of 6.0 (saliva has pH 6.7 so pH~pKa)

Question 33

Histatins definition

Answer 33

Class of proteins in saliva that are rich in histadine.

Question 34

pH of blood

Answer 34

7.4

Question 35

Buffers in blood

Answer 35

Bicarbonate and proteins

Question 36

Importance of pH homeostasis in blood and cells

Answer 36

preserve enzyme and membrane (proteins) function