Acids, bases and buffers 03/10 Flashcards
Acid definition
A molecule or ion that tends to donate protons (H+) in solution
HX <–> H+ + X-
How do protons exist in water?
hydronium ion, H3O+ (written as H+ for ease)
Base definition
A molecule or ion that tends to acquire (mop up) protons in solution
X- + H+ <–> HX
e.g. NH3 + H+ <–> NH4+
Equation for the dissociation constant
Ka = [H+] [X-] / [HX]
pKa equation
pKa= -log Ka
Why is water pH7?
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)
Importance of carbonic acid
Formed when CO2 dissolves in blood tissue fluids. CO2 + H2O –> H2CO3 <–> H+ + HCO3- . Acts as a buffer at physiological pH
Lactic acid equation, formation and removal
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)
Role of hydrochloric acid in the body
HCl <–> H+ + Cl-
Secreted into the stomach in the digestive process
Under what conditions do oral bacteria produce acids?
Anaerobic
Examples of acids produced by oral bacteria
Lactic acid, formic acid, acetic acid, propionic acid, butyric acid.
Equilibria for dissolution of calcium phosphate (hydroxyapatite) by acids
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.
How does saliva/certain foods aid remineralisation?
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.
Tooth erosion cause
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.
Chelating agents
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.
Effect of chelating agents
Bind to Ca2+ which are no longer available to counteract calcium solubilisation. Contribute to tooth erosion
Two ways of making a buffer
- neutralising excess acid with OH-
e.g. CH3COOH + NaOH –> CH3COONa + H2O - mixing a solution of acid and a salt
When does maximum buffering capacity occur?
When the acid has dissociated 50%.
So [HX] = [X-]
Ka = [H+]
pKa = pH
Henderson-Hasselbalch equation
pH = pKa + log10[X-]/[HX]
2 buffer solution mixtures
Either a weak acid and one of its salts, or a weak base and one of its salts e.g. NH3 and NH4Cl
Range of effective buffering capacity
Within +- 1pH unit of the buffer’s pKa
Main buffer in saliva
HCO3 - (bicarbonate)
2 other buffers in saliva
Phosphate and protein
Where does bicarbonate in saliva come from?
- blood
- carbonic anhydrase in the salivary glands that catalyses formation of carbonic acid
Equation for formation of carbonic acid
H2O + CO2 –> H2CO3
Carbonic acid buffer equilibrium
H2CO3 <–> H+ + HCO3- (pKa 6.37)
pH of saliva
around 6.7
How does phosphate act as a buffer?
H3PO4 can donate H+ ions to counteract an increase in alkalinity and it can regain the H+ ions to counteract an increase in acidity
How many pH ranges can phosphate act as a buffer? What are the equilibria?
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
Location on a titration curve where buffering is occurring
Point of inflexion - smallest gradient
Feature of proteins that enable buffering
acidic or basic side chains in many amino acids e.g. aspartic and glutamic acids contain carboxyl group in side chains.
Feature of histidine that gives buffering capacity
Imidazole side chain with a pKa of 6.0 (saliva has pH 6.7 so pH~pKa)
Histatins definition
Class of proteins in saliva that are rich in histadine.
pH of blood
7.4
Buffers in blood
Bicarbonate and proteins
Importance of pH homeostasis in blood and cells
preserve enzyme and membrane (proteins) function
