Acids, Bases and Buffers Flashcards
What is an acid?
A molecule or ion which tends to give up protons in solution
What is a base?
A molecule or ion which tends to acquire protons in solution
What is Ka?
Acid dissociation constant
Direct measure of strength of an acid
What is the acid dissociation constant equation?
Ka = [H+].[X-]/[HX]
What does a low pKa mean?
The lower the pKa, the stronger the acid
How do you work out pKa from Ka?
pKa = -log10Ka
When does a buffer work maximally?
When it is half dissociated
What is the pH of water?
7
What does a pH < 7 mean?
Acidic solution
What does a pH > 7 mean?
Basic solution
Which acids do oral bacteria produce when breaking down carbohydrates?
Lactic acid, formic acid, acetic acid, propionic acid, butyric acid
What is the pKa of hydrochloric acid?
pKa <1
What is the equation for pH?
pH = -log10 [H+]
How is carbonic acid formed in the body?
When carbon dioxide dissolves in the blood
When is lactic acid formed?
During anaerobic respiration - during exercise (in muscle during rapid movement)
What is lactic acid converted to in the liver in the “Cori” cycle?
Glucose by gluconeogenesis
Which component of teeth gives enamel its rigidity?
Hydroxyapatite
Describe the dissolution of calcium phosphate (hydroxyapatite) by acids
Ca10(PO4)6(OH)2 + 8H+ 10Ca2+ + 6HPO42- + 2H2O
- An increase in H+ causes the equilibrium to shift to the right, increasing the solubilisation of calcium phosphate
- An increase in calcium or phosphate concentration causes the equilibrium to shift to the left, inhibiting solubilisation and promoting re-mineralisation
What is tooth erosion caused by?
The direct effect of acids in the diet on teeth
What is a chelating agent?
Agents which bind strongly to divalent an trivalent cations
Which chelating agent is present in orange juice?
Citric acid
Citric acid is a chelating agent. What does this mean?
Chelating agents bind strongly to divalent and trivalent cations. Citric acid binds to Ca2+, preventing remineralisation of teeth
What is the definition of a ‘buffer’?
A solution which resists changes in pH when an acid or alkali is added to it/compounds that resist pH changes when exposed to extremes of pH
What is a buffer solution made up of?
Mixtures of either a weak acid and one of its salts, or a weak base and one of its salts
What is the Henderson-Hasselbalch equation?
pH = pKa + log10 [base]/[acid]
Allows you to estimate the pH of a buffer/to determine the amount of acid and conjugate base needed to make a buffer solution of a certain pH
Within which range does a buffer have effective buffering capacity?
Within +/- 1 pH unit of its pKa
When do buffers have greatest buffering capacity?
When 50% dissociated (pKa = pH)
In a buffer, when does pKa = pH
When the buffer is 50% dissociated
What is the main buffer in saliva?
Bicarbonate (HCO3-)
Name 3 buffers present in saliva
- Bicarbonate
- Phosphate
- Protein
Where does bicarbonate in saliva come from?
Bicarbonate in saliva comes partly from the blood and partly from the action of carbonic anhydrase, an enzyme in the salivary glands that forms carbonic acid. Carbonic acid dissociates into hydrogen ions and bicarbonate
What is the pH of saliva?
pH 6.7
Which pH does bicarbonate buffer optimally?
pH 6.7 - the pH of saliva
What do water and carbon dioxide form and which enzyme is involved?
Carbonic acid
Carbonic anhydrase catalyses the reaction
H2o + CO2 H2CO3
Why does phosphate act as a buffer at 3 different pH ranges?
There are 3 different equilibria involved
- H3PO4 H+ + H2PO4-
- H2PO3 H+ + HPO42-
- HPO42- H + PO43-
Explain how proteins in the saliva can act as buffer
- Many amino acids in proteins contain side chains which are acidic or basic
- e.g. aspartic and glutamic acids contain carboxyl groups in their side chains
- Histidine has an imidazole side-chain with a pKa of 6.0
Which protein is an important buffer the saliva?
Histatins - rich in histidine
Help to buffer the saliva against the effects of acids
What type of proteins are good buffers?
Proteins containing acidic or basic amino acids e.g. aspartic acid, glutamic acid, lysine, arginine, histidine
What pH is blood maintained at?
pH 7.4
Why is it important that blood is maintained at a pH of 7.4?
Cells require pH in the cytoplasm and organelles to be closely controlled to preserve enzyme and membrane function
Which buffers are important in maintaining the pH of blood (pH 7.4)?
Bicarbonate and protein buffers
What is the pH of saliva?
pH 6.7
Explain how phosphate acts as a buffer in the saliva in resisting the pH as it becomes more alkaline
Phosphate donates H+ ions to resist the increasing alkalinity
Explain how phosphate acts as a buffer in the saliva in resisting the pH as it becomes more acidic
Phosphate accepts H+ ions as the pH becomes more acidic
Which side chain does histidine have which has important buffering capacity in saliva?
Imidazole
Which amino acids in proteins are the main acidic buffers?
Aspartic acid
Glutamic acid
