Acids, Buffers and Bases Flashcards
What is an acid?
A molecule or ion which tends to give up protons in solution;
water itself: H2O <—> H+ + OH- (a very weak acid)
acetic acid: CH3COOH <—> H+ + CH3COO-
H+ is actually H30+
What is a base?
A molecule or ion which tends to acquire protons in solution:
X- + H+ HX
hydroxyl ion: OH- + H+ <—> H2O
acetate ion: CH3COO- + H+ <—> CH3COOH
ammonia: NH3 + H+ <—> NH4+
amines: R-NH2 + H+ <—> R-NH3+
What is the conjugate acid and what is the conjugate base in this reaction?
HX <—> H+ + X-
Con. Base- X-
Con. Acid- H+
What is the acid dissociation constant?
How to calculate pKa from Ka?
pKa= -logKa
Relationship between pKa and acid strength?
The lower the value of pKa the stronger the acid, The HIGHER the value of Ka the stronger the acid
Name 3 Important acids in biology?
Carbonic Acid
Lactic Acid
Hydrochloric Acid
Describe carbonic acid in biology;
Eqn?
pKa?
How is it formed?
Function/Role?
Carbonic acid: H2CO3 <—> H+ + HCO3-
pKa = 6.37
-Formed when CO2 dissolves in blood tissue fluids.
-Acts as a buffer at physiological pH.
Describe Lactic Acid acid in biology;
Eqn?
pKa?
How is it formed?
Causes?
Lactic acid: CH3CH(OH)COOH <—> H+ + CH3CH(OH)COO-
pKa = 3.86
-Formed by human cells from glucose during anaerobic respiration, especially in muscle during rapid movement.
-Causes cramp.
-Is converted to glucose by gluconeogenesis in the liver (the “Cori cycle”).
In what conditions are acids produced by bacteria?
Anaerobic Conditions
Give some acids produced by oral bacteria and their pKa
Lactic acid: CH3CH(OH)COOH pKa = 3.86
Formic acid: HCOOH pKa = 3.75
Acetic acid: CH3COOH pKa = 4.73
Propionic acid: CH3CH2COOH pKa = 4.87
Butyric acid: CH3CH2CH2COOH pKa = 4.82
Describe the dissolution of calcium phosphate (hydroxyapatite) by acids
-Ca10(PO4)6 (OH)2 + 8 H+ <—> 10 Ca2+ + 6 HPO42- + 2 H2O
-Ca10(PO4)6 (OH)2 + 14 H+ <—> 10 Ca2+ + 6 H2PO4- + 2 H2O
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
Saliva contains calcium and phosphate to aid remineralisation
Calcium-containing foods such as milk help to protect teeth from decay
Describe Hydrochloric Acid;
Eqn?
pKa?
Role?
HCl <—> H+ + Cl-
pKa = <1
-Secreted into the stomach as part of the digestive process.
-Major component in gastric use
What acid of the acids produced by oral bacteria is produced in the largest numbers?
Lactic Acid
How do oral bacteria produce acids?
By fermenting carbohydrates (mainly dietary)
What does excess amounts of the acids produced by oral bacteria cause?
Tooth decay
-Excess amounts of these acids that are not removed in the oral cavity following fermentation of dietary sugars and carbohydrates by oral bacteria, will cause dental caries if not cleared by saliva
What does calcium assist?
Remineralisation process by shifting equilibrium to the left
What does dental caries occur due to?
The production of acid by plaque bacteria
What is tooth erosion caused by?
The direct effect of acids in the diet on teeth
What drinks often cause tooth erosion?
Many soft drinks as they have pHs as low as 2 and 3
How can fruit juices cause tooth erosion?
As the contain organic acids (e.g. citric acids) which act as chelating agents
What are chelating agents?
Agents which bind strongly to divalent and trivalent cations
How to chelating agents contribute to tooth erosion?
As they bind to calcium ions so calcium ions are no longer available to counteract calcium solubilisation
What do buffers do?
resist changes in pH when exposed to acids or bases
How can buffers consisting of similar concentrations of conjugate acid and conjugate base me made?
By either neutralising an acid with OH- or mixing together a solution of acid and one of its salts with an appropriate Ka
When is buffering capacity at a maximum?
pH=pKa 50% dissociated
What are buffer solutions mixtures of?
Buffer solutions are mixtures either of a weak acid and one of its salts, or a weak base and one of its salts
Give an example of two components that could make up a buffer
Ammonia and ammonium chloride
For weak acids, how can the pH of a mixture of the acid and one of its salts be calculated?
pKa + log10 [salt] / [acid]
This is known as the Henderson-Hasselbalch equation
At what value does a buffer generally have effective buffering capacity?
+/- 1 pH unit of its pKa
When do buffers have greatest buffering capacity?
When 50% dissociates (pKa=pH)
What is the main buffer in saliva?
HCO3- BICARBONATE
Apart from bicarbonate, what else contributes to salivas buffering capacity?
Phosphate and protein
Where does the bicarbonate in saliva come from?
Partly from the blood and partly from the action of carbonic anhydrase, an enzyme in the salivary glands that forms carbonic acid
H2O + CO2 <—> H2CO3
How does carbonic acid form carbonic acid?
what does this provide?
Carbonic acid dissociates into hydrogen ions and bicarbonate:
H2CO3 <—> H+ + HCO3- (pKa = 6.37)
This provides the major buffering capacity at the pH of saliva (around pH 6.7)
Why is phosphate in particular a very effective buffer in saliva?
As it can act as a buffer at three different pH ranges, since three equilibria are involved
What are the equations for each phosphate dissociation and the pKa?
H3PO4 <–> H+ + H2PO4- pKa = 2.16
H2PO4- <–> H+ + HPO42- pKa = 7.21
HPO42- <–> H+ + PO43- pKa = 12.32
*Here, pH=pKa as the buffers are at optimum activity
Describe how phosphate act as a buffer.
As the pH increases the phosphate forms more H+ ions to counteract increasing pH, restricting pH change and acting as a buffe
What is the equivalence point of a titration curve?
Number of moles of base= Number of moles of acid
Where is point of inflection on a titration curve? What happens at this point?
The midpoint of any flat part of the titration curve
At the point of inflection the buffer has maximum buffering capacity as pH= pKa
What else in saliva can act as buffers
Salivary proteins can also act as buffers
Many amino acids in proteins contain what?
Side chains that are acidic or basic foe instance, aspartic and glutamic acids contain carboxyl groups in their side chains
How does the side chain in histamine contribute to buffering?
Histidine has an imidazole side-chain with a pKa of 6.0, and this has important buffering capacity in proteins in saliva (pH around 6.7
Name a class of PROTEIN in saliva that buffers against the effect of acids.
HISTATINS which are rich in histamine
Name some acidic and basic proteins that are good buffers
ACIDIC-
Aspartic Acid
Glutamic Acid
BASIC-
Lysine
Arginine
Histamine
Apart from in the oral cavity, where else is buffering important?
In blood and cells
What pH is blood maintained at and by what buffers?
7.4
By bicarbonate and protein buffers
Why do cells require the pH of organelles and the cytoplasm to be closely controlled?
To preserve enzyme and membrane function
What will happen if the pH of cells and blood is not maintained?
Cellular functions cannot occur, leading to cell death
