pH and Buffering

Question 1

Define pH

Answer 1

pH = -log [H+]
It defines how acidic/alkaline a solution is

Acidity depends on ONLY free H+ ions- NOT on those bound to anions

Question 2

What is the living range of pH in the blood?

Answer 2

7.0-8.0

however ideally its between 7.35-7.45

Question 3

Why is regulating the blood pH important?

Answer 3

It is important due to proteins- the [H+] and [OH-] that can affect the bonds that stabilise the protein ie. VdW, ionic, electronic interactions

Question 4

What is the definition of acidosis ?

Answer 4

excessively acidic condition

Question 5

What is the definition of alkalosis ?

Answer 5

excessively alkali condition

Question 6

Where does acid come from in the body?

Answer 6

breakdown of protein
Some acids enter in foods eg. lemon and vinegar
incomplete oxidation fats/glucose
Loading and transport of CO2 in the blood- carbonic acid

Question 7

What statements can be made about concentrations at neutral pH and room temperature

Answer 7

[H+] = [OH-] = 10^7

Question 8

How is acid-base equilibria regulated in the body?

Answer 8

The lungs-expelling CO2
Kidneys- regulate blood pH
Systems in the blood known as chemical buffers

Question 9

What are Chemical Buffers?

Answer 9

Buffers resist abrupt and large swings in the pH of body fluids- by releasing H+ ions when pH is too high and by binding to H+ when pH is too low. Both counteract the deviation.

A buffer is a combination of the acid and the conjugate base

Question 10

a strong acid-

Answer 10

proton donator that fully dissociates in solution

Question 11

a strong base -

Answer 11

proton acceptor that fully dissociates in solution

Question 12

a weak acid-

Answer 12

proton donator that slightly dissociates in solution- it is able to manipulate it so it dissociates fully by increasing pH or not at all by decreasing pH

Question 13

a weak base-

Answer 13

proton acceptor that slightly dissociates - it is able to manipulate it so it dissociates fully by increasing pH or not at all by decreasing pH

Question 14

Ionisation of water

Answer 14

the idea that water is a very very weak acid- hence will dissociate slightly.
Pure water is 55.6 M
[H+]{OH] = 10^-14

Question 15

Why does urine have a greater range of pH than bloody (5-8)

Answer 15

Kidney regulates the blood pH therefore what the urine composition is like reflects what the kidney has done

Question 16

The pKa of ethanoic acid is 4.75, why isn’t this the ideal to be used as a buffer

Answer 16

As blood pH is 7.4 (approx) hence would mean that using ethanoic acid would result in the blood pH falling below the lower range of pH.

Question 17

List the dissociations of phosphoric acid

Answer 17

H3PO4—> H2PO4 -
H2PO4- —-> HPO4 2-
HPO4 2- ——> PO4 3-

Question 18

Which 2 acids would be most commonly used in the body

Answer 18

H2PO4 - and HPO4 2-

Question 19

What is the definition of pKa

Answer 19

pKa= -log [Ka]

Ka = [H+][OH-] / [HA]

At the half neutralisation point pH= pKa

The Lower the pKa the higher the Ka hence the stronger the acid

Question 20

What is Henderson- Hasselbalch equation?

Answer 20

pH= pKa + log [A-] / [HA]

Question 21

What are physiologically important buffers?

Answer 21

H2CO3 —> HCO3-
H2 PO4 2- —-> HPO4 -
Protein —-> Protein -
Protein + —–> Protein

Question 22

What determines whether a protein would be a good buffer?

Answer 22

The R groups of the individual amino acids?

eg. Histidine

Question 23

Describe the example of carbonic acid

Answer 23

H2CO3 ---> HCO3-  pKa=6.1
[H2CO3] is proportional to the pCO2
 the HH equation helps you distinguish the cause
eg. with high pH
low [A-] means diabetic causes
high [acid] means respiratory causes

Question 24

Why is haemoglobin useful in terms of acid and base

Answer 24

it is a good buffer due to large number histidine residues

However pKa of histidine in Hb is different to that of free His due to the neighbouring residues affecting the pKa

Question 25

Why is the pKa of oxyhaemoglobin and deoxyhaemoglobin different

Answer 25

Due to the conformational change in the His positions- which changes the structure

Question 26

Which of haemoglobin is a better buffer?

Answer 26

deoxyhaemoglobin-
pH = 7.35 and pKa(deoxyhaemoglobin)=7.8

Hb is more basic therefore accepts H+ ions
BACKWARD reaction favoured.
Decreases the concentration of [H+] which increases pH so it reaches ph=7.8
Whilst not ideal it is still within the living pH range

Question 27

Why is aspirin able to diffuse more easily at pH 2 rather than pH 7

Answer 27

Asparin is a weak acid (pKa =4-5)
At pH 2: asparin is more basic- accepts the H+ ions
Few charged ions so it can more easily pass through phospholipid bilaye
At pH 8- aspirin is more acidic so releases H+ ions
More charged ions so it is more difficult to pass through the phospholipid bilayer