L4: Maintenance of pH Flashcards
What is the ‘solubility of water’ (Kw)?
- In aqueous solutions the product of H+ and OH- concentrations is constant
- This equates to 10^-14 mole squared metres squared
What happens in an acidic solution to the Kw?
The H+ concentration increases and the OH- decreases
What happens in a basic solution to Kw?
The H+ concentration decreases and the OH- increases
Define pH
What would the H+ concentration be in a neutral solution?
10^-7
What is important to remember about proton and hydroxide concentrations?
The sum of the concentrations is not constant, but the product of them is
What is the range of pH values?
0-14
What is the H+ concentration in water?
- Water has an Mr of 18 - this is the molecular weight
- 1000g of water in a litre
- 1000/18 = 55 moles per litre = concentration of water
What pH range are most physiological processes within cells between?
5-7.6
How is pH measured?
- Litmus paper (most basic form)
- Universal indicator (range of indicators that change colour)
- Natural biological indicator is red cabbage
- pH metre (most sophisticated form)
How does a pH metre work?
- The reference electrode (usually a platinum wire immersed in silver or silver chloride
- This is immersed in potassium chloride solution
- Sensing membrane within the glass membrane
- Different amount of protons associated with the glass membrane depending on pH of the solution
- This determines the potential difference between the two electrodes
How is the voltage (potential difference) of the pH metre converted into a pH value?
- Have to calibrate the electrode
- Have to use 3 precision pH metres - one acidic, one neutral and one basic
- Displays the pH reading after
When can a pH metre not produce a reading? Why?
- At extreme pH readings below 0 and above 14
- Process becomes non-linear
- There is normally a linear charge of protons associated with a glass membrane - restrict reading to within the normal 0-14 range
Define a strong and weak acid
- Strong acids will fully dissociate in an aqueous solution e.g. HCl
- Weak acids will partially dissociate in an aqueous solution e.g. carboxylic acids
What is the conjugate acid and what is the conjugate base in this equilibrium equation?
- HA is the conjugate acid
- A- is the conjugate base
What is the Henderson-Hasselbach equation?
Describe the graph for lactate
- Start point at pH 0
- Initially the addition of the base means a quick rise in pH from 0-2
- Linear range in pH with very small change in the mid-section
- Lactate acts as a pH buffer in the middle - keeps the pH of the solution constant
- This is the same for the ammonia solution as well (but at different pHs)
What is a good buffer around pH 7, which is more physiological?
Phosphate buffer
What does the pK value (where 50% of the base is added) tell you?
Half of the weak acid or base is dissociated and half is not dissociated
What does a high and low pH for an acid tell you from the graph?
- More dissociated weak acid for a high pH
- Less dissociated weak acid for a low pH
When is the charged or uncharged version of the lactate shown on the graph?
- The protonated version is uncharged
- Above the pK value (50%) for lactate is the charged version
- Below the pK value (50%) for lactate is the uncharged version
When is the charged or uncharged version of the ammonia shown on the graph?
- The protonated version is charged
- Above the pK value (50%) for ammonia is the uncharged version
- Below the pK value (50%) for ammonia is the charged version
What is an example of a biological molecule which can be both basic and acidic?
- Amino acids
- Have both an amine and carboxylic acid group
What has to happen to amino acids in order to demonstrate pH dependency on a graph?
Have to add 2 base equivalents because there are 2 sites which need to be protonated or deprotonated