pH and Buffers Flashcards
What is pH?
measure of Hydrogen ion concentration
Tells you (acidity or alkalinity of a solution).
- log [H+]
What does acidity of sol depend on?
only free hydrogen ions in plasma - Not those still bound to anions.
Why is blood pH regulation important?
Blood is in contact with nearly every body cell.
What is normal blood pH range?
7.35 to 7.45
What type of scale is pH scale?
Log scale – going up each pH unit is 10x
What is living range?
pH 7.0-7.8
What happens if pH is below living range?
Acidosis
What happens if pH is above living range?
alkalosis
Where do acids in the body come from?
in foods
Meat eater – blood Ph below 7.4 due to aa in protein + S in disulphide bonds gen acids
What are 3 things acids in body gen by?
- breakdown of proteins
- incomplete oxidation of fats or glucose,complete ox – get CO2 (which is acidic) and H2O but get more acid from incomplete
- loading and transport of carbon dioxide in the blood.
What are 3 e.g.s of everyday things with low pH?
Lemon juice + gastric juice (pH2)
Coffee (ph5)
What are 3 e.g.s of everyday things with fairly wide pH range/neutral pH?
Urine (pH5-8)
Saliva, milk (pH6.5)
What are 3 e.g.s of everyday things with neutral pH?
Distilled water (pH7) Human blood + semen (pH7.4)
What are e.g.s of everyday things with high pH?
Household bleach (pH 12) Oven cleaner (pH 13.5)
What are 3 things that regulate acid –base balance in body?
- the lungs – when [H+] increases H2CO3 can dissociate into H+ + HCO3- but lungs shift equ other way: H2CO3 CO2 + H2O – breathe out CO2
- the kidneys
- systems in the blood known as chemical buffers
What is a buffer?
resist abrupt and large swings in the pH of body fluids – can’t work over large range but a certain range
How do buffers work?
releasing H+ (acting as acids) when the pH begins to rise – OH- increasing react with H+ - H2O so pH doesn’t change
binding H+ (acting as bases) when the pH drops – high [H+] – picks up free H+ - no change in pH
What is an acid?
proton (H+ ) donor
What is a base?
proton acceptor
What are weak acids?
dissociate incompletely but depending on pH of sol its in – dissociate diff amounts. e.g. in H2O – not that much but if adding base e.g. in titration – eventually dissociates completely
What is % of water in an infant and why?
73% or more (low body fat and low bone mass)
What is % of water in old age and why?
45%
What is % of water in a healthy young man?
60%
What is % of water in a healthy young man?
50%.
What is conc of pure water?
55.6M solution
What is dissociation of H2O like?
dissociates to a very small extent – most water in 1l is water but tiny amount dissociates (behaves like weak acid) to H+ + OH-
What is Keq (equ constant)?
1.8 x 10-14
What is ionic product of H2O?
in any aq sol, [H+] x [OH-] = 10-14 M2
Why is pH of water 7 at neutrality?
[H+] =[OH-], conc of both = 10-7M
pH = -log[10-7] = 7
When [H+] = 10 -2 M, what is pH?
2
when [H+] is 10-2, what is [OH-]?
10-12
when [H+] is 10-4, what is [OH-]?
10-10
if 10 to power 1 = 10 what is log?
if 10 to power 2 = 100, what is log?
of 10 to power 3 = 1000. what is log?
1
2
3
If blood pH is 7.4 what is [H+]?
[H+] = 3.98 x 10-8 M
What is e.g. of strong acid dissociation?
HCl→ H+ + Cl-
What is e.g. of weak acid dissocation?
H2CO3 →H+ + HCO3-
Which is more effective proton acceptor - strong/weak base?
Strong
Which is more effective proton donor - strong/weak acid?
Strong
If ur titrating ethanoic acid with NaOH, how many protons can ethanoic acid lose?
2 by gradually adding NaOH to get CH3COO-
At beginning of titration, how much CH3COOH is there and how much does it red over progress of titration?
100% at beginning
Around halfway – half of mol are CH3COOH (weak acid) and other half are CH3COO- (conjugate base) – at midpoint of buffering region
End - no acid left, only base present
What does flat line on titration curve mean and why?
buffering is happening bc the point where there’s equal conc of CH3COOH and CH3COO- - if base added then CH3COOH dissociates to CH3COOH and if acid added, CH3COO- accept H+ –> CH3COOH
What does sharp increase in pH on titration curve mean?
not buffering – no acid left, only base present
What is pKa?
pH at which the acid is half dissociated - there’s equal amounts of base and acid
–log Ka
What does pKa tell u?
which range of Ph each weak acid will buffer best. can act as a buffer as there’s half acid and half base - equal amounts of undissociated acid and its conjugate base
What is equ point?
pH when added equal vol/molecules of base as the acid
The lower the pka… and why?
the stronger the acid – as it can dissociate over greater Ph range – its acting more like a a strong acid
What is Henderson-Hasselbalch equation?
pH = pKa + log [A-]/[HA] A- = conjugate base HA = acid
Why is HH equation important?
A convenient way to relate the pH of a solution, the pKa of a weak acid and the relative amounts of dissociated and non-dissociated (unprotonated and protonated) forms of the acid.
What is H2CO3 in blood proportional to?
partial pressure of CO2 so conc of H2CO3 relates to amount of CO2 in blood
What does it mean for a patient with low pH?
either HCO3- low and normal H2CO3 so CO2 normal = metabolic acidosis – give insulin
What does it mean for a patient with high pH?
HCO3- normal and H2CO3 high so CO2 high = respiratory acidosis - ventilate
How does pKa relate to buffering?
Buffers are mixtures of weak acids and their conjugate bases
Buffering is the ability of a solution to resist a change in pH when acid or alkali is added
At the pKa there are equal amount of dissociated and non dissociated forms of the acid (conjugate base and acid)
At the pKa buffering is best
Why does pH not change with buffer?
If H+ are added they can be picked up by the conjugate base
If OH- are added the acid can donate a proton and form H2O
In this way, the pH does not change
What are phys important buffers blood, saliva and other body fluids?
H2CO3→ HCO3-
H2PO4- → HPO42-
Protein → protein-
protein+ → protein
Which is best buffer in blood?
Protein
What is pKa of H2CO3→HCO3-?
6.1
What is pKa of H2PO4- → HPO42-?
6.8
What is met alkanosis?
high bicarbonate
What is resp acidosis?
CO2 low
Why can gly never be in form 2HN-CH2-COOH?
the Ph that allows diss of NH3 is so high the COOH would have already diss
How many Hs can be diss from gly and from where?
2 - one from COOH and NH2 group as COOH is stronger acid (pKa = 2.34) than NH3+ (9.66) so will dissociate at low pH.
Why is there no buffering by the zwitterion?
buffer needs weak acid and its own conjugate base but NH3+ and COO- are not conjugate to each other
Which bit of aa involved in buffering and why?
R groups as alpha carboxyl and alpha amino groups are not good physiological buffers
(they are involved in the peptide bond anyway)
Why is cys not good buffer?
H can be lost from SH but SH in disulphide bond anyway so u can’t remove it
Which protein is buffer in blood?
Hb
Why is Hb a good buffer?
presence of a large number of histidine residues – 38 per chain . When its put in protein/aa – pKa of side chain in His changes depending on charges of groups around it so it goes up and its nearer to blood pH.The pKa of histidine in Hb is different from that of free His.
Neighbouring groups affect the pKa
What is range weak acid/aa will buffer?
narrow range about pKa maybe 1 unit either side
What is pKa of His R group?
6
What is pKa of his in oxyHb?
6.8
What is pKa of his in deoxyHb and why is it diff to oxyHb?
7.8 – goes up due to conformational change as Hb drops O2 so His in Hb exposed to diff env
Below pKa, which is more present: acid/conjugate base?
Acid
Why is it diff for drugs to diffuse across lipid mem?
Charged groups are diff to diffuse through mem – lots of drugs are weak acids so to get them across mem they need to be in uncharged state
Why doesn’t aspirin diffuse well at all pH values?
bc it will be charged at some PHs and not others
What is a low blood pH certainly?
acidaemia
What happens in a solution with a pH equal to the pKa of a buffer, what happens to any applied change in pH?
occurs at a slower rate
What is most important buffer in blood?
carbonic acid
