pH + buffers Flashcards
why is water essential to understanding pH
- defines pH
- 70% mass most living creatures
- all biological reactions occur in aqueous medium
- cell structure + function is adapted to chemical + physical properties of it + its ionisation products
what is water, what does this mean
polar solvent
- dissolved most charge molecules
- dissolves most salts (hydrates + stabilises cations and anions by weakening electrostatic interactions)
what are hydration spheres
molecules of h2o form a sphere around ions
- the delta +ve on water (hydrogens) associated with -vely charged ions
- delta -ve on water (oxygen) associated with +vely charged ions
what may be formed around certain gas molecules, what are these
CLATHRATE CAGES
- water molecules bond under certain conditions
- form complex networks of molecules forming cage like structures (these encapsulate gas)
what does the organisation of water do to the charged moiety
reduce its electronic potential
how is solvent property of water why we get its saturation point ie with salt and where is this used in biochemistry
if there isnt enough water in the salt it will stop dissolving
used in process of “salting out” to get proteins out of solution
which type of reactions to water molecules undergo to yield their constituent ions and what are these ions
reversible ionisation
H+ and OH-
which 2 factors give the degree of ionisation of water at equilibrium
1) Keq (equilibrium constant of water)
2) ion product of water (1x10^-14 at 25 degrees)
what is the equation for keq
keq = [H+][OH-] / [H2O]
how does the equation for keq form the basis for sorensens pH scale
- when conc of H+ and OH- is equal = neutral pH
- when conc of H+ and OH- is constant an increase in one is compensated by a decrease in the other
explain the equilibrium of NaCl
- forward reaction (K1) occurs more readily than reverse reaction
- eqm so far to the right that its in favour of the dissolution so ion solution (Na+ + Cl-)
- tiny amount of NaCl always remains as its in eqm
explain what would be observed if we put HAp in water
- nothing visible to eye
- HAP dissolves until forward and reverse reactions are equal
what is the formula for and ions of HAp
- Ca10(PO4)6OH2
- 10Ca^2+
- 6PO4^3-
- 2OH-
what is the equilibrium of HAp
- eqm so far to left that it massively favours preservation of the HAp crystal
- forward reaction much smaller than reverse
what happens to strong acids (HCl, H2SO4) and strong bases (NaOH) in aqueous solution
- completely ionised (fully dissociate)
- eqm in favour of products
why dont strong acids exist natively in nature or biological systems
have an affinity for water
what happens to weak acids and bases in terms of eqm
- only partly dissociate
- eqm mixture of undisassociated and disassociated species
- at some point forward and reverse reactions reach eqm
what happens to acetic acid (weak acid) in equilibrium
CH3COOH ->
- only partially ionises (dissociates in water)
- at some point reaction reaches eqm (forward+reverse equal)
- initially carboxylic group on left is protonated (still has a hydrogen) but if its made to be aqueous the acetate ion is formed by losing this H
what is the equilibrium constant? explain it for the dissociation of acetic acid
Ka (defined in same way as KEQ of water) and tells us the degree of dissociation (strength of the acid/base)
Ka = (product of concentration of acetate ion + hydrogen ion) / (conc of undissociated acetic acid)
give the Ka equation for acetic acid
Ka = (product of concentration of acetate ion) / (conc of undissociated acetic acid)
Ka = [H+][CH3COO-] / [CH3COOH]
what is the value of Ka for acetic acid at 25 degreesC and what does this mean
- 76x10^-5
- small
- means it dissociates slightly so we can define it as a weak acid
what happens to hydrochloric acid (strong acid) in equilibrium
HCl ->
- preference for forward reaction (so more ions formed)
- dissociates to a very large extent
what is the KA equation for HCl and what is the value of Ka, what does this mean
Ka = [Cl-][H+] / [HCl]
= 1.3x10^6
- large
- strong acid
what is Ka converted to, how and why
- pKa
- pKa = -log Ka
- makes large no easier to work with
- it flips the r/ship so pKa value is small for strong acids and large for weaker
how is pH calculated
-log[H+]
what equation is useful to help estimate the pH of a buffer solution
henderson-hasselbach
pH = pKa + log ([conjugate base / acid])
what is a buffer
- molecules that can resist pH changes that would otherwise occur by additions of acid / base
- DO NOT completely stop pH changes BUT DO reduce magnitude of the change
what are buffers commonly in lab + food industry
weak acid / base and their conjugate acids / bases
what happens to the equilibrium mixture if we add
a) acid (H+)
b) base (OH-)
c) what does this mean in this system
a) acid associates to acetate ion
b) base associates to acetic acid forming H2O with H on carboxyl group
c) no pH change
H+ + OH- buffered to equal extents
how does titration curve of acetic acid with hydroxide ions added in demonstrate buffering
- a buffering region (acetic acid only has 1 H+ to give up / 1 spot to receive a H so only one region)
- pH rises a little initially
- at midpoint pH = pKa ([CH3COOH]=[CH3COO-])
- instead of a massive jump = extended inflection points where no of OH- ions doesnt greatly affect pH (NOT a plateau)
- at endpoint pH rises rapidly
what is the general rule for buffering regions of buffers
SO acetate (pKa = 4.75) is a useful buffer in which region
- will buffer to one pH unit either side of their pKa
- 3.8 to 5.8
how does titration curve of phosphoric acid with hydroxide ions added in demonstrate buffering
- 3 buffering regions (has 3 protons to lose) each with an endpoint defined by a sharp increase
- 3 plateau regions where OH- addition causes only small pH inc
how do buffers work
- take up added H+
- neutralise added OH-
what is the buffer system when conc of conjugate acid = conc of acid
- high concs of buffer components allow buffer to handle large additions of acid / base
- buffer components are equal conc so added acid / base = buffered well
what is the buffer system when conc of conjugate base = higher than conc of acid
- conjugate base buffer component bigger than acid
- SO more effective at buffering adding acid
what is the buffer system when conc of conjugate base = lower than conc of acid
- acid component bugger than conjugate base
- more effective at buffering added base
what does the inherent pH of buffer solutions depend on and which equation describes this
- ratio of acid component to conjugate base component
- dissociation characteristics of the buffer
- Henderson hasselbach equation
1) pH = pH of buffer
2) pKa = defining dissociation characteristics
3) log [A-]/[HA] = defines ratio of conjugate base [A-] to acid
what intra + extra cellular pH do animals generally try maintain
close to 7.4 (deviation of 1 pH unit could be catastrophic)
why is acidosis a bigger threat than alkolysis
- we produce various acids through various metabolic processes (ie breathing, running)
how do we protect ourselves against acidosis
- lots of phosphates intracellularly (ie ATP -> tri+di phosphates) and proteins containing histidine (only amino acid with pKa value of physiological pH)
where does buffering occur in proteins
- imidizol ring at position 1 (N) OR 3 (NH) because these electrons are delocalised and it can exist in a resonant form
- pH largely buffered by phosphate and histidine side chains in the proteins
what happens when an acid and conjugate base pair are buffered
- conjugate base takes up H+ forming an acid
- acid gives up a H+ to form water + conjugate base
what system is extracellular pH buffered by and how does it work
bicarbonate buffer system
- CO2 generated in tissues as byproduct
- CO2 dissolves in plasma
- dissolved CO2 = CO2(d)
- CO2(d) is in eqm with carbonic acid which is in eqm with bicarbonate (pKa between these = 6.4)
what is the equation for CO2(d) equilibrium
CO2(d) + H2O ->
what happens in bicarbonate buffer system when it is acid challenged (H+ increases)
- eqm driven to left
- conc of CO2(d) increases
- the increase causes more CO2 production and excess CO2 is lost through lungs to atmosphere
what happens in bicarbonate buffer system when it is challenged by a base (OH- increases)
- eqm driven to right as H+ used to neutralise OH-
- depletes CO2 in blood which is compensated for by reduced lung ventilation rate
define Le Chateliers Principle
when an external constraint is placed upon a system in eqm the system will move in such a way as to oppose the external constraint
why is salivary buffering vital
to prevent dissolution of tooth surface (why xerostomia is a problem as cannot get substituted HAp)
what is enamel composed of and where is this found
- substituted HAp
- in eqm with its constituent ions in saliva
what does le chateliers principle predict will happen when we remove hydroxide and phosphate from right of equation (by adding H+)
drive more HAp into solutions (dissolve teeth)
what would happen if we introduce acid into the mouth (ie orange juice)
- first ion to buffer = OH- (becomes water)
- more acid means phosphate will try to buffer acid
- eqm now in favour of rhs as H+ removing hydroxide AND phosphate from rhs = external constrict
- the system wants to oppose it so drives more HAp from its constituent ions + into solution
why is coke not as bad as OJ for teeth
- coke is a common ion in phosphate
- so doesnt drive eqm so far right
what are the 3 buffer systems in saliva
1) bicarbontate (most imp in stimulated)
2) phosphate
3) proteins
what happens in bicarbonated buffer system in saliva if H+ is added
- reaction driven to left to produce CO2 + H2O
- carbonic anydrase enzyme catalyses uptake of proton to bicarbonate + eliminates protons increasing efficiency of system
how is carbonic anhydrase retained in the pellicle
- carbonic anhydrase + bicarbonate sequestered from saliva into pellicle
- eliminates H+ produced by cariogenic bacteria
what are the 2 main offenders which challenge enamel structure
1) plaque acid
2) consumption of acidic food/drink (lead to dental erosion)
how is fermentable and acid food/drink removed quickly
mechanical stimulation associated w mastication + chemical stimulation of taste buds helps inc flow rate increasing washing action
what increases with flow rate
1) pH
2) bicarbonate concentration (carbonic acid stays constant)
how is salivary buffering adaptive
- buffering capacity increases in response to potential increases in plaque acid conc
which curves demonstrate the importance of salivary bicarbonate buffer
stephen curves
- quantative method for evalutating chemical + physical agents that modify production of acids in bacterial plaque
- measures plaque pH as function of time following consumption of carbs with + without saliva being present
what do stephen curves demonstrate WITH saliva present
- saliva pH drops = 7.4 -> 6.5
- still above critical pH (5.5)
what do stephen curves demonstrate WITHOUT` saliva present
- pH below critical pH
- enamel at risk
- dissolution can occur
