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pH + buffers Flashcards

1
Q

why is water essential to understanding pH

A
  • 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
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2
Q

what is water, what does this mean

A

polar solvent

  • dissolved most charge molecules
  • dissolves most salts (hydrates + stabilises cations and anions by weakening electrostatic interactions)
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3
Q

what are hydration spheres

A

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
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4
Q

what may be formed around certain gas molecules, what are these

A

CLATHRATE CAGES

  • water molecules bond under certain conditions
  • form complex networks of molecules forming cage like structures (these encapsulate gas)
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5
Q

what does the organisation of water do to the charged moiety

A

reduce its electronic potential

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6
Q

how is solvent property of water why we get its saturation point ie with salt and where is this used in biochemistry

A

if there isnt enough water in the salt it will stop dissolving

used in process of “salting out” to get proteins out of solution

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7
Q

which type of reactions to water molecules undergo to yield their constituent ions and what are these ions

A

reversible ionisation

H+ and OH-

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8
Q

which 2 factors give the degree of ionisation of water at equilibrium

A

1) Keq (equilibrium constant of water)

2) ion product of water (1x10^-14 at 25 degrees)

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9
Q

what is the equation for keq

A

keq = [H+][OH-] / [H2O]

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10
Q

how does the equation for keq form the basis for sorensens pH scale

A
  • 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

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11
Q

explain the equilibrium of NaCl

A
  • 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
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12
Q

explain what would be observed if we put HAp in water

A
  • nothing visible to eye

- HAP dissolves until forward and reverse reactions are equal

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13
Q

what is the formula for and ions of HAp

A
  • Ca10(PO4)6OH2
  • 10Ca^2+
  • 6PO4^3-
  • 2OH-
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14
Q

what is the equilibrium of HAp

A
  • eqm so far to left that it massively favours preservation of the HAp crystal
  • forward reaction much smaller than reverse
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15
Q

what happens to strong acids (HCl, H2SO4) and strong bases (NaOH) in aqueous solution

A
  • completely ionised (fully dissociate)

- eqm in favour of products

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16
Q

why dont strong acids exist natively in nature or biological systems

A

have an affinity for water

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17
Q

what happens to weak acids and bases in terms of eqm

A
  • only partly dissociate
  • eqm mixture of undisassociated and disassociated species
  • at some point forward and reverse reactions reach eqm
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18
Q

what happens to acetic acid (weak acid) in equilibrium

CH3COOH ->

A
  • 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
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19
Q

what is the equilibrium constant? explain it for the dissociation of acetic acid

A

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)

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20
Q

give the Ka equation for acetic acid

A

Ka = (product of concentration of acetate ion) / (conc of undissociated acetic acid)

Ka = [H+][CH3COO-] / [CH3COOH]

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21
Q

what is the value of Ka for acetic acid at 25 degreesC and what does this mean

A
  1. 76x10^-5
    - small
    - means it dissociates slightly so we can define it as a weak acid
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22
Q

what happens to hydrochloric acid (strong acid) in equilibrium

HCl ->

A
  • preference for forward reaction (so more ions formed)

- dissociates to a very large extent

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23
Q

what is the KA equation for HCl and what is the value of Ka, what does this mean

A

Ka = [Cl-][H+] / [HCl]

= 1.3x10^6

  • large
  • strong acid
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24
Q

what is Ka converted to, how and why

A
  • 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
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25
Q

how is pH calculated

A

-log[H+]

26
Q

what equation is useful to help estimate the pH of a buffer solution

A

henderson-hasselbach

pH = pKa + log ([conjugate base / acid])

27
Q

what is a buffer

A
  • 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
28
Q

what are buffers commonly in lab + food industry

A

weak acid / base and their conjugate acids / bases

29
Q

what happens to the equilibrium mixture if we add

a) acid (H+)
b) base (OH-)
c) what does this mean in this system

A

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

30
Q

how does titration curve of acetic acid with hydroxide ions added in demonstrate buffering

A
  • 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
31
Q

what is the general rule for buffering regions of buffers

SO acetate (pKa = 4.75) is a useful buffer in which region

A
  • will buffer to one pH unit either side of their pKa

- 3.8 to 5.8

32
Q

how does titration curve of phosphoric acid with hydroxide ions added in demonstrate buffering

A
  • 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
33
Q

how do buffers work

A
  • take up added H+

- neutralise added OH-

34
Q

what is the buffer system when conc of conjugate acid = conc of acid

A
  • 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
35
Q

what is the buffer system when conc of conjugate base = higher than conc of acid

A
  • conjugate base buffer component bigger than acid

- SO more effective at buffering adding acid

36
Q

what is the buffer system when conc of conjugate base = lower than conc of acid

A
  • acid component bugger than conjugate base

- more effective at buffering added base

37
Q

what does the inherent pH of buffer solutions depend on and which equation describes this

A
  • 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
38
Q

what intra + extra cellular pH do animals generally try maintain

A

close to 7.4 (deviation of 1 pH unit could be catastrophic)

39
Q

why is acidosis a bigger threat than alkolysis

A
  • we produce various acids through various metabolic processes (ie breathing, running)
40
Q

how do we protect ourselves against acidosis

A
  • lots of phosphates intracellularly (ie ATP -> tri+di phosphates) and proteins containing histidine (only amino acid with pKa value of physiological pH)
41
Q

where does buffering occur in proteins

A
  • 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
42
Q

what happens when an acid and conjugate base pair are buffered

A
  • conjugate base takes up H+ forming an acid

- acid gives up a H+ to form water + conjugate base

43
Q

what system is extracellular pH buffered by and how does it work

A

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)
44
Q

what is the equation for CO2(d) equilibrium

A

CO2(d) + H2O ->

45
Q

what happens in bicarbonate buffer system when it is acid challenged (H+ increases)

A
  • eqm driven to left
  • conc of CO2(d) increases
  • the increase causes more CO2 production and excess CO2 is lost through lungs to atmosphere
46
Q

what happens in bicarbonate buffer system when it is challenged by a base (OH- increases)

A
  • eqm driven to right as H+ used to neutralise OH-

- depletes CO2 in blood which is compensated for by reduced lung ventilation rate

47
Q

define Le Chateliers Principle

A

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

48
Q

why is salivary buffering vital

A

to prevent dissolution of tooth surface (why xerostomia is a problem as cannot get substituted HAp)

49
Q

what is enamel composed of and where is this found

A
  • substituted HAp

- in eqm with its constituent ions in saliva

50
Q

what does le chateliers principle predict will happen when we remove hydroxide and phosphate from right of equation (by adding H+)

A

drive more HAp into solutions (dissolve teeth)

51
Q

what would happen if we introduce acid into the mouth (ie orange juice)

A
  • 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
52
Q

why is coke not as bad as OJ for teeth

A
  • coke is a common ion in phosphate

- so doesnt drive eqm so far right

53
Q

what are the 3 buffer systems in saliva

A

1) bicarbontate (most imp in stimulated)
2) phosphate
3) proteins

54
Q

what happens in bicarbonated buffer system in saliva if H+ is added

A
  • reaction driven to left to produce CO2 + H2O
  • carbonic anydrase enzyme catalyses uptake of proton to bicarbonate + eliminates protons increasing efficiency of system
55
Q

how is carbonic anhydrase retained in the pellicle

A
  • carbonic anhydrase + bicarbonate sequestered from saliva into pellicle
  • eliminates H+ produced by cariogenic bacteria
56
Q

what are the 2 main offenders which challenge enamel structure

A

1) plaque acid

2) consumption of acidic food/drink (lead to dental erosion)

57
Q

how is fermentable and acid food/drink removed quickly

A

mechanical stimulation associated w mastication + chemical stimulation of taste buds helps inc flow rate increasing washing action

58
Q

what increases with flow rate

A

1) pH

2) bicarbonate concentration (carbonic acid stays constant)

59
Q

how is salivary buffering adaptive

A
  • buffering capacity increases in response to potential increases in plaque acid conc
60
Q

which curves demonstrate the importance of salivary bicarbonate buffer

A

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
61
Q

what do stephen curves demonstrate WITH saliva present

A
  • saliva pH drops = 7.4 -> 6.5

- still above critical pH (5.5)

62
Q

what do stephen curves demonstrate WITHOUT` saliva present

A
  • pH below critical pH
  • enamel at risk
  • dissolution can occur

IOE (22 decks)

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