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pH + buffers Flashcards

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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
how is pH calculated
-log[H+]
26
what equation is useful to help estimate the pH of a buffer solution
henderson-hasselbach pH = pKa + log ([conjugate base / acid])
27
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
28
what are buffers commonly in lab + food industry
weak acid / base and their conjugate acids / bases
29
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
30
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
31
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
32
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
33
how do buffers work
- take up added H+ | - neutralise added OH-
34
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
35
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
36
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
37
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
38
what intra + extra cellular pH do animals generally try maintain
close to 7.4 (deviation of 1 pH unit could be catastrophic)
39
why is acidosis a bigger threat than alkolysis
- we produce various acids through various metabolic processes (ie breathing, running)
40
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)
41
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
42
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
43
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)
44
what is the equation for CO2(d) equilibrium
CO2(d) + H2O ->
45
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
46
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
47
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
48
why is salivary buffering vital
to prevent dissolution of tooth surface (why xerostomia is a problem as cannot get substituted HAp)
49
what is enamel composed of and where is this found
- substituted HAp | - in eqm with its constituent ions in saliva
50
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)
51
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
52
why is coke not as bad as OJ for teeth
- coke is a common ion in phosphate | - so doesnt drive eqm so far right
53
what are the 3 buffer systems in saliva
1) bicarbontate (most imp in stimulated) 2) phosphate 3) proteins
54
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
55
how is carbonic anhydrase retained in the pellicle
- carbonic anhydrase + bicarbonate sequestered from saliva into pellicle - eliminates H+ produced by cariogenic bacteria
56
what are the 2 main offenders which challenge enamel structure
1) plaque acid | 2) consumption of acidic food/drink (lead to dental erosion)
57
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
58
what increases with flow rate
1) pH | 2) bicarbonate concentration (carbonic acid stays constant)
59
how is salivary buffering adaptive
- buffering capacity increases in response to potential increases in plaque acid conc
60
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
61
what do stephen curves demonstrate WITH saliva present
- saliva pH drops = 7.4 -> 6.5 | - still above critical pH (5.5)
62
what do stephen curves demonstrate WITHOUT` saliva present
- pH below critical pH - enamel at risk - dissolution can occur
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