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Biochemistry: Cellular Composition > Week 1 practical - pH and buffers > Flashcards

Week 1 practical - pH and buffers Flashcards

1
Q

acidity

A

measure of H+ concentration

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2
Q

pH definition

A

pH = -log10[H+]

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3
Q

[H+]

A

[H+] = 10^-pH

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4
Q

[H+] of pure water

A

1.0x10^-7

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5
Q

why do we use pH to measure acidity as opposed to H+ concentration?

A

H+ is small and varies widely

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6
Q

acids

A

proton donators

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7
Q

bases

A

proton acceptors

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8
Q

what type of acids are carboxylic acids?

A

weak

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9
Q

what do pipettes give their maximum volume in?

A

micro litres

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10
Q

what do we use to set the volume on a pipette?

A

thumbwheel

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11
Q

how do we know which tip to use on a pipette?

A

match the tip colour with the colour of the push button

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12
Q

what would we set the pipette to to measure 2 micro litres?

A

P20

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13
Q

how many microlitres in 1ml?

A

1000

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14
Q

what is a P20 pipette used to measure?

A

2-20 microlitres

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15
Q

what is a P200 pipette used to measure?

A

20-200 microlitres

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16
Q

what is a P1000 pipette used to measure?

A

between 200-1000 microlitres

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17
Q

what is a P5000 pipette used to measure?

A

1000-5000 microlitres

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18
Q

on which pipette do we need to twist the tip to tighten?

A

P5000

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19
Q

extra step when using P5000 pipette

A

twist tip to tighten

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20
Q

what must we do when not using the glass electrode bulb?

A

keep it moist

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21
Q

how many microlitres in 1ml?

A

1000

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22
Q

steps to calibrating a pH meter

A

-place electrode in the pH 7 standard buffer first
-completely immerse glass electrode bulb for an accurate reading
-agitate probe for faster reading
-reading to within 0.02 units of the pH 7 using “buffer offset”
-rinse then place electrode in the pH 4 standard buffer - adjust reading to within 0.02 units of the pH of the chose buffer using “slope %”
-probe back in pH 7 and shake gently
= meter is calibrated
=reading within 0.05 pH units of 7 (repeat if not)
+ remember to replace the calibrated electrode in the storage solution (bulb dries out = damaged)

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23
Q

steps to taking a measurement with a pH meter

A

-rinse and blot electrode before using (to avid contamination of measured solution)
-completely immerse glass electrode bulb and shake
-stabilise to 0.02 units before recording

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24
Q

what do pipettes give their maximum volume in?

A

micro litres

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25
Q

how many pH units should a reading be within once it’s been calibrated?

A

0.05

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26
Q

on which pipette do we need to twist the tip to tighten?

A

P5000

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27
Q

what do we do before using the electrode on a pH meter and why?

A

rinse with distilled water to avoid contamination

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28
Q

what do we use to set the volume on a pipette?

A

thumbwheel

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29
Q

what must we do when not using the glass electrode bulb?

A

keep it moist

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30
Q

what type of acids are carboxylic acids?

A

weak

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31
Q

what would we set the pipette to to measure 2 microlitres?

A

P20

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32
Q

Ka equation + define symbols

A

Ka = [A-][H+]/[HA]

HA = weak acid
A- = conjugate base
H+ = proton

33
Q

what’s the difference between a conjugate acid and conjugate base?

A

the presence of an additional proton

34
Q

what is the only thing the Henderson-Hasselbalch equation can be used for?

A

for conjugate acid-base pairs

35
Q

pKa equation

A

pKa = -logKa

36
Q

can the henderson-hasselbalch equation be used for acids and bases that react together?

A

no, only for conjugate acid-base pairs

37
Q

conjugate acid and base pair equation

A

acid –><– base + H+

38
Q

Henderson-Hasselbalch equation

A

pH = pKa + log10 [A-]/[HA]

39
Q

derivation of Henderson-Hasselbalch equation

A

Ka = [A-][H+]/[HA]

take logs of both sides

log Ka = log [A-][H+]/[HA]

rearrange

log Ka = log [A-]/[HA] + log [H+]

rearrange

-log[H+] = -logKa + log [A-]/[HA]

definitions of pH and pKa

pH = pKa + log [A-]/[HA]

40
Q

Uses of Henderson-Hasselbalch equation

A
  • describe how concentrations of conjugate acid and base in a buffer solution change at different solutions
  • find pKa of a buffer (mix known amounts of conjugate acid + base + see how concentration changes)
    -pH = pKa –> concentrations of conjugate acid + base are equal
    -predict how a solution will respond to adding a strong acid or base
41
Q

how do we know if the concentrations of conjugate acid and base are equal?

A

pH = pKa

42
Q

what does it mean if pH = pKa?

A

the concentrations of conjugate acid and base are equal

43
Q

what are biochemical reactions sensitive to?

A

pH

44
Q

why is the pH of the body tightly regulated?

A

biochemical reactions are sensitive to pH

45
Q

acidosis

A

when the pH of the blood plasma is less than 7.35

46
Q

when the pH of the blood plasma is less than 7.35

A

acidosis

47
Q

symptoms of acidosis

A

headaches, confusion, breathlessness, coma

48
Q

what are there to control intracellular pH?

A

proteins that pump protons out of cells
buffers

49
Q

major buffer in blood and interstitial fluid

A

bicarbonate buffer system

50
Q

where is the bicarbonate buffer system found?

A

it’s a major buffer in blood and interstitial fluid

51
Q

bicarbonate buffer system equation

A

CO2 + H2O –><– H2CO3 –><– HCO3- + H+

52
Q

why is there CO2 in the bicarbonate buffer system equation?

A

produced in the tissues

53
Q

carbonic acid

A

H2CO3

54
Q

what happens to the carbonic acid in the bicarbonate buffer system?

A

it ionises immediately

55
Q

bicarbonate ions

A

HCO3-

56
Q

how do clinicians estimate the concentration of HCO3- in patient’s plasma?

A

from…
pH
[CO2]
HH equation
use a pKa of 6.1

57
Q

name a buffer commonly used in biochemistry

A

phosphate

58
Q

why is phosphate a buffer that’s commonly used in biochemistry?

A

3 ionisable groups = 3 pKas and 4 different chemical forms

59
Q

ionisation

A

gain or lose electrons

60
Q

how much of the ionic species is remaining at 2 pH units from the pKa and why?

A

1/100
pH is a log function

61
Q

what happens to the phosphate buffer at different pHs?

A

different concentrations of the different forms of phosphate in the buffer

62
Q

forms of phosphate in the buffer

A

H3PO4
H2PO4-
HPO4^2-
PO4^3-

63
Q

at which pH is phosphate in the H3PO4 form in the buffer?

A

low pH

64
Q

at which pH is phosphate in the PO4^3- form in the buffer?

A

high pH

65
Q

what do we have when pH = pKa?

A

equal concentrations of conjugate acid and base
(careful with H2)

66
Q

what is true when we have equal concentrations of conjugate acid and base?

A

pH = pKa
(careful with H2)

67
Q

when is a buffer most resistant?

A

when [conjugate acid] = [conjugate base]

68
Q

describe the buffer when [conjugate acid] = [conjugate base]

A

most resistant

69
Q

what does a greater difference between the pH and the pKa lead to?

A

a greater difference between the relative concentrations of acid and conjugate base

70
Q

when do buffers function best?

A

within one pH unit of their pKa

71
Q

why do buffers function best within one pH unit of their pKa?

A

because pKa is the -log of Ka

72
Q

when do we have a greater difference between the relative concentrations of acid and conjugate base?

A

when there’s a greater difference between the pH and the pKa

73
Q

which buffer should we choose and why?

A

one with a similar pKa to the pH of the solution you want to use
buffers function best within one pH unit of their pKa

74
Q

what gives a buffer a greater buffering capacity?

A

greater concentration of the buffer

75
Q

buffer concentration often used by biochemists

A

10-100mmol.L-1

76
Q

why are buffers used a lot?

A

near-physiological pKa
lack of toxicity
tend not to interfere with biochemical reactions

77
Q

examples of buffers used + pKa

A

MES 6.1
phosphate 6.9
HEPES 7.5
Tris 8.1

78
Q

What would we set the pipette to to measure 450 micro litres? Explain

A

P45
On P1000, bottom number is 10 micro litres

Biochemistry: Cellular Composition (1 decks)

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