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Foundations in medicine design > ionic equlibria, pH and buffers > Flashcards

ionic equlibria, pH and buffers Flashcards

1
Q

what is an electrolyte solution

A

an electrolyte solution is one which conducts electricity, therefore there are ions present, often aqueous solutions of ionic compounds.

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2
Q

what happens when ionic compounds dissolve

A

he ions become separated from one another by water molecules, ions are not formed when an ionic solid dissolves but become free to move apart in the presence of water

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3
Q

solute in strong electrolyte solution

A

the solute is present entirely as ions- difficult to measure equilibrium constant

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4
Q

solute in weak electrolysed solution

A

solute is only incompletely ionised in solution, i.e. some molecules survive- able to express dissociation by the law of mass action in terms of the equilibrium constant

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5
Q

equilibrium constant

A

if equilibrium lies to RHS, the numerator will be grater than 1, K will be greater than 1
if equilibrium lies to LHS, K will be less than 1

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6
Q

what is autoprotolysis

A

a reaction where one molecule transfers a proton to another molecule of the same kind is called autoprotolysis

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7
Q

autoprotolysis constant

A

Kw=ionic product or autoprotolysis constant for water

= 55.51M

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8
Q

autoprotolysis constant in relation to acids and alkalis

A

in pure water [H]=[OH]
[H]=square root Kw which is 1x10-7mole l-1

solutions with [H] more than 10-7 mol l-1 are acidic and less than this value are alkaline

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9
Q

what is pH defined as

A

the negative log (to the base of 10) of the hydrogen ion concentration

the higher the [H] the lower the pH

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10
Q

ionic strength

A

concentration of other ionic species ((and magnitude of their change) in the solution is called the ionic strength
presence of these ions tend to limit mobility of H thereby decreasing activity of H

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11
Q

how to measure pH

A

standard hydrogen electrode
Nernst equation
Nernst factor=slope factor
Nernst factor will change with temp. must calibrate pH meter before doing analysis to allow for any changes, the slope will therefore also change with temperature

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12
Q

pH sensor

A

a pH electrode is made up of; measuring electrode, reference electrode
pH is a measurement of two half cell electrode potentials; one form pH sensitive electrode and other from reference electrode

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13
Q

reference electrode

A

at the reference electrode, there is a solid/solution interface
this allows chemical reaction to occur, enables electrical current to flow through and measures pH

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14
Q

measuring electrode

A

voltage produced here depends on ionic activity of the ion
at pH 7, output in 0mV- this is called the isopotential point for a perfect electrode
point at which both electrodes give the same reading
as pH increases mV becomes more negative
as pH decreases mV becomes more positive

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15
Q

reference electrode

A

both electrodes will produce a pH measurement when connected to a measuring device
to achieve an accurate value from the measurement electrode, the reference electrode must have a constant and stable potential
consists of a silver wire coated with silver chloride immersed in an electrolyte solution
also has a salt bridge junction which enables a physical and electrical interface with solution being measured, this completes the path from the glass measuring electrode to the reference electrode

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16
Q

calibration of pH sensor

A

must calibrate pH sensor to maintain accurate readings, compensates for changes in potential within and between the measuring and reference electrodes
electrodes are matched at the factory to produce a 0mV reading in zero solution
to calibrate- check pH of pH 7 buffer solution
check slope of measuring electrode
to do this always use a buffer value at least 3pH units from zero buffer e.g pH4 should pick buffers close tot that of solution being measured

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17
Q

Bronsted-lowry theory

A

acid=proton donor

base=proton acceptor

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18
Q

weak acids

A

weak acids in aqueous solutions are not completely ionised, they are in equilibrium with the undissociated acid
as is the case for water which is a very weak acid.

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19
Q

dissociation of a weak acid

A

HA(reversible arrow) H+A
A=conjugate base

Ka=ionisation or dissociation constant
Ka=[H][A]/[HA]

If numerator > denominator, Ka is large, acid is strong
if numerator < denominator, Ka is small, acid is weak

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20
Q

dissociation of weak base

A

for any weak base A, A9reversible arrows) HA+OH
HA=conjugate acid
Kb=ionisation or dissociation constant
Kb=[HA][OH]/[A]
If numerator > denominator, Kb is large, base is strong.
If numerator < denominator, Kb is small, base is weak

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21
Q

how to calculate pH

A

for weak acids; set ub equilibrium eq and determine [H] by using acidity constant Ka then use pH=-log[H]

for weak bases use equilibrium equation to calculate pOH from Kb then convert pOH to pH by subtracting from 14

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22
Q

pKa

A

useful to express strengths of acids and bases using same term, done by considering equilibria between an acid and its conjugate base

acid and base dissociation constant related through ionic product of water
therefore a strong acid has a weak conjugate base and a weak base has a strong conjugate acid. this relationship allows the strength of acid to be expressed in terms of the dissociation constant pKa

pKa=-logKa

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23
Q

relationship between pKa and acid strength

A

since a negative log, the lower the value of pKa the stronger the acid

24
Q

reaction between an acid and a base

A

neutralisation reaction producing an ionic compound (salt)

25
Q

what is a salt

A

any ionic compound formed b the neutralisation of an acid with a base

26
Q

usual properties of salts

A

ionic, water soluble, high m.p., crystalline solids, often used to increase water solubility of a drug

27
Q

hydrolysis of salts

A

when a salt dissolves in water the compound dissociates completely to give solvated anions and cations

28
Q

cation

A

positively charged species

29
Q

anion

A

negatively charged species

30
Q

hydrolysis

A

raking a bond by action of water

31
Q

effect hydrolysis can have on a strong acid/strong base solution

A

salt dissociates to give free ions surrounded by water molecules
salt is neutral

32
Q

effect hydrolysis can have on a strong acid/weak base

A

acidic salt

33
Q

effect hydrolysis can have on a weak acid/strong base

A

basic salt (alkaline solution)

34
Q

effect hydrolysis can have on a weak acid/weak base

A

equal concentrations of ions yields a neutral salt

35
Q

amphiprotic salts

A

more complex- can function both as an acid and a base, both can donate or accept H ions in solution

36
Q

how to calculate pH of an amphiprotic salt

A

pH=1/2(pKa1+pKa2)

37
Q

what are buffer solutions used for

A

calibration of pH meters, to culture bacteria, control of pH of solutions for chemical reactions, administering intravenously to critically ill patients

38
Q

reason why pH in pure substances changes so easily

A

this occurs because pure substances have no mechanism to neutralise incoming acidic or basic impurities

39
Q

buffers

A

solutions that resist changes in pH when small amounts of acid or base are added are called buffers

40
Q

what do buffers consist of

A

consist of a weak acid (to supply protons to a strong base) and its conjugate base (to accept protons from a strong acid) or a weak base (to accept protons from a strong acid) and its conjugate acid (to transfer protons to a strong base)
a buffer will stabilise the pH of a solution by providing a source or sink for protons

41
Q

when do buffers work best

A

buffers work best at a pH=pKa of acid or base from which they are made

42
Q

what can Henderson hasselbalch equation be used for

A

can be used to calculate [acid], [base] or [salt], can also be used to determine pKa of acid or base in lab, requires potentiometric titration with pH meter

43
Q

how to determine pKa of weak acid and weak base

A

to determine pKa of weak acid; plot pH curve during a titration with strong base, identify the pH halfway to the stoichiometric point

weak base; titrate with strong acid, find pka then use pKa+pKb=pKw

44
Q

buffer capacity

A

number of moles of strong monobasic acid or monobasic base required to produce an increase or decrease of 1 unit of pH scale
buffer capacity=no moles/change in pH

45
Q

high buffer capacity

A

can maintain its buffering action after addition of more strong acid or base than one with a small capacity
high capacity generally when amount of base present is aboun10% more of the amount of acid
or acid about 10% or more of the amount of base
otherwise gets used up too quickly

46
Q

how is the bloodstream and intracellular fluid buffered to pH of 7.4
(biological buffers)

A

3 main buffers; dissolved CO2, dihydrogen phosphate, protein macromolecules

47
Q

amino acids

A

compound with COOH and NH2 groups in same molecule. all chiral except glycine. exist as internal salt or zwitterion in solution
amphiprotic, in solution, both functional groups ionise to form salt
ionised at all values of pH

48
Q

isoelectric point (pl)

A

pl is the average of the two pKa values on either side of the zwitterion

represents minimum solubility of protein- at which migration of protein in an electrical field is slowest, used in electrophoresis to separate mixtures of proteins

49
Q

%ionised for acids and bases

A

weak acids- %ionised=100/1+anti log (pKa-pH)

weak bases- %ionised =1+anti log(pH-pKa)

50
Q

approx. rule of thumb for if pH=pKa for acidic and basic drugs

A

acidic-
if pH=pKa+1, drug 90%ionised
if pH=pKa+2 drug 99%ionised
if pH=pKa+3, drug 99.9%ionised

basic-
if pH=pKa-1, drug 90%ionised
if pH=pKa-2, drug 99%ionised
if pH=pKa-3, drug 99.9%ionised

51
Q

how to determine acidic or basic drug

A

important to remember that pKa values do not tell you if a compound is acidic or basic, it is only the negative log of the dissociation constant. same rules apply for pH

52
Q

acidic drugs

A

acidic substance that ionises to donate protons to its surrounding, structure becomes resonance stabilised through loss if hydrogen. distributes the negative charge around the anion
majority tent to be carboxylic acids
phenol groups also tent to be acidic

53
Q

basic drugs

A

base is a proton acceptor
pharmaceutical and biological sciences are concerned mainly with behaviour of drugs in aqueous systems. in these situations, drugs are basic if they contain; a nitrogen with atom with available lone pairs of electrons e.g. amine
but not all drug which contain nitrogen are basic- only basic if lone pairs of electrons are available for reaction e.g. amides contain nitrogen but are usually neutral

54
Q

pH indicators

A

end point of titrations determined by colour change by indicator, indicators used in pH titrations are themselves weak acids or bases, colour depends on if they are ionised or not
Important to select an indicator with an end point where
pHin=pH9stoichiometric point) equal to or plus 1

55
Q

what is the end point of a reaction defined as

A

point at which [acid] and [base] forms of indicator are equal

56
Q

predicting end point

A

to predict pH at end point of titration, most acid-base reactions are over when ratio of ionised form to unionised form is 1000:1

hence when pH-pKa+3 for acid of pH=pKa-3 for base

Foundations in medicine design (9 decks)

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