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AH chem > Volumetric Analysis > Flashcards

Volumetric Analysis Flashcards

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1
Q

qualitative analysis

A

the process of identifying what is in a chemical sample

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2
Q

quantitative analysis

A

the process of measuring how much is in the sample

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3
Q

standard solution

A

accurately known concentration

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4
Q

solution in the burette

A

standard solution

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5
Q

solution in the conical flask

A

the analyte

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6
Q

types of titrations

A

acid-base titration
redox titrations
complexometric titrations

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7
Q

principal requirements of a reaction which can be used for titration?

A

it goes to completion and proceeds rapidly

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8
Q

what is a standard solution made from?

A

from a solute called a primary standard

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9
Q

primary standard requirements

A

be available in a high state of purity
must be stable in air and in solution
soluble
have a reasonably high GFM

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10
Q

why would NaOH not be used as a primary standard

A

unstable in the air
- doesn’t meet the requirements of a primary standard

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11
Q

how to prepare a standard solution

A
  1. calculate the mass of primary standard required given the volume and concentration of solution you desire
  2. sample of primary standard must be dried to remove water (placed in a crystallising basin and stored in desiccator)
  3. weigh accurately the approximate mass of substance to make the dried solution
  4. glass used used to stir solution while dissolved in deionised water
  5. solution transferred to a volumetric flask with rinsings
  6. flask stoppered and inverted several times to ensure a uniform concentration
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12
Q

how is an airtight seal maintained in a dessicator

A

lightly greasing the ground-glass surfaces on lid and base

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13
Q

why are primary standards not dried by heating?

A

risk of them decomposing if temperature is too high

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14
Q

‘weighing by difference’

A

weigh empty tray
weigh tray + substance
substance transferred
reweigh empty tray
difference is the accurate mass of primary standard transferred

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15
Q

‘weighing accurately approximately’

A

the mass of primary standard has to be recorded exactly
but it doesnt need to be exactly what was calculated

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16
Q

what should be done with the stopper when preparing a standard solution?

A

only removed when necessary
decreases chance of absorption

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17
Q

what is an analyte

A

solution of unknown concentration

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18
Q

why should equipment be rinsed before used in a titration?

A

ensures all impurities adhering to inner surface are washed away

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19
Q

what is the purpose of the first, rough titration?

A

see the colour change and to provide an approximate titre volume

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20
Q

concordant results

A

within 0.1cm3 of each other

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21
Q

what do we try to obtain in a titration?

A

the equivalent or stoichiometric point

22
Q

why can the end point and equivalence point never coincide?

A

the end point overshoots the equivalence very slightly

23
Q

complexometric titration

A

complexes are formed -EDTA is an important complexometric reagent and can be used to determine the concentration of metal ions in solution

24
Q

what is a ligand

A

an ion or molecule attached to a metal atom by coordinate bonding

25
Q

when are titrations not direct

A

during a back titration

26
Q

back titration description

A

adding a known excess mass of one standard reagent to a known mass of substance being determined
after the reaction, the excess of standard reagent is determined by titration against a 2nd standard reagent

27
Q

when are back titrations used

A

if no suitable indicator is available for a direct titration
the end-point of the back titration is clearer than the direct
the reaction between the standard reagent + analyte is slow
the analyte is insoluble

28
Q

what are indicators

A

compounds that allow us to detect end-points of titrations

29
Q

how do indicators work?

A

it has one colour in its acid form and a different colour in its conjugate base form

30
Q

properties of an acid-base indicator

A

a weak organic acid that will dissociate an aqueous solution

31
Q

types of indicators

A

bromophenol blue
methyl red
bromothymol blue
phenol red
phenolphthalein

32
Q

bromophenol blue pH colour range

A

3 - 4.6

33
Q

methyl red pH colour range

A

4.2 - 6.3

34
Q

bromothymol blue pH colour range

A

6 - 7.6

35
Q

phenol red pH colour range

A

6.8 - 8.4

36
Q

phenolphthalein pH colour range

A

8.3 - 10

37
Q

strong acid / strong base
equivalence point

A

7

38
Q

weak acid / strong base
equivalence point

A

> 7

39
Q

strong acid / weak base
equivalence point

A

< 7

40
Q

weak acid / weak base
equivalence point

A

~ 7

41
Q

indicator for a weak aicd / weak base titration

A

no suitable indicator since the pH change at the equivalence point is gradual and relatively small

42
Q

why is it important that only a few drops of indicator is used?

A

they are weak acids themselves so are capable of being neutralised themselves

the more indicator used the greater volume of base is needed to neutralise it

43
Q

what can a redox indicator be

A

an oxidising or reducing agent

44
Q

how can an oxidising/reducing agent be used as an indicator?

A

it signals the end the titration as it has different colours in its reduced/oxidised state

45
Q

what does a redox and acid-base indicator respond to

A

acid-base = pH
redox = redox potential

46
Q

why are redox titrations unusual

A

some cases, no indicator is needed as reagent acts as one

47
Q

why is iodine not self-indicating in practice

A

the colour change is gradual and difficult to notice
iodine molecules must be present in relatively high concentrations before the colour is discernible

48
Q

why does starch allow iodine to self-indicate

A

starch forms a blue-coloured complex with iodine molecules
even when iodine concentration is low, the blue colour is evident

49
Q

why must starch be fresh when used in a titration

A

it decomposes to form glucose which is a reducing agent so will react with iodine causing the titre volume to be higher/lower than expected

50
Q

complexometric titrations

A

metal ion indicators

51
Q

what are metal ion indicators

A

organic dyes which form coloured complexes

52
Q

what must a suitable indicator do in a complexometric titration

A

bind less strongly with metal ions than the complexing agent

AH chem (3 decks)

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