Unit 4 - Volumetric Analysis / standard solutions and dilutions

Question 1

What is a standard solution

Answer 1

A standard solution is a solution for which an accurate concentration is known. It can be prepared directly from a solute , if that solute is a primary standard.

Question 2

What is a primary standard /what are the requirements

Answer 2

To be a suitable primary standard , a substance must have the following properties :

• have high purity
• stable in air / in solid / when in solution ( not reactive)
• be soluble in solvent (usually soluble in water)
• solubility should be high enough that solutions of relatively high concentrations can be prepared
• have high GFM : reduces uncertainty in the mass of substance weighed out

Question 3

Examples of Substances that can be used as primary standards

Answer 3

• oxalic acid (H2C2O4.2H20)
• sodium carbonate (Na2CO3)
• potassium iodate (KIO3)
• potassium dichromate (K2Cr2O7)
• potassium hydrogen phthalate ((KH(CH4O4))

Question 4

Is sodium hydroxide suitable to use as a primary standard ?

- explain why or why not

Answer 4

No.
Substances such as NaOH (sodium hydroxide) are not suitable to use as primary standards because
- it has a relatively low Gfm
- unstable as a solid and solution (absorbs moisture / water and carbon dioxide from the atmosphere)

Question 5

What can be done to sodium hydroxide to make it suitable to be used in volumetric analysis

Answer 5

Sodium hydroxide must be standardised before being used in volumetric analysis

Question 6

How to prepare a standard solution

Answer 6

• mass of solute (primary standard) required to make concentration of solution is weighed accurately by difference.
• dissolve solute in small amount of deionised water in a beaker
• transfer the solution carefully into a standard flask , along with several washings from my beaker to ensure all solute is completely transferred
• make solution up to the mark with more deionised water
• invert stoppered flask several times to ensure thorough mixing

Question 7

What does volumetric analysis actual involve ?

Answer 7

• volumetric analysis involves using a solution of known concentration (standard solution) in a quantitative reaction to determine the concentration of the other reactant

Question 8

What procedure/s is used to carry out volumetric analysis ?

Answer 8

A titration : whether I’m form of 
- standard titration 
( redox titration , acid / base titration)
- compleximetric titration 
- back titration

Question 9

How is a standard titration carried out

Answer 9

• titrations involve measuring one solution quantitatively into a conical flask using a pipette
• the other solution is added from a burette into the conical flask (whilst swirling the conical flask) until a permanent colour change of an indicator is seen in conical flask
• results are recorded , titration should be repeated until, concordant result(+or-0.1cm3) is obtained

Question 10

What is a rough titration

Answer 10

A “rough” titration is carried out first , followed by more accurate titrations until concordant titre values (+or-0.1cm3 of each other)
- the mean / average value of the concordant titres is used in calculations

Question 11

Diluting standard solutions steps

Answer 11

standard flasks are used when diluting one of the standard solutions before titration is carried out. Dilution involved the following stages :

• pipetting a known volume of standard solution into a clean standard flask
• adding deionised water to the flask until the mark is just just below the graduation mark
• using a dropper to add deionised water very slowly until the bottom of the meniscus is on the graduation mark
• putting a stopper in the standard flask and then inverting it to mix it thoroughly

Question 12

What is the dilution factor

Answer 12

The dilution factor is the amount the original solution is diluted by

Question 13

Dilution factor equation

Answer 13

Dilution factor = total volume after dilution / initial volume added

Question 14

When a concentrated solution is diluted the amount of solute doesn’t change , only the ______

Answer 14

When a concentrated solution is diluted the amount of solute does not change , only the amount of solvent does

Question 15

Dilution questions and equation c1v1 = c2v2

Answer 15

Look at page 8 in notes

Question 16

• What are Acid / base titration ?

- what are the requirements and how is end point determined

Answer 16

Acid / base titrations are neutralisation reactions (the reaction of an acid with a base that results in the Ph moving towards 7)

• an indicator is required and the choice of indicator depend on the Oh equivalence point(point at which the reaction is just complete)
• the experimenter is looking for a permanent colour change in conical flask (usually due to presence if indicator) this is called the end point.
• the ideal situation is where the equivalence point and the end point are exactly the same

Question 17

What is equivalence point

Answer 17

Equivalence point is the point at which the reaction is JUST complete.
The permanent colour change due to indicator indicates the end point
, ideally the equivalence point and end point are exactly the same.

Question 18

Redox titration - what are they and what happens in them

Answer 18

• redox titrations are based on redox reactions.
• the reducing agent is an electron donor + is itself oxidised during the reaction
• the oxidising agent is an electron acceptor and is reduced during the reaction

Question 19

Good oxidising and reducing agents

Answer 19

Oxidising agents include : 
- potassium permanganate 
- sodium dichromate 
These two are always easily reduced 
Reducing agents include : 
- sodium sulfite 
- potassium iodide 
- any alkali metal 
These are all easily oxidised

Question 20

Useful reagent : acidified potassium permanganate

- explain how it is used

Answer 20

• this is an excellent oxidising agent as it has the advantage of acting as it’s own indicator ( self indicating)
• purple permanganate is usually placed in the burette
• the reducing agent (plus sulfur if acid to provide H+ ions) is placed in the conical flask
• the purple permanganate ions change to colourless Mn2+ ions as they are added do the reducing agent (potassium permanganate is decolourised)
• the end point is observed when the reducing agent is used up and purple MnO4 ions no longer react - end point indicated by a very pale pink ,almost colourless colour caused by small excess of Mn (VII)

Question 21

What is a compleximetric titration

- what are these types of titrations useful for

Answer 21

• a compleximetric titration is a form of volumetric analysis in which the formation of a coloured complex is used to indicate the end point of a titration. (Based in reactions in which complexes are formed)
• compleximetric titrations are useful for the determination of metal ions in a solution

Question 22

A useful reagent for compleximetric titrations - EDTA

Answer 22

• EDTA is a useful reagent - it is generally used as it forms complexes with metal ions in a 1:1 ratio

Question 23

Why is EDTA use limited,

Talk of indicators and requirements eg displacement

Answer 23

EDTAs use can be limited as appropriate indicators need to be used, and are or always available

• the indicator has to form a complex with the metal ion BUT , the indicator must bind less well with the metal than EDTA , (EDTA must ind better with it to form the coloured complex) as EDTA is added the indicator needs to be displaced
• when all indicator attached to metal ions has been displaced , a colour change is observed indicating the end point
• the most common indicator used is murexide which is an excellent indicator for the titration of calcium and nickel ions

Question 24

Back titration : what are they and why are they used

Answer 24

A back titration / indirect titration - allows the concentration of an unknown solution to be determined, by reacting it with a known excess of a reagent. The quantity of the excess reagent is determined by a titration with a second reagent.
- the concentration of the unknown solution can’t be found out just by a standard titration, this could be for a number of reasons eg ( it’s solid or impure , it’s slow reacting , it’s an insoluble salt or it’s volatile etc) this is why this is used.

Question 25

Use of a control : what is a control and why is it used

Answer 25

• the use of a control in an experiment validates a technique and may consist of a determination of a solution of known concentration .
• in the determination of the percentage of acetyl salicylic acid in commercial aspirin tablets, a sample of pure aspirin (100%) would also be analysed to validate the technique. A sample of pure aspirin of accurately known mass say 1g could be treated in the same manner as the . If the experimentally determined quantity of aspirin, is very close to the known value of 1 g then it could be concluded that this method of determining the aspirin content is valid and therefore any results obtained by this method will be reliable

Question 26

how to convert

• mol l-1 to gl-1
• mol l-1 to ppm
• what is 1 ppm

Answer 26

• x gfm
• x gfm x 1000
• 1 ppm is 1 mg per kg ( 0.001g per kg)
  so 0.3ppm is 0.3 mg per kg