8.1 b: Ion testing (cations and anions)

Analysis of Inorganic Substances: Ion testing

​What we will cover

Open Ended

Why do we monitor the environment?

Hint: think about the different types of pollution

​Monitoring the environment:

• manage level of pollutants - things that have an undesired effect on the environment

• sticking to guidelines and parameters - national guidelines for safety levels

• many different things can be pollutants

  • chemical spills: industrial standards on disposing of waste and transportation. Can impact soil, water, and air quality. Impact health and wellbeing of organisms.

  • fertilisers: excess use causes pollution, makes their way into large waterways and kills marine life.

    • eutrophication occurs dur to excess nitrates and phosphates and causes excess algae to be formed, which prevents photosynthesis by blocking sunlight, which prevents oxygen from being formed

​Examples:

Sources of water pollution can include: 

• Point source water pollution comes from a discrete source, such as a pipe or drain flowing from an industrial activity. This can include release of strong acids and bases, heavy metals or other toxic chemicals.

• Diffuse source water pollution arises from a range of diverse urban and rural land uses across a catchment, including from the overuse of fertilisers and pesticides.

Air pollution can be caused by:

• natural sources such as bushfires, dust storms, sea salt and pollen

• domestic activities such as burning wood fires, including using wood heaters, fuel-powered garden equipment, and portable fuel containers

• commercial businesses such as spray painters, printers, quarries, service stations

• industrial activities such as coal mining, oil refining and power generation

• on-road motor vehicles such as buses, cars and trucks

• off-road vehicles and equipment such as dump trucks, bulldozers and marine vessels

Soil contamination generally occurs at sites that have been used for heavy industry or intensive agriculture or for chemical storage, such as service stations and dry cleaning sites.

​Identifying ions in solution:

• when monitoring the environment, it is necessary to qualitatively and quantitatively determine levels of particular ions - to determine whether they are in an appropriate range

• we can test through use of precipitation titrations, gravimetric analysis, and other instrumental techniques

​Cation identification:

Rewind back to Year 11: Flame Tests

Used to qualitatively distinguish between different metal ions.

When the ion is subjected to heat, then the electrons in the lowest energy level move from this ground state to a higher energy level (excited state). Eventually they lose this energy and fall back down to their ground state. The excess energy is released as a wavelength of light, and gives a characteristic colour emission of light.

Multiple Choice

When a solution containing the sodium ion is heated in a flame it gives off this color

• yellow

• red

• orange

• green

Multiple Choice

Shorter wavelengths of electromagnetic radiation have ______ energy than longer wavelengths.

• less

• more

Multiple Choice

When a solution containing copper ions is heated in a flame it gives off this color

• red

• yellow

• blue/green

• purple

Multiple Choice

Strontium flame test?

• red

• yellow

• orange

• blue

Multiple Choice

Barium flame test?

• red

• yellow

• orange

• Green

You need to memorise the following metals and colours:
Copper - blue-green

Potassium - light purple 

Strontium - scarlet

Calcium - brick red

Barium - pale green

​Using precipitation reactions:

​Cation identification: complexation reactions

• complex ions - formed when one or more small molecules/ions attach themselves to a central cation

  • the central atom is often a transition metal

  • the surrounding atoms are called ligands and must have a lone pair

• co-ordinate covalent bond is produced where one of the atoms does not have a full outer shell - acts the same as a covalent

• usually causes a colour change

• common ligands: water, ammonia, hydroxide

​Cation identification: complexation - copper complex ions

• copper salts usually dissolve in water to form a blue solution and form a complex, hexaaquacopper(II) - [Cu(H₂O)₆]²⁺

• when hydroxide ions are added (i.e. through sodium hydroxide solution) the OH ions remove H ions from the water ligands that are attached to the copper ion

• Once a H is removed, the molecule has no charge and that is insoluble in water - therefore, precipitate forms

• [Cu(H2O)6]2+(aq) + 2OH−(aq) → [Cu(H2O)4(OH)2](s) + 2H2O(l)

  This is usually written more simply as:

  Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s)

• when ammonia is added to a copper complex solution, the ammonia replaces water as a ligand which forms tetraamminediaquacopper(II) ions

• only 4 out of the 6 water molecules are replaces

• [Cu(H₂O)₆]2+(aq) + 4NH₃(aq) ⇌ [Cu(H₂O)₂(NH₃)₄]²⁺(aq) + 4H₂O(l)

Multiple Choice

I form a pale green ppt when either sodium hydroxide or ammonia is added.

• copper

• iron (III)

• iron (II)

• aluminum

Multiple Choice

I produce a pale blue ppt with sodium hydroxide and ammonia. Adding more sodium hydroxide has no effect. But when you add more ammonia to me, I dissolve into a deep blue solution.

• Ammonia

• Sodium

• Copper (II)

• Calcium

Multiple Choice

I produce a brown ppt when sodium hydroxide is added.

• Copper ions

• iron (II) ions

• zinc ions

• iron (III) ions

Multiple Choice

Dilute sodium hydroxide can be added to an unknown solution to indicate the presence of....

• iron(II)

• iron(III)

• copper

• all of the above

Multiple Choice

Why can't you use a flame test to distinguish between silver and lead?

• silver is colourless

• lead is toxic

• they are the same colour

• both 1 & 2 are correct

Multiple Choice

A red precipitate will form when potassium dichromate is added to a precipitate sample containing....

• silver

• lead(II)

• barium

• calcium

​Anion identification:

​Anion identification: carbonates, acetates, hydroxides

• carbonate, hydroxides and acetates are all basic

  • can use litmus paper will identify the presence of these ions

• carbonates:

  • will produce carbon dioxide (limewater milky) in a neutralisation rxn

• hydroxides:

  • neutralise an acid to produce salt and water

• acetates:

  • do not precipitate, so will be identified by an absence of reaction in other tests

  • also, they smell vinegary

  • can do a test with iron(III) chloride, which will turn it red

​Anion identification: chlorine, bromine, iodine

• all form a precipitate when mixed with silver nitrate solution

  • chlorine = white

  • bromine = cream

  • iodine = yellow

• hard to tell the difference, so a confirmation test with ammonia is used

  • solubility of the silver halides in ammonia solution is as follows; AgCl>AgBr>AgI

​Anion identification: sulfates and phosphates

• Barium will form a precipitate with them both under specific pH conditions

• when the solution is acidified with pH < 2, then phosphates will not form a precipitate

• we can tell this also from their Ka values:

  • SO₄^- = 10^-2

  • PO₄^2- = 4.8 x 10^-13

• HSO4 will be a much stronger acid, therefore more SO4 ions in solution, therefore enough of it to form a precipitate

• adding in ammonia to shift the pH allows a phosphate precipitate to be formed (shifts equ to the right)

Multiple Choice

Which ion turns red litmus paper blue?

• chloride

• sulfate

• hydroxide

• phosphate

Multiple Choice

Adding barium nitrate to an unknown solution and a white precipitate forming will indicate the presence of....

• sulfate

• phosphate

• carbonate

Multiple Choice

Silver nitrate and ammonia are used to test for....

• metals

• noble gases

• halogens

• metal oxides

Multiple Choice

Test for sulphate ions is...

• Add acid or barium chloride

• Add barium chloride or barium nitrate

• Add silver nitrate or barium nitrate

• Add barium chloride or silver nitrate

Multiple Choice

Dilute hydrochloric acid is added to a colourless solution. Bubbles appear immediately and this gas turns limewater cloudy. What ion was present in the solution?

• Carbonate

• Sulfate

• Nitrate

• Chloride

Open Ended

​Up next:

Gravimetric analysis and precipitation titration