Qualitative analysis

Question 1

What does Qualitative analysis rely on?

Answer 1

Observation measurements

Question 2

What does Quantative analysis rely on?

Answer 2

Numerical measurements (e.g. titrations)

Question 3

Describe the carbonate test

Answer 3

Unknown compound (suspected to contain Co3 2-) is reacted with nitric acid. if Co3 2- ions were present carbon dioxide gas would be produced.
Bubble the carbon dioxide gas through limewater, which should turn cloudy if CO3 2- ions were present.

Question 4

Describe the sulfate test

Answer 4

Make a solution with the unknown compound. Add Barium nitrate (to produce Ba 2+ ions) to the solution. If sulfate ions were present a white precipitate should be produced BaSO4, as barium sulfate is not very soluable.

Test for SO4 2-

Test for SO4 2-

Question 5

Describe the Halide test

Answer 5

Prepare a solution of the suspected halide ions, then add Silver Nitrate. A precipitate should form. White for Chloride ions, cream for Bromide ions, yellow for Iodide ions.
As the precipitates colours are similar aqueous ammonia is then added to test the solubility of each precipitate formed.
Silver Chloride is soluble in dilute ammonia
Silver Bromide is soluble in conc ammmonia
Silver Iodide is not soluble in conc ammonia

test for Cl- Br- I-

test for Cl- Br- I-

Question 6

Describe the sequence of the anion test

Answer 6

1. Carbonate test
2. Sulfate test
3. Halide test

Question 7

why is the sequence for halide tests as it is?

Answer 7

A false positive cannot be achieved with the carbonate test as sulfate / halide ions do not react with nitric acid.
A false positive can be achieved if the sulfate test was before the carbonate as Barium ions react with CO3 2- to make a white precipitate.
If the halide test was before the sulfate or carbonate tests a false positive could be given as the Silver ions react with CO3 2- and SO4 2- to produce insoluble precipitates.

Question 8

Difference when conducting an anion test with a mixture of anions

Answer 8

Add nitric acid until no further bubbling occurs to react with all CO3 2- ions in the carbonate test.
Add exces Barium nitrate to react with all SO4 2- ions in the sulfate test. (Barium nitrate must be used not Barium chloride as the chloride ions react in the halide test)
Carry out the Halide test as usual.

Question 9

Test for cations ( NH4 +)

Answer 9

Add Aqueous sodium hydroxide to a solution of suspected ammonium ions. Heat the solution to produce ammonia gas. The ammonia gas should turn damp pH paper blue (alkaline).

Solution must be heated as ammonia is very soluable in water.