136 GROUPS I AND IITESTS FOR GROUP I CATIONS- All the cations of Group I produce a characteristic colour in a
flame (lithium, red; sodium, yellow; potassium, violet; rubidium,
dark red; caesium, blue). The test may be applied quantitatively by
atomising an aqueous solution containing Group I cations into a
flame and determining the intensities of emission over the visible
spectrum with a spectrophotometer (flame photometry). - The larger cations of Group I (K, Rb, Cs) can be precipitated
from aqueous solution as white solids by addition of the reagent
sodium tetraphenylborate, NaB(C 6 H 5 ) 4. Sodium can be precipitated
as the yellow sodium zinc uranium oxide ethanoate (sodium zinc
uranyl acetate). NaZn(UO 2 ) 3 (CH 3 COO) 9. 9H 2 O. by adding a clear
solution of^4 zinc uranyl acetate' in dilute ethanoic acid to a solution
of a sodium salt.
TESTS FOR GROUP II CATIONS
Calcium, strontium and barium produce characteristic flame colours
like the Group I cations (calcium, orange; strontium, red; barium,
green) and flame photometry can be used for their estimation. All
give insoluble carbonates in neutral solution.
Magnesium is slowly precipitated as the white magnesium
ammonium tetraoxophosphate(V), MgNH 4 PO 4. 6H 2 O. when a
solution of disodium hydrogentetraoxophosphate(V) is added to a
solution of a magnesium salt in the presence of ammonia and
ammonium chloride.
QUESTIONS
- Relatively little is known about the chemistry of the radioactive
Group I element francium. Ignoring its radioactivity, what might
be predicted about the element and its compounds from its position
in the periodic table?
(Liverpool B.Sc.. Part I) - The elements in Group II of the Periodic Table (alkaline earth
metals) are. in alphabetical order, barium (Ba). beryllium (Be),
calcium (Ca). magnesium (Mg), radium (Ra) and strontium (Sr),
(a) Arrange these elements in order of increasing atomic numbers.
(b) Write down the electronic configurations of any two of the