Group V Cation

GROUP V CATION

CATIONS: NH4+, Na+, K+ and Mg2+

Precipitating agent: There is no precipitating agent for this group.

PROCEDURE V - 1

Test of NH4+ should be done on original solution. Place 5 to 10 drops of the original solution in a small beaker, add equal volume of water, then add 1 ml of NaOH. Immediately cover beaker with a watch glass to which adheres moist red litmus paper (see Figure 1) and warm (do not boil) the solution. Even shading of the litmus paper from red to blue confirms the presence of NH4+.

watch glass

moist red litmus paper

50 ml beaker

unknown + NaOH

PROCEDURE V - 2

To 3-5 drops the solution of group V cations add 1 M HOAc until the solution is acidic, then add 5 drops of sodium reagent (zine uranyl acetate), shake and set aside for about one hour. A slow yellow precipitate confirms the presence of Na+ .

The presence of Mg2+, K+, and NH4+ do not interfere with this test.

PROCEDURE V - 3

To another 10 drops of Group V solution, add 6 M NH4OH until it is alkaline to litmus, then add one drop in excess. Now add 3 drops of Na2HPO4. White precipitate (Often slow in forming) confirms the presence of Mg2+ .

PROCEDURE V - 4

To solution from Procedure V - 3, add 4 drops of concentrated HNO3 in a casserole and evaporate to dryness. (Avoid splattering--it reduces the concentration of the sample), then heat intensely for a few minutes. After the casserole is cool, add 2 drops of 1 M HCl and 5 drops of HOAc to the residue and dissolve (heat if necessary--do not evaporate all of the solution). Now add 5 drops of Na3Co(NO2)6. A yellow precipitate confirms the presence of K+.