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Saturday, December 1, 2012

The Systematic Analysis of Cation Group 5 - The Soluble Group - Na+, K+, Mg++, NH4+

Cation Group 5 - The Soluble Group

The  ions of this analytical group are left in solution after after separation of all the other cations. The position in the Periodic Table of the three metals and of the less common members of the Soluble Group are shown at the right.
Na+, K+, Mg2+, NH4+
In the experiment, we use three reaction to analysis the existences of each cations.

Magnesium ion 
To separate magnesium ion from the solution, we use ammonium ions and phosphate ions to form precipitation with magnesium ion.:

 Mg2+(aq) + NH4+(aq) + PO43-(aq) + 6H2O(l) -----> MgNH4PO4˙6H2O(s)

We can separate the magnesium precipitation from the solution, and then add some acid to dissolve the salt to release magnesium ions out again.

MgNH4PO4˙6H2O(s) + H+ -----> Mg2+(aq) + NH4+(aq) + H2PO4-(aq) + 6H2O(l)

 We use a special reagent, S. and O. reagent(Suitsu and Okuma 's reagent). It can be prepared by dissolve 0.5 g of the dye (p-nitrobenzeneazoresorcinol) in 100 ml of 0.25 M NaOH. 

Mg2+(aq) + 2OH-(aq) -----> Mg(OH)2(s)
 The dye would be absorbed on the magnesium hydroxide precipitation and give a blue lake.

Potassium ion
 Identification. Reagent: Sodium cobaltinitrite
 Potassium ion could form stable precipitation with the giant anion, cobaltinitrite.

2 K+ + Na+ +  Co(NO2)63+ --- > K2NaCo(NO2)6 (s)

 The precipitation is a very bright yellow, just like the color of cadmium sulfide. So it is very easy to observe.

 Sodium ion
 Identification. Reagent: Uranyl magnesium acetate
 Uranyl magnesium acetate is also a giant, heavy anion. It could form precipitation with sodium ion. 

 Na+ + Mg2+ + 3 UO22+ + 9 C2H3O2- + 9 H2O ------ > NaMg(UO2)3(C2H3O2)9˙9 H2O(s)

 The precipitation is pale yellow green color, and is fluorescent.
 Note: Uranyl magnesium acetate is a low radioactive substance. Treat as a possible carcinogen. 
 Uranyl magnesium acetate is yellow
Original Sample:

 Analysis for Na+, K+, Mg2+
Divide the solution into three equal parts.

Na Test

Add some drops uranyl magnesium acetate in to one part of the sample which is without ammonium. Wait the reaction for 5 minutes.

 The 1/3 original sample

 Added uranyl magnesium acetate after seconds

 Five minutes passed, some fluffy precipitation form.

Yellow Green Precipitation:
NaMg(UO2)3(C2H3O2)9˙9 H2O(s)

Potassium Test

Add 1 drop 6M HOAc, some NaNO2, and add some Na3Co(NO2)6 (aq) . Warm the solution for a while and cool it down.

Yellow Precipitation:
K2NaCo(NO2)6 (s)

Magnesium Test

Add 1 drop 6M NH4Cl, few drops Na2HPO4. Let it react for 5 minutes. Centrifuge it.Take the precipitation. Dissolve the precipitation in HCl. Add 1 drop S.&O. reagent. Alkalize the solution with 6M NaOH solution.

The color of upper layer is the dye, and the color of 
lower layer is the complex of magnesium hydroxide and the dye

The blue color is the complex of magnesium hydroxide and the dye


1. Qualitative Analysis and Electrolytic Solution

2. Introduction to Semimicro Qualitative Analysis (8 ed.)


  1. I'm curious to know if Sodium carbonate will react with Sodium cobaltnitrite because I assume it'd be no reaction, but when I tested it in the lab it gave a reaction. So I think either my sodium carbonate sample might have been contaminated or somethings going on that I can't see. Please help if you can, that would be greatly appreciated! :)

  2. your sodium carbonate is contaminated