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) + PO₄^3-_(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

  Outline

  
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

References:

1. Qualitative Analysis and Electrolytic Solution
2. Introduction to Semimicro Qualitative Analysis (8 ed.)