The coloured glassy mass formed in the borax bead test is due to the formation of ?

The borax bead test is a traditional qualitative analysis method used in inorganic chemistry to identify metal cations based on the colour they produce when heated with borax (Na₂B₄O₇·10H₂O). When borax is strongly heated on a loop of platinum or nichrome wire, it loses its water of crystallization and undergoes decomposition in two steps:

1. 
   

   Na₂B₄O₇·10H₂O → Na₂B₄O₇ + 10H₂O

   
   


2. 
   

   Na₂B₄O₇ → 2NaBO₂ + B₂O₃

   
   

Here, boric anhydride (B₂O₃) acts as a flux and reacts with metallic oxides to form metal metaborates (M(BO₂)₂). These metaborates dissolve in the molten borax bead to give a transparent, coloured glassy mass. The colour depends on the metal ion present, as transition metals form differently coloured borates due to d–d electronic transitions.

Examples include:

• 
  

  Cu²⁺ (Copper): Blue-green bead

  
  


• 
  

  Cr³⁺ (Chromium): Green bead

  
  


• 
  

  Co²⁺ (Cobalt): Deep blue bead

  
  


• 
  

  Ni²⁺ (Nickel): Brownish bead

  
  


• 
  

  Fe³⁺ (Iron): Yellow to brown bead

  
  

These characteristic colours help in the identification of metal ions in laboratory testing. The reaction producing the coloured glassy mass is:
MO + B₂O₃ → M(BO₂)₂

The bead appears colourless if the metal ion does not form a coloured compound, such as in the case of Zn²⁺ or Al³⁺.

The test demonstrates the principles of coordination chemistry and oxidation states of transition metals, showing how d-orbital interactions lead to visible colours.

Hence, the coloured glassy mass observed in the borax bead test is due to the formation of metal metaborates, which serve as the basis for identifying transition metal ions in qualitative inorganic analysis