Are metaphosphoric acid and phosphonic acid the same? I have been told that they're same BUT the structures online are different.

Metaphosphoric acid has formula: $\ce{(HPO3)_n}$

phosphonic acid has the formula: $\ce{H3PO3}$

Is this correct?


1 Answer 1


Metaphosphoric acids general formula is (HPO3)n ; where n denoted number of phosphoric acid units present in the ring with n being greater or equal to 3. These acids are actually the phosphoric acid units bonded together in rings (cyclic structures), hence forming metaphosphoric acid molecules.

In Metaphosphoric acids, each phosphorus has an oxidation state of +5. Here I am giving you the example of trimetaphosphoric acid i.e. when n is 3.

Trimetaphosphoric acid

Phosphonic acid or Orthophosphorous acid is H3PO3 with the oxidation state of phosphorous being +3. Here is its structure:

Orthophosphoric acid

  • $\begingroup$ Not sure phosphorous is +3. Hydrogen attached to it might be hydridic? $\endgroup$ Commented Mar 22, 2017 at 10:43
  • 1
    $\begingroup$ Yeah, P claims the two electrons from the P-H bond to give 5-2=3. $\endgroup$ Commented Mar 22, 2017 at 20:38

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