In this post, we will determine the oxidation numbers in the phosphate ion – PO43-.

To determine the oxidation number of an atom(s) in a molecule or an ion, start with the known oxidation numbers and the rules summarized below:



Keep in mind that the summary is zero for neutral molecules and is equal to the charge for ions.

For PO43-, the element with a standard oxidation state is oxygen which is -2, and we need to determine the oxidation state of the sulfur. 

Rule 8 – The oxidation of oxygen is -2:

Assign x for the oxidation state of P and set up an equation:

x + 4(-2) = -3, x = +5

The oxidation number of the element in the given ion is the same for any salt containing that ion. For example, we can confirm that the oxidation number phosphorus in Ca3(PO4)2 is +5 because it is +5 in the phosphate ion.

Based on rule 2, the oxidation number of calcium in Ca2+ is +2, and that of oxygen is -2 according to rule 8. So, we can set up the following equation: 

3(+2) + 2(x) + 2 x 4 x (-2) = 0

x = +5 

To confirm the answer:


3(+2) + 2(+5) + 2 x 4 x (-2) = 0


Further Reading


More examples of the oxidation state in this multiple-choice quiz:


Oxidation State and Redox Reactions Quiz


Reactions in Aqueous Solutions Quiz



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