The charge of phosphate : 3-
Further explanation
Oxidation is an increase in oxidation number, while reduction is a reduction in oxidation number.
The formula for determining Oxidation Numbers in general:
1. Single element atomic oxidation number = 0.
Group IA (Li, Na, K, Rb, Cs, and Fr): +1
Group IIA (Be, Mg, Ca, Sr and Ba): +2
Group IIIA(Al, B) : +3
H in compound = +1, except metal hydride compounds (Hydrogen which binds to groups IA or IIA) oxidation number H = -1, for example, LiH, MgH₂, etc.
2. Oxidation number O in compound = -2, except OF2 = + 2 and in peroxide (Na₂O₂, BaO₂) = -1 and superoxide, for example KO₂ = -1/2.
3 The oxidation number in an uncharged compound = 0,
Total oxidation number in ion = ion charge, Example NO₃⁻ = -1
Compound AlPO₄ :
The oxidation number in an uncharged compound = 0
Since the charge of Al as group 3A (or group 13) is +3, while the AlPO₄ compound is a polyatomic compound (consisting of polyatomic ions PO₄ and Al), the PO₄ charge:
charge Al + charge PO₄ = 0
+ 3 + charge PO₄ = 0
charge PO₄ = -3