Aqueous redox chemistry:

Oxidation State (Frost) diagrams. 

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For the reaction of element X in formal oxidation state n,   X(n) + ne^-  X(0)  DG^o  nFE^o   If DG^o/F is plotted against the formal oxidation states, the higher the value the less stable, ie. more oxidizing, the species will be.    

For nitrogen at aH⁺ = 1

NO3- + 6H+ + 5e-  1/2N2 + 3H2O	Eo = 1.25V
1/2N2O4 + 4H+ + 4e-  1/2N2 + 2H2O	Eo = 1.36V
HNO2 + 3H+ + 3e-  1/2N2 + 2H2O	Eo = 1.45V
NO + 2H+ + 2e-  1/2N2 + H2O	Eo = 1.68V
1/2N2O + H+ + e-  1/2N2 + 1/2H2O	Eo = 1.77V
1/2N2 + 2H+ H2O + e-  NH3OH+	Eo = -1.87V
1/2N2 + 5/2H+ + 2e-  1/2N2H5+	Eo = -0.23V
1/2N2 + 4H+ + 3e-  NH4+	Eo = 0.27V

    With G^o/F in units of volts and defining DG^o as per mol of nitrogen atoms, the coordinates will be:  

N(V)	NO3-	5x1.25, 5
N(IV)	N2O4	4x1.36, 4
N(III)	HNO2	3x1.35, 3
N(II)	NO	2x1.68, 2
N(I)	N2O	1x1.77, 1
N(-I)	NH3OH+	-1x(-1.87), -1
N(-II)	N2H5+	-2x(-0.23), -2
N(-III)	NH4+	-3x0.27, -3

    A plot of DG^o/F against the formal oxidation state is called an oxidation state diagram very useful for comparative chemistry in aqueous solution.     Figure 2: The oxidation state diagram of nitrogen and phosphorus in acidic conditions.