Using compounds in two equations

In some volumetric analyses the known substance and the unknown substance do not appear in the same balanced equation (that is, they are not related directly). For example, the amount of the oxidant OCl^– in a given volume of bleach solution can be determined by reacting the bleach solution with I^– in acid. The HOCl formed reacts with I^– to form I₂ (equation 1). The I₂ is then titrated (equation 2) with a solution of Na₂S₂O₃of known concentration.

(1) H⁺ +			HOCl	+ 2I^– H₂O +		I₂	+	Cl^–
			unknown			formed

	n(I₂)	=	n(HOCl)
	1		1

(2)	I₂	+	2S₂O₃^2–	2I^– + S₄O₆^2–
	reacts		known

	n(S₂O₃^2–)	=	n(I₂)
	2		1

The amounts in moles of the known (S₂O₃^2–) and unknown substances (HOCl) are related through the substance (I₂) which is formed in the first reaction and consumed in the second. Thus, the relationship to the right of equation (2) above is applied before the relationship to the right of equation (1) above, as shown below.

Known
c(S₂O₃^2–)
V(S₂O₃^2–)

n = cV

Step 1

n(S₂O₃^2–)

n(S₂O₃^2–)	=	n(I₂)
2	=	1

Step 2

n(I₂)

n(I₂) = n(HOCl)

Step 3

n(HOCl)

c	=	n
		V

Step 4

Unknown
c(HOCl)