In some volumetric analyses the Known and the Unknown are in the same balanced equation. Thus, 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 Na2S2O3 of known concentration.
The amount in moles of the known (S2O32–) and unknown substances (HOCl) can be related through a substance (I2) formed in equation (1) and consumed in equation (2).
Thus, the relationship between amounts for equation (2) above is applied before the relationship between amounts for equation (1).
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 Na2S2O3 of known concentration.
| (1) | H+ + | HOCl | + 2I–  | |
| unknown | ||||
| H2O + | I2 | + | Cl– | |
| formed | ||||
| n(I2) | = | n(HOCl) |
| 1 | 1 |
| (2) | I2 | + | 2S2O32– | | |||
| reacts | known | ||||||
| 2I– + S4O62– |
| n(S2O32–) | = | n(I2) |
| 2 | 1 |
The amount in moles of the known (S2O32–) and unknown substances (HOCl) can be related through a substance (I2) formed in equation (1) and consumed in equation (2).
Thus, the relationship between amounts for equation (2) above is applied before the relationship between amounts for equation (1).
| Known c(S2O32–) V(S2O32–) | n = cV Step 1 | n(S2O32–) |
 Step 2 | n(I2) |
| n(I2) = n(HOCl) Step 3 | n(HOCl) |
Step 4 | Unknown c(HOCl) |