Measuring a second order reaction rate with reactants A and B can be problematic: The concentrations of the two reactants must be followed simultaneously, which is more difficult; or measure one of them and calculate the other as a difference, which is less precise. A common solution for that problem is the pseudo-first order approximation.
If the concentration of one relative reactant remains constant because it is supplied in great excess, its concentration can be absorbed at the expressed constant rate, obtaining the pseudo first order reaction constant, because in fact, it depends on the same concentration of only one of the two reactants. If, for example, [B] remains constant, then: