Intermediate Conc. for 1st Order Series Rxn is the concentration of product of first step or intermediate, of second step of first order irreversible rxn. And is denoted by CR,1st order . Intermediate Conc. for 1st Order Series Rxn is usually measured using the Mole per Cubic Meter for Molar Concentration. Note that the value of Intermediate Conc. for 1st Order Series Rxn is always positive.