The primary KIE is typically measured by comparing the reaction rates of isotopically labeled and unlabeled substrates. The ratio of these rates gives the kinetic isotope effect. For example, if the reaction rate with protium (H) is k_H and the rate with deuterium (D) is k_D, the primary KIE is given by:
KIE = k_H / k_D
A significant difference in these rates indicates a substantial primary KIE, suggesting that the bond involving the isotope is broken or formed during the rate-determining step.
