Only when substrate B is all used up or depleted, species C can build up in amount causing the color to change.
The basis of the reaction is similar to substrate-depletive clock reaction, except for the fact that rate k2 is very slow leading to the co-existing of substrates and clock species, so there is no need for substrate to be depleted to observe the change in color.
[2] The reaction is irreproducible in each run due to the initial inhomogeneity of the mixture which result from variation in stirring rate, overall volume as well as geometry of the reactors.
Repeating the reaction in the statistically meaningful manners leads to the reproducible cumulative probability distribution curve.
After a delay, a dark blue color suddenly appears due to the formation of a triiodide-starch complex.