  Chemically reacting systems exhibiting a repeatable delay period before a
visible and sudden change are referred to as clock reactions; they have a long
history in education and provide an idealisation of various biochemical and
industrial processes. We focus on a purely substrate-depletive clock reaction
utilising vitamin C, hydrogen peroxide, iodine and starch. Building on a recent
study of a simplified two-reaction model under high hydrogen peroxide
concentrations, we develop a more detailed model which breaks the slow reaction
into two steps, one of which is rate-limiting unless hydrogen peroxide levels
are very high. Through asymptotic analysis, this model enables the effect of
hydrogen peroxide concentration to be elucidated in a principled way, resolving
an apparent discrepancy with earlier literature regarding the order of the slow
reaction kinetics. The model is analysed in moderate- and high-hydrogen
peroxide regimes, providing approximate solutions and expressions for the
switchover time which take into account hydrogen peroxide concentration. The
solutions are validated through simultaneously fitting the same set of
parameters to several experimental series, then testing on independent
experiments across widely varying hydrogen peroxide concentration. The study
thereby presents and further develops a validated mechanistic understanding of
a paradigm chemical kinetics system.
