  Chemical clock reactions are characterised by a relatively long induction
period followed by a rapid `switchover' during which the concentration of a
\emph{clock chemical} rises rapidly. In addition to their interest in chemistry
education, these reactions are relevant to industrial and biochemical
applications. A substrate-depletive, non-autocatalytic clock reaction involving
household chemicals (vitamin C, iodine, hydrogen peroxide and starch) is
modelled mathematically via a system of nonlinear ordinary differential
equations. Following dimensional analysis the model is analysed in the phase
plane and via matched asymptotic expansions. Asymptotic approximations are
found to agree closely with numerical solutions in the appropriate time
regions. Asymptotic analysis also yields an approximate formula for the
dependence of switchover time on initial concentrations and the rate of the
slow reaction. This formula is tested via `kitchen sink chemistry' experiments,
and is found to enable a good fit to experimental series varying in initial
concentrations of both iodine and vitamin C. The vitamin C clock reaction
provides an accessible model system for mathematical chemistry.
