  A mathematical model to describe the growth of an arbitrarily large number of
nanocrystals from solution is presented. First, the model for a single particle
is developed. By non-dimensionalising the system we are able to determine the
dominant terms and reduce it to the standard pseudo-steady approximation. The
range of applicability and further reductions are discussed. An approximate
analytical solution is also presented. The one particle model is then
generalised to $N$ well dispersed particles. By setting $N=2$ we are able to
investigate in detail the process of Ostwald ripening. The various models, the
$N$ particle, single particle and the analytical solution are compared against
experimental data, all showing excellent agreement. By allowing $N$ to increase
we show that the single particle model may be considered as representing the
average radius of a system with a large number of particles. Following a
similar argument the $N=2$ model could describe an initially bimodal
distribution. The mathematical solution clearly shows the effect of problem
parameters on the growth process and, significantly, that there is a single
controlling group. The model provides a simple way to understand nanocrystal
growth and hence to guide and optimise the process.
