  Sharp chemical discontinuities can occur in protoplanetary discs,
particularly at `snow-lines' where a gas-phase species freezes out to form ice
grains. Such sharp discontinuities will diffuse out due to the turbulence
suspected to drive angular momentum transport in accretion discs. We
demonstrate that the concentration gradient - in the vicinity of the snow-line
- of a species present outside a snow-line but destroyed inside is strongly
sensitive to the level of turbulent diffusion (provided the chemical and
transport time-scales are decoupled) and provides a direct measurement of the
radial `Schmidt number' (the ratio of the angular momentum transport to radial
turbulent diffusion). Taking as an example the tracer species N$_2$H$^+$, which
is expected to be destroyed inside the CO snow-line (as recently observed in TW
Hya) we show that ALMA observations possess significant angular resolution to
constrain the Schmidt number. Since different turbulent driving mechanisms
predict different Schmidt numbers, a direct measurement of the Schmidt number
in accretion discs would allow inferences about the nature of the turbulence to
be made.
