I don't know of any English (or French) word "louch", the closest I've
 found being "louche", (the same in either language), which means,
 loosely, "dodgy", i.e. an adjective.
However, your question suggests a noun. There is a French noun,
 "louche", which means a "ladle" (in English), (or possibly "spatula"?
 - you know, the common metal ones used in chemistry labs)
Alternatively, and I'm really scraping the barrel here, but I
 half-remember, from school chemistry days (but don't quote me)
 something like this term was applied to "the small amount of liquid
 which was used to wash out, and hence dilute, the vessel in which
 one was making up a solution of a particular strength, using a
 known mass of solute, and measured volume of solvent (e.g. water).
 I.e. the "louch" was a small part of this measured volume, which
 was kept aside for rinsing out. Although, on second thoughts,
 I might be confusing this with "douche".
If neither of these sound likely, please send a bit more context/detail
, and I'll try some deeper research (which might take a little while
 longer - e.g. trip to library etc. :) )