|A fairly standard sight glass|
Credit: John C. Ernst
Occasionally, the interface is so clean that it is impossible to see. In such cases, a conductivity measurement can be most useful to distinguish between the aqueous and the organic phase. It is also useful to sprinkle a pinch of Celite (diatomaceous earth) into the batch. The Celite tends to collect at the interface and helps to accentuate it. The Celite of course must be removed later by polish filtration, and so this is not recommended during the final processing steps.One of the things that I've mentioned before is that sometimes, it's actually quite difficult to see the reaction in the plant, because you can't see through the reactor walls like you can with a separatory funnel. You have all sorts of visual cues for a separation through a all-glass separatory funnel: you can typically watch the little bubbles of phases, the vortices, you can shine a flashlight into it (great trick, works like a charm.)
But when you're working with a large enough reactor, the walls are not going to be made of glass -- then, you're going to have to rely on a sight glass.* Sight glasses are smallish glass cylinders attached to the bottom valve of the reactor that your solutions flow through. Experienced operators have told me that they've blinked as they're pumping solutions through the sight class, missed the phase cut and had to send the entire 2,000 gallon batch back around through the reactor (which, of course, costs time and money, etc.) That Celite trick might help a little -- I'll have to try it sometime.
*Unless, like Mr. McConville notes, you're relying on conductivity measurements or a density flow meter.