Mohr method
/ MORE METH-od /
Picture a race where two finish lines sit close together: the front-runner crosses first, and only after it does the second runner cross and raise a coloured flag. The Mohr method uses that staged timing — a second, slightly redder precipitate forms only after the main one is finished, signalling the end with a colour change.
Specifically, it is a precipitation titration of chloride (or bromide) with silver nitrate, using a small amount of chromate ion as indicator. Silver first ties up all the chloride as white silver chloride; only when that is exhausted does silver start to form red silver chromate, whose appearance marks the end point.
It matters as a classic, cheap way to measure chloride in water and food. Its catch is that it only works in roughly neutral solution: too acidic and the chromate indicator fails, too basic and silver hydroxide precipitates, so the pH must be controlled within a narrow band.
Titrating salty water with silver nitrate and a pinch of potassium chromate, the mixture stays milky white until, at the end point, the first lasting reddish-brown tinge of silver chromate appears.
The end point is the first permanent reddish tint of silver chromate.
Because some excess silver is needed before the red colour shows, the Mohr method slightly overshoots; a blank titration on chromate-only water corrects for this small, consistent error.