Volhard method
/ FOLE-hart METH-od /
Imagine paying for something with a banknote that is too large, then counting your change to work out the price. The Volhard method does the same with silver: you deliberately add too much silver, then measure the leftover to figure out how much was used up.
Specifically, it is an argentometric back-titration. A known, excess amount of silver nitrate is added to precipitate the halide; the unreacted silver is then titrated with thiocyanate, using an iron salt as indicator that turns blood-red the instant all the leftover silver is consumed.
It matters because, unlike the Mohr method, it works in acidic solution, which avoids many interferences and lets it measure bromide and iodide cleanly. Its main subtlety: for chloride, the silver chloride precipitate can slowly redissolve and give back chloride, so it is filtered off or coated before the back-titration to avoid an error.
To measure chloride by Volhard, excess silver nitrate is added in acid, the silver chloride is filtered off, and the leftover silver is back-titrated with potassium thiocyanate until a faint blood-red iron-thiocyanate colour persists.
Excess silver is added first, then the leftover is back-titrated to a red end point.
It is the standard acidic-solution alternative to the Mohr method. The red colour comes from an iron-thiocyanate complex, not from any silver precipitate, which is a common point of confusion.