back titration
/ BAK ty-tray-shun /
Imagine pouring a known number of buckets of water onto a fire, then measuring how much spills back into a tank afterward; the water that did not spill back must have gone into the fire. Back titration uses that same trick of measuring the leftover.
In a back titration you deliberately add an excess of one reagent of known amount to the sample, let it react fully, and then titrate the unreacted leftover with a second standard solution. The amount that reacted with the sample is simply the amount added minus the amount left over.
It matters because it rescues situations where a direct titration fails — when the reaction is too slow, the end point too unclear, or the analyte a stubborn solid. Its cost is extra steps and two known solutions, so errors from each can add up if you are not careful.
To measure calcium carbonate in an antacid tablet, a chemist adds excess standard hydrochloric acid to dissolve it, then back-titrates the leftover acid with standard sodium hydroxide; the acid consumed by the tablet gives the carbonate content.
Measure the leftover, then subtract to find what reacted.
Back titration needs two accurately known solutions instead of one, so it carries roughly twice the chance for volumetric error. Use it only when a clean direct titration is not practical.