complexometric titration
/ kom-pleks-oh-MET-rik ty-TRAY-shun /
Imagine the metal ions in a glass of hard water as small magnets floating around, and you slowly add a molecule shaped like a tiny claw that grabs each magnet and locks it up. A complexometric titration is exactly that: you drip in a reagent that wraps around the metal ions until every last one is caught, then read off how much reagent it took.
Precisely, it is a volumetric method in which the titrant forms a soluble complex with the metal ion being measured. The amount of titrant needed to react with all the metal — the end point, usually signalled by a colour-changing indicator — tells you how much metal was present, because the metal and titrant combine in a fixed, known ratio.
It matters because it is the standard way to measure metals such as calcium, magnesium, zinc, and many others quickly and cheaply, without fancy instruments. Its main demand is control of conditions: pH, masking of interfering metals, and a clean indicator change must all line up, or the result drifts.
To find the calcium in tap water, you add a buffer to fix the pH, a metal-ion indicator that turns wine-red, then titrate with EDTA until the solution snaps to pure blue — the volume of EDTA gives the calcium content.
A classic EDTA titration of calcium, read by an indicator colour change.
Almost every modern complexometric titration uses EDTA, because it binds metals one-to-one regardless of their charge, which keeps the arithmetic simple. The word 'chelometric' means the same thing.