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Explain the principle of EDTA method.

Mumbai University > First Year Engineering > Sem1 > Applied Chemistry 1

Marks: 3M

Year: Dec 2014

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EDTA stands for Ethylene Diamine Tetra Acetic acid. As it is insoluble in water, we use its disodium salt.

Structure of EDTA:

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By nature, Eriochrome Black T indicator is blue in colour. When EBT indicator is added to water sample, it forms a wine red coloured unstable Ca-Mg-EBT complex.

This reaction is carried out under a basic PH of 8- 10 using ammonia buffer.

$Ca^{2+} / Mg^{2+}$ in water + EBT [Ca / Mg – EBT] unstable wine red complex

When EDTA is titrated against the complex, EDTA replaces all the EBT and forms a stable Ca / Mg –EDTA complex. The liberated EBT indicates the end point as steel blue.

$[Ca / Mg \ \ ^– EBT] + EDTA → [Ca / Mg^– \ \ \ EDTA] + EBT$

(Wine red/unstable) $ \ \ \ \ \ \ \ $ (Stable) $ \ \ \ \ \ \ $ (Steel blue)

So, the end point is the colour change from wine red to steel blue.

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