Jawaharlal Nehru Technological University Hyderabad B.Tech I ENGINEERING CHEMISTRY Previous Question Paper (June - 2022 - R16)

The principle of the EDTA titration method is based on the formation of stable, water-soluble, colorless chelate complexes between EDTA (Ethylenediaminetetraacetic acid) and metal ions like Ca²⁺ and Mg²⁺. Since EDTA is a hexadentate ligand, it forms a 1:1 complex with these ions. The titration uses an indicator, typically Eriochrome Black T (EBT), which forms a wine-red complex with metal ions at pH 10. As EDTA is added, it displaces the indicator, causing a color change from wine-red to steel-blue at the endpoint.

The estimation procedure involves:

1. Prepare a standard hard water sample and adjust its pH to 10 using an ammonium buffer (NH₄Cl/NH₄OH).
2. Add a few drops of EBT indicator, turning the solution wine-red.
3. Titrate against a standard EDTA solution until the color changes sharply to steel-blue.
4. The volume of EDTA consumed corresponds to the total hardness (Ca²⁺ + Mg²⁺).
5. To determine calcium hardness separately, perform a titration at pH 12 using Murexide indicator, where Mg²⁺ precipitates as Mg(OH)₂.

The Nalgonda technique is a cost-effective method for defluoridation of water, primarily involving the addition of alum (aluminum sulfate) and lime (calcium hydroxide) to water followed by flocculation, sedimentation, and filtration. While the standard technique relies on chemical precipitation, monitoring the efficiency of fluoride removal is accurately achieved using an Ion Selective Electrode (ISE) method.

The ISE method for fluoride determination operates on the principle of a potentiometric sensor. The electrode consists of a lanthanum fluoride (LaF₃) crystal membrane that allows only fluoride ions to pass through, creating a potential difference proportional to the logarithm of the fluoride ion activity in the sample, as described by the Nernst equation:

In practice, a Total Ionic Strength Adjustment Buffer (TISAB) is added to the water sample to maintain a constant ionic strength, mask interfering ions like Al³⁺ or Fe³⁺, and maintain an optimal pH (around 5.0 to 5.5) to prevent the formation of HF or OH^- interference. The potential is measured against a reference electrode, and the fluoride concentration is determined by comparing the measured potential against a calibration curve prepared from standard fluoride solutions.