Question

The reagent commonly used to determine hardness of water titrimetrically is: (a) Disodium salt of EDTA (b) Sodium thiosulphate (c) Sodium citrate (d) Oxalic acid

Short answer

(a) Disodium salt of EDTA.

Step-by-step solution

Understand the Concept

Hardness of water is primarily due to the presence of calcium and magnesium ions. To determine the hardness, these ions need to be titrated using a complexometric titration, typically involving a chelating agent like EDTA.

Identify the Suitable Reagent

The most common reagent used for this type of titration is one that can effectively bind with the calcium and magnesium ions. Among the given options, the disodium salt of EDTA is well-known for forming stable complexes with these ions.

Compare the Reagents

Let's compare each reagent: - Disodium salt of EDTA: Forms stable complexes with calcium and magnesium. - Sodium thiosulphate: Commonly used for iodine titrations; not suitable here. - Sodium citrate: Does not effectively titrate calcium or magnesium ions. - Oxalic acid: Used for calcium analysis, but not complexometric titration of multiple ions.

Choose the Correct Answer

Based on the information, the reagent that is specifically used for determining the hardness of water by titrating calcium and magnesium ions is the disodium salt of EDTA.

Key concepts

EDTA

EDTA, short for Ethylenediaminetetraacetic acid, is a powerful reagent widely used in complexometric titration, especially for determining water hardness. Complexometric titration involves forming a complex between a metal ion and a ligand. Here, EDTA acts as the ligand. It has the unique ability to form very stable complexes with metal ions in solution. This feature makes it ideal for binding with calcium and magnesium ions, which contribute to water hardness.

EDTA comes in various salt forms, and the most commonly used in titrations is its disodium salt. This salt dissolves well in water, making it handy for preparing a titration solution. When EDTA is added to a solution containing calcium and magnesium ions, it binds strongly to these ions, forming stable complexes. This reaction allows for the quantification of these ions in a solution. The endpoint of the titration can usually be determined using an indicator that changes color when all the metal ions have formed complexes with EDTA.

Therefore, EDTA is indispensable in both industrial and laboratory settings for analyzing samples where the concentration of ions, like calcium and magnesium, needs to be accurately measured.

Water Hardness

Water hardness is a measure of the concentration of certain metal ions, particularly calcium and magnesium, present in water. These ions occur naturally and often end up in water supplies through soil and rock erosion.

Hard water is not harmful to human health, but it can cause several practical issues. It can leave mineral deposits on plumbing fixtures and appliances, reduce soap effectiveness, and result in limescale build-up in water heaters and boilers. Because of these issues, monitoring and managing water hardness is crucial in both domestic and industrial settings.

Complexometric titration, typically using EDTA, is a standard method for determining water hardness. This process precisely measures the total concentration of calcium and magnesium ions. By analyzing these concentrations, water treatment facilities can adjust their treatment processes to manage water hardness, ensuring optimal water quality for a variety of uses.

Calcium and Magnesium Ions

Calcium and magnesium ions are two common culprits responsible for the hardness of water. These ions are naturally present in groundwater owing to their abundance in the Earth's crust. When water percolates through soils and rocks, it dissolves these mineral ions, which then become part of the water supply.

Calcium ions come from compounds like limestone (calcium carbonate) and chalk, while magnesium ions often originate from rocks like dolomite and serpentine. Both ions interact in water to form stable complexes, contributing to the degree of water hardness.

In the context of a complexometric titration with EDTA, calcium and magnesium ions play pivotal roles. EDTA selectively binds to these ions, removing them from the solution. By comparing the amount of EDTA used in the titration to bind these ions, one calculates the water's hardness level. This process allows for a thorough assessment of water quality, facilitating the management of water sources to prevent the adverse effects of hard water on infrastructures and appliances.