What is the theory yof determining the hardness of water by EDTA titration?

Question

What is EDTA?
How can I calculate the concentration of the metal ions?

Answer 1

Na2EDTA is a complex molecule. Its name stands for ethylenediaminetetraacetic acid – disodium salt. The formula is Na2C10H14N2O8. Even though its name and structure are complex, you only need to know that: EDTA reacts with Ca+2 and Mg+2 in a one to one ratio as follows:
:Ca+2 + EDTA-2 -----> CaEDTA

Mg+2 + EDTA-2 -----> MgEDTA
The formula mass of Na2EDTA dihydrate is 372.24 g/mole.Na2EDTA comes as a dihydrate(absorbs water from the atmosphere) which is efflorescent (literally means "bursting into flower" but in this case just means that it forms bubbles), you cannot reliably weigh out the desired number of grams. The salt may be fully, or only partially, hydrated at the time you weigh it. The result is that you cannot be certain that you have prepared the proper strength reagent of EDTA..therefore: you need to standardize the solution The results of the titration is reported as mg of CaCO3/liter.
To convert Calcium plus Magnesium (meq/L) Hardness as CaCO3 (mg/L) 50
Of course since Fe and Mg and Ca and other metals are also present in water it is better (more accurate) to titrate for each metal species individually and calculate the hardness using the results of the individual results. EDTA hardness is sometimes referred to as "Total hardness"and is reported as " Hardness as mg/l CaCO3."

Answer 2

Determination Of Water Hardness

Answer 3

The hardness of water is defined as the total concentration of alkaline earth metals present. This is usually determined by measuring the total amount of magnesium and calcium that is present, since these two make up the majority of metals present. The accepted practice for reporting the hardness of water is by milligrams of calcium carbonate.
EDTA is used as a titrant since it forms a complex with metal ions, thereby measuring the amount of calcium and magnesium present. The amount of EDTA used in the tirtation is proportional to the amount of calcium and magnesium present. Once you know the volume of EDTA used for the titration, this value can be converted into milligrams of calcium carbonate or PPM (part per million) calcium carbonate, therefore determining the hardness of water.

EDTA: ethylenediaminetetraacetic acid, a chelating agent that can donate electrons, Lewis Base
usually prepared by dissolving disodium EDTA in water

Answer 4

EDTA is used to bind metal ions, such as calcium, lead etc,. If you add a known amount of EDTA to water, and measure the free (unbound) EDTA, then you have a measure of how much mineral is in the water- the lower the free EDTA, the higher the mineral content of the water. Mineral concentration is typically what we term hardness of water.