Total Hardness of Water Determination
Introduction
The main cause of water hardness is dissolved salts of calcium and magnesium. Additionally, the other ions like Strontium, Iron, Barium and Manganese also contribute to water hardness. Traditionally, total hardness is measured by the amount of soap that is required to produce leather. Hardness affects the amount of soap that is needed to produce foam or lather. Hard water requires more soap, because the calcium and magnesium ions form complexes with soap, preventing the soap from foaming. Hard water can also leave a film on hair, fabrics, and glassware. Hardness of the water is very important in industrial uses, because it forms scale in heat exchange equipment, boilers, and pipes. Some hardness is needed in plumbing systems to prevent corrosion of pipes.
To determine the Total Hardness of Water, EDTA (Ethylenediamine tetra acetic acid) is the easiest method because EDTA reacts with all metal without alkali metal and the proportion is 1:1. EDTA forms colorless stable complexes with Ca2+ and Mg2+ ions present in water at pH = 9 - 10. Erichrome black – T dye (E.B.T) is used for this test.
At pH 10, EDTA forms colorless, water soluble stable complexes with calcium and magnesium ions. When the indicator Erichrome black T dye is added into the hard water, then the indicator forms unstable complex with calcium and magnesium ions and the solution turn into wine red. If there is no hardness the color becomes blue which is original color of indicator. Now, when this solution is titrated against EDTA, then the calcium and magnesium ions started to form a stable metal-EDTA complex. After all the free calcium and magnesium ions are consumed, the EBT is replaced by EDTA from the unstable complex and liberates the free Erichrome Black-T. Then the water color change from wine red to blue that indicates the end point.
Reactions
The reactions that take place during the titration are:
Types of Hardness
There are two types of hardness:
To determine the Total Hardness of Water with EDTA method, an inorganic acid is initially added to convert temporary hardness into permanent hardness. Ca(HCO3)2 + 2HCl → CaCl2 + H2O + 2CO2
Objective(s) of the Experiment
This test is carried out in order to estimate the amount of total hardness present in the given sample of water by EDTA titration method.
Equipments and Materials Needed
Apparatus:
Chemicals:
Safety Precaution
Avoid skin contact with chemicals. Clothing contaminated with NaOH solution should be carefully removed. Spillage adhering to skin should be immediately washed with plenty of water.
Procedures
Results and Calculations
Tabulate the readings as shown below:
References
Credits: Samuel Alalade. Obafemi Awolowo University, Ile-Ife, Osun State, Nigeria.