Published on March 14, 2014
Determination of Hardness of Water
What is Hardness..? • Generally hardness of water is defined as the measure of capacity of water to precipitate soap i.e., the capacity of the water to form lather with soap. • Hard water contains dissolved minerals such as Ca2+, Mg2+, Fe3+, SO4 2- ,etc., • The degree of hardness is measured in Parts Per Million(ppm) or Grams per Gallon(GPG). • Water with different degrees of hardness is used for different purposes such as household and industrial purposes.
Why determine Hardness..? • It is essential to know the hardness of the water since it defines the purpose of it. • Hard water is not a health hazard but it cannot be used in industrial applications such as in boilers,pipes,etc., • It has the tendency of forming CaCO3 precipitate which later results in sludge/scale formation.
Determination of Hardness • The hardness of water can be estimated by methods such as gravimetric analysis, EDTA titration, atomic absorption, etc., • In the above methods, EDTA titration is the most inexpensive and simple way of determining the hardness. • There are two types of hardness:Temporary and Permanent. Temporary Hardness is due to the bicarbonate ions(Ca,Mg bicarbonates) being present in the water. It can be removed by heating the water. Eg:Ca (HCO3 )2 ------> CaCO3 +CO2 +H2O Permanent hardness is due to chlorides and sulphates of Ca2+, Mg+2, Fe3+ and SO4 - ions and it cannot be removed by boiling.
EDTA Titration • Permanent hardness is usually determined by titrating it with a standard solution of ethylenediamminetetraacetic acid, EDTA. • The EDTA is a complexing, or chelating agent used to capture the metal ions. This causes the water to become softened, but the metal ions are not removed from the water. • This method includes a series of titrations to determine the total,permanent,temporary,Ca, Mg hardness of the given water sample. • First the EDTA soln. is standardized by titrating it against a standard CACl2 solution and its normality is found out. End point is appearance of steel blue color.
Colors of the soln before and after the titrations • Then the total hardness is found out by titrating the water sample(added buffer soln and EBT indicator) against the standardized EDTA solution with end point as the same steel blue color. Hardness=N x 50 x 1000 (N-Normality) • The permanent hardness is found out by titrating a boiled and filtered hard water sample against EDTA solution. • Temporary hardness is found by subtracting the permanent hardness from the total hardness. • The Ca hardness is found by titrating the water sample(added 2N NaOH, murexide indicator) against EDTA solution. Here the end point is the appearance of purple color. If necessary the Mg hardness is found by subtracting the Ca hardness from total hardness.
Mechanism… • EDTA is a versatile chelating agent. A chelating agent is a substance whose molecules can form several bonds to a single metal ion. Chelating agents are multi-dentate ligands. A ligand is a substance that binds with a metal ion to form a complex ion.Multidentate ligands are many clawed, holding onto the metal ion to form a very stable complex. EDTA can form four or six bonds with a metal ion, forming a stable complex. • Ca2+ + EBT ------> [Ca-EBT] Unstable complex(Wine red) [Ca-EBT] + EDTA -------> [Ca-EDTA] + EBT Stable complex (Blue) (Colorless) Same reaction occurs incase of Mg2+ also. • At the end of the titration EBT is released free, resulting in the formation of blue color.
• The figure shows the structure of Ca-EDTA complex where, > carbons are black >hydrogens are white >oxygens are red >nitrogens are blue >calcium ion is the green ball. • The EDTA complexes with the metal ions,while EBT is set free. • In determining the Ca hardness, the water sample is heated to remove temporary hardness and the Mg ions are precipitated by addition of 2N NaOH solution and it is titrated against EDTA using murexide indicator. • Necessary condition to be maintained while doing the titration is to keep the pH at about 10.
Pros and Cons ADVANTAGES: • It is a very simple and fairly accurate method. • It also very cost efficient as only a burette and a conical flask are required. • It also very quick and time efficient process when compared to gravimetric analysis. DISADVANTAGES: • A disadvantage is that there might be some human errors due to the solutions made up by hand. • EDTA grabs all the metal ions in the water, not just the Ca2+ ions. This gives us a value that is not truly the concentration of Ca2+ ions. This causes an experimental error of about 1%, but that is acceptable.
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