Complexometric Determination of Water Hardness

ABSTRACT The average concentration of CaCO3 obtained was 212 ppm, with a standard deviation of 1 ppt. The results indicate that the unk B tap water can be considered as hard water. INTRODUCTION Hard water is due to metal ions (minerals) that are dissolved in the ground water. These minerals include Ca 2+, Mg2+, Fe3+, SO42-, HCO3-. When this water evaporates or boils, the difficult to dissolve metal salts remain as a scaly residue. Hard water inhibits the effectiveness of soap and detergents. Calcium ions typically make the most significant contribution to water hardness. This is why hardness is measured in terms of mg CaCO3/L of solution. Hardness is also reported in units of parts per million (ppm). Water with a hardness value of < 60 ppm is considered “soft” and water with >200 ppm is considered “hard.” The purpose of this lab was to determine the water hardness of a sample of unk B tap water. The disodium salt of EDTA was used to determine the concentration of M 2+ metal ion impurities in hard water by a complexometirc titration. 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. EDTA acts as a chelating agent because each nitrogen and one oxygen from each of the four carboxylic acid groups has an electron pair to , donate (ligand- a substance that binds with a metal ion to form a complex ion) to a metal ion center, making EDTA a hexadentate ligand which forms an octahedral complex. An indicator, Eriochrome Black T enables the detection of when the EDTA has completely chelated the metal impurities. In the presence of a metal cation, Eriochrome Black T forms a pink complex. H2In- represents the anion of the free, solvated indicator, and M2+ represents Mg2+ or Ca2+.
H2In-3 (aq) + M2+ (aq)  MIn- (aq) + 2H+ (aq)
Blue pink

As you add EDTA solution from a buret, the metal ions preferentially