General Chemistry II Lab
“Determination of the Hardness of Water”
The purpose of this practical is to determine a hardness of water by titrating with chelating agent EDTA.
Water is supposed to be hard when it contains dissolved minerals especially Ca2+, Mg2+, Fe3+, SO42-, and HCO3-. Hard water can be formed when underground water percolates through limestone or when water dissolves calcium sulfate or magnesium sulfate. The concentration of Ca2+, (and Mg2+) generally exceeds that of any other metal ions, consequently, hardness is expressed in terms of CaCO3. The determination of water hardness is a useful test that provides a measure of the quality of water for household and industrial uses. Hard water has no obvious health effects, but these ions form insoluble compounds with soap. Results are unsightly "scum" on clothing and so-called "bath-tub ring". Water hardness is important to industry because hard water, upon being heated, precipitates CaCO3, which then clogs boilers and pipes. It results in the equipment malfunction or damage and expensive cleaning. Elimination of water hardness is referred to as "water softening". Hardness due to bicarbonate (HCO3-) can be eliminated by boiling, to expel CO2. Thus, bicarbonate hardness is classified as temporary hardness. Hardness arising due to the presence of Ca2+, Mg2+, Fe3+, and SO42-, cannot be eliminated in the same way as temporary hardness. Water containing these ions is said to be permanently hard. Permanent hardness is usually determined by titration with a standard solution of ethylene diammine tetraacetic acid, EDTA. EDTA is a ligand that has more than one point of attachment and complexes most metal ions in aqueous solution. It is weak acid that loses protons. They react in 1:1 ratio, so the titrations reach the end point when all the metal ions reacted. Eriochrome Black-T (Erio-T) indicator will be used, since both EDTA and Ca+2 colorless. This is a molecule that...
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