CHEM 01LC Lecture Notes - Lecture 6: Calcium Hydroxide, Ethylenediaminetetraacetic Acid, Titration
CHEM 01LC Experiment 6: Determination of Water Hardness
●Experimental purpose
○The purpose of this experiment is to use a standardized EDTA solution to
determine the water hardness of an unknown sample
●General Terminology
○Hard water: water that contains magnesium and/or calcium ions
■Ions react with soap to prevent the formation of a lather
○Temporary hard water: water that contains less than two moles of bicarbonate ion
for every magnesium or calcium to be removed
○Permanent hard water: water that contains less HCO3
-
■Must precipitate ions as carbonates, phosphates, or borates
●Softening of Hard Water
○Softening: the removal of the magnesium and calcium ions from solution
○Two methods:
■Boiling
:
●Ca2+ + 2HCO3
- → CaCO3 + H2O + CO2
■Treatment with slaked lime
:
●Ca2+
+ 2HCO3
- + Ca(OH)2 → 2CaCO3 + H2O
●Water Hardness
○Water hardness: the total concentration of Ca2+ and Mg2+ ions in water
■Expressed in ppm CaCO3 (mg CaCO3/ L water)
○Hardness is determined through titration with a chelating agent
■EDTA (ethylenediaminetetraacetic acid)
●Can lose four protons
●Indicator
○The indicator used in this experiment is calmagite
■Complexes with magnesium to form MgIn- (wine red)
■Indicator reverts to acid form HIn2- (sky blue)
Document Summary
Chem 01lc experiment 6: determination of water hardness. The purpose of this experiment is to use a standardized edta solution to determine the water hardness of an unknown sample. Hard water : water that contains magnesium and/or calcium ions. Ions react with soap to prevent the formation of a lather. Temporary hard water : water that contains less than two moles of bicarbonate ion for every magnesium or calcium to be removed. Permanent hard water : water that contains less hco 3 . Must precipitate ions as carbonates, phosphates, or borates. Softening : the removal of the magnesium and calcium ions from solution. Water hardness : the total concentration of ca 2+ and mg 2+ ions in water. Expressed in ppm caco 3 (mg caco 3 / l water) Hardness is determined through titration with a chelating agent. The indicator used in this experiment is calmagite. Complexes with magnesium to form mgin - (wine red)
