Complexometric Determination of Water Hardness
CHM152LL LAB MANUAL
COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS
Complexometric Determination of Water Hardness
Introduction
Complex ions When a neutral molecule or anion (a Lewis base) donates electron pairs and attaches itself to a metal ion center (a Lewis acid), the resulting cluster, or complex, of atoms becomes a single complex ion. When such complexes form, the electron donating groups (called ligands) form coordinate covalent bonds through empty orbitals on the metal ion. An example appears in Figure 1, where the copper(II) ion complexes with four ammonia molecules (the electron donor ligands) to form the square planar, copper-ammine complex cation. NH3 | H3N— Cu2+ —NH3 | NH3 Figure 1. The complex ion tetraamminecopper(II) [Cu(NH3)4]2+. Some polyatomic ligands have multiple lone pairs of electrons available for bonding to the central metal ion. When such a ligand (with more than one binding site) forms a complex with a metal ion, we call the process chelation, and the ligand used in the complex the chelating agent. Chemists often employ chelation to make the metal more soluble–or less soluble–in a solvent of choice. For example, toxic heavy-metal waste cleanup and remediation uses a selected chelating agent to form complexes with heavy-metal ions, which eases the metal’s removal from soil. Doctors use a similar method to chelate metal toxins in the human body for therapeutic applications. A common chelating agent is ethylenediaminetetraacetic acid (EDTA). 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 to a metal ion center, making EDTA a hexadentate (“six-toothed”) ligand which forms an octahedral complex.
Fig. 2. Ethylenediaminetetraacetic acid (EDTA).
Fig. 3. EDTA4– chelating a calcium ion.
REV 2012-02-13
COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS
PAGE 1 OF 4
CHM152LL LAB MANUAL
COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS
COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS
Complexometric Determination of Water Hardness
Introduction
Complex ions When a neutral molecule or anion (a Lewis base) donates electron pairs and attaches itself to a metal ion center (a Lewis acid), the resulting cluster, or complex, of atoms becomes a single complex ion. When such complexes form, the electron donating groups (called ligands) form coordinate covalent bonds through empty orbitals on the metal ion. An example appears in Figure 1, where the copper(II) ion complexes with four ammonia molecules (the electron donor ligands) to form the square planar, copper-ammine complex cation. NH3 | H3N— Cu2+ —NH3 | NH3 Figure 1. The complex ion tetraamminecopper(II) [Cu(NH3)4]2+. Some polyatomic ligands have multiple lone pairs of electrons available for bonding to the central metal ion. When such a ligand (with more than one binding site) forms a complex with a metal ion, we call the process chelation, and the ligand used in the complex the chelating agent. Chemists often employ chelation to make the metal more soluble–or less soluble–in a solvent of choice. For example, toxic heavy-metal waste cleanup and remediation uses a selected chelating agent to form complexes with heavy-metal ions, which eases the metal’s removal from soil. Doctors use a similar method to chelate metal toxins in the human body for therapeutic applications. A common chelating agent is ethylenediaminetetraacetic acid (EDTA). 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 to a metal ion center, making EDTA a hexadentate (“six-toothed”) ligand which forms an octahedral complex.
Fig. 2. Ethylenediaminetetraacetic acid (EDTA).
Fig. 3. EDTA4– chelating a calcium ion.
REV 2012-02-13
COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS
PAGE 1 OF 4
CHM152LL LAB MANUAL
COMPLEXOMETRIC DETERMINATION OF WATER HARDNESS