Page 718 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Precipitation 673

and the six NH 3 molecules bonded to the central Equivalent: Molecular weight of a given compound divided
Co 3þ are ligands. The coordination number is the by its electric charge; for example, the equivalent
number of ligand atoms bonded directly to the cen- weight of calcium as Ca 2þ is 40 g=mol=2 ¼ 20
tral metal ion; thus, the coordination number for g=mol.
[Co(NH 3 ) 6 ] 3þ is 6 because 6 ligand atoms, that is, Hardness: Cations that consume soap and which may cause
NH 3 , are bonded to the Co 3þ ion. Common ligands precipitates that may deposit as scale in boilers or
in coordination compounds include NH 3 ,F ,Cl pipes, etc.

[:C N:] etc. Heavy metals: Some metals that are toxic also have a high

See also chelate from Greek chela, crab’s claw atomic weight; common ones in this category
(Silberberg, 1996, pp. 988–990). include As, Pb, Cd, and Hg, which are also toxic.
Coordination compound: One or more central atoms or Some authorities (e.g., SenGupta, 2002, p. 1) have
central ions, usually metals, with a number of ligands suggested that the term ‘‘heavy metal’’ is a misnomer
attached. and the designation should be ‘‘toxic metals’’ as a
Coprecipitation: In the formation of an amorphous precipi- more accurate description of the particular elements
tate, for example, Fe 2 O 3 H 2 O, trace elements (both of concern.
dissolved and suspended) may be adsorbed onto and Hydrolysis:
trapped within the precipitate (Banerjee, 2002, 1. A ligand exchange reaction of hydrated metal
p. 182). ions with an acid or base. Example of stepwise
Covalent bond: Interatomic forces between two atoms are hydrolysis of aquoaluminum(III) is
due to the attraction of the two nuclei to the mutually
shared electrons. The latter forces are stronger than
Al(H 2 O) 6 3þ þ H 2 O
the repulsive forces between the nuclei, giving a net
attractive force between the atoms, for example, H 2 . ! Al(H 2 O) 5 OH 2þ þ H 3 O þ (21:G:1)
Such bonding may occur also within polyatomic
2 Al(H 2 O) OH 2þ þ H 2 O
ions, for example, carbonate, CO 3 (Silberberg, 5
1996, p. 62). The shared electron pair, or bonding ! Al(H 2 O) 4 (OH) 2 þ H 3 O þ (21:G:2)
þ
pair, is represented by a pair of dots or a line, for
example, H:H or H–H (Silberberg, 1996, p. 62). The Al(H 2 O) 4 (OH) 2 þ H 2 O
þ
bond energy depends on the compound and the
! Al(H 2 O) (OH) (s) þ H 3 O þ (21:G:3)
3
3
number of bonds, for example, H–H bond energy
is 432 kJ=mol; C¼¼C bond energy is 614 kJ=mol
Al(H 2 O) 3 (OH) 3 (s) þ H 2 O
(Silberberg, 1996, p. 336).
! Al(H 2 O) (OH) þ H 3 O þ (21:G:4)
Covalent compound: Covalent compounds form when elem- 2 4
ents share electrons, which usually occurs between
nonmetals. The foregoing reactions illustrates that hydrolysis
Crystal: A solid that is ‘‘ordered’’ at the molecular level; an of metal ions is a stepwise replacement of coord-
ordered appearance is also visible since the order inated molecules of ‘‘water or hydration’’ by
extends to the external appearance of the solid hydroxyl ions. In the reactions shown, this occurs
(Silberberg, 1996, p. 428). by transfer of protons from waters of hydration to
Crystal lattice: (1) Three-dimensional framework of par- free water molecules to from hydronium ion. The
ticles that form a crystal. (2) An array of points that species concentrations are functions of pH since
forms a regular pattern that exists throughout the they are proton transfers.
crystal; within the array, a ‘‘unit-cell’’ is the simplest 2. Another definition of hydrolysis is the reaction of
arrangement of points that, when repeated, gives the a salt constituent, cation, and anion, with water.
lattice. The ‘‘particles’’ may be atoms, molecules, or Within the framework of the Bronsted theory,
ions (Silberberg, 1996, pp. 430–435). the term hydrolysis is no longer necessary say
Electron activity: The idea of electron activity, {e }, is Stumm and Morgan (1996, p. 91), since in prin-

analogous to proton activity, {H }. While pH is ciple there is no difference in the proteolysis
þ
of a molecule and that of a cation or anion to
þ
defined as pH log{H }, pe is defined as pe
log{e } (Snoeyink and Jenkins, 1980, p. 339). The water.

pe term is a measure of the availability of electrons in Ionic bond: The central theme of ionic bonding is the transfer
solution, although no free electrons exist in solution. of electrons from metal to nonmetal to form ions that
For a half-reaction, ox þ ne ! red, pe ¼ pe8 come together into a solid ionic compound (Silber-
(1=n)log[(red)=(ox)] and pe8 ¼ (1=n)log K, in which berg, 1996, p. 328).
K is the equilibrium constant for the half reaction. Langelier index: Professor W. F. Langelier devised an
The pe is related to a half-cell potential, E H , as, expression from the acid–base equilibrium statement,
pe ¼ 16.9E H (Snoeyink and Jenkins, 1980, p. 339). which provides an ‘‘index’’ as to whether a water is