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The Islamic University of Gaza Faculty of Engineering Civil Engineering Department Environmental Engineering (ECIV 4324) Chapter 2 – Chemical Water Quality. Water Quality: Definition, Characteristics, and Perspectives. Contents. Chemical water – quality parameters.

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The Islamic University of GazaFaculty of EngineeringCivil Engineering DepartmentEnvironmental Engineering(ECIV 4324)Chapter 2 – Chemical Water Quality

Water Quality: Definition, Characteristics, and Perspectives

chemical water quality parameters
Chemical water – quality parameters
  • Water is called a universal Solvent
  • Chemical parameters of water are:
      • Total dissolved solids,
      • Alkalinity,
      • Hardness,
      • Fluorides,
      • Metals,
      • Organics,
      • Nutrients.
chemistry of solutions
Chemistry of solutions
  • Atom the smallest unit of element
  • Molecules of elements or compounds are constructed of atoms.
  • Molecular mass is the sum of the atomic mass of all atoms in molecule
  • For example :
  • Atomic mass of oxygen (O) = 16 , for Hyrogen (H) =1
  • Molecular mass for water (H2O) =18
A mole of an element or compound is its molecular mass in gm.

For example :

one mole of oxygen (O2) = 32, water (H2O) =18

  • One mole of a substance dissolved in sufficient water to make one liter of solution is called a one molar solution.
  • The charged species is called ions. Produced when compounds dissociate in water.
  • +ve ions called cations, -ve ions called anions
  • Neutrality means the number of cations = the number of anions.
The valence is the number of charges on an ion.

The valence of (Na+)=1,

The valence of (Cl-)=1

  • The equivalence of an element is the number of hydrogen atoms that element can hold in combination or can replace in a reaction (= valence in most cases)
  • An equivalent of an element is its gram molecular mass (mole) divided by its equivalence.
  • A milliequivalent of an element is its milligrams molecular mass divided by its equivalence.
example 2 2
Example 2-2

How many grams of calcium will be required to combine with 90 g of carbonate to form calcium carbonate?


1- One equivalent of Carbonate (CO32-)

2- One equivalent of Calcium(Ca2+)

3- The no. of equivalents of Carbonate must equal to the no. of equivalents of calcium.

  • 4- No. of equivalents of calcium

5- So, we need 3 equiv of carbonate = 3 equiv X 20 g/equiv = 60 g.

The concentration of substance A can be expressed as an equivalent concentration to substance B as the following:
  • Generally, the constituents of dissolved solids are reported in terms of equivalent calcium carbonate concentrations
example 2 3
Example 2-3

What is the equivalent calcium carbonate concentration of:

  • 117 mg/L of NaCl?
  • 2x10-3 mol/L of NaCl?

1- One equivalent of Calcium Carbonate (CaCO3)

2- One equivalent of Sodium Chloride (NaCl)

One mole of a substance divided by its valence is one equivalent 

ionization of solids substances with crystalline structure
Ionization of solids substances with crystalline structure
  • Water is a reactant
  • Water is not a reactant
  • Dissolution : the solid form may be dissociated into its ionic component
  • Precipitation: the ionic components may be recombining into the solid form.
Dynamic Equilibrium is reached when the rate of dissolution and the rate of precipitation is exactly equal.
  • Mass action equation

Solid compound

Ionic components

K is an equilibrium constantfor a given substance in pure water at given temperature

The brackets [ ] indicate molar concentration.

At equilibrium, the solid phase does not change concentration (precipitation = dissolution)

Where Ksp is the solubility product.

If the concentration of either or both of the ions is increased , the product of the ionic concentration will exceed the Ksp and precipitation will occur to maintain the equilibrium conditions.

See table 2-3 (solubility product values)

example 2 4
Example 2-4

The solubility product for the dissociation of Mg(OH)2 is 9x10-12. Determine the concentration of Mg2+ and OH- at equilibrium, expressed as milligrams per liter CaCo3.

  • The no of moles of Mg2+ resulting from dissociation = x,
  • Then there are 2x moles of OH-