Peter S. Cartwright, PE, CWS-VI
The basic chemistry fundamentals presented every month is not intended to be a comprehensive chemistry course, but rather basic instruction on chemistry as it relates to water and water treatment. It is hoped that your interest will be piqued and induce you to want to learn more. The desired outcome is that it will help you become a more effective and valuable water treatment professional. Please get back to us with any questions or concerns; we welcome your input!
The pH helps in predicting corrosion and scaling tendencies, but is not the only measure for these. At low pH (acidic) levels, the solution tends to be corrosive.
Acids are compounds that release hydrogen ions (H+) in solution (e.g: hydrochloric acid, sulfuric acid, etc.). It is the H+ ion that gives acids their properties; the greater the H+ ion concentration, the stronger the acid.
HCl → H+ + Cl–
H2SO4 → 2H+ + SO42–
Bases are compounds that release hydroxide (OH–) ions in solution (e.g: calcium hydroxide [slaked lime], sodium hydroxide, ammonium hydroxide, etc.). They produce a form of alkalinity.
Ca (OH)2 → Ca2+ + 2OH–
NaOH → Na+ + OH–
NH4OH → NH4+ OH–
Acids
Strong acids completely separate into their ions and stay separated. This is indicated by a single arrow pointing in one direction.
HCl → H+ + Cl–
Weak acids and bases are in a continuous process of ionization, but free ions are also continuously recombining to form molecules.
CH3COOH (vinegar) ↔ H+ + CH2COOH–
(99.5 percent separated)
At any one time, only a portion of the acid or base is present as ions. This equilibrium process is indicated by arrows pointing in both directions.
Strong/weak acids
Strong acids ionize 100 percent and include the following
examples:
• HCl (Hydrochloric)
• HBr (Bromic)
• HI (Hydriodic)
• HNO3 (Nitric)
• HClO4 (Perchloric)
• H2SO4 (Sulfuric)
Weak acids do not ionize completely and include the
following examples:
• HC2H3O2 (Acetic)
• HCN (Hydrocyanic)
• H2B4O7 (Boric)
• HNH2SO3 (Sulfamic)