Alkalinity

Alkalinity is the ability of water to neutralize acid. Alkalinity tests are used to control boiler blowdown and to predict the potential for calcium scaling in cooling water systems. Alkalinity in boilers causes:

  • Foaming and carryover of solids in steam
  • Embrittlement of boiler steel
  • Corrosion of condensate lines

Carbon dioxide dissolves in rain water as a gas. The dissolved carbon dioxide reacts with water molecules and forms carbonic acid:

CO2 + H2O = H2CO3

Trace amounts of carbonic acid are formed, but it is acidic enough to lower pH from the neutral point of 7. Carbonic acid is a weak acid, and does not lower pH below 4.3. However, a pH of 4.3 is low enough to cause significant corrosion of system metals.

In the ground the pH of the water rises and the carbonic acid ion (H2CO3) changes to bicarbonate ion (HCO3).

When the pH reaches 8.3, all the carbonic acid has been changed to bicarbonate. If the pH continues to rise, the bicarbonate ion (HCO3) is changed to a carbonate ion (CO3). The three ions carbonic acid (H2CO3), bicarbonate HCO3), and carbonate (CO3) can be converted from one to another by changing the pH of the water.

Changes in pH can be reduced through “buffering”. When acid or caustic is added to water containing carbonate/bicarbonate ions, the pH of the system does not change as quickly as it does in pure water. Much of the acid or caustic added is consumed as the carbonate/bicarbonate ratio changes. Alkalinity in water exists at a pH above 4.4.

Alkalinity is measured by a double titration with acid. Acid is added to the sample to the Phenolphthalein end point (pH 8.3) and the Methyl Orange end point (pH 4.4). Titration to the Phenolphthalein end point (the P-alkalinity) measures hydroxide (OH) and ½ of the carbonate alkalinity (CO3). Titration to the Methyl Orange end point (the M-alkalinity) measures all the bicarbonate ions (HCO3), carbonate ions (CO3), and hydroxide ions (OH). The results are expressed as CaCO3.

Alkalinity may be removed from water by:

  • Reverse osmosis
  • Demineralization
  • Anion ion exchange dealkalization
  • Cation ion exchange in hydrogen cycle
  • Acid addition
  • Lime ad lime soda softening
  • Hot lime softening

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