Alkalinity.

a. Sources of alkalinity.

The three basic sources of alkalinity in water are: alkalinity resulting from the bicarbonate ion (HCO3 - ), the carbonate ion (CO3 = ), and the hydroxyl ion (OH - ). The amount of each of these in water can be determined by titrating with an acid to certain pH levels (end points) using phenolphthalein (P alkalinity) and a methyl orange (M alkalinity) end points. The relationship between pH and these alkalinities is shown in figure.

b. Relationship of P, M and OH alkalinities.

Test procedures for determining the P and M alkalinities are included in commercially available test kits. The OH alkalinity can be determined by a specific test or it can be calculated from the P and M alkalinities.

The OH alkalinity is the result of the hydroxyl ion (OH - ) in the water, and is also known as "caustic alkalinity" or "causticity".

The relationship between the measured P and M alkalinities and level of hydroxyl, carbonate, and bicarbonate forms of the alkalinity is shown in table below, and is discussed further.

Alkalinity in boiler water essentially results from the presence of hydroxyl and carbonate ions.

(1) Hydroxyl alkalinity (causticity) in boiler water is necessary to protect the boiler against corrosion. Proper dosage of sodium hydroxide is covered in paragraph 3-19. Too high a causticity causes other operating problems, such as foaming. Excessively high causticity levels can result in a type of caustic attack of the boiler called "embrittlement".

(2) With the phosphate control program phosphate in the boiler water combines with calcium to precipitate calcium phosphate, removing calcium from the boiler water.

 Calcium phosphate, under proper conditions, forms a finely divided, fluid sludge, which can be carried by the boiler circulation and can, in general, be readily removed by blowdown.

Because calcium phosphate is the least soluble of the calcium salts in boiler water, phosphate control prevents formation of other calcium scales, such as calcium carbonate, calcium sulfate, or calcium silicate.

The pH in the boiler must be kept above 11.0 to prevent possible formation of a sticky sludge that adheres to the boiler surfaces.

(3) Causticity in boiler water is the result of the addition of sodium hydroxide (NaOH) or the breakdown of bicarbona-te alkalinity in the feedwater under the influence of heat to produce sodium carbonate, carbon dioxide and water:

2NaHCO3 -> NaCO3 + CO2 + H2 O

Sodium                         Sodium Carbon Water

Bicarbonate   Carbonate Dioxide   

Carbonates react with the hot boiler water, although not as fast or as complete as the bicarbonate reaction, to form carbon dioxide and sodium hydroxide which increases the causticity in the boiler and the acidity of the steam.

Na2 CO3 + H2 O -à 2NaOH + CO2

   [ Main ] [ Up ]