An intermediate look at pool and hot tub water chemistry and testing

Lithium hypochlorite

Lithium hypochlorite is a white, relatively odourless granular solid made by mixing chlorine gas with caustic soda, then with lithium hydroxide, and then drying to a powder.

When added to water, lithium hypochlorite has the following chemical reaction:

Li(OCl) + H₂O → HOCl + Li⁺ + OH^–

Advantages

• safest chlorine chemical to handle;
• no premixing required;
• completely soluble; and
• dissolves quickly at normal pool water temperatures and will not bleach vinyl liners.

Disadvantages

• creates strong acid demand by raising pH;
• most costly form of inorganic chlorine; and
• high total dissolved solids (TDS) contribution due to inert content (71 per cent).

Trichloroisocyanuric acid (Trichlor)

Trichlor, an organic chlorine, is a slow-dissolving, white solid generally sold in 25 mm (1 in.) and 76.2 mm (3 in.) tablets for automatic erosion or floating feeders.

When added to water, trichlor has the following chemical reaction:

C₃N₃O₃Cl₃ + 3H₂O → 3HOCl + C₃H₃N₃O₃

Advantages

• good for feeders due to slow dissolution; and
• convenient to handle, but use of gloves is recommended.

Disadvantages

• lowers total alkalinity;
• creates strong base demand due to low pH; and
• steadily increases the cyanuric acid (CYA) level, which, if not watched carefully, may result in over-stabilized (chlorine lock): 7-ppm increase/lb/37,854 L (10,000 gal).

Sodium dichloroisocyanurate (Dichlor)

Dichlor is a rapidly dissolving white granular solid generally sold in 25-mm (1-in.) and 76.2-mm (3-in.) tablets for automatic erosion or floating feeders. This type of chlorine is also available in granular form. Its impact on pH is minimal.

When added to water, dichlor has the following chemical reaction:

NaC₃N₃O₃Cl₂ + 3H₂O → 2HOCl + C₃N₃O₃ + NaOH

Advantages

• easily handled; and
• little impact on acid or base demand due to small pH impact on water.

Disadvantages

• most costly ‘stabilized’ chlorine; and
• will steadily increase the CYA level (rapidly, if used as a shock) which, if not watched carefully, may result in over-stabilized (chlorine lock): 6- to 7-ppm increase/lb/37,854 L (10,000 gal).