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

Chlorine gas

Chlorine gas is normally a pale greenish-yellow and has a pungent odour, which can burn the eyes, nose, throat, and lungs if handled improperly. Its use in North America has diminished greatly in the past few decades.

When added to water, chlorine gas has the following chemical reaction:

Cl₂ + H₂O → HOCl + HCl

Advantages

• least expensive source for creating HOCl

Disadvantages

• high capital costs for application equipment;
• highly regulated (with associated costs);
• lowers pH and total alkalinity (TA) dramatically at application site; and
• meticulous safety procedures required (no known injuries from licensed applicators).

Sodium hypochlorite

Sodium hypochlorite, an inorganic chlorine, is stable only in solution, but loses strength over time, degrading to 10 to 12 per cent strength at point of use. It is also often referred to incorrectly as ‘liquid chlorine.’

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

NaOCl + H₂O → HOCl + Na⁺ + OH^–

Advantages

• completely soluble in water;
• after chlorine gas, it is the least costly source of HOCl; and
• leaves no residue in pool water.

Disadvantages

• bulky and heavy; ≈ 90 per cent water and caustic;
• creates strong acid demand by raising pH due to use of sodium hydroxide to manufacture and stabilize the bleach; and
• decomposes quickly in storage.

Even though this is commonly referred to as ‘bleach,’ it must be noted that laundry-grade bleach is not acceptable for use in pools and hot tubs due to the high level of contaminants found in these products.

Calcium hypochlorite

Calcium hypochlorite is a white, granular powder made by mixing chlorine gas with calcium hydroxide, then drying to the resulting powder.

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

Ca(OCl)₂ + 2H₂O → 2HOCl + Ca⁺⁺ + 2OH^–

Advantages

• compact and easily handled; and
• minimal decomposition in storage.

Disadvantages

• creates strong acid demand by raising pH;
• increases calcium hardness by 4 ppm/lb/37,854 L (10,000 gal); and
• may create cloudy water due to inert insoluble substances or sudden rise of pH causing formation of a CaCO₃ ‘cloud.’