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

By Wayne Ivusich

To ensure the health of bathers and the life of a pool or hot tub, it is necessary to monitor a handful of water-quality factors on a regular basis.
To ensure the health of bathers and the life of a pool or hot tub, it is necessary to monitor a handful of water-quality factors on a regular basis.

Dealing with pool and hot tub water chemistry issues is sometimes confusing and occasionally frustrating because of the many critical components involved. A basic understanding of this topic is often not enough. Sometimes it requires ‘diving in’ a little deeper and gaining a better understanding of what it takes to maintain healthy, balanced water. The first key players in attaining this goal are sanitation and oxidation.

Sanitation and oxidation

Although many service technicians think ‘sanitation’ and ‘oxidation’ are one in the same, there is a distinct difference between the two. Sanitation is the complete destruction of bacteria, germs, viruses, algae, etc., that may enter the water. These nasty components can be potentially harmful to swimmers. Sanitizing them (i.e. killing them) removes this possibility. Oxidation is the process of oxidizing (i.e. burning off) organics that may get into the water such as hair, oils, lotions, sunblock, leaves, etc. By ridding the water of these contaminants, it eliminates the main source of eye, nose, and skin irritation. The good news is with proper sanitation and oxidation these problems do not exist.

Consider the following:

  • the average person can introduce more than 50 billion bacteria into pool water;
  • the average-sized dog is equal to 50 people—that means 250 billion bacteria enter the water when fido decides to jump in for a swim.

Although there are many products available for sanitizing and oxidizing pool/hot tub water, there are actually only two chemicals that can perform both of these processes at the same time—chlorine and bromine.

First, here is a look at the six forms of chlorine currently available:

  • Chlorine gas (Cl2)
  • Sodium hypochlorite (NaOCl)
  • Calcium hypochlorite [Ca(OCl)2]
  • Lithium hypochlorite (LiOCl)
  • Trichloroisocyanuric acid, a.k.a. Trichlor (C3N3O3Cl3)
  • Sodium dichloroisocyanurate, a.k.a. Dichlor (NaC3N3O3Cl2)

All chlorine products react with water to produce the same acid—hypochlorous acid (HOCl):

H2O + Cl2 → HOCl + OCl

HOCl is a very powerful sanitizer/oxidizer, whereas the OCl (hypochlorite) portion has only one per cent of HOCl’s ability to kill germs.

When considering which product to use for the pool and/or hot tub being serviced, it is important to know that each of these products has its own unique characteristics. One of the primary issues with chlorine is the difference in the available chlorine content (ACC) in the product and the effect it has on the water’s pH. (see Figure 1 below)

Product Available Chlorine Content (ACC) pH
Chlorine 100 %
Sodium Hypochlorite 10 to 12 % 11 to 13
Calcium Hypochlorite 45 to 78 % 8.5 to 11.8
Lithium Hypochlorite 35 % 10.8
Dichlor 55 to 63 % 6.7
Trichlor 90 % 2.8 to 3.5

Figure 2 (below) shows how the water’s pH determines the proportions of HOCl and OCl.

pH % HOCl % OCl-
7.0 79.3 20.7
7.2 70.7 29.3
7.5 54.8 45.2
7.8 37.8 62.2
8.0 27.7 72.3
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