The Mysterious pH Factor
Table 1: pH
What pH Is
We use pH as an index to express how acidic or basic a solution is. (The scientific definition of pH is "the negative logarithm of the hydrogen ion concentration".) A pH greater than 7.0 is basic, and a pH lower than 7.0 is acidic. An extremely basic substance such as sodium hypochlorite (liquid chlorine) has a very high pH, while muriatic acid has a very low pH. Ordinary drinking water typically has a pH near neutral, around 7.0 on the pH scale.
In pools and spas, it is important to maintain the water in the slightly basic range of 7.2 to 7.8. The National Spa and Pool Institute (NSPI), the industry association in the United States, has set a standard of 7.2 to 7.6 as the ideal pH.
You should know one more important point: the pH scale is logarithmic. That means that one unit of difference on the pH scale indicates an increase (or decrease) of a factor of 10. (The Richter Scale uses the same relationship to represent the relative strength of earthquakes.) For example, a pH of 8.2 is 10 times more basic than a pH of 7.2; 9.2 is 100 times more basic than 7.2.
Why pH Is Important
Losing control of pH in the water unleashes a whole series of problems. It affects the comfort and safety of swimmers and bathers. The pH can also damage metal equipment and gel-coat if it gets out of balance. The pool or spa owner should know about the following potential side effects if he or she neglects the pH.
If pH Is Low
- The water can corrode surfaces, metal equipment or fixtures.
- Swimmers and bathers can experience discomfort from burning eyes and itchy skin.
- The chlorine may dissipate more quickly.
- The water may cause blistering and etching of the gel-coat.
If pH Is High:
- Calcium and metals tend to come out of solution (the opposite of dissolving) at high pH levels, creating the potential for stain and scale formation. The calcium and metals will actually create deposits and discoloration on pool walls and equipment.
- Swimmers and bathers can experience discomfort from burning eyes and itchy skin.
- High pH can contribute to cloudy water.
- As the pH increases, chlorine becomes less efficient as a sanitizer. (If the pH is too high, the chlorine won't form enough hypochlorous acid, the form of chlorine that does the best work as a sanitizer.)
pH Control
Increasing pH - You can use sodium carbonate (soda ash) to increase pH when levels are too low. Other chemicals that can raise the pH are sodium bicarbonate and sodium sesquicarbonate. The amount required varies from pool to pool; it depends on the chemical used.
Decreasing pH - Sodium bisulfate and muriatic acid can lower the pH when it gets too high. Sodium bisulfate is popular because it is granular, making it safe to handle and easy to use. Muriatic acid is a concentrated liquid acid that can cause severe burns in direct contact with skin.
Factors Affecting pH
- Sanitizers
- Gas chlorine, trichlor and bromine tabs will all reduce pH when added to the water.
- Sodium hypochlorite (liquid chlorine), calcium hypochlorite (Cal-Hypo) and lithium hypochlorite will all raise the pH of the water.
- Cyanuric acid (used to stabilize chlorine and prevent it from dissipating) has a pH of about 4.0, so it will tend to decrease the pH of the pool.
- Environmental factors such as rain, dirt, leaves, fertilizer over-spray and animals can adversely affect the pH of the water.
- Contaminants introduced by swimmers and bathers, including urine, sweat, oils and cosmetics can affect the pH of the water.
Table 2: NSPI pH Standards
| Minimum | Ideal | Maximum |
|---|---|---|
| 7.2 | 7.2-7.6 | 7.8 |
Testing pH
The pH is the most important balancing parameter for pool and spa water. You should check the pH once per week at a minimum. Ideally, you should test after every use of the water. You should be sure to test after heavy bather load or after any change in environmental conditions; rain, dust, leaves and tree pollen can all change the pH. The pH in turn can affect the sanitizer and other components of the water chemistry.
