Pool Care Guide PH


Every substance has a pH: water, juice, dirt, leaves, salt, food, shampoo. All of these items have a certain amount of acid in them. The amount of acid in a substance is measured by the pH. Vinegar is also known as acetic acid. Citrus fruits and juices contain large amounts of citric acid. Baking soda contains a very small 1 amount of acid, and is thus referred to as a base, or alkali. In swimming pools, a certain pH range is required to ensure proper efficiency of other chemicals, as well as the comfort of those using the pool. Proper care must be taken to ensure that the pH of the pool does not reach a level which could be harmful to swimmers, or which could damage equipment or the pool itself.


pH: pH stands for "power of hydrogen" and is so named because it is the presence of hydrogen, which determined the acidity of a substance. The more hydrogen, the more acidic the substance is, and thus the lower the pH. It is often a misunderstanding that as the pH increases this means that the amount of acid in the water increases, when in fact the opposite is true.

Acid: Acid is a substance, either liquid or granular, which has the ability of lowering the pH of water. In liquid form it is quite concentrated, and thus only a small amount is required to lower the pH of an average swimming pool. Examples of common acids are muriatic (hydrochloric) acid, sulfuric acid, and sodium bisulfate (dry acid).

Base (or Alkali): A base is a substance, usually a granular product, which has the ability of raising the pH of water. Examples of common bases are soda ash and baking soda.


Acid lowers the pH of swimming pool water. As a general rule, the pH of pool water in Arizona tends to always increase, although there are a few cases where the opposite occurs. Therefore acid is a commonly used chemical in Arizona pools. There are three different types of acids used in pools, and they will be discussed here, along with testing methods and some-other factors, which affect the pH level of the pool water.

Types Of Acid

a. Muriatic (hydrochloric) Acid

Muriatic is the most common type of swimming pool acid, and it is also the strongest (most highly reactive) type of acid. It is also called hydrochloric acid, as it is made of only hydrogen and chlorine. The highest concentration available to the consumer is only 38%, but even at this relatively low concentration it is a very dangerous chemical. Care should be taken to avoid contact with the skin, clothing, and every 'other surface capable of corrosion. Do not ever add water to acid ... always add acid to water. Adding water to acid can cause the escape of harmful, perhaps fatal fumes and also the possibility of fire. Always keep all acids in a well-ventilated area outside of all living areas.

b. Sulfuric Acid

Sulfuric acid is one of the most commonly recognized acids in the public sector. It is, however, a much less reactive acid than muriatic acid, and is therefore referred to as a "weak" acid. Because of this, it can be found in much higher concentrations, as high as 93%. At this concentration, one cup of sulfuric acid will do the same job as 1 quart of muriatic acid. Care should be taken to avoid overdosing of the swimming pool using concentrated sulfuric acid. Even though it is less reactive than muriatic acid, the same care and precautions should be taken as with muriatic acid.

c. Sodium Bisulfate (dry) (Not Recommended!)

Dry acid is a granular acid commonly used to lower the pH of fiberglass pools and spas, vinyl-lined pools, painted pools, and all pools whose surface could be damaged by strong liquid acids. Although it is a relatively weak acid, it can be a powerful irritant and thus the same safety precautions should be followed as with muriatic and sulfuric acid.

C. BASE (alkali)

A base raises the pH of swimming pool water. In most cases, routine addition of a base is not necessary, but a small amount of one, such as soda ash, may be needed in case too much acid is added either by the user or by nature.

Types Of Bases

a. Soda Ash

Soda ash is a chemical usually granular, which neutralizes acid. It is made from sodium, carbon, and oxygen, and can be as harmful and dangerous as acid, so care must be taken to avoid contact with skin, clothing, and anything subject to corrosion. Never add water to soda ash ... always add soda ash to water. In addition, all of the same safety precautions taken with acid should also be taken with soda ash.

b. Baking Soda

Baking soda is a very common chemical found in virtually all households. It is also used in swimming pools in a limited capacity. Baking soda is a "weak" base, and because of this, it is an excellent means to control total alkalinity (discussed in Section 4). Unlike acid and soda ash, baking soda is relatively mild and generally does not irritate human tissues. However, since it does react with acid in a somewhat violent manner, care should be taken to avoid mixing baking soda and acid in any environment other than the swimming pool.


The pH Scale

On a standard test kit, the pH scale is graduated from 6.8 to 8.2. These numbers are not in ppm. The numbers refer to the concentration of hydrogen (acid) in the water but the scale is somewhat in reverse. The scale actually ranges from a low of zero to a high of 14, but only the range from 6.8'to 8.2 is of concern for swimming pools. Water with a pH of zero is said to be pure acid, while water with a pH of 14 is said to be pure base (or alkali). A 7.0 pH indicates neutral water, meaning it has neither too much acid nor too much base. Muriatic acid has-a pH of approximately 1.0, while soda ash has a pH of approximately 13.0. Water with a pH of 6.0 is 10 times more acidic than water with a pH of 7.0, and the same is true for the alkaline side of the scale. Thus it is imperative that the pH be kept within proper levels to ensure the safety of the swimmers, and the longevity of the pool equipment.

Ideal Levels

The ideal level for pH is generally the same for all swimming pools, regardless of climate, amount of use, temperature, or pool surface. This ideal level falls between 7.4 and 7.6 for all pool surfaces. There are two important reasons for this ideal level. First, chlorine is most efficient at a pH of 7.0, when there is neither too much acid nor too much base to interfere with its function. However, the pH of human blood is approximately 7.42, and thus a 7.0 pH would cause discomfort to swimmers. Thus it is recommended to keep the pH as close to 7.4 as possible. Also, to ensure that no corrosion of the pool surface takes place, a slightly higher pH is needed, and as a result the ideal range for pH is between 7.4 and 7.6. For most pools in Arizona, it is generally not too difficult to keep the pH within this range, and only slight amounts of acid and/or soda ash are required.

Time And Frequency Of Testing

During the warm summer months, and also during the late-summer monsoon season, the pH of the swimming pool can change rapidly. For this reason it is advisable to test the pH level daily during these times. The specific time of day at which the pH should be tested is not as critical as that for chlorine, but for convenience, both should be tested at the same time, namely late afternoon.


In addition to the addition of acid and soda ash by the user, there are a number of other factors, which can vary the pH of a swimming pool. Among these are the presence of citrus plants, rain, dirt and debris, and the type of pool surface present.

Citrus Plants

As mentioned earlier, citrus plants contain a powerful acid called citric acid. Once this acid is introduced to the pool water, it lowers the pH, frequently to a large degree. In some cases, the amount of citric acid added to the water in this way is severe. In this case, it is advisable to keep as many leaves and citrus fruits out of the pool water as possible, whether by eliminating or relocating the plant, or by adding a cover to the pool.


In urban areas where a significant amount of air pollution exists, the rain, which falls, brings with it a significant amount of sulfuric acid. The pH of the rainwater in our area has been measured as low as 4.0, and this level is easily enough to lower the pH of the swimming pool an appreciable amount. It is important to test the pH as soon as possible after a rainstorm to ensure that it does not remain below ideal levels for an extended period of time.

Dirt And Debris

Some soils, especially those in the dry land area, are very alkaline. Since alkaline substances neutralize acid and raise the pH of water, the presence of large amounts of dirt and debris in the swimming pool can increase the pH significantly. In cases where large amounts of dirt and debris are entering the pool water, it is advisable to keep the pool covered as much as possible.

Pool Surface

The type of surface used on a swimming pool can also affect the pH of the pool water. Plaster or plaster based pool surfaces such as Pebble Tec, are porous, alkaline materials. They can absorb and neutralize acid in pool water, thereby raising its pH. In this case, small amounts of acid are generally needed to maintain a proper pH balance. Fiberglass, vinyl, and painted plaster pools, on the other hand, do not have porous surfaces, and thus do not absorb or neutralize acid in the water. Here, small amounts of soda ash or baking soda may be needed to keep the pH at a proper level after acid is added in the form of rain, citrus leaves, etc.

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