We are managing the chemical treatment of the swimming pool water. Proper chemical treatment is needed in order to prevent a wide range of potential problems including scale and stain formation
Chemical Factors – pH
PH is the term used to refer to the degree of activity of an acid or base in the water and is the most important chemical factor in swimming pools. PH is measured on a scale from 0 to 14 with 7 being neutral. A pH value between 0 and 7 is considered acidic with 0 being the greatest acid activity and getting weaker as it approaches a value of 7. A value of 7 to 14 is considered basic with 14 being the greatest base activity. Another word for basic is alkaline; however, this is not to be confused with total alkalinity.
PH and total alkalinity are not the same. Pool water pH is best kept in the range of 7.2 to 7.8. When pH remains below 7.2, the water is considered to be corrosive. This means etching of plaster and metals in equipment such as heat exchangers will result. In addition, it is more difficult to keep chlorine in the pool because while more effective as a sanitizer at the low pH, chlorine is also much less stable resulting in the consumption of larger quantities of chlorine than would be used at normal pH levels. Maintaining the pH higher than 7.8 will increase the tendency to form scale or cloudy water. Calcium, the major component in scale, is a relatively unstable mineral and when the pH is high, the calcium is not as soluble and it will have a greater tendency to precipitate or “fall out” of solution resulting in cloudiness or scale. High pH will also reduce chlorine effectiveness resulting in the need to maintain higher chlorine levels to achieve maximum sanitization. If the pH is low, sodium carbonate, otherwise known as pH up or soda ash, is added to raise the pH. If the pH is high, pH Down is used. PH Down comes in two forms: liquid acid (muriatic acid) or dry acid (sodium bisulphate). Changes in the pH of pool water can be caused by many factors but the most significant cause is the sanitizer used. Since the sanitizer is the most frequently added chemical in pools, it can have a powerful impact on pH and overall water quality. Of the sanitizers typically used in pools, chlorine is the most common. Chlorine comes in a variety of forms and varies widely in pH. For example, most tableted forms of chlorine have a very low pH and will tend to lower pH over time, while liquid chlorine is very high in pH and will tend to raise pH values. This will be covered in more depth in the Biological section of this book. Changes in pH due to sanitizers or other factors can be minimized and controlled by the proper maintenance of the next chemical factor, total alkalinity.
This is the most desirable form and is the form responsible for the actual sanitization activity in the water. It is measured using a free chlorine test kit and its level is critical in the pool. If this form is not present, little or no sanitizing can take place. Free chlorine is actually composed of two types of compounds: HOCl (hypochlorous acid) and OCl¯ (hypochlorite ion). This is important because they exist together in a condition or state known as equilibrium. This means that together they make up 100% of the free chlorine content, but that content consists of some of each. For example, if 25% of the free chlorine is HOCl, then the OCl¯ level will be the other 75%. It is important to note that only the HOCl component is effective as a sanitizer. Therefore, it seems logical that we would want as much of the free chlorine as possible made up of the HOCl. However, the level of HOCl and OCl¯ present is dependent upon the pH. This is one of the critical Chapter 3: Biological Factors – Sanitization 21 Notes: reasons that the proper pH level in pool water is so important. As the pH goes up or down, the relative amount of HOCl vs. OCl¯ also increases or decreases. The following chart shows how much of each of these two compounds are present at different pH levels.
pH 6.0 7.0 7.5 8.0 9.0
%HOCl 97 75 50 23 3
% of OCI¯ 3 25 50 77 97
As the chart shows, at pH 7.5 only about half of the free chlorine exists in the desirable form of HOCl. The level of HOCl will increase as the pH goes down and it must also be pointed out that as the pH decreases, so does the stability of the chlorine. As pH rises, the stability of the free chlorine will increase, but its activity as a sanitizer diminishes. In order to get the most effective and economic benefit of chlorine, keep it in the desirable pH range of 7.2-7.8. Lower pH will be detrimental to pool surfaces and equipment, while higher levels will render chlorine ineffective as a sanitizer.
Total alkalinity refers to the ability of the pool water to resist a change in pH. The key purpose total alkalinity serves is to help manage or control the pH in the pool. It does this by acting as a buffer so that when materials are added to a pool that would cause the pH to go up or down, these changes are controlled and do not results in severe changes to pool water balance.
Chemical Factors – Total Alkalinity
When a substance is added to pool water that could affect the pH, total alkalinity will react to neutralize it and help keep the pH in the desired range. Total alkalinity does not determine what the pH will be, but rather acts to help keep the pH in the range desired. Total alkalinity is measured in parts per million (ppm) using a total alkalinity test kit. Total alkalinity is best kept in the range of 80-120 ppm. When the value is less than 80 ppm, the water can become aggressive and the pH can swing easily upward and downward and back again. If the value is higher than 120 ppm, the water can become cloudy and scale forming and the pH will tend to drift upward. In adjusting total alkalinity downward, the same acids used to lower pH are employed. When reducing total alkalinity, it is best to add small amounts of acid, either liquid or dry, over a period of several days as opposed to making large adjustments rapidly. Each time the acid is added the pH will drop initially and then the total alkalinity will neutralize it. This results in the pH returning to the previous level and the total alkalinity value will drop. Simply repeat the process daily until the desired level is reached. Adding too much acid at once may result in lowering the pH so severely that corrosion of pool surfaces and equipment may result and the existing total alkalinity may not be sufficient to raise the pH back to the normal level. When raising total alkalinity, sodium bicarbonate is the chemical of choice and the required amount can be added all at once. For more information on how to adjust total alkalinity, consult Section – Pool Care Guidelines, Formulas and Calculations. It is possible in freshly filled pools to find that both total alkalinity and pH need to be adjusted. It is recommended that you adjust the total alkalinity before the pH. However, in rare circumstances, you may have a condition where one factor is high and the other very low. In such a situation, you may consider first adjusting whichever factor is lower. If you find widely varying values for both pH and total alkalinity in a freshly filled pool, it may be worthwhile to wait about 24 hours before making any adjustments. This wait will generally result in some natural balancing of the water without added chemicals and is sometimes referred to as allowing the water to come into equilibrium. If additional adjustment is still needed, it will require far less time or chemical. In all cases, never add acid to the pool water if the pH is less than 7.2, even if the total alkalinity is high. Instead, wait for the pH to rise first before proceeding. If the pH does not come up by itself after a day or two, you will need to add some pH up before proceeding.
TOTAL DISSOLVED SOLIDS:
Total dissolved solids (TDS) are normally the least worrisome factor.TDS is the sum of all materials dissolved in the water and normally runs in the range of 250 ppm and higher. There is much discussion over what levels are considered too high, but there is no real lower limit. TDS is comprised of many different chemical compounds, which means that the issue of how much is too much actually depends more on what they consist of than how much there is. For example, sodium chloride or ordinary salt is extremely soluble and is therefore unlikely to cause a problem, whereas, as we have seen, calcium compounds can be a problem even at fairly low levels. In general, when the TDS exceeds approximately 1500 ppm, problems may begin to occur. It must be pointed out that pools whose sanitizing systems are based on chlorine or bromine generation equipment (salt generators) will likely have much higher TDS levels. These pools actually have salt in one form or another added to the pool. The salt used is highly soluble and does not cause the type of problems normally associated with high TDS, but never the less, it does add to the TDS level in the pool. When testing water in this type of pool for TDS, the salt intentionally added to the pool needs be taken into account. At elevated levels, TDS can lead to cloudy or hazy water, difficulty in maintaining water balance, reduction in sanitizer activity and foaming. Unfortunately, the only way to reduce TDS is to drain a portion of the water and replace it with fresh water. Sequestering agents do not help when high TDS levels are causing cloudy water.
Biological Factors – Sanitization:
Shock oxidation treatment and algae control are the key elements in maintaining clean water. The previous two chapters dealt with keeping the water clear while here we will address how to keep it clean. I. Sanitization The process of controlling bacteria in the water is known as sanitization. Sanitizaion is not to be confused with the control of algae in the pool water, as algaecides are best used for that purpose. While a wide variety of methods for sanitizing pools are available, the two most common methods are chlorine and bromine. Other processes have also gained more attention, including PHMB (biguanide), ozone and ionizers. Each has its strengths and weaknesses. CHLORINE The most widely used sanitizer for pool sanitization is chlorine, and it is available in a number of forms. The following table lists the most commonly used forms of chlorine and some of their characteristics: CHLORINE PRODUCT FORM CONTENT pH Sodium Hypochlorite (liquid bleach, liquid chlorine) Liquid 10-12% 13-14 Dichlor (granular stabilized chlorine) Granular 56-62% 6-7 Trichlor (tableted stabilized chlorine) Tablets, Pucks & Sticks 90% 2-3 Calcium Hypochlorite (granular chlorine, unstabilized) Granular 47-75% 11-13 Regardless of which form of chlorine is used, all produce the same active sanitizer when added to water. Once the chlorine has been added to the water, it can react with contaminants and then take several different forms, not all of which are desirable.