Chemistry 101
Keeping a clean, clear, safe pool requires maintaining many different aspects and having a firm grip on several technologies. Traditionally, one of the most difficult to understand is chemistry. This is true for many reasons. Chemistry itself is a complex science. The interactions between chemicals are always affected by several factors, it is rarely a "what you see is what you get" transaction. What's more, the chemistry in your pool is always changing, and most people don't find joy in sitting around hour after hour monitoring these changes.
Sanitization
First and foremost, pool water must be sanitized. You will hear people talk about sanitizers, oxidizers, biguanides, and other "sanitizers". There is a major difference between sanitizing and oxidizing your water. Sanitizing "kills" viruses and bacteria. Oxidizing does not. Oxidizing "burns out" organic material in your water. You can think of sanitizers as "poison" to virii and bacteria, and oxidizers as "fire" to the non-living substances that make your water cloudy. The most common among sanitizers is chlorine. Bromine is also often used; especially in hot water. Oxidizers include specialty chemicals such as Oxy-Brite (chemically known as potassium peroxymonosulfate), and hydrogen peroxide. Chlorine and Bromine also have strong oxidizing properties, so these qualify as both sanitizers and oxidizers, which is why they work so efficiently in water. Non "chemical" forms of oxidizing agents include UV (Ultraviolet) light, and Ozone.
You may have heard of "chlorine alternatives". These have existed for several years now. A market appeared for these chemicals because many people have incorrectly identified chlorine as a nuisance in pool water. Nothing could be more untrue; however the mismanagement of any sanitizer or oxidizer can cause problems both for the pool and the swimmer. Chlorine alternatives are used only in residential pools; as they are not and have never been approved for commercial use. The main reason for this is one of safety. Chlorine alternatives do not actually kill bacteria and virii, they chemically "coat" these organisms in hopes to make them large enough for a filter to remove them. This is a hit or miss science and has not been proven to be safe enough for use in public pools. Therefore, we do not recommend it for use in any pool. Also, since these chemicals have no oxidizing properties, an extra chemical, at a large additional cost, must be added to "clean" the water. The system as a whole is many times more expensive than chlorine. For the purpose of our discussion, we will focus on the approved method, which is the chlorine system.
Chlorine exists in many forms. As a chemical element, it exists as a very toxic gas. Some large, older pools still use chlorine gas. The gas is compressed into cylinders, where under great pressure it turns into a liquid. When the cylinder is opened, the pressure expels the chlorine back into the atmosphere (or more commonly directly into water), where it becomes a gas again. Because this gas is pure 100% chlorine, it is becoming more and more common to switch for safety reasons to other, less potent and less dangerous to handle forms of chlorine. Also, because it is 100% chlorine, it is the least expensive form of chlorine per gallon of water treated. Gas chlorine has a very low pH.
The most common and safest form of chlorine is Sodium Hypochlorite, also known as "liquid chlorine". It is commonly manufactured at 15-17% available chlorine, and shipped to end user as 12.5% available chlorine. This is not because there is any dilution of the chlorine before it is shipped; rather it is a property of chlorine as it exists in water. Much like water flows to reach equilibrium, chemicals make changes in order to reach a stable state. Sodium Hypochlorite is not a very stable product. It begins to lose its strength from the moment it is manufactured. It loses strength quickly at first, then slower as it reaches a state of equilibrium. For this reason, it is recommended that Sodium Hypochlorite be used before it loses much of its strength. Therefore, ordering more than can be used within a reasonable time (3-6 months) is not advisable. Sodium Hypochlorite is still a very pure form of chlorine. Liquid chlorine has a very high pH.
Another "pure" form of chlorine is Calcium Hypochlorite. This exists as a dry product, either in tablet form or as a granular substance. While available in other strengths, it is most commonly available as 65% available chlorine strength. Calcium Hypochlorite does degrade over time, however nowhere near as fast as Sodium Hypochlorite. It is rather economical to store Calcium Hypochlorite for up to a year or longer. Calcium Hypochlorite has a rather high pH, though not as high as Sodium Hypochlorite.
Lithium Hypochlorite is sometimes used, most commonly for "shocking" or raising chlorine levels very high for temporary periods when the pool is not being used. Lithium Hypochlorite is not very popular because it is very expensive.
Other popular forms of chlorine are actually mixtures of chlorine and a chemical used to stabilize chemical levels in water. This chemical is known as Cyanuric Acid, commonly called just "stabilizer". They are used in residential pools and less often in commercial. They are known to the residential market as "chlorine tablets" or "sticks", among other various trade names. This product is popular because it slows the reaction of the suns' UV rays with chlorine, which degrades it very quickly. Because of this reaction, less chlorine is used. Like most things, however, this comes with drawbacks. First of all, it is a very expensive product, much more expensive than the two chemicals added separately would cost. Secondly, as you always need to replenish chlorine, you are always adding cyanuric acid along with it. While chlorine degrades, cyanuric acid does not. Therefore, each time you add more of this product, you are adding more cyanuric acid. Stabilizer has a "beneficial" effect at controlled levels; specifically 40-70ppm. Higher levels of stabilizer begin to slow the reaction of chlorine down to a point where it is inhibited from being effective as a sanitizer and oxidizer compelely. The only way to remove stabilizer is to drain water and refill with fresh, which becomes a nuisance and a cost in itself. For this reason, and others, we don't recommend using stabilizer. If our customers insist, we like to keep it at low, controlled levels, such as 40-50ppm. You have to remember that while the reaction to the sun is being inhibited, so is the reaction with bacteria and virii. Therefore, higher levels of chlorine are recommended to be available to kill these organisms when present.
Many pool owners swear by the use of stabilizer, however, we feel safety is more important than cost, and will always recommend the avoidance of its use.
The action of chlorine as an oxidizer and as a sanitizer is the same no matter what form of chlorine is chosen. To put it in simplest terms, when chlorine is introduced into water, it splits into two chemicals, Hypochlorous Acid (HA) and the less effective Hypochlorite Ion (HI). Because HA is the chemical that will be doing all the "killing" and oxidizing, we are only concerned with its properties.
There is a major factor that influences how much HA will be formed when Chlorine is added to water. That factor is pH. pH will determine the ratio at which Chlorine is split between HA and HI. For example, at a high pH, more HI is formed than HA. At a low pH, just the opposite is true. Since HA is the chemical we want, it would then make sense that we would rather keep the pH low. In "chemical world", this is true, however in the real world, swimmers could not tolerate the low pH levels, so we choose a happy medium somewhere between pH 7.2 and 7.8. This yields enough HA that we can typically feel safe with known measurements of chlorine in the water.
Now that we have sufficient HA in the water, it goes to work killing bacteria and viruses, some very quickly, others take quite some time to kill. Some of these organisms are E. Coli and Giardia. These cause infections of the gastrointestinal tract and are very infectious. Fortunately, these organisms are not necessarily present every day in pool water, but their mere existence presents problems for swimmers. Therefore, we take weekly test for traces of these organisms, and, if present, employ methods of destroying them with higher levels of chlorine while the pool is closed to swimmers.
HA is inhibited by contaminants in water. It will become bonded with ammonia and nitrogen to form compounds known as "chloramines". Chloramines and other compounds formed are commonly known as "combined" chlorine. It is said that HA that is not inhibited by these contaminants is free to kill harmful organisms, therefore we call it "free" chlorine. Daily tests are taken for both levels in the pool. When combined chlorine levels become high, it is necessary to destroy the bond that has combined the chlorine. This is done one of two ways; either by a method called "breakpoint chlorination" or by means of other oxidizing agents that will destroy the contaminant that bonded with the chlorine and make it free to sanitize again. Breakpoint chlorine is typically done by adding chorine at a rate of 10 times the amount of combined chlorine. This proves sufficient to break the bond, oxidize the contaminant, and "free up" all the chlorine in the pool. This is a popular method in residential pools that don't have heavy use, however in a commercial pool, it can be difficult, if not impossible, to raise the chlorine levels that high and have them back to a safe level by the time swimmers will use the pool again. Therefore, there are other methods to break these bonds.
One popular method is to use potassium peroxymonosulfate. This is an oxidizer that at low levels will break these bonds without having any adverse effects on swimmers. It accomplishes the same results as breakpoint chlorination or "shocking". Another method growing in popularity is to us UV light and ozone. These methods are very effective, as they destroy the contaminants almost as fast as they form, and as oxidizers, also disappear instantly after doing their job. The major drawback of this system is its upfront cost and maintenance costs, which can be prohibitive to some clients. However, there is no better way of eliminating this problem.
Besides inhibiting chlorine's effectiveness, combined chlorine will often release harmful chlorine gas into the air. This is a by-product of the mixture of ammonia and chlorine. Though the gases may exist at seemingly low levels, repeated and prolonged exposure to this contaminated air has proven harmful to humans. This becomes a real problem for indoor pools, and even moreso when air circulation systems are not functioning properly or not sized properly to expel the harmful air and replace it with fresh air.
Therefore, the ideal situation is to have a constant level of chlorine in the water, existing in a free state. This can be difficult to accomplish, but must be addressed consistently.
pH
pH is the measurement of hydrogen ions in water. More specifically, it is a logarithmic scale of the negative hydrogen ion concentration. The scale has a range of 0-14, where a pH of 7 is called "neutral". Any substance with a pH measuring below 7 is called acidic, and any substance with a pH greater than 7 is said to be "basic" or "alkaline".
pH is important for several reasons. As noted above, it has quite a bit to do with the effectiveness of chlorine in water. It also must be monitored for bather comfort and safety, and also for properties (in the extremes) that can be harmful to the pool and its equipment. A low pH will be corrosive to swimmers and equipment. It will cause rashes and itching, as well as red eyes and other effects. It will also corrode metals, most notably copper (used in pool heaters and boilers). Not monitoring pH can be a very expensive mistake, as anyone who has had to replace a heat exchanger can attest to.
High pH can be just as harmful to bathers. In water, it tends to make minerals precipitate, or come out of solution. A simple way to explain this phenomenon is by comparing it to sugar in a cup of coffee. Sugar is the mineral, coffee is the water. When the pH gets high enough, the sugar that was dissolved in the coffee becomes UNDISSOLVED and falls to the bottom of the cup. The same is true in a swimming pool. When the pH is high, calcium will come out of solution and make deposits on the pool walls, bottom, and in extremes can even clog up pipes and harm equipment. Any iron or copper present in the water can also come out of solution, leaving it exposed to the oxidizing properties of chlorine. When this happens, it is like air oxidizing metal and forming rust - only it takes place in water and not in air. Water will become discolored - iron turning the water red, brown, or green, and copper turning the water green. Other chemicals must then be employed to remedy this situation, and the pH must be returned to normal.
Managing pH is done through the use of acids and alkalines. pH in water will climb or fall in direct proportion to the pH of the elements introduced in it. For example, gas chlorine has a low pH. Using this product tends to lower the overall pH of the water over time, requiring the use of a pH-raising chemical. On the other hand, liquid chlorine has a very high pH, and will raise the overall pH, requiring the use of an acid to return it to normal.
Chemicals commonly used to raise pH are: Sodium Carbonate (Soda Ash), Sodium Bicarbonate (Baking Soda), Sodium Hydroxide (Caustic), and Potassium Hydroxide (liquid soda ash). Chemicals that lower pH are: Hydrochloric Acid (Muriatic Acid), Sodium Bisulfate (Dry Acid), and Carbon Dioxide (CO2 gas). Each of these chemicals has its advantages and disadvantages depending on many factors that may be present in the water. Almost all of them will affect some other form of pool water chemistry. It can take quite some time and experience to learn the complex relationships between these chemicals and the water they are treating. There are no hard-and-fast rules, there are only typical relationships which usually tell us what we can expect by using one chemical over another. Those relationships are too complex for this article, but feel free to ask any one of our techs for information on which chemical is right for your pool.
Total Alkalinity
Sometimes just called "alkalinity" for short, this is a measurement of the total alkaline quality of the water. Where pH measures just the hydrogen properties of the water, total alkalinity takes into account other forms of alkalinity/acidity. These are not always as directly harmful to bather, however, prolonged incorrect levels of total alkalinity will wreak havoc on pool equipment and surfaces.
Alkalinity is also commonly describes as a "pH buffer". This is due to the fact that low levels of total alkalinity tend to make hydroxl alkalinity (or pH) fluctuate acutely with the addition of even small amounts of pH-changing products. Having correct levels of alkalinity will prevent this swing in pH, and so therefore is thought of as a buffer for the pH. Conversely, you can have too much of a good thing. High levels of alkalinity will make adjustments to pH very difficult, as pH will tend to increase to match the higher levels of alkalines in the water even after addition of acids.
Total alkalinity is measured in parts per million, and the proper range can be anywhere from 80-120 ppm or more, depending on your situation. Again, the complex relationships of alkalines and acids is more that we can address quickly in this article.
Raising alkalinity is almost always done with sodium bicarbonate (or baking soda). Lowering alkalinity is always done with Hydrochloric (or Muriatic) acid. Adjusting alkalinity is not accomplished as quickly as pH changes are; this tends to be a result of several slow additions of these chemicals over time, which will make the proper adjustment.
Calcium Chloride (Hardness)
Often overlooked as an element of proper water balance, calcium levels in the water are very important. It can be said that water is always seeking a natural balance of calcium. If it has too little, it will be corrosive and seek to balance itself by etching concrete and other metals in search of balancing its need for calcium. If there is too much calcium, water will leave deposits which can become quite a nuisance to eliminate. Therefore, calcium levels play an important role in overall balance of water, and again, the overall balance of water will play a part in how calcium acts in water. It is another complex relationship, but it is important to remember that the proper levels will eliminate these problems. That level, measured in parts per million, is usually between 200-400 ppm depending on other water quality factors.
It is also important to remember that we can raise calcium levels using calcium chloride. We cannot, however, easily lower calcium levels without destroying much of the chemical balance that exists otherwise. Therefore, it is almost always advisable to lower calcium levels by dilution, or draining and refilling the pool with fresh water. This is only applicable, of course, when the fill water has a much lower calcium level than the pool water, otherwise it would be a waste of time and water.
If lowering the calcium level is not practical or possible, there are other ways of compensating for high calcium levels through complex relationships to total alkalinity and pH. Please ask a tech what is right for your situation.
Temperature
While its role may be small, it is worth noting that water temperature does have an effect on chemistry. Chemical relationships typically produce heat or consume it, and the overall temperature of water can inhibit or synergize these reactions to an extent. Most noteworthy, is the retarding of chlorine effectiveness at high temperatures. For this reason, in hot water such as whirlpools, it is advisable to keep elevated levels of free chlorine, or switch to bromine, which is not affected by temperature differences found in bathing water.
Total Dissolved Solids
While TDS is a minor factor in water balance, it does have a lot to do with clarity when high levels are reached. Much like the comparison to a cup of coffee and sugar above, when TDS levels are too high, solid matter (although microscopic) will remain suspended in water because the water becomes supersaturated and cannot absorb any more solids. Many things contribute to TDS over time; chemical additions, contaminants introduced by swimmers, and even elements found in the air. Sometimes TDS can be managed through dilution; other times levels rise so quickly that it is preferred to replace all the water and "start fresh".
The comparison to the coffee cup and sugar is such that while you can add two or three spoonfuls of sugar to your coffee and they dissolve, adding more sugar will not dissolve and results in solid, wet sugar lying on the bottom of your coffee cup. While you could continue to stir your coffee and keep the sugar "suspended" in the coffee (much like a pool is being stirred by its circulation system), the sugar will never, ever dissolve unless more coffee is added. In a pool, this can cause clarity problems that no filtration system can clear. While some people have tried to publish guidelines as to how high the TDS level should be before replacing the water, we have found that TDS is "not a problem until it's a problem". Every pool is somewhat different, and some pools can be cloudy with 2500 ppm of TDS, while other can carry 4500 ppm and still be clear.
Final word
While all these relationships can be complex and confusing, understanding the basics will go a long way towards getting to know them better. There exists another scale which attempts, with relatively accurate results, to determine the overall balance of water. This is known as the Langlier Saturation Index, or LSI. It takes into account pH, alkalinity, calcium levels, temperature, and TDS and assigns values to all these levels. By performing a mathematical equation on the values, we come up with a number that tells us if the water is balanced (0.0), corrosive (lower than 0.3), or alkaline (higher than 0.3). We can then look at the individual levels that made up these values, and make proper adjustments to balance the water without taking any one level too far out of safe range. Please ask a tech if you would like to have an LSI performed on your pool.
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