WATER CHEMISTRY BASICS
Everyone knows water is H2O, but from there not all water is the same. Water is known as the universal solvent. This means that practically any element found in nature can be dissolved into solution. There could be many different elements, compounds and living organisms. There are dissolved gasses such as carbon dioxide and nitrogen. There are minerals such as calcium and sodium; trace metals such as iron, copper and manganese; living organisms such as algae, fungus, mold and bacteria. Then there’s everything that we put into it, intentional or unintentional. These are the challenges we face when trying to keep pool and spa water clear and blue.
Balancing the water is key to ensuring water stays clear and blue. This is also true for swimmer comfort. So when water is UNBALANCED it can either be corrosive or scale forming. Corrosive water is aggressive and attempts to dissolve everything it comes in contact with. [i.e. pitting and etching pool surfaces, eating away at anything metal (lights fixtures, handrails, or copper heat exchangers) and destroying seals and o-rings in pumps and filters] Scaling water does the opposite by trying to deposit or precipitate the calcium carbonate out of solution. This can be in unsightly deposits on the pool finish or by building up in and clogging plumbing, filters and heat exchangers. While BALANCED water will not scale or corrode.
This is accomplished by the parameters that determine the water balance:
- pH
- Calcium Hardness (CAL)
- Total Alkalinity (TA)
- Temperature
- Total Dissolved Solids (TDS).
pH
Acceptable Range: 7.2 – 7.8
Ideal Range: 7.4 – 7.6
pH is the measured on a scale from 0-14, with a pH of 7 being neutral. Below 7, the water is acidic and above 7 the water is basic (or alkaline). The pH scale is logarithmic, meaning that every whole unit increase is 10 times its predecessor. Therefore, a pH of 6 is 10 times more acidic than a pH of 7and a pH of 3 is 10,000 more acidic than a pH of 7!
The pH number represents… the negative logarithm of the potential Hydrogen (pH) ion concentration.
So remember, the lower the pH the more acidic the water is and the higher the pH the more basic the water is.
pH can also affect water clarity. Typically, a high pH can promote cloudy water and a low pH will promote clearer water. A low pH can be a bigger problem because the water may not give any visual signs that there may be a problem. If the water is clear the homeowner may think “If it looks good, it must be good” and they may not realize the damage that is being done to there pool surface and equipment. Where as a high pH is generally handled sooner because the problem can be more visible.
pH will also affect the efficiency of the sanitizer’s ability. Chlorinating products are the most vulnerable to pH. The lower the pH the more active the sanitizing part of the chlorine is but it leaves the water prematurely. At a pH of 7.5 only 50% of the chlorine is in its killing form and at 8.0 it is down to only 22%.
Calcium Hardness (CAL)
Ideal Range: Plaster Type 200-400ppm
Fiberglass Type 350-500ppm
Vinyl 200-300ppm
Water containing little to no calcium is called soft. Water containing high amounts of calcium is called hard. In Pool and Spa water, Calcium Hardness is an important characteristic of the water balance. Water that contains too little calcium is likely to become aggressive. Aggressive Water seeks to satisfy its mineral appetite by dissolving calcium into water from whatever source it comes into contact with. While water containing too much calcium has the potential to scale. The only way to remove calcium from pool water is to do a partial drain and refill with fresh water. But before doing this it is always wise to test the source water first. Most municipal water sources have a lower calcium level (around 150 ppm, but always test). While well water can have vastly different levels (typically higher).
Some other facts about Calcium Hardness
· Well water generally has higher calcium/mineral levels.
· Plaster Type pools (i.e. Marcite, Krystal Krete, …) but especially Exposed Aggraget type and dark color finishes are at much higher risk to scale.
· New Plaster/Exposed Aggregate type finishes have a very high potential to scale.
· Fiberglass type finishes will actually consume calcium, so it is very important to closely monitor. Low calcium will result in the deterioration of finish.
· The waters appetite for calcium changes with the temperature of the water.
TOTAL ALKALINITY (TA)
Ideal Range: 80 – 150ppm
Total Alkalinity is the measure of the ability of water to resist changes in pH…that is … the ability to “buffer” water from wide pH swings. Total Alkalinity serves two purposes: it is the governor of pH and it is the third and last major factor that affects water balance.
When we test for Total Alkalinity we are measuring the concentration of carbonates, bicarbonates, silicates, borates, and other chemical compounds that contribute to Total Alkalinity. Some of these chemical compounds contribute to the test but don’t necessarily contribute significantly to the pH buffering capacity. In most situations, a higher TA, will result in a greater resistance to pH change. But too high a TA will lead to a high pH and may contribute to scaling. While little to no TA will result in a low pH and aggressive water. When the TA is within Ideal Ranges it helps buffer pH and prevent pH bounce – a rapid fluctuation of pH levels with only small amounts of acid, base (alkalinity) or other pH altering agents.
When measuring TA in pool water we need to also take into account the Cyanuric Acid (CYA) factor. For more information refer to your Total Alkalinity dosage chart or page 12 and page 60 table K in the Taylor Book #2004B
TEMPERATURE
In the pool industry most averages that are talked about have been developed by looking at the industry as a hole. “Average” pool temperatures is around 78* - 82* F. In Florida average summer water temperatures can run into the high 90’s and in winter can drop into the low 50’s. But you can generally assume most pool water is going to be the same as Gulf temps.
How does that affect us? Hotter water has a greater tendency to scale or precipitate and colder water has a greater tendency to corrode.
TOTAL DISSOLVED SOLIDS (TDS)
Ideal Range: Below 2500ppm (accept in salt chlorine generator pools)
With water being the universal solvent it will dissolve or put into solution as much as it can hold. Almost everything water comes in contact with goes into solution, (not to mention everything we put into it like chemicals, perspiration and body waste, environmental debris and more.) These tiny solids and particles are too small to be caught by the filter. We refer to the entire group of dissolved substances (not observable, suspended matter) as TDS or Total Dissolved Solids. TDS will increase over time and water becomes more corrosive as TDS rises. As well as other problems like water becoming cloudy or hazy and filtration is ineffective. It’s harder for sanitizing agents to attack and kill bacteria and inhibit algae growth. While pool owners add more chemicals to fight a problem that can’t be solved chemically. The only way to remove TDS is to do a partial drain and refill with fresh water.
So what does this all mean? Plainly put, when water balance parameters are above the proper range, scaling may occur. Even if one or more factors are high while the remainders are within range. Likewise when parameters are below their proper ranges, water becomes corrosive, even if one or more factors are low while the remainder are within range. So when all factors are within there proper range the water is balanced. But what if two factors are to high and two are to low?
Well how high is too high, and how low is to low? Are all factors are weighted the same? These questions can only be answered by a calculation called the Saturation Index. This saturation index has been adjusted for the pool water conditions and made into a formula that determines whether the water is in balance. But again, if all factors are within recommended ranges these calculations may not be needed for every pool. The easy way to calculate this is by using the Taylor WaterGram. For more information on how to use this, please see the Taylor Book #2004B pages 13-14.
Now that we’ve talked about how to balance pool water to protect the pool and equipment, next we will discus how to protect bathers by keeping pool water safe.
SANITATION & OXIDATION
Sanitation is the reduction of the level of microorganisms by significant numbers. Microorganisms present may include algae, bacteria, and other pathogens. Chlorine is the most popular sanitizer because it is the only sanitizer with oxidizing properties. Oxidation is the breaking down, or “burning up” of organic compounds in the pool water. These organic compounds come from multiple sources, such as bather waste, yard debris, and other microorganisms. As the chlorine does its job sanitizing and oxidizing it gets used up, thus creating a chlorine demand.
CHLORINE / BLEACH 10.5% in Florida
Ideal Range: Free Chlorine: 2 - 4ppm
Total Chlorine: 0ppm
Super Chlorination: 30ppm
There are many different types of chlorine out there to choose from. When added to water, all chlorine products produce hypochlorous acid (HOCl). This is the chemical species that controls microorganisms and oxidizes organics.
“There are four main reactions of HOCl that affect pool sanitation, they are:
1. The dissociation (or break up) of HOCl - …HOCl will dissociate (break up) into the hydrogen ion (H+) and the hypochlorite ion (OCl-). HOCl is a very effective sanitizer while OCl- has very little bacteria-killing ability. So this means hypochlorous acid in swimming pools exists in two forms, bacteria-killing and non-killing. As the pH decreases chlorine’s killing ability increases dramatically. That’s why pools run at a high pH have more difficulty with algae.
2. Reaction with microorganisms and organics – HOCl, when killing microbes and oxidizing organics, loses its properties and becomes an ordinary (inactive) chloride ion (Cl-) that is not detected when testing for chlorine.
3. Reaction with ammonia – Ammonia is one of the end (waste) products of the body’s metabolism. The reaction of HOCl with ammonia produces compounds called chloramines or combined chlorine. These chloramines irritate the eyes and mucous membranes and are often confused with pool chlorine because of their “chlorine smell”. Also, combined chlorine has very little sanitation and oxidation abilities, and the presence of combined chlorine can use up the free chlorine (or HOCl) much faster than desired.
4. Reaction with sunlight – In the presence of sunlight, HOCl is reduced to inactive chloride ions by the ultraviolet rays of the sun. On a bright sunny day up to 90% of the chlorine can be destroyed by sunlight within two hours.”
When any one of these reactions takes place it creates a chlorine demand on the water. To satisfy this chlorine demand we must superchlorinate or shock the water. Shock refers to rapidly increasing the chlorine level to 15ppm in a short period of time. Superchlorination (or “super-shock”) refers to rapidly increasing the chlorine level to 30ppm in a short period of time. In some cases the chlorine demand is so great that it can actually create a chlorine deficit. When this happens, you can add even large amounts of chlorine and still get no free chlorine residual. At this point the addition of more chlorine can actually raise the chlorine demand rather than satisfy it, so a Chlorine Demand Test is needed to find out what the true demand is, so it can then be satisfied.
The presence of chloramines can also cause a chlorine demand, when this happens all the free chlorine gets used up rapidly trying to remove this “spent” chlorine, and thus only adds to the problem. To remove these chloramines we must do a Breakpoint Chlorination. In order to do breakpoint chlorination we must first test for the Total Chlorine (the sum of the free chlorine and the combined chlorine). Breakpoint chlorination is achieved by increasing chlorine 10 x the combined chlorine.
TC=FC+CC or CC=TC-FC 10 x CC = BPC
By learning more about all the different types of chlorine products you will be better able to determine which chlorine is suited for the task at hand. To learn more about these different types you can refer to the BioGuard Pro Guide Book or The Taylor Book #2004B.
CYANURIC ACID (CYA)
Ideal Range: 30 – 80 ppm
As mentioned earlier, HOCl is dramatically affected by the UV rays of the sun. To slow down this degradation we use cyanuric acid (also known as CYA, chlorine stabilizer, or conditioner) to serve as a “sunscreen” for the free chlorine. Without the presence of CYA the free chlorine residuals are burned up three to five times faster in the presence of sunlight. CYA reaches peek performance at around 50ppm, however as it protects the free chlorine it slows it down. At levels over 100ppm the free chlorine is now working so slow, that without the introduction of chlorine demand causing agents it may last indefinitely with minimal efficiency. While it has never been proven that excessively high levels of CYA can cause “chlorine lock”, the more CYA dissolved into the water the more the overall effectiveness of the chlorine is diminished. The only way to lower CYA is to brush heavily and do a partial drain. CYA also contributes to TDS.
