As a proud pool owner one of the most important jobs you have is to maintain the correct pool chemistry in you pool water.
Once the pool chemistry is balanced and the water is adequately sanitized with chlorine your family will have crystal clear and safe water for swimming.
If you can afford the budget you may choose to have someone visit your home and service your pool.
However, it’s not that difficult or time-consuming to maintain the pool yourself. Please read our pool chemistry course and learn how to maintain your own pool.
Table of Contents
Pool Chemistry & Water Analysis
What is pH?
pH is a scale in chemistry to indicate the level of acidity or alkalinity of an aqueous (water-based) solution. The symbol pH means “power or potential of hydrogen”.
The pH scale is an inverse logarithmic scale indicating the concentration of hydrogen ions in solution. The lower the pH the higher the concentration of hydrogen ions and the more acid the solution. The higher the pH the lower the concentration of hydrogen ions and the more alkaline the solution.
Examples of pH
Strong acid is pH 0
Pure water, at 25C, is neutral at pH 7.
Strong base (or alkaline) is pH 14.
However, pH fluctuates depending on temperature. Pure water at 0C is pH 7.4.
Sea water is normally between pH 7.5 and 8.4.
Swimming pool water should normally be between pH 7.2 and 7.6 which is slightly alkaline and never acidic.
Pool water chemistry must be at the correct pH to ensure the effective operation of your salt water chlorinator. Then the chlorinator can produce adequate chlorine levels to kill algae and bacteria. The correct pH protects the pool surfaces and metal finishes from deterioration.
If you don’t have a salt water pool chlorine must be administered by a floating chlorine dispenser or manually added to the pool every day.
Sanitizing Pool Water
All swimming pools must have the water regularly sanitized with the addition of chlorine, bromine, Biguanide or use ozone or ion treatment. Without water treatment the pool will turn green with algae within one week and be unsafe for swimming.
What is Chlorine?
Chlorine (Cl) is a fundamental chemical element of nature. At room temperature (25C), pure Chlorine is a pale yellow-green gas that is extremely reactive, poisonous and toxic.
However, it is not found as a free element in nature because it is too reactive and always combines with other elements. It is abundantly found in nature in combination with metals to form chloride salts, such as sodium chloride (NaCl) which is common table salt.
The highly reactive and oxidizing characteristic of Chlorine led to the development of commercial bleaches and disinfectants to sanitize bathrooms, hospitals and other public areas.
Chlorine is commonly used to sanitize bacteria and microbes in water purification processes in water treatment plants.
Chlorine, in the form of sodium hypochlorite (NaClO), is used to sanitize swimming pool water. Sodium hypochlorite solution is unstable and rapidly decomposes releasing aqueous chlorine which kills bacteria, algae and other microbes in your pool water.
Sodium hypochlorite is strongly alkaline and corrosive so you need to protect your skin by wearing gloves when handling this chemical.
Free Chlorine (1 to 3 ppm)
Free chlorine is a pool chemistry term to describe the chlorine available to neutralize contaminants in the water. The effective range for free chlorine is 1 to 3 ppm. Keep the level at 3 ppm for maximum effectiveness.
Please read more about What is Free Chlorine in Your Pool
Combined Chlorine
When chlorine is dissolved in your pool water it kills bacteria and any other organic contaminants. Once the free chlorine has done its job it becomes inactive and combined with other elements so it can no longer sanitize the pool water.
Total Chlorine
Total chlorine is the total of free chlorine and combined chlorine in the pool water.
Stabilized Chlorine
Stabilized chlorine is simply chlorine with cyanuric acid added to it.
When you add plain chlorine, either manually, floater or salt water chlorinator, to an outdoor pool the UV light and heat from the sun will burn off the unstabilized chlorine quickly.
To prevent the chlorine from being burned off cyanuric acid or pool stabilizer is added to the pool water. This pool chemical binds to the chlorine preventing it evaporating and therefore stabilizes the chlorine making it last three times longer. The stabilized chlorine is much more effective at sanitizing the water especially in the hot summer period when you need it most.
Unstabilized Chlorine
Unstabilized chlorine is simply plain chlorine without added stabilizer.
If you use normal chlorine without cyanuric acid (stabilizer) the sun will evaporate the chlorine quickly and reduce its sanitizing capacity. You will have to add more chlorine or turn up the chlorinator and pump to generate more chlorine costing more money.
You have the choice of using unstabilized chlorine and adding cyanuric acid manually, or use stabilized chlorine that already has cyanuric acid mixed in.
Mostly, if you are manually adding chlorine to a pool you would want stabilized chlorine otherwise you have to add cyanuric acid separately. However, you have to be careful that you are not overdosing the pool with cyanuric acid (stabilizer) because this will reduce the effectiveness of the chlorine.
Most pool shock products are unstabilized chlorine because your pool would already be stabilized and you don’t want to add any more cyanuric acid when you shock the pool.
If you have an indoor swimming pool you will not have to worry about stabilizer because the pool is protected from the harmful UV light and heat of the sun. So only add unstabilized chlorine to an indoor pool.
What is Shocking Your Pool?
The term “shocking your pool” is pool industry slang language for super chlorinating your pool. All it means is giving your pool an extra dose of chlorine to ensure the pool is adequately sanitized by killing bacteria, viruses and algae that may exist in the water.
Please read more about How to Shock Your Swimming Pool
What are Chloramines?
Chloramines are the chemical residue after chlorine has reacted and bound with dirt, sweat, sunscreen, urine, bird droppings, decaying insects, bacteria, algae and anything else that invades your pool water. If you pool water smells like bleach then you have too many chloramines in the water and it’s time to shock the water.
Chloramines build up in the water making it easier for bacteria and algae to multiply and take over the pool. Shocking the pool water (super chlorinating) eliminates chloramines and keeps your pool water sanitized.
Best Way to Shock Your Pool
Before you handle any chemicals make sure that you are using protective clothing and gloves. I recommend a mask and eye protection as well. Do not to add chemicals to your pool when there is a breeze. You might end up wearing the chemicals.
Most shock treatments are a powder or granular chemical that comes packaged in a one dose pack which is the correct size for the average backyard pool. Normally, you can spread the shock around over the pool surface. Carefully read the manufacturer’s instructions to ensure you are applying the shock correctly for your pool.
Test the water the next day to make sure the pool is still balanced, otherwise adjust levels as required.
Shock treatments can cause the pool water to look cloudy for a few days. This is normal and will dissipate over the next few days as the free chlorine reacts to other compounds in the pool.
Which Shock Treatment to Use?
The most widely used shock treatment is calcium hypochlorite.
If your calcium level is already high don’t use shock containing calcium hypochlorite as this will make it worse. Use lithium hypochlorite or sodium hypochlorite instead.
Some products can be directly added to the pool water whereas others are best mixed into a bucket of water first. Always follow the instructions on the packet.
Alternatives to Chlorine Pool Chemistry
What is Bromine?
You may already know from your high school chemistry lessons that Bromine is a chemical element directly underneath chlorine on the periodic table of elements. So Bromine has similar properties to chlorine in that it is used to sanitize water by killing bacteria, algae and other contaminants.
Bromine exists as a red-brown liquid at room temperature and is less reactive and a weaker oxidizing agent than chlorine. However, bromine is still a relatively reactive chemical and toxic.
Bromine vs Chlorine Pool Chemistry
Bromine works better and is more stable at higher temperatures than chlorine so it is commonly used in spa pools and hot tubs.
The use of Bromine produces less chemical odor than chlorine which suits small water pools and spas or people who cannot stand the smell of chlorine.
Free chlorine has a short lifespan in the pool. Once free chlorine combines with contaminants it rapidly loses its chemical reactivity and will no longer work to sanitize the water, whereas bromine has a longer lifespan in the water and keeps the water sanitized for longer.
When chlorine reacts with contaminants in the water it forms chloramines which smell and are ineffective against bacteria. The chloramines disintegrate with shock treatment and dissipate over time.
Bromine reacts with contaminants in the water to form bromines which are still active against bacteria and the smell is almost odorless.
Bromine is less reactive on your skin and is recommended for people with sensitive skin. However, chlorine and bromine have similar chemical properties so if you are allergic to chlorine using bromine will not be an alternative sanitizer.
However, bathers really need to wash their skin and swimwear after bathing in a bromine spa or hot tub because the chemical smell tends to stay on your skin and swimwear.
The main disadvantage of bromine is that it costs much more than chlorine. As a result pool owners prefer to use chlorine in a swimming pool but may use bromine in the spa or hot tub.
Bromine cannot be stabilized with cyanuric acid like chlorine so it’s less useful in an outdoor pool as the UV light will render it ineffective. Use bromine in hot tubs and spa pools because of the higher temperature, small volume of water and less reactivity to your skin.
Switching from Chlorine to Bromine
If you have a salt water chlorinated pool you will need to replace your equipment with bromine feeder equipment and drain the salt water and refill with fresh water.
Otherwise, switching from chlorine to bromine is simply a matter of stop using chlorine and start using bromine. Some pool care specialists say it’s safe to use the same pool water. However, we recommend to change the water to be on the safe side. If you have a hot tub or spa you should change the water every season to eliminate chemical build-ups.
If you have an automatic feeder it will need to be replaced to guarantee there is no chlorine residue to mix with bromine because the mixing of the two chemicals is not safe and may cause a dangerous chemical reaction.
Switching from Bromine to Chlorine
Switching back the other way requires you to drain the pool water and refill to eliminate all bromide ions from the pool. If you have a hot tub or spa you should change the water every season to eliminate chemical build-ups.
Do not mix chlorine into a bromine pool as it will react.
If you had a brominator, replace it with a new chlorine dispenser.
Biguanide Pool Chemistry
What is Biguanide?
Biguanide (polyhexamethylene biguanide) is a chlorine-free and bromine-free alternative sanitizing product that can be used in swimming pools. This product was originally developed for use as a surgical disinfectant but has been developed for the pool industry.
It works by combining with contaminants in the water to produce insoluble particles that are trapped by the filter. It requires hydrogen peroxide to be added to the pool to work.
The advantages of Biguanide:
- unaffected by pH
- not degraded by sunlight
- not degraded by higher water temperatures
- chemically stable in water
- no chloramines
- no odors
- gentler on skin, hair and eyes
The disadvantages of Biguanide:
- more expensive than chlorine
- not effective as chlorine or bromine
- can cause cloudy pool water
- requires hydrogen peroxide which is a bleach
- uses higher concentration than chlorine to sanitize water
- needs special test kit or strips
- requires more filter cleaning
Some pool owners love using Baquacil, which is the most common product on the market that contains biguanide. Many have reported that there pool water is clean, clear and perfect for swimming. It seems to be used more for fresh water above ground pools than other types of pools.
Please read our post on Biguanide Pool Chemistry
What is Cyanuric Acid (Pool Stabilizer)?
Cyanuric Acid (CNOH) is manufactured as a white, odorless solid compound and is most commonly used as a stabilizer for swimming pool water. Chlorine is a highly reactive element and dissipates rapidly in water. The acid binds to free chlorine in pool water slowing down the reaction of chlorine which extends the useful life of chlorine in the water.
On a hot summer day ultra-violet radiation can quickly burn off free chlorine from pool water. This leaves the water unsanitized and less safe for swimming. Adding the correct amount of cyanuric acid to pool water ensures the free chlorine in your pool lasts longer. And it dissipates slowly keeping the water sanitized for a longer period.
It is essential to add cyanuric acid to the water to stabilize and extend the chlorine’s useful life in the pool. Stabilized chlorine will last 3 to 4 times longer than unstabilized chlorine.
The normal level of Cyanuric Acid (Pool Stabilizer) is 30 – 80 ppm
Note: Do not add cyanuric acid to hot tubs or spa pools that are sanitized with bromine. Bromine is not able to be stabilized and is only recommended for spas and hot tubs because it would quickly burn off in the sun and it works well at the higher temperatures in hot tubs.
Do I Need to Add Stabilizer to my Pool?
If you have an indoor pool that is fully shaded from the sun you do not need to add cyanuric acid (pool stabilizer) to your pool. The free chlorine in the water will stay active longer than an outdoor pool because there is no UV light to dissipate the chlorine.
If you have a salt water outdoor pool you will need to manually add stabilizer to the pool. The salt chlorinator will produce chlorine when the pump is running. But without stabilizer the chlorine will evaporate and dissipate quickly.
If you have a fresh water pool you will need to add chlorine and stabilizer. You can add them separately or use a stabilized chlorine product that contains cyanuric acid.
Note: The danger with stabilized chlorine products is that you can overdose the pool with too much stabilizer which will reduce the effectiveness of chlorine. Always test your water to ensure you have the correct chemical levels.
How to Lower Cyanuric Acid in a Pool
When you test your pool water and find the cyanuric acid (stabilizer) level is too high you need to dilute the level. You have three options:
- Buy a Stabilizer reducer product.
- Drain some water from your pool and refill with fresh water to dilute the water.
- Wait for rain to dilute the pool water.
When draining pool water you should only need to drain off the level down to the bottom of the skimmer box and refill. This should reduce the cyanuric acid level by 5 to 10%.
Note: When you refill a pool with tap water or water from another source it may unbalance the other chemical components of your pool water. You need to retest everything and adjust the levels as required.
Note: If you are finding your cyanuric acid level is high I recommend you don’t use stabilized chlorine products so that you are in full control of your cyanuric acid level.
How to Raise Cyanuric Acid in a Pool
Simply add more pool stabilizer to the pool water to increase the level of cyanuric acid to the recommended level of 30 – 80 ppm. Please follow the instructions on the product label.
If it’s the off season and you don’t swim in the pool because it’s too cold then you are safe with a low level of stabilizer at 30 or 40ppm. Use a pool cover to protect the chlorine from the sun. Then you will only need a lower level of stabilizer.
During the hot summer season increase the level to 70 or 80ppm to ensure the chlorine is protected against the sun. The best time of day to add any chemical to pool water is in the early evening just as the sun is going down. This will ensure the chemical is not affected by UV light and heat giving the chemical time to work overnight. Also you need to ensure that the pump is going to run for at least three or four hours to thoroughly circulate the water and mix in the chemicals.
The most important thing to remember is to slowly increase the level and retest the water after it has dissolved and circulated around the pool. I always add half the recommended dosage of stabilizer, retest and later add some more. It’s better to be safe than sorry because if you add too much it is difficult to reduce the level. You normally have to drain some water from the pool, refill and rebalance all the other chemicals.
What is Total Alkalinity?
Alkalinity and pH are not the same thing but they correlate together.
pH is a scale in chemistry to indicate the level of acidity or alkalinity in a solution.
Swimming pool water should normally be between pH 7.2 and 7.8 which is slightly alkaline and never acidic.
Total Alkalinity in pool water is the measure of all alkaline compounds dissolved in the water. It is a measure of the water’s ability to resist changes in pH by providing capacity to neutralize acid. The right amount of alkalinity helps to buffer your pool water from changes in pH.
In the swimming pool industry, total alkalinity is a measure of the dissolved carbonates and bicarbonates in water in parts per million (ppm).
The correct level of alkalinity depends on many other factors such as water hardness, pH and temperature.
Total Alkalinity in pool water should be between 80 – 120 ppm.
Vinyl Lined Pools should have a level 100 to 150 ppm
Fibreglass Pools should be 80 to 100 ppm
Concrete Pools with a pebble aggregate lining should be 100 to 200 ppm
High Alkalinity Level in Pool Chemistry
A high total alkalinity can occur when too much chemical, such as sodium bicarbonate or chlorine shock, has been added to the water too quickly. Always add small quantities of chemical, wait a while and retest the water before adding further amounts of chemical. Yes, it is a slower process but the only way to guarantee that too much chemical is not added resulting in unbalanced pool water.
High total alkalinity can be caused by the water supply to your pool.
If your pool water test is indicating total alkalinity is too high you will need to lower alkalinity in your pool water. Otherwise, you will encounter the following problems:
- High and unstable pH level.
- High calcium level and scaling
- Cloudy water
- Chlorine becomes less effective resulting in algae growth
- Filter frequently clogging up
- Irritated eyes and itchy skin for swimmers
Low Alkalinity Level in Pool Chemistry
A low total alkalinity can occur after heavy rainfall especially acid rain.
A low total alkalinity in pool water may lead to the following issues:
- Stains forming on the pool surfaces
- Pitting, etching or erosion of the pool surfaces
- Irritated eyes and itchy skin for swimmers
- Green water
- Rapid changes in pH
- Corrosion of metal equipment parts (ladder, vacuum pole etc.)
How to Increase Total Alkalinity in Pool Water
It is more common to encounter low Total Alkalinity than high Total Alkalinity, however both problems need to be corrected by rebalancing your pool water chemistry.
Use sodium bicarbonate to raise total alkalinity in pool water. This is simple process of adding the required quantity to the water. Simply follow the instructions on the container. Be careful not to add too much chemical as you may raise the alkalinity too high. Raise the level in small increments. Re-test over several days to prevent raising the level to high.
If you need to raise both Total Alkalinity and pH together use sodium carbonate (soda ash). However, sodium carbonate can cause cloudy pool water so only add small amounts at a time.
How to Decrease Total Alkalinity in Pool Water
If the pH level is normal but Total Alkalinity is still too high use sodium bisulphate to lower the Total Alkalinity as this will not adversely change the pH. Please ensure you carefully follow the instructions on the container using the correct dosage for your volume of pool water. Lower the level in small increments with re-testing over several days to prevent adding to much chemical to the pool.
Add Acid to the Pool Water
If the pH level is already high, then the simplest way to reduce Total Alkalinity is to add small amounts of acid to the pool. This will be a slow process as you will need to test every day and add more acid until the Total Alkalinity and pH has been reduced to an acceptable level.
Begin by adding one cup of hydrochloric acid or muriatic acid into the pool water. Ensure you read the instructions on the container and practice safe handling of chemicals. Make sure you have spread the acid around in the pool and not poured it into one location as it may begin damaging the pool surfaces or tile grouting. The best time to add the acid is just before the pool pump is due to come on. Otherwise, you will need to run the pool pump for several hours to mix in the acid. After mixing or the next day, test the pool water. If the Total Alkalinity and pH are both still too high repeat the above process until the levels are normal.
When either the pH or Total Alkalinity has reduced to the normal level stop the process. You may need to further adjust pH or Total Alkalinity separately with another process to correctly balance the water.
Note: If you need to adjust both pH and Total Alkalinity, always adjust the Total Alkalinity level first as the pH may change.
What is Calcium Hardness?
Calcium Hardness is the measure of all calcium compounds dissolved in the pool water. If you have a salt water chlorinated pool do not use calcium chloride to raise hardness or calcium hypochlorite (dry granular chlorine) to raise the free chlorine level. These chemicals will change the calcium hardness of the water.
The recommended level of calcium is as low as possible to minimize any problems with calcium type deposits forming on the pool surfaces or in the salt water chlorinator cell.
Calcium Hardness 200 – 400 ppm
Warning: Water sources from bores, dams and rivers can be high in mineral and organic contaminants resulting in faster mineral deposit formation and poor chlorine production. These water sources will require additional chemical treatment before the water is suitable for a swimming pool.
What is Salinity in Pool Chemistry?
A salt water pool contains a level of dissolved salt that is pumped through the salt water chlorinator cell to produce chlorine. The typical salinity of a salt water pool is around the same as the salinity of human tears and only 15% of seawater salinity.
A low salinity level will restrict the capacity of the chlorinator cell to produce an adequate amount of chlorine to sanitize your pool.
During the hot summer season, a higher level of salinity is recommended to ensure that maximum chlorine production is maintained to counteract the evaporation of chlorine from the pool water.
Normal Salinity is 3000 to 7000 ppm. Recommend 6000+ ppm in summer.
Note: The actual salinity depends on the manufacturer’s specifications for your brand of salt water chlorinator.
Total Dissolved Solids (TDS) in Pool Chemistry
Total Dissolved Solids (TDS) is a measure of all materials dissolved in the pool water. This includes all pool chemicals, all contaminants, salt, carbonates & bicarbonates, and metal ions – phosphate, iron, copper, manganese. Swimming in the pool adds contaminants such as sweat, cosmetics, hairspray, sunscreen, etc. Over time the TDS level in your pool water will increase as more contaminants and chemicals dissolve into the water.
Pure water has a TDS of 0 ppm.
Drinking water has a maximum TDS of 500 ppm.
Fresh water swimming pools have a TDS up to 2000 ppm.
Salt water pools have a TDS of 3000 to 6000 ppm, including the dissolved salt.
What causes high TDS in pools?
All substances added to the pool water increases the total dissolved solids TDS in your pool. This includes all the normal pool chemicals such as chlorine, shock, pH reducer, algaecide, etc. and sweat, cosmetics, hairspray, sunscreen, etc. from swimmers. Over time the TDS level will increase and eventually the water will be over saturated with dissolved solids.
What is TDS in a salt water pool?
Salt water pool chemistry will have a higher TDS because of the dissolved salt. A typical freshwater pool will have a TDS from 0 to 2000 ppm. However, when you add salt to the water the TDS will increase to over 3000 ppm. The usual level of TDS in a salt water pool is 3000 to 6000 ppm depending on the required salinity level for your salt chlorinator.
Why Do I Need to Lower the TDS?
A high TDS level in your pool water will reduce the effectiveness of chlorine and other chemicals added to the pool. Your pool water will be more difficult to keep chemically balanced and sanitized resulting in:
- Scaling & stains on the pool surface
- calcium deposits
- hard water
- salty tasting water
- cloudy or haziness
- green water as the chlorine is ineffective
- running the pump longer using more electricity
Your pool chemistry may be perfectly balanced but you still get the above problems because the TDS is too high. The addition of more chlorine or other chemicals will not resolve the problem.
How to Test for TDS in Pool Chemistry
There are two methods to test for total dissolved solids:
- Electronic Digital Chemistry test meter which measures pH, free chlorine, TDS and other indicators. These are expensive but highly accurate and easy to use.
- TDS test strips are cheap and simple to use.
How Do I Get Rid of TDS in my Pool?
There is no product that you can add to the pool water to lower the level of total dissolved solids (TDS) in the water. When the TDS is too high you will need to drain or partially drain your pool and refill with fresh water.
Drain, clean and refill your swimming pool every 5 years to resolve this issue.
Metal Ions in Pool Chemistry
Metal ions accumulate in swimming pool water. The most common ions are Phosphate, Iron, Copper and Manganese.
What is an ion?
An ion is a term from chemistry.
When an atom (or group of atoms) gains or losses one or more electrons it becomes a charged particle, called an ion.
What Metal Ion is in the Pool?
When you add chlorine to the pool and the water changes color you probably have a very high metal ion level. Too much metal in the pool chemistry will cause stains or spots on the pool surface.
Green water indicates copper or iron.
Brown water indicates iron.
Dark Purple water indicates manganese.
How Do Metal Ions Get in the Pool Chemistry?
Water Source
If you filled the pool with a water source other than your town water supply you may have introduced metal ions into the pool. Water from bores, dams, streams, rainwater tanks and other sources may contain high levels of metals. These metals will raise your total dissolved solids (TDS) level and may cause problems sanitizing and balancing the water chemistry.
Metal Pipe Corrosion
Copper or iron may have come from a corrosive water pipe or metal fitting in the water heater or filtration system. Normally mains water pipes are well maintained and cause no problems. Councils have to provide drinking water with total dissolved solids (TDS) less than 500 ppm so there shouldn’t be any metal ions in your drinking water.
Algaecide
Some brands of algaecide contain copper ions. Too much use of these algaecides over time can add copper to the pool chemistry increasing the TDS.
How to Remove Metal Ions from the Pool Chemistry
There are three ways to remove metal ions from pool water.
Shock Treatment
Add shock to the pool water to oxidize the metal ions in to harmless molecules that will sink to the bottom where they can be picked up by the pool cleaner or manually vacuumed away.
Sequestering Agent
Add a sequestering or chelating agent to the pool. These chemicals form complex metal molecules that sink to the bottom where they can be vacuumed away.
Drain & Refill the Pool
Swimming pool water accumulates contaminants, metals and other dissolved solids over time which cannot be chemically removed from the water. Replace the pool water every 5 years to combat this problem. While the pool is empty take the opportunity to make repairs or renovate the pool surface.
What is Langelier Saturation Index (LSI)
The Langelier Saturation Index (LSI) is an important pool chemistry measurement that checks the complete state of the pool water. It was first developed by the chemist Dr Wilfred Langelier. It’s a parameter that is calculated to predict the calcium carbonate stability of water which shows how far the water is from a neutral state. When the LSI is too low the pool water is corrosive, when too high the water is scale forming where you get white rough build-up on pool surfaces.
In pool chemistry, LSI is the difference between the actual pH and the saturation pH.
LSI = pH – pHs
The LSI is calculated from pH, temperature, calcium hardness and total alkalinity.
However, it’s mostly affected by pH and total alkalinity levels.
LSI Index must be between -0.3 and +0.3 for Balanced Water
An LSI reading of zero is the ideal result for swimming pool water.
LSI less than -0.3, water is corrosive
The pool chemistry is unbalanced and may cause pitting, etching, dissolving and staining of pool surfaces, tile grout and plumbing pipes.
LSI greater than 0.3, water is scale forming
Scale and other deposits may form on the pool surfaces, equipment and inside pipes. Scale appears as rough, white deposits on the walls. If scale builds up inside the plumbing and filtration system it will reduce the water flow. This will cause higher pressure and more strain on the pump motor. The filter will not perform effectively leaving your pool unclean.
How to Calculate the Langelier Saturation Index (LSI)
If you take your water sample to a pool shop for analysis their software will usually calculate the LSI. The following formula computes LSI with indices of temperature, calcium and alkalinity.
LSI = pH + Ti + Ci + Ai -12.1
Ti = temp index, 60F/16C = 0.4, 66F/19C = 0.5, 76F/24C = 0.6, 84F/29C = 0.7
Ci = calcium index, 100ppm = 1.6, 150ppm = 1.8, 200ppm = 1.9, 250ppm = 2.0, 300ppm = 2.1 etc.
Ai = alkalinity index, 75ppm = 1.9, 100ppm = 2.0, 125ppm = 2.1, 150ppm = 2.2, 200ppm = 2.3, 250ppm = 2.6
Example
pH = 7.6
Ti = 0.6 (76F or 24C)
Ci = 1.9 (200ppm)
Ai = 2.0 (100ppm)
LSI = 7.6 + 0.6 + 1.9 + 2.0 – 12.1 = 0.0
The pool water is neutral and ideal for swimming.
How to Decrease Langelier Saturation Index (LSI)
Add muriatic acid to decrease LSI
Other factors that decrease LSI:
- Adding stabilizer (cyanuric acid)
- Increased total dissolved solids (TDS)
How to Increase Langelier Saturation Index (LSI)
Add sodium bicarbonate (baking soda) to increase LSI
Other factors that increase LSI:
- Temperature increases
- Increase the Total Alkalinity
- pH increase
- Increased Calcium Hardness
Algaecide
Many pool owners who have a green pool make the mistake of adding algaecide to the water. If your pool water is already green with an algae bloom it’s too late to add algaecide to the water. You need to correct your pool chemistry by heavily shocking the water with a high chlorine dose.
Adding algaecide to a pool is a preventative control to help stop an algae bloom occurring if the free chlorine level drops unexpectedly. Add a dose of algaecide to the pool water at the beginning of the summer period or when you open your pool for the year to kill algae before it can have the opportunity to grow and bloom. The algaecide dissolves in the water and lasts for many months giving you lasting protection throughout the summer period.
Clarifier
If your pool is only slightly cloudy where you can see the bottom at the shallow end use a clarifier.
When pool water becomes cloudy it contains a build-up of very fine particles that your filter cannot remove from the water. Sometimes these particles are so fine that they never sink to the bottom but stay suspended in the water.
The use of a clarifier pool chemical will help to combine these tiny particles together into larger particles that can be filtered out or fall to the bottom and can be vacuumed manually or by a robot cleaner. Of course, you will need to backwash or hose your filter after this process to remove the particles.
Flocculent
If your pool is very cloudy where you cannot see the bottom use a flocculent.
A flocculent chemical works in the same way as clarifier by clumping together fine particles into larger particles. However, the particles will be heavy enough to sink to the bottom where they can be vacuumed away.
You will need to manually vacuum to the waste drain otherwise you would be clogging your filter. As you vacuum to waste the water level will drop so you need to refill the lost water with a garden hose.
How to Add Chemicals to a Swimming Pool
Pool chemicals are dangerous and reactive. Please store all pool chemicals in a cool, dry location and kept out of the reach of children. Wear gloves and protective clothing when handling chemicals.
Always read the label on the product and follow the instructions for adding the chemicals to pool water. Some products are poured or spread directly into the pool. Other products recommend you add the chemical to a bucket of water first, mix it and spread around the pool. Make sure the bucket is clean and doesn’t contain any chemical residue as it may react with the product you are mixing in the bucket.
Run the pump for a few hours after adding chemicals to pool water to ensure the product is thoroughly mixed in. The best time to add chemicals is right before the pump is due to operate. So look at the timer and add chemicals when the pump comes on.
NEVER MIX CHEMICALS TOGETHER
Many people have suffered chemical burns, explosive reactions or breathed in toxic chlorine and burnt their lungs when mixing pool chemicals. You should never mix pool chemicals together as they may react leading to fire, explosion or release of toxic chlorine gas. Even mixing dry chlorine with liquid chlorine may cause an explosion.
NEVER ADD WATER TO CHEMICALS, ALWAYS ADD CHEMICALS TO WATER
You must never add water to chemicals because the chemical may react with the water causing an explosion or the release of toxic chlorine gas that has the potential to damage your lungs. Always add a small amount of the chemical to a large amount of water to minimize any unwanted reaction. For example, mix the chemical into a clean full bucket of water or pour the chemical directly into the pool.
When to Add Chemicals to a Pool
The best time to balance your pool chemistry and add chemicals to a pool is when your family has finished using the pool for the day. So late afternoon or early evening is the ideal time because you cannot swim in a pool when you have just added chemicals. Also, the pump takes time to mix in the chemicals.
Adding the chemicals in the evening prevents UV sunlight from inactivating the chemicals and ensures the chemicals have the longest amount of time to work before the UV light returns the next day. Adding chemicals, especially chlorine, in the middle of a hot summer day is least effective as it will evaporate quickly and become ineffective by UV radiation.
Ensure that your pump is going to operate for at least 3 or 4 hours after you have added the chemicals to mix and circulate the product thoroughly around the pool.
Congratulations
Now that you have completed the Pool Chemistry Course you’re a pool chemistry expert, well maybe!
Truly, you will know more than the average pool owner. And you will be in a better position to care for your pool without spending money on unnecessary callouts to pool professionals.