Salt System Science
By Joe Sweazy
In recent years, nothing has been embraced by the swimming pool industry like in-line saltwater chlorine generation. This amazing technology uses common salt to sanitize a swimming pool. While this technology is not new—in fact, it has been around for decades—in-line saltwater chlorine generation has been experiencing tremendous growth in the United States over the past few years. While most people at least know about in-line saltwater chlorine generation, many do not fully understand the chemistry behind the phenomenon.
How it works
In-line saltwater chlorine generation starts with common salt like rock salt, water softener salt or even good old table salt. Several chemical companies now offer specialized salt containing buffers, stabilizer and other ingredients beneficial to a swimming pool. Whatever the source of salt, the process of turning it into chlorine is the same. Just make sure to use a high-purity salt (generally at least 99 percent pure) to avoid potential problems such as staining.
Swimming pool water containing the right amount of salt, generally 2,500 to 3,500 ppm (mg/L), is passed through a low-voltage electrical current. The electrical current from the in-line saltwater chlorine generator (often referred to as a chlorinator) converts the chloride ions from the salt into chlorine. The chemistry is fairly complex, but let’s boil it down to the basics: sodium chloride (NaCl) in water (H2O) with electrical current yields free hypochlorous acid (HOCl), which is the sanitizing form of chlorine in swimming pool water. Any other form of chlorine, such as liquid chlorine (bleach), granular chlorine or tablets, will produce the same active chlorine or hypochlorous acid.
The chlorine sanitizes the pool water and oxidizes the waste to remove it from the water. Eventually the chlorine gets used up, but it doesn’t go away. In fact, it is transformed right back into sodium chloride—salt! The salt is recycled again and again, being transformed from salt to chlorine.
However, do not assume that because of the way this system works, it is possible to ignore the salt level. On the contrary, the salt level can change. Any time fresh water is added to make up for water lost in splashing, back-washing or evaporation, the salt level decreases; rainwater will also decrease the level.
On the other hand, any time chlorine is added to the water by any means other than generation, the salt level will increase. Remember, chlorine that is used up will become salt. For example, if it is necessary to shock the water to get the chlorine level back up after a party, the chlorine added will raise the salt level in the pool once it has been used up. Other chemicals in the pool may contain chlorides as well and so they may form salt in the swimming pool eventually. Therefore, it is important to monitor the salt level by testing at least once per month.
How to test salt levels
There are several methods available for testing the salt level, but tasting the water is not one of them. Everyone tastes salt differently so that the taste of the pool water is not a reliable indicator of what the level of salt is in the water. Generally, the salt level is tested by one of three methods: test strip, titration kit or conductivity meter.
Test strips are a widely accepted method for measuring salt in swimming pools because of their accuracy and ease of use. Test strips usually require matching a color to a set standard and some salt test strips do utilize this technology. They can be convenient for determining if the salt is in the right range, but they generally lack precision. There is another type of test strip, however, called a salt or chloride titrator, which provides both precision and convenience. This method requires a small sample of water and three to five minutes to complete a reaction with the titrators. The titrator strips absorb a specific amount of water into the column of impregnated paper. The paper then forms a colored peak that is proportional to the amount of chloride in the water, so sodium chloride can then be calculated. These strips also tend to be less expensive than other methods.
Titration is another proven way of analyzing sodium chloride levels. A small water sample and two reagents are needed to conduct the test. The first reagent is added. The color of the water sample changes if salt is present. You then add a second reagent dropwise, counting drops until a distinct color change takes place. The number of drops is then multiplied by a factor to get the salt level. The technique in this method is a bit more complicated and the kits tend to be slightly more expensive.
Another commonly used method for salt measurement is conductivity. Conductivity is a measure of water’s ability to conduct electrical current. Salts, minerals and even dissolved gases contribute to the conductivity of a solution. Therefore, conductivity can be used as an indicator of the amount of dissolved materials in a solution. The concentration of dissolved ions (salt content) will factor into determining conductivity of the water. The higher the salt content, the higher the conductivity will be. This allows for a good approximation of the salt content in swimming pool water; if salt were the only chemical in water, this approximate measurement would be a great estimate.
But salt is not the only dissolved solid in swimming pool water. Other dissolved materials can influence the conductivity of the water and that makes calibration of the instrument critical to measuring salt alone. A sodium chloride standard, not the ‘442 standard’, is used to calibrate the instrument so that it can react the same way to salt in the swimming pool.
Since swimming pool water is not pure salt water, other factors can interfere with this method. Conductivity varies with the motion of ions in solution (sodium and chloride ions). This property of ionic motion is referred to as mobility. Higher temperature of the water causes ions to move faster, causing the measured conductivity (and measured salt value) to be biased high. Lower temperature causes to ions to slow down, leading to a low bias in measured salt value. Therefore, temperature needs to be taken into consideration when measuring the salt with conductivity. Hardness and pH can also influence the conductivity and should be within specified limits to get precise conductivity measurements for salt content.
Unfortunately, no swimming pool system is maintenance-free and in-line saltwater chlorine generators are no exception. Here are a few important things to know about the water chemistry of in-line saltwater systems.
- The chlorine and pH still need to be tested regularly to ensure that the generator is working properly and to keep up with the demand of the water. As with any other system, the chlorine should be ideally maintained in the range of one to three ppm (mg/L). The pH should also be monitored to ensure bather comfort and chlorine effectiveness; the ideal range is the same as other sanitizer systems, pH 7.2 to 7.8. Total alkalinity and hardness are also important factors that should be tested regularly, but these can be tested less frequently than pH and chlorine.
- Cyanuric acid, often referred to as stabilizer or conditioner, is very important to the effectiveness of in-line saltwater chlorine generation servicing outdoor pools. In-line saltwater chlorine generators produce pure chlorine that is not stabilized. Therefore, UV rays will cause this chlorine to degrade quickly. Cyanuric acid will help to protect the chlorine from the UV degradation. Since no cyanuric acid is added regularly in salt pools, it is recommended that the cyanuric acid level be maintained slightly higher than the average pool. Instead of the usual 30 to 50 ppm, the ideal level is 60 to 80 ppm. The cyanuric acid level will also drop with the addition of fresh water as does the salt level, so test the cyanuric acid level regularly (monthly at least).
- Many in-line saltwater chlorine generators have their own internal salt monitor that measures the pool’s salt content. Keep in mind that this is a conductivity measurement and will have the limitations discussed in the conductivity section above. For this reason, the salt should still be monitored regularly in these systems to ensure that the meter is reading accurately. After all, if the actual salt level is different than what the generator system ‘thinks’ it is, the in-line saltwater chlorinator will not be generating chlorine adequately, if at all.
The bottom line
In-line saltwater chlorine generation is going to continue to grow in popularity in the swimming pool industry. Therefore, understanding these systems now is important. The science behind this technology is a bit complex, but understanding how to use it is not. Although it is not quite the ‘set it and forget it’ technology that some might believe, salt chlorine generation can be an improvement to everyday pool maintenance if used properly.
About the author
Joe Sweazy is Technical Sales and Services Manager for HACH Company/ETS. He has published more than a dozen articles on pool and spa water chemistry and has presented numerous seminars at conferences of the National Spa and Pool Institute (APSP). He can be reached by email at firstname.lastname@example.org
About the company
HACH Company/ETS manufactures AquaChek, the world’s #1 brand of pool and spa test strips and other water quality products that are in use around the world, simplifying analysis with reliable, accurate results. For more information about AquaChek and AquaTrend products, call toll free 1-888-AquaChek (1-888-278-2243) or log onto www.AquaChek.com