A chlorine molecule or ion kills bacteria, algae, and disease-causing organisms by moving through the cell membranes and deactivating the cell's essential enzymes and structures. All chemical forms of chlorine provide oxidation, but different forms vary in effectiveness depending on how easily they move through the cell membranes. The most effective form of chlorine for disinfection is Hypochlorous Acid; the Hypochlorite Ion form is next most effective; Chloramines are less effective.
Free chlorine shows the level of disinfecting chlorine available to keep your pool sanitary. FC should be tested and chlorine added daily, unless you have an automatic feeder , in which case you can test it every couple of days. FC is consumed by sunlight and by breaking down organic material in your pool. The level of FC you need to maintain depends on you and how much your pool is used.
Hypochlorous Acid + Hypochlorite Ion) Free Available Chlorine or free chlorine (FAC or FC) is the measure of chlorine's active disinfecting power. The free chlorine reading is the sum of the hydrochlorous acid and the hypochlorite ion residual concentration levels in the pool/spa water in parts per million (ppm).
Hypochlorous Acid is the most active disinfectant form of chlorine in the water.
When chlorine reacts with ammonia and nitrogen compounds in the water (urine and other organic wastes) it forms chloramines and nitrogen. Chloramines cause an unpleasant chlorine odor and eye irritation to swimmers.
Combined chlorine is an intermediate breakdown product that is created in the process of sanitizing the pool. CC causes the "chlorine" smell many people associate with chlorine pools. If CC is above 0.5 you should shock your pool. CC indicates that there is something in the water that the FC is in the process of breaking down. In an outdoor pool, CC will normally stay at or near zero as long as you maintain an appropriate FC level.
Potassium monopersulfate (a common non-chlorine shock) will often show up on tests as CC.
How to test Chlorine
How to lower chlorine
Replace some of the pool water with fresh water. This solution will also have the positive side-effect of lowering total dissolved solids and cyanuric acid.
Do nothing. The chlorine will eventually drop to the desired level. High amounts of sunlight and a high bather load will speed up the process.
You can add chemicals to the water. Sodium Thiosulfate and Sodium Sulfite are the most common. Commercial chlorine neutralizer is the same thing except in a pretty small bottle and many times more expensive. Another possibility is peroxide. Pool stores carry peroxide in 5gal jugs for UV sanitation systems. Either way be careful not to add too much; if you do, it may be very hard to maintain a proper chlorine level for some time, the residual in the water will keep taking the chlorine down.
pH indicates how acidic or basic the water is. pH should be tested daily at first. Once you gain experience with your pool, less frequent monitoring may be appropriate depending on your pool's typical rate of pH change. pH levels between 7.2 and 7.8 are suitable for swimming, while levels between 7.4 and 7.6 are ideal. pH below 7.2 tends to make eyes sting or burn. pH below 7.0 can cause damage to metal parts, particularly pool heaters with copper heat exchange coils. High pH can lead to calcium scaling.
How to test pH
How to adjust pH
TO LOWER PH: To lower both total alkalinity and pH, you will need to add an acid. Liquid muriatic acid (hydrochloric acid diluted to 20 percent) and dry sodium bisulfate are among the most popular choices.
The acid combines with the water, thereby increasing the concentration of hydrogen ions and lowering the pH.
Similarly, the acid reacts with bicarbonates in the water and reduces the water's total alkalinity in the process.
You could also look for a generic “pH decreaser,” “alkalinity decreaser,” or “combination decreaser” chemical at a pool supply store.
TO RAISE PH: Add soda ash (sodium carbonate) to raise the pH of your pool water. Soda ash may be labeled by many different manufacturer names. Ensure the base ingredient of the product is sodium carbonate.
pH of water will tend to rise naturally and with increased usage.
Total alkalinity indicates the water's ability to buffer pH changes. Buffering means you need to use a larger quantity of a chemical to change the pH. At low TA levels, the pH tends to swing around wildly. At high TA levels, the pH tends to drift slowly.
How to test Alkalinity
How to Adjust Alkalinity
To raise the level of TA, we use sodium bicarbonate; it is the only chemical which will do this without increasing the pH very much.
Lowering the total alkalinity is also a slow process. Acid, either liquid or dry, is added to the deepest part of the pool with the filter off. The acid should only be added a little at a time, diluting it before pouring it into the pool. Wait 3 days between applications. It could take days or even weeks to reduce the TA if it is very high.
Calcium hardness indicates the amount of calcium in the water. Over time, water with low calcium levels will tend to dissolve calcium out of plaster, pebble, tile, stone, concrete, and to some extent fiberglass surfaces. You can prevent this from happening by keeping the water saturated with calcium.
How to test Calcium Hardness
How to adjust Calcium Hardness
Calcium hardness can easily be increased using calcium chloride.
Reducing calcium hardness is more difficult. Either replace some or all of the water in the pool, or add chemicals that will keep the calcium in solution and prevent it from depositing out.