Pool Chemistry is fairly simple, for chemists. We provide the chemical equations related to adding and using chlorine that occur in a pool to educate our members, and provide transparency that no pool store, or maintenance can, or will supply. Adding bleach is a basic process (Raises pH); it is the using up of bleach (chlorine) that is an acidic process (lowers pH) so the net result is almost neutral. Chlorine usage means the process by which chlorine gets used up by sanitizing the water.
Liquid Chlorine Pool Chemistry Equations
Adding Liquid Chlorine
NaOCl + H2O → Na+ + OCl-
OCl- + H2O ⇌ HOCl + OH-
Sodium Hypochlorite (liquid chlorine or bleach) combines with water to produce sodium ions (part of regular table salt) plus disinfecting chlorine plus hydroxyl ion. The hydroxyl ion makes this a basic reaction that raises pH, but because the disinfecting chlorine is a weak acid this overall reaction raises the pH by less than a strong base would. Note that there is a small amount of extra base in the form of Sodium Hydroxide (lye or caustic soda) that comes with Sodium Hypochlorite and is there to help preserve it, but this amount is rather small.
Using Up Of Chlorine
Breakdown of Chlorine by Sunlight (UV)
2HOCl → O2(g) + 2H+ + 2Cl-
2OCl- → O2(g) + 2Cl-
Chlorine breaks down in the presence of ultraviolet radiation, such as found in sunlight, and forms oxygen gas and chloride ion (and hydrogen ion, if starting with HOCl hypochlorite). Because a hydrogen ion is produced, this is an acidic process, but since disinfecting chlorine is a weak acid, only some of it breaks down in a way that lowers pH as shown above (i.e. only HOCl produces H+; OCl- does not). During the process of chlorine breakdown by sunlight, there are hydroxyl (OH•), oxygen anion (O-•) and chlorine (Cl•) radicals that are also produced as short-lived intermediates (technical details in this post Pool Water Chemistry). This can help oxidize organics in the pool.
Net Chlorine To Breakpoint (Ammonia "Oxidation")
2NH3 + 3HOCl → N2(g) + 3H+ + 3Cl- + 3H2O
OCl- + H+ → HOCl
The disinfecting form of chlorine (HOCl) combines with ammonia through a series of reactions (that are not shown) with the net result being the production of nitrogen gas plus hydrogen ion and chloride ion. Though by itself this would be a strong acid reaction, there is also OCl- present that will combine with hydrogen ion to form more HOCl since the ratio of HOCl to OCl- will remain constant (and is about 50/50 at pH 7.5). Therefore, the net reaction is acidic, but not strongly so. Further technical details are in this post.
Overall combination of adding chlorine and having it used up The net reactions are as follows if you combine the ones shown above.
2NaOCl → 2Na+ + 2Cl- + O2(g)
3NaOCl + 2NH3 → 3Na+ + 3Cl- + N2(g) + 3H2O
The overall net reaction of adding sodium hypochlorite to your pool and having it used up in its most typical ways is simply to produce salt (yes, sodium chloride or table salt, dissolved in water, of course) and either oxygen or nitrogen gas (and water). That is why using liquid chlorine in a pool will raise the salt level…a chlorine pool is a salt pool.
Other things that could happen
If you do not have enough chlorine in your pool relative to your bather load (ammonia demand), then the chlorine may not completely oxidize ammonia and instead you will get chloramines (first, monochloramine). This reaction is basic. However, sunlight (UV) may break down monochloramine which will result in the rest of the breakpoint process which overall is acidic (same as shown above)
It is also possible for chlorine to combine with organic compounds to form chlorinated organics that are hard to breakdown. When people talk about the health problems with chlorine, it is usually about some of these chlorinated organics (Disinfection By-Products, DBPs) known as Tri-Halo-Methanes(THMs) such as chloroform. Also, some chloramines such as nitrogen trichloride (NCl3) not only smell bad but can cause health problems (especially in indoor pools with poor air circulation). In an outdoor pool exposed to sunlight and with a good residual of chlorine you typically don't get these "bad" compounds.
Salt (SWG) Pool
In a salt water pool you produce chlorine through the following reactions:
At the anode (positive plate): 2Cl- → Cl2(g) + 2e-
At the cathode (negative plate): 2H2O + 2e- → H2(g) + 2OH-
which nets out to the following where the chlorine gas dissolves in water:
2H2O + 2Cl- → Cl2(g) + H2(g) + 2OH-
Cl2(g) + H2O → HOCl + H+ + Cl-
H+ + OH- → H2O
2H2O + Cl- → HOCl + OH- + H2(g)
or equivalently
H2O + Cl- → OCl- + H2(g)
Note that the products of HOCl and OH- are exactly the same as you get when you add liquid chlorine or bleach (ignoring sodium ion). This process is partly basic, but not strongly so due to the HOCl weak acid. Therefore, the overall net result in a salt pool is simply the production of oxygen or nitrogen gases. The disinfecting chlorine that was created from chloride ion gets converted back to chloride ion as it is used to sanitize.
The net reactions in an SWG pool for chlorine addition from the SWG and then breakdown from sunlight and oxidation of ammonia are as follows:
2H2O → O2(g) + 2H2(g)
2NH3 → N2(g) + 3H2(g)
The chlorine is not "seen" in the above net reactions because the chloride that became chlorine goes back to being chloride again. The oxygen gas comes from water when chlorine gas dissolved in it (i.e. from hypochlorite ion or hypochlorous acid) while the nitrogen gas comes from the ammonia (the oxygen or hydroxyl in the chlorine reverts back into water in this case, using the hydrogen from the ammonia to do so).
If you have a salt pool and don't use CYA (this isn't normal or recommended) then you could also outgas chlorine in the same way that CO2 is outgassed. This is more likely if you are aerating the water (e.g. have water features, slides, fountains, jets pointed up, lots of splashing, ...). This process is strongly basic and greatly increases the pH (HOCl + Cl- --> Cl2(g) + OH-). The reason this would tend to only happen in a salt pool without CYA is that a high concentration of both chloride ion (Cl-) and disinfecting chlorine (HOCl) are needed, and it occurs more readily at lower pH.
Lowering pH by adding Muriatic Acid
HCl (aq) −→ H+ (aq) + Cl- (aq)
When adding muriatic acid (hydrochloric acid, HCl) to a swimming pool, the aqueous (aq) acid dissociates into hydrogen ions (H+) and chloride ions (Cl-) when mixed with water, lowering the pH of the pool water.
pH = -log10[H+]
pH is the negative log base 10 of hydrogen ion activity. Therefore, when you add H+ (Hydrogen ions), you lower pH, as pH is a measure of hydrogen ion activity. Adding muriatic acid also adds Cl-, which is, chloride ions…exactly how salt is measured in a pool. So when you add muriatic acid, you are adding salt.
How Muriatic acid lowers Total Alkalinity (TA)
HCl → H+ + Cl−
H+ + CO32- → HCO3-
H+ + HCO3 → H2CO3
H2CO3 → H2O + CO2(g)
Muriatic acid, when added to the pool, completely dissociates into Hydrogen and Chloride ions. The Hydrogen ions (H+) will react with carbonate ions (CO32-), if they are present in the pool, converting them into bicarbonate ions (HCO3−). The bicarbonate ions then react with more hydrogen ions from the acid to form carbonic acid (H2CO3). Carbonic acid is unstable and breaks down into water (H2O) and carbon dioxide (CO2). Total alkalinity is lowered as Carbonic acid breaks down. Total alkalinity is a measure of the total concentration of alkaline substances (like carbonates and bicarbonates) in the pool water.
Raising pH with 20 mule team borax
Dissolution of Borax:
Na2B4O7⋅10H2O −→ 2Na+ + B4O72− + 10H2O
Hydrolysis of Tetraborate Ions:
B4O72− + 7H2O −→ 4H3BO3 + 2OH−
Net Reaction:
Na2B4O7⋅10H2O + H2O −→ 2Na+ + 4H3BO3 + 2OH−
20 Mule team Borax is the sodium tetraborate decahydrate (Na2B4O7 · 10H2O) that, when dissolved in water, is hydrolyzed to Sodium ions, boric acid and Hydroxide ions (OH−). The converse of H+ ions in measuring pH are OH- ions. A higher concentration of OH- anions results in a higher pH.
Raising Total Alkalinity with Baking Soda
Dissolution of Baking Soda:
NaHCO3 + H2O → Na+ + HCO3
Reaction of Bicarbonate Ion with Water:
HCO3− + H2O ⇌ H2CO3 + OH-
Overall Reaction
NaHCO3 + H2O ⇌ Na+ + OH− + H2CO3
When baking soda (sodium bicarbonate, NaHCO3) is added to water, it dissolves, and results in a rise in sodium ions (Na+), Hydroxide ions (OH-) which raises pH, and H2CO3 which raises Total Alkalinity. As a result, baking soda raises TA and pH.
Raising CH with Calcium Chloride
CaCl2(s) −→ Ca2+(aq) + 2Cl−(aq)
Calcium hardness is a measure of the concentration of calcium ions (Ca2+) in water. Increasing Ca2+ will raise calcium hardness (CH).
Lowering CH
There is no way to reduce CH except replacing water, or using reverse osmosis.
Dissolution of Cyanuric Acid in Water
H3CY(s) → H+ + H2CY-(aq)
Cyanuric acid in its dissolved form exists in water as a weak acid, but it doesn't fully dissociate into ions. At pool water typical pH levels, it's mostly present as H2CY-.
Interaction with Chlorine
Cl2 + H2CY- ⇌ HClCy- + H2O
Cyanuric acid primarily acts to stabilize free chlorine (such as Cl2, HOCl, and OCl−) in pool water. This stabilizing effect is achieved through the formation of a cyanuric acid chlorine complex. This complex slows down the degradation of chlorine by UV light, extending its effectiveness as a disinfectant. The reaction is in equilibrium, meaning the chlorine can still be used for sanitation, but it is protected from photodegradation.
Cyanuric Acid as a Weak Acid
Though cyanuric acid is a weak acid, it does not significantly lower the pH of pool water when added in typical concentrations. It has a pKa around 4.0, which means that it only slightly dissociates to produce hydrogen ions (H+) in water, lowering pH. This dissociation is minimal, and its primary role is as a chlorine stabilizer rather than as an acid that significantly impacts water chemistry.
Increasing salt
NaCl(s) −→ Na+(aq) + Cl−(aq) When sodium chloride (NaCl) dissolves in water, the polar water molecules surround the individual Na+ and Cl− ions, causing them to separate and disperse evenly in the solution. The salt simply dissociates, and no new chemical bonds are formed or broken. Cl- is the primary measure used in salt tests for pool water.
Decreasing salt
The only way to remove salt is to replace water or use reverse osmosis.
