Pool Chemistry

Pool Chemistry is fairly simple for chemists. We provide the chemical equations related to adding and using chlorine 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 (it Raises pH); using up bleach (chlorine) is an acidic process (it lowers pH), so the net result is almost neutral. Some of this content was originally posted in the thread "Pool Water Chemistry" by user "Chem Geek". The thread is linked at the bottom of this page. Chlorine usage is the process of using chlorine to sanitize the water.

What your test kit measures

• Free Chlorine (FC) measures all the forms of chlorine in the pool, HOCl (hypochlorous acid) + OCl^- (hypochlorite ion) and all the chlorine that is bound with cyanuric acid, in ppm.
• Combined Chlorine (CC) measures the amount of free chlorine that has bound itself to a contaminant or organic material, such as ammonia or other contaminants, and is no longer available to disinfect, in ppm. This is done by measuring chloramines Monochloramine (NH₂Cl), Dichloramine (NHCl₂), and Trichloramine (NCl₃).
• pH measures the concentration of hydrogen ions (H⁺) in the water. More hydrogen ions are acidic (lower pH), less are basic (higher pH).
• Total Alkalinity (TA) measures bicarbonate (HCO₃^-), carbonate (CO₃^2-), and hydroxide (OH^-) ions, and 1/3 of the CYA, in ppm.
• Calcium Hardness (CH) measures calcium carbonate (CaCO₃), in ppm.
• Cyanuric Acid (CYA) measures cyanuric acid, in ppm.
• Salt measures the concentration of Chloride Cl^- ions, then converts that measurement into the concentration of sodium chloride (NaCl) reported as ppm of NaCl.

Chlorine Pool Chemistry Equations

Notations: (s) = solid (aq) = aqueous (g) = gas (l) = liquid

Liquid Chlorine

Adding Liquid Chlorine

Sodium hypochlorite in the liquid form (liquid chlorine) is a mix of sodium ions Na⁺, hypochlorite ions OCl^-, and a little hydroxide (OH^-) in Water (H₂O).

When liquid chlorine is added to pool water.

HOCl ⇌ H⁺ + OCl^-

The hypochlorite ions (OCl-) react with hydrogen ions to produce hypochlorous acid (HOCl) which is the active sanitizing agent in chlorine. It is a weak acid and can easily penetrate cell walls, killing bacteria and algae. Hypochlorous acid and the hypochlorite ion are in an equilibrium reaction. This reaction and the concentrations on both sides are dependent on the pH. The pKa of this reaction is given as 7.54 which is the pH point where the concentrations of both are equal.

This graph shows the relationship in the percentages of HOCl and OCl^- at various pH levels, which, because they are in equilibrium, will always total 100%.

The hydroxyl ion makes this a basic reaction that raises the pH, but because hypochlorous acid is a weak acid the 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.

Breakdown of Chlorine by Sunlight (UV)

2HOCl + UV → O₂(g) + 2H⁺ + 2Cl^-

2OCl^- + UV → O₂(g) + 2Cl^-

Chlorine breaks down in the presence of ultraviolet radiation, such as 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.

Technically it is only the hypochlorite ion that is (mostly is) degraded by UV. HOCl is also reduced in concentration because OCl- and HOCl remain in a rapid equilibrium. As OCl^- is degraded HOCl becomes OCl^- + H⁺ and there is more OCl^- to be degraded by UV until both are all used up.

The overall net reaction of adding sodium hypochlorite to your pool and having it broken down by UV adds Cl^-. This is what the Taylor reagents test for when testing salt. As a result chlorine in a pool will raise the salt level…a chlorine pool is a salt pool.

Net Chlorine To Breakpoint (Ammonia "Oxidation") or CC elimination

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.

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 to produce salt (yes, sodium chloride or table salt, dissolved in water) and 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.

Combined Chlorine (CC), also call chloramines, are formed by the reaction of free chlorine with nitrogen-containing contaminants from bathers and the environment. If your pool smells like chlorine, it is due to high CCs (Dichloramine and Nitrogen Trichloride) in the oxidation process. CCs are usually the product of heavily used pools. CCs are also a primary cause of eye and mucous membrane irritations.

Other things that could happen

Suppose you do not have enough chlorine in your pool relative to your bather load (ammonia demand). In that case, 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 break down. 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 smells foul but can cause health problems (especially in indoor pools with poor air circulation). In an outdoor pool exposed to sunlight and with a good chlorine residual, you typically don't get these "bad" compounds.

Salt (SWG) Pool

In a saltwater 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^-

This nets out to the following where the chlorine gas dissolves in water:

2H₂O + 2Cl^-(aq) + 2e^- → Cl₂(g) + H₂(g) + 2OH^-(aq)

The chlorine gas reacts with water to produce hypochlorous acid:

Cl₂(g) + H₂O → HOCl(aq) + H⁺(aq) + Cl^-(aq)

Some of the hypochlorous acid will disassociate to form hypochlorite (dependent on pH):

HOCl(aq) ⇌ H⁺(aq) + OCl^-(aq)

A small amount of water will stay in equilibrium with both proton and hydroxide ions:

H₂O ⇌ H⁺(aq) + OH^-(aq)

The net reactions are:

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).

SWG should be installed in a proper fashion according to manufacturer specification. When they are installed properly, the vast majority of the chlorine dissolves. If not installed properly, there is always a slight chance of Cl₂ venting to the atmosphere. And when it is vented, the Cl₂ that is produced in a SWG is not converted to HOCl and HCl, which changes the balance of H⁺ to OH^- in favor of the hydroxide ion which will cause an increases in pH.

pH and TA Adjustments

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⁺ + CO₃^2- → 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 (CO₃^2-), 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.

Natural pH rise

CO₂ + H₂ ⇌ H⁺ + HCO​₃^-

Equilibrium of carbon dioxide, hydrogen ions and Carbonic acid in the pool. When CO₂ outgasses the equilibrium shifts left, reducing the amount of Hydrogen ions resulting in pH rise. For more details on why CO₂ outgasses, see PH TA Relationship. ​

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^-(aq)

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.

Calcium Adjustments

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.

Cyanuric Acid (CYA)

To simplify the discussion, we will use the dominate species of H₂CY^- cyanuric acid and the strongly disinfecting and oxidizing form of chlorine, HOCl Hypochlorous acid. OCl^- will act the same way as HOCl. There are many species of cyanuric acid that exist at various pKa. See O'Brien in the references for a deeper dive into the cyanuric acid chlorine complexes.

Dissolution of Cyanuric Acid in Water

CyA(s) → CyA(aq)

CyA(aq) ⇌ H⁺ + H₂CY^-(aq)

Cyanuric acid (CyA), commonly just referred to as CYA, has the chemical formula C3H3N3O3 or (CHNO)3 and can also be referred to as CyA or H₃Cy where the Cy refers to the (CNO)3 cyclic structure. When dissolved in swimming pool water it exists as the weak acid and the first disassociated cyanurate ion (H₂CY^-) where the pKa is given as 6.88. At typical pool water pH levels CYA is mostly present as the cyanurate ion until it reacts with free chlorine. CYA testing measures both the acid and the cyanurate ion.

Interaction with Chlorine

HOCl + H₂CY^- ⇌ HClCy^- + H₂O

Cyanuric acid primarily acts to stabilize free chlorine 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, until the HOCl is used on the left side of the equation, freeing more HOCl from the cyanuric acid chlorine complex.

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.

Salt

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.

Boric Acid

Boric Acid dissolved in water

H₃​BO₃​(s) → H₃​BO₃​(aq)

At normal pool pH, most boric acid stays as H₃BO₃, with some converting to tetrahydroxyborate ion B(OH)₄^⁻

Weak acid equilibrium (buffering):

H₃​BO₃​ + H₂O ⇌ B(OH)₄^- + H⁺

This creates a buffering system that resists upward pH drift.

Boric acid reducing pH rise in a salt cell

At the cathode (negative plate):

2H₂O + 2e^- → H₂(g) + 2OH^-

This increases OH⁻, which raises the local pH inside the salt cell.

Without Borates, the hydroxide converts bicarbonate to carbonate.

HCO³ + OH^- → H₂ + CO₃^2-

Then the carbonate connects to calcium and you get calcium carbonate, which is what forms scale the in cell.

Ca² + CO₃^2- → CaCO₃

As indicated earlier, boric acid participates in the buffering equation.

H₃​BO₃​ + H₂O ⇌ B(OH)₄^- + H⁺

When OH⁻ builds up (like in a salt cell), boric acid reacts with that OH⁻ to form borate. This reduces the pH in the call and reduces the formation of calcium carbonate reducing the potential for scaling within the cell.

H₃​BO₃ + OH^- → B(OH)₄^-

Pool water testing chemical equations

FC FAS-DPD Test

FAS-DPD Chlorine test measures:

Free Chlorine: HOCl, OCL^-, and free chlorine bound to CYA, in ppm.

Combined Chlorine: the presence of chloramines: Chloramine (NH₂Cl), Dichloramine (NHCl₂) and Nitrogen Trichloride (Trichloramine)(NCl₃), in ppm.

Free Chlorine

Free chlorine (HOCl or OCl⁻) reacts with DPD (N,N-diethyl-p-phenylenediamine) to form a pink-colored dye (a redox reaction). The amount of DPD⁺ created is proportional the the amount of free chlorine.

HOCl + DPD → DPD⁺ (Wurster dye (pink)) + Cl^- + H⁺

OCl^- + DPD → DPD⁺ (Wurster dye (pink) + Cl^-

Ferrous ammonium sulfate (FAS) (R-0871) is FAS in water:

Fe(NH₄)₂(SO₄)₂ · 6HO (s) + H₂O → Fe²⁺ (aq) + 2 NH₄⁺ (aq) + 2 SO₄^2- (aq) + 6 H₂O (l)

The iron ion is used to titrate the pink DPD radical back to its colorless form.

DPD⁺ (Wurster dye (pink) + Fe²⁺ → DPD(aq) + Fe³⁺

When you titrate with ferrous ammonium sulfate (FAS), the iron(II) ions (Fe²⁺) reduce this pink oxidized form of DPD back to its reduced (colorless) form. The endpoint is when the pink color disappears — that indicates that all the oxidized DPD has been reduced, which corresponds stoichiometrically to the amount of chlorine that was present.

Combine Chlorine

If Combined Chlorine (Chloramine (NH₂Cl), Dichloramine (NHCl₂) and Nitrogen Trichloride (Trichloramine)) are also present, after the free chlorine is measured, DPD+ created is proportional the the amount of Choramines when R-0003 containing an potassium iodide is added (R-0003). Potassium iodide in water forms potassium and iodide ions.

KI(s) + H₂ → K⁺ + I^-

Those iodide ions react with chloramines to form iodine.

​Chloramine

NH₂Cl + 2I^- + 2H⁺ → NH₃ + Cl^- + I₂ + H₂

Dichloramine

NHCl₂ + 3I^- + 2H⁺ → NH₄⁺ + Cl^- + I₂

Nitrogen Trichloride

NHCl₃ + 3I^- + 3H⁺ → NH₄⁺ 3Cl^- + I₂

The liberated iodine reacts in the same way as the free chlorine with DPD:

I₂ ​+ DPD (colorless) → DPD (pink/red) + 2I^-

Ferrous ammonium sulfate (FAS) is used to titrate the pink DPD radical back to its colorless form.

DPD⁺ (Wurster dye (pink) + Fe²⁺ → DPD(aq) + Fe³⁺

Which corresponds stoichiometrically to the amount of chloramines that were present.

pH Test

Phenol red (H₂In) (R-0004 [Taylor] or R-0014 [TFTestKits]) changes color based on pH, transitioning from yellow (H₂In) to red (HIn^-) to violet (In^2-) over pH 6.8–8.2.

First Dissociation (Yellow to Red):

H₂In (yellow) ⇌ HIn^- (red) + H⁺ (pKa ~7.9)

At pH ~6.8: yellow (H₂In dominates).

At pH ~7.4: red (HIn^- dominates).

Second Dissociation (Red to Violet):

HIn^- (red) ⇌ In^2- (violet) + H⁺

At pH > 8.2: violet/purple hues may appear.

Total Alkalinity

R-0007 - Dechlorinating the sample

Why dechlorinate before alkalinity testing?

Chlorine (free chlorine like HOCl and OCl⁻) is an oxidizer.

If you use pH indicators like R-0008 in chlorinated water:

• The chlorine can oxidize the indicator dye.
• This leads to false color changes and inaccurate alkalinity readings.

Dechlorination using Sodium Thiosulfate (R-0007), if chlorine is present:

4 HOCl + Na₂S₂O₃ → 2 NaHSO₄ + 4 HCl

R-0008 - Adding a dye indicator

HIn (red) ⇌ H⁺+In⁻ (green)

As acid is added in the next step and pH drops, the dye shifts color with an endpoint pH ~4.5.

R-0009 - Neutralizing Alkalinity

R-009 is a weak sulfuric acid, which contains hydrogen ions H⁺ which are used to neutralize the alkalinity, and ultimately shift the color of the dye.

Hyrdoxide neutralization:

OH⁻ + H⁺ → H₂O

Carbonate neutralization:

First protonation: Carbonate

CO₃²⁻ + H⁺ → HCO₃⁻

Second protonation: Bicarbonate

HCO₃⁻ + H⁺ → H₂CO₃

And carbonic acid (H₂CO₃) decomposes:

H₂CO₃ ⇌ CO₂(g) + H₂O

Cyanuric acid neutralization:

HCy²⁻ + H⁺ → H₂Cy⁻

• The CYA species that’s measured (i.e., contributes to alkalinity) is mainly HCy²⁻.
• Most CYA in pool water exists as H₂Cy⁻ and H₃Cy, which don’t significantly react during the alkalinity test at pool pH.
• As a result about 1/3 of cyanurates show up in alkalinity.

The end point of the titration is about a pH of 4.5 when the color makes the final change. To get to a pH of 4.5, the titrating agent is being used to neutralize all the forms of alkalinity. If it takes more drops, it means that there were more carbonate, bicarbonate, carbonic acid and cyanurates to neutralize before the acid can shift the dye to red with a pH of 4.5, indicating all the TA has been neutralized.

Calcium Hardness (CH)

The test uses EDTA titration with Eriochrome Black T (EBT) as the indicator to measure calcium ions (Ca²⁺).

R-0010 - Raising the pH:

A sodium hydroxide buffer (R-0010) raises pH to ~10–11, keeping Ca²⁺ soluble.

Ca²⁺ (aq) + OH^- (aq) → No precipitation (Ca²⁺ remains soluble)

R-0011 - Indicator Binding to Calcium

EBT (HIn^2-, blue) binds Ca²⁺, forming a red complex.

Ca²⁺ + HIn^2- (blue) → CaHIn (red)

R-0012 - Titrating with EDTA

EDTA (H₂EDTA^2-, from R-0012) binds Ca²⁺ more strongly, releasing EBT to its blue form at the endpoint.

CaHIn (red) + H₂EDTA^2- → CaEDTA^2- (colorless) + HIn^2- (blue) + H⁺

The amount of EDTA required to release EBT in its blue form is a direct measure of the calcium ions in solution.

Cyanuric Acid (CYA)

Melamine (C₃H₆N₆) reacts with cyanuric acid ((CHNO)3) in water to form a hydrogen-bonded, insoluble precipitate (melamine-cyanurate, C₆H₉N₉O₃).

C₃H₆N₆ (aq) + C₃H₃N₃O₃ (aq) → C₆H₉N₉O₃ (s)

The melamine in R-0013 (C₃H₆N₆) reacts with all available CYA (both free and that released from chlorinated isocyanurates) to form an insoluble melamine-cyanurate precipitate, causing turbidity.

The precipitate’s turbidity is proportional to the total CYA concentration, measured by the volume needed to obscure a black dot in the test tube.

Why the Test Measures Total CYA

• Weak Bonding: The chlorine-CYA bond in chlorinated isocyanurates is not covalent but a weak coordination complex. In aqueous solution, the equilibrium favors partial dissociation, ensuring enough free CYA is available to react with melamine.

• Test Conditions: The test is conducted at pool water pH (~6–7.8), where CYA (pKa ~6.9) is partially deprotonated, and chlorinated isocyanurates dissociate readily. No additional reagents (e.g., acids or bases) are needed to break the chlorine-CYA bond.

• Excess Melamine: The R-0013 reagent contains a high concentration of melamine, driving the precipitation reaction to completion (Le Chatelier’s principle), capturing all CYA, including that released from bound forms.

• Rapid Reaction: The melamine-CYA reaction is fast (complete within ~2 minutes, as per Taylor’s updated K-1720 instructions), and the equilibrium of chlorinated isocyanurates adjusts quickly, ensuring all CYA is accounted for.

Salt

this section is UNDER CONSTRUCTION R-0637 - adding the indicator:

K₂CrO₄ (aq) → 2 K⁺ (aq) + CrO₄^2-

The potassium chromate (K₂CrO₄) dissolves in the sample, releasing chromate ions (CrO₄^2-). These ions give the solution a yellow color and remain unreactive until the titration with silver nitrate (R-0718) reaches the endpoint.

No Reaction with Chloride: At this stage, the chromate ions do not interact with the chloride ions (Cl^-) from NaCl in the pool water. Their role is to form a red precipitate (Ag₂CrO₄) with excess Ag⁺ after all Cl^- is precipitated as AgCl.

The primary reaction upon adding R-0718 is the precipitation of silver chloride as silver nitrate reacts with chloride ions in the sample:

AgNO₃ (aq) + Cl^- (aq) → AgCl (s) + NO₃^- (aq)

Ionic equation: Ag⁺ (aq) + Cl^- (aq) → AgCl (s)

(R-0637): Potassium chromate (K₂CrO₄) reacts with silver ions (Ag⁺) to produce a red precipitate of silver chromate (Ag₂CrO₄) at the endpoint. Precipitation of Silver Chloride Silver nitrate (AgNO₃) reacts with chloride ions (Cl^-) to form insoluble silver chloride (AgCl), a white precipitate. This occurs during titration until all chloride is consumed. Reaction: AgNO₃ (aq) + Cl^- (aq) → AgCl (s) + NO₃^- (aq) Ionic equation: Ag⁺ (aq) + Cl^- (aq) → AgCl (s)

References:

• Pool Water Chemistry Thread
• O'Brien, 1974
• Wahman 2019
• Wojtowic 1996
• Pickens 2023
• Pickens 2019
• Falk