Pool Chemistry Reference: Complete Parameter Guide and Glossary (2026)

Algaecide

A chemical that prevents or kills algae in pool water. Three main types: polyquat 60 (polyquaternary ammonium chloride — non-foaming, non-staining, best for maintenance), copper-based (most effective against black algae but can stain surfaces), and sodium bromide (winter/opening use). Algaecide prevents algae re-establishment after shocking; it does not clear a green pool.

Target: add weekly at 2-4 oz per 10,000 gallons when FC is below 4.0 ppm.

  Alkalinity — see Total Alkalinity

See Total Alkalinity (TA) below.

  Available Chlorine

The amount of active chlorine in a product, expressed as a percentage. Trichlor tablets: 90% available chlorine per PHTA/EPA standard. Cal-hypo 68%: 65% available chlorine. Dichlor (dihydrate): 56% available chlorine. Available chlorine is the figure that determines how much FC a given product adds to pool water.

Note: ‘available chlorine’ on a label ≠ the percent of the bag that is active ingredient.

  Backwash

The process of reversing water flow through a sand or DE filter to flush trapped debris out through the waste line. Backwash when the pressure gauge reads 8-10 psi above the filter’s clean baseline reading. One backwash cycle for a sand filter uses approximately 200-500 gallons of water.

Cartridge filters do not backwash — remove and rinse the cartridge instead.

  Breakpoint Chlorination

The point at which enough free chlorine has been added to destroy all combined chlorine (chloramines) in the pool. The generally accepted formula from the Certified Pool Operator Handbook is to raise FC by 10 times the combined chlorine level. Below breakpoint, adding chlorine worsens the chloramine problem before improving it — the dose is all-or-nothing.

Example: CC = 0.8 ppm. Raise FC by 8 ppm to reach breakpoint.

  Cal-Hypo  (Calcium Hypochlorite)

The active ingredient in most pool shock products. Standard residential pool shock is 68% calcium hypochlorite, yielding 65% available chlorine. Does not add CYA to pool water. Does add calcium — relevant in pools with existing high calcium hardness. Sold in 1-lb bags, case quantities, or bulk buckets.

PHTA standard: 68% cal-hypo = 65% available chlorine. Source: PHTA Cal-Hypo Fact Sheet 2021.

  Calcium Carbonate  (CaCO3)

The scale compound that forms when pool water is oversaturated with calcium. Appears as white deposits on tile, grout, waterline, and equipment surfaces. Also the primary mineral component of plaster pool surfaces — low calcium water pulls CaCO3 from plaster to satisfy its mineral demand (causing etching). Controlled through calcium hardness, pH, and alkalinity balance (LSI).

  Calcium Hardness  (CH)

The concentration of dissolved calcium in pool water, measured in ppm. Target range: 200-400 ppm for plaster/concrete pools; 175-225 ppm for vinyl liner and fiberglass pools (PHTA / industry standard). Low CH: water is corrosive — etches plaster, grout, and metal fittings. High CH: water deposits calcium scale on surfaces and equipment. No chemical removes calcium — lower CH only by partial drain and refill.

Adjustment: 1.25 lbs calcium chloride per 10,000 gal raises CH by ~10 ppm.

  Chloramine  (CC)

The byproduct formed when free chlorine reacts with ammonia, nitrogen compounds, or organic matter from bathers. Measured as combined chlorine (CC = total chlorine minus free chlorine). Above 0.5 ppm per PHTA guidelines: shock the pool. Causes the strong ‘chlorine smell’ most people incorrectly attribute to too much free chlorine. Chloramines are irritants — they are the opposite of clean water.

Test: Total Chlorine – Free Chlorine = Combined Chlorine.

  Combined Chlorine  (CC)

See Chloramine above. Measured by subtracting free chlorine from total chlorine. PHTA guideline: shock the pool when CC exceeds 0.5 ppm. A pool that smells strongly of ‘chlorine’ almost always has elevated combined chlorine, not excess free chlorine.

Target: below 0.5 ppm. Source: PHTA guidelines.

  Cyanuric Acid  (CYA)

A UV stabilizer that protects chlorine from sunlight degradation. Enters pool water from trichlor tablets and dichlor granules (each adds ~6-7 ppm CYA per lb per 10,000 gallons) or as a direct additive. PHTA APSP-11 Standard: target range 30-50 ppm; maximum 100 ppm. Above 80 ppm: chlorine effectiveness drops substantially. No chemical removes CYA — only dilution by partial drain and refill reduces levels.

Also called: pool stabilizer, pool conditioner, isocyanuric acid.

  Dichlor  (Sodium Dichloroisocyanurate)

A fast-dissolving stabilized chlorine with 56% available chlorine (dihydrate form) or 62% (anhydrous form). Near-neutral pH (~6.5-7.0). Adds CYA at the same rate as trichlor (~6-7 ppm per lb per 10,000 gallons). Best for spas, hot tubs, pools needing quick chlorine addition without large pH impact. Not recommended as a primary pool sanitizer due to rapid CYA accumulation.

PHTA Dichlor Fact Sheet confirms 56% available chlorine (dihydrate).

  Diatomaceous Earth  (DE)

A white powder made from fossilized remains of diatoms (microscopic algae) used as filter media in DE-type pool filters. Filters pool water to 2-5 microns — the finest residential pool filtration available. Must be recharged after each backwash. Respiratory hazard — wear a dust mask when handling dry DE powder.

  FC/CYA Ratio

The relationship between free chlorine and cyanuric acid that determines actual chlorine effectiveness. As CYA increases, more free chlorine must be maintained to keep the same concentration of active hypochlorous acid (HOCl) available for sanitization. Industry standard: maintain FC at a minimum of 7.5% of CYA level. At CYA = 50 ppm: minimum FC = 3.75 ppm. At CYA = 80 ppm: minimum FC = 6 ppm.

This is why a pool with CYA at 100 ppm and FC at 3 ppm may have algae despite ‘normal’ FC readings.

  Free Chlorine  (FC)

The active, sanitizing chlorine available in pool water — the portion not yet reacted with contaminants. Consists of hypochlorous acid (HOCl) and hypochlorite ion (OCl-). PHTA target range: 1.0-4.0 ppm for residential pools. Below 1.0 ppm: bacterial and algae growth risk. Above 4.0 ppm: do not swim. Free chlorine is what your test kit measures in Step 1 (before the second reagent).

Source: PHTA Cal-Hypo Fact Sheet; EPA-FIFRA product label standard.

  Hardness — see Calcium Hardness

See Calcium Hardness (CH) above.

  HOCl  (Hypochlorous Acid)

The active killing form of free chlorine. HOCl is a small, neutral molecule that penetrates algae and bacterial cell walls and destroys them. In unstabilized pools (no CYA), the percentage of FC that exists as HOCl depends primarily on pH: ~66% at pH 7.2, ~50% at pH 7.5, ~20% at pH 8.0. In stabilized pools with CYA, most chlorine is bound to CYA and the FC/CYA ratio (not pH) is the primary determinant of HOCl concentration.

HOCl is 80-100x more effective as a sanitizer than OCl- (hypochlorite ion).

  Langelier Saturation Index  (LSI)

A calculation that predicts whether pool water is balanced, scale-forming (positive LSI), or corrosive (negative LSI). Uses pH, total alkalinity, calcium hardness, water temperature, and total dissolved solids. Balanced water (LSI near zero) neither etches surfaces nor deposits scale. The LSI is the reason all three of pH, TA, and CH must be considered together rather than in isolation.

LSI = pH + TF + CF + AF – 12.1 (where TF = temperature factor, CF = calcium factor, AF = alkalinity factor)

  Liquid Chlorine  (Sodium Hypochlorite)

Pool-grade liquid chlorine at 10-12.5% sodium hypochlorite concentration. Does not add CYA. Raises pool pH when added. Degrades with heat and UV exposure — buy in small quantities and use promptly. 10 oz of 10% liquid chlorine raises FC approximately 1 ppm in 10,000 gallons. Preferred for saltwater pools, vinyl liner pools, and high-CYA pools where no additional stabilizer is desired.

PHTA Liquid Chlorine Fact Sheet: EPA-specified FC range 1.0-4.0 ppm.

  Muriatic Acid  (Hydrochloric Acid (HCl))

A liquid acid used to lower pH and total alkalinity in pool water. More aggressive and less expensive per dose than sodium bisulfate (dry acid). Always dilute in a bucket of pool water before adding — never add concentrated muriatic acid directly to pool water near the surface. Always add acid to water, never water to acid. Produces chlorine gas if mixed with chlorine-based chemicals.

Approximate equivalence: 1 quart muriatic acid (31%) ≈ 1.25 lbs sodium bisulfate.

  OCl-  (Hypochlorite Ion)

The less-active form of free chlorine present at higher pH levels. OCl- is 80-100x less effective at killing pathogens than HOCl (hypochlorous acid). As pH rises above 7.5, a larger proportion of free chlorine exists as OCl- rather than HOCl. At pH 8.0 in unstabilized pools, approximately 80% of FC exists as the less-effective OCl-. This is why high pH reduces chlorine effectiveness.

  ORP  (Oxidation-Reduction Potential)

An electronic measurement (in millivolts) of water’s oxidizing capacity — a proxy for chlorine effectiveness. Higher ORP generally indicates more effective sanitization. Useful for automated controller systems. CYA interferes with ORP readings because it buffers HOCl concentration, making ORP readings unreliable in stabilized pools. ORP is not a substitute for directly measuring FC, CC, and CYA.

  pH

A measure of how acidic or basic pool water is, on a scale of 0-14. PHTA target range: 7.2-7.8 (slightly alkaline). Below 7.2: water is corrosive — etches surfaces, causes eye irritation, corrodes equipment. Above 7.8: chlorine effectiveness drops, scale forms, swimmer eye irritation. pH drifts upward naturally in most pools from aeration, bather load, and trichlor chemistry (trichlor has a pH of 2.8-3.0 but the overall pool pH still rises from other sources).

Adjustment: raise with soda ash (6 oz per 10,000 gal = +0.2 units); lower with sodium bisulfate.

  Polyquat 60

Poly(oxyethylene(dimethyliminio)ethylene(dimethyliminio)ethylene dichloride) at 60% concentration. The recommended maintenance algaecide for residential pools. Non-foaming, non-metallic, non-staining. Effective against green and yellow/mustard algae. For black algae: copper-based products are more effective. Weekly dose: 2-4 oz per 10,000 gallons. Add only when FC is below 4.0 ppm.

  ppm  (Parts Per Million)

The standard unit of measurement for pool chemistry parameters. 1 ppm = 1 mg/L = 1 milligram of substance per liter of water. In pool chemistry: 1 ppm in a 10,000-gallon pool = approximately 0.083 lbs (1.3 oz) of pure substance. All target ranges in this guide are expressed in ppm unless otherwise noted.

  Sanitizer

A chemical that destroys or inactivates bacteria, viruses, and other microorganisms in pool water to safe levels. The primary pool sanitizer is free chlorine (from trichlor tablets, cal-hypo shock, liquid chlorine, or a salt chlorine generator). Bromine is an alternative sanitizer used primarily in spas. Biguanides (Baquacil, Soft Swim) are non-chlorine sanitizer alternatives. Per PHTA and EPA guidelines, maintain FC at 1.0-4.0 ppm for safe residential pool sanitation.

  Shock

The practice of temporarily raising free chlorine to 10-30 ppm to destroy chloramines, kill algae, and oxidize organic contaminants that routine sanitation cannot handle. Standard maintenance shock: 1 lb cal-hypo 68% per 10,000 gallons raises FC ~10 ppm. Green pool treatment: 3-7 lbs per 10,000 gallons depending on severity. Shock after sunset. Swim when FC drops back to 1-4 ppm. Shock is not a substitute for regular chlorination — both are needed.

Source: PHTA Cal-Hypo Fact Sheet 2021.

  Sodium Bicarbonate  (Baking Soda / Alkalinity Increaser)

The chemical used to raise total alkalinity in pool water. Chemically identical to baking soda (NaHCO3) — pool store ‘alkalinity increaser’ is the same product at a higher price per pound. Standard dose: 1.5 lbs per 10,000 gallons raises TA by approximately 10 ppm. Also raises pH slightly. Set TA before adjusting pH.

  Sodium Bisulfate  (Dry Acid / pH Down)

A granular acid used to lower pH and total alkalinity in pool water. Safer to handle and store than muriatic acid. Active ingredient: sodium bisulfate (NaHSO4). Standard dose: approximately 1 lb per 10,000 gallons lowers pH by 0.2-0.4 units depending on starting pH and TA. Broadcast near return jets; never add through skimmer.

  Sodium Carbonate  (Soda Ash / pH Up)

The chemical used to raise pH in pool water. Sold as ‘soda ash’ or ‘pH increaser.’ More concentrated than sodium bicarbonate — raises pH more significantly per ounce. Standard dose: 6 oz per 10,000 gallons raises pH approximately 0.2 units. Pre-dissolve in a bucket before adding to avoid localized cloudiness.

  Stabilizer — see Cyanuric Acid

See Cyanuric Acid (CYA) above.

  Superchlorination — see Shock

See Shock above.

  TDS  (Total Dissolved Solids)

The cumulative concentration of all dissolved substances in pool water — minerals, chemicals, organic compounds, and their byproducts. TDS increases continuously as chemicals are added and water evaporates. The only fix for high TDS is dilution by partial drain and refill. Industry standard: action threshold when TDS exceeds 1,500 ppm above the fill water baseline. High TDS does not directly cause safety issues but can make water chemistry harder to balance and more corrosive.

  Total Alkalinity  (TA)

A measure of the water’s buffering capacity — its ability to resist pH changes. Consists of bicarbonates, carbonates, and hydroxides dissolved in the water. PHTA target range: 80-120 ppm. Below 80 ppm: pH is unstable and swings with every chemical addition or rainstorm. Above 120 ppm: pH is buffered upward and resists lowering even with acid additions. Set TA before adjusting pH — it is the chemical foundation that makes pH stable.

Adjustment: sodium bicarbonate raises TA; muriatic acid or sodium bisulfate lowers TA.

  Total Chlorine  (TC)

The sum of free chlorine (FC) and combined chlorine (CC). TC = FC + CC. Measured by adding a second reagent to the FC test in a DPD test kit. The difference between TC and FC is combined chlorine. If TC reads 3.5 ppm and FC reads 3.0 ppm, combined chlorine is 0.5 ppm — at the PHTA shock threshold.

  Trichlor  (Trichloro-S-Triazinetrione)

The active ingredient in 3-inch chlorine tablets and some granular chlorine. 90% available chlorine per PHTA and EPA standards. Stabilized — contains cyanuric acid (~54% by weight), which protects chlorine from UV degradation and accumulates in pool water with each tablet dissolved. pH 2.8-3.0. One pound per 10,000 gallons raises FC approximately 8-9 ppm and adds approximately 6-7 ppm CYA.

PHTA Trichlor Fact Sheet confirms 90% available chlorine and trichloro-S-triazinetrione chemical name.