What are pool chemicals? Product identity, function and storage

The table lists the products most often met on site, grouped by function. The available chlorine values given for trichlor and dichlor are theoretical upper limits calculated from the molecular formula; the commercial label value may be lower.

The most decisive column in the table is what the product leaves behind in the water. Calcium hypochlorite adds calcium with every dose and hardens the water; Wikipedia notes that sodium hypochlorite is preferred for general-purpose disinfection for this reason. Trichlor and dichlor add cyanuric acid with every dose. Sodium hypochlorite adds neither, but it degrades with light and heat: a 13.6% solution has been recorded losing 17% of its strength in one year at 7 °C.

The distinction most often confused on site is between inorganic chlorine (calcium hypochlorite, sodium hypochlorite, salt electrolysis) and organic chlorine (trichlor, dichlor). Both form hypochlorous acid in water; the difference lies in the carrier of the chlorine. In inorganic products the carrier is calcium or sodium, and it stays in the water. In organic products the carrier is the cyanuric acid ring; as trichlor reacts, the cyanuric acid concentration in the pool accumulates.

The practical consequence is this: pool water run on organic chlorine reaches a high cyanuric acid level within a few months even if no stabiliser is ever added; the only remedy is to drain part of the water and replace it with fresh. With inorganic chlorine the stabiliser is a separate product and the level is under the operator's control. When a pool project is being prepared, the disinfectant type is treated as an input that affects the fresh-water make-up rate.

The second confused pair is sodium bisulfate and hydrochloric acid. Sodium bisulfate is a dry granule that can be handled and stored safely; the anhydrous form is hygroscopic and cakes in an open package. Hydrochloric acid is a liquid; at 25% and above it is classified GHS H314 (severe skin burns and eye damage), and its vapour is hazardous to the respiratory tract. In indoor pool plant rooms granular acid is common; in outdoor pools hydrochloric acid is common for cost reasons.

Most incompatibilities between pool chemicals rest on two mechanisms: the hypochlorite + acid reaction and the organic chlorine + inorganic chlorine reaction. Wikipedia gives the reaction for calcium hypochlorite explicitly: Ca(ClO)2 + 4 HCl → CaCl2 + 2 Cl2 + 2 H2O. The same source states that sodium hypochlorite solution releases toxic chlorine gas when mixed with an acid such as hydrochloric acid or vinegar.

The rule covers the dosing sequence as well as the bucket: putting a trichlor tablet and then calcium hypochlorite into the same skimmer basket means the two products meet with a little water. In automatic dosing the acid and hypochlorite lines are not connected to the same injection point; enough distance is left between them for the water to mix.

Storage rules derive from the chemistry of the product. Calcium hypochlorite is not stored wet or hot; it is not placed next to acid, organic matter or metal; in humid air it slowly decomposes and gives off a chlorine smell. Sodium hypochlorite degrades with light and heat; in production the solution is kept below 40 °C, on site the drums are kept in a cool place out of the sun and used up within the season. Sodium bisulfate, calcium chloride and sodium carbonate are hygroscopic; an open package turns to stone. Trichlor tablets are kept dry in a closed container, away from wet scoops.

The first value to look for on the label is the active ingredient percentage. For calcium hypochlorite, 65-70% available chlorine is the commercial standard; a lower value means filler or a degraded product. For sodium hypochlorite the pool solution is 10-25%; 3-8% household bleach does not do the same job. For hydrochloric acid the 20-32% range defines the grade. On algaecide and winterising liquid, the active ingredient line (quaternary ammonium percentage or copper content) is what to read.

Test kits are also part of the purchasing decision. DPD tablet kits measure free chlorine (DPD 1) and total chlorine (DPD 3); the difference is combined chlorine. Phenol red measures pH. Test strips show the same parameters roughly on a colour scale and are not sufficient for operating records. A photometer reads the DPD colour intensity optically and gives chlorine, pH, alkalinity, cyanuric acid and calcium hardness as numbers. In commercial pools the daily pool maintenance record is kept with a photometer.

The seasonal consumables list is the purchasing side of the pool maintenance plan. The list gives product types, not quantities; quantity depends on volume, bather load and climate.

The history of chlorine disinfection in pools comes from drinking water treatment. Calcium hypochlorite became widespread as the 70% available chlorine product that replaced bleaching powder (20% available chlorine) and came to be known as HTH. Sodium hypochlorite has been standardised as a 12% solution in water utilities and 15% in wastewater plants. Trichlor and dichlor are obtained by chlorinating cyanuric acid; cyanuric acid entered pool chemistry first as the stabiliser that extends the free chlorine half-life 4-6 times, then as the carrier skeleton of these products. Salt electrolysis produces the same hypochlorous acid at the pool; 3,000-5,000 ppm salt is one tenth of seawater.

In terms of the pool project and specification, the counterpart is these items: disinfectant type and the fresh-water make-up rate that depends on it; whether the pH reducer is liquid or granular, and the store ventilation flow rate; a shelf with separate spill trays for acid and chlorine; the distance between the acid and chlorine injection points in automatic dosing; a chemical store that is a cool, ventilated room separate from the pump room; the DPD photometer and the recording frequency. If these rooms do not enter the project at the pool construction stage, the chemicals pile up in the pump room and the ground is laid for the field incidents above.

The pool water target range (pH 7.2-7.8; alkalinity 80-120 ppm; calcium hardness 200-400 ppm; cyanuric acid 20-50 ppm) appears in the specification as a separate table; the product used to reach that range is chosen from the classification table. Product selection does not change between a concrete pool and a liner pool; only the calcium hardness target differs.