Sodium dichloroisocyanurate (NaDCC, CAS 2893-78-9) is the solid chlorine donor that replaced bleach in most of the places where bleach was inconvenient: emergency water treatment, pool shock, farm biosecurity, hospital spill kits, cooling towers. It carries about 56% available chlorine against 10 – 15% for liquid hypochlorite, it stores dry for a year or more without losing strength, and it ships without paying freight on water. This article covers the chlorine-release chemistry, why the numbers on a specification sheet are oxidising equivalents rather than purities, the manufacturing route, dissolution and dosing, the cyanuric acid balance that decides whether a pool stays sanitised, comparison with TCCA, calcium hypochlorite and chloramine T, and the safety and regulatory points that matter at purchase.
1. What NaDCC Is and Where It Fits
NaDCC is the sodium salt of dichloroisocyanuric acid: a six-membered triazine ring carrying two N–Cl bonds and one N–Na. It is a white granular solid, supplied at 56% or 60% available chlorine, that dissolves to release free available chlorine. Its position in the market is defined by three practical advantages over liquid hypochlorite and one over the trichlorinated analogue:
- Concentration: about 56% available chlorine in the solid form, against 10 – 15% for sodium hypochlorite solution.
- Stability: the dry solid retains strength for 12 – 24 months under good storage; a bleach solution loses several percent per month and faster when warm.
- Freight and handling: no water to ship, no liquid containment, smaller storage footprint per unit of chlorine.
- Neutral solution: unlike TCCA, which gives an acidic solution, NaDCC solutions sit near neutral, which is gentler on dosing equipment and easier to balance in treated water.
| Identity | Value |
|---|---|
| Chemical name | Sodium dichloroisocyanurate; sodium 1,3-dichloro-1,3,5-triazinane-2,4,6-trione |
| CAS number | 2893-78-9 |
| Synonyms | NaDCC; SDIC; DCCNa; troclosene sodium (INN); sodium dichloro-s-triazinetrione |
| Molecular formula | C3Cl2N3NaO3 (anhydrous); C3Cl2N3NaO3·2H2O (dihydrate) |
| Molecular weight | 219.94 (anhydrous); 255.97 (dihydrate) |
| InChIKey | MSFGZHUJTJBYFA-UHFFFAOYSA-M |
| Canonical SMILES | C1(=O)[N-]C(=O)N(C(=O)N1Cl)Cl.[Na+] |
| Related compounds | Trichloroisocyanuric acid, TCCA (CAS 87-90-1); cyanuric acid (CAS 108-80-5) |
2. Available Chlorine Is an Oxidising Equivalent, Not a Purity
This is the concept that decides whether a buyer understands the product. Available chlorine expresses oxidising power as if it were chlorine gas. One N–Cl bond accepts two electrons when reduced to chloride – exactly as one molecule of Cl2 accepts two electrons when it becomes two chloride ions. Therefore:
one N–Cl bond ≡ one mole of Cl2 ≡ 70.906 g of available chlorine.
| Compound | Formula | MW | N–Cl count | Available chlorine | Reality check |
|---|---|---|---|---|---|
| NaDCC, anhydrous | C3Cl2N3NaO3 | 219.94 | 2 | 141.812 / 219.94 = 64.48% | the ceiling; no NaDCC can exceed this |
| NaDCC dihydrate | C3Cl2N3NaO3·2H2O | 255.97 | 2 | 141.812 / 255.97 = 55.40% | below 56%, so a 56% grade cannot be a pure dihydrate |
| TCCA | C3Cl3N3O3 | 232.41 | 3 | 212.718 / 232.41 = 91.53% | matches the familiar “90% chlorine” label for TCCA |
| Sodium hypochlorite | NaOCl | 74.44 | 1 effective | 70.906 / 74.44 = 95.3% on a pure basis | but commercial bleach is 10 – 15% because it is mostly water |
Two very useful conclusions follow. First, the grade percentages are not purities: 56% available chlorine corresponds to about 86.9% NaDCC content, and 60% to about 93.1%; the remainder is moisture and inorganic salts. Second, the dihydrate ceiling of 55.40% and a moisture specification of 5 – 10% together prove that a commercial 56% grade is an anhydrous-dominant material with partial hydration – a full dihydrate would carry about 14.1% water and could not reach 56% chlorine in any case. If a supplier’s certificate shows 56% chlorine and 14% moisture simultaneously, one of the two numbers is measured differently than it is labelled.
3. Chlorine Release and the pH Dependence
Dissolved NaDCC hydrolyses to release hypochlorous acid, the actual disinfecting species:
NaDCC + 2 H2O → cyanuric acid + 2 HOCl + NaOH
Hypochlorous acid then dissociates with a pKa of about 7.5:
HOCl ↔ H+ + OCl–
HOCl is a small, uncharged molecule that crosses cell walls; OCl– is charged and much less effective at the same total chlorine. The fraction of chlorine present as HOCl therefore depends almost entirely on pH, and this is why the pH of the product solution matters as much as the percentage on the label.
| pH of the treated water | HOCl share of free chlorine | Practical consequence |
|---|---|---|
| 5.5 | about 99% | maximum disinfection efficiency, but aggressive to metals and to skin |
| 6.0 | about 97% | excellent efficiency, manageable corrosion in most systems |
| 7.0 | about 78% | the usual operating point for drinking water and pools |
| 7.5 | 50% | the crossover point; efficiency starts to fall quickly |
| 8.0 | about 22% | most of the chlorine is present as the weak OCl- form |
| 8.5 | about 8% | dosing has to be raised substantially to hold the same residual |
The specification range of pH 5.5 – 7.0 for the product solution is therefore not arbitrary: it is the range in which the delivered chlorine is mostly in the effective HOCl form. A plant that doses at pH 8.5 and blames the chlorine for poor results is fighting the equilibrium, not the product.
4. Manufacture
| Stage | Chemistry | Control point |
|---|---|---|
| 1. Cyanuric acid supply | cyanuric acid (C3H3N3O3) from urea or from hydrocyanic acid routes | purity, particle size and residual nitrogen compounds |
| 2. Chlorination | cyanuric acid or its sodium salt is chlorinated with chlorine gas and sodium hydroxide in aqueous medium | chlorine stoichiometry and pH: these decide the balance between the dichloro product and the trichloro product (TCCA) |
| 3. Crystallisation | the sodium salt is crystallised from the reaction liquor | crystal form and hydration: this is where the 56% versus 60% chlorine difference originates |
| 4. Separation and drying | centrifuging and controlled drying | moisture: over-drying risks decomposition, under-drying costs chlorine per tonne of freight |
| 5. Granulation and screening | granules cut to 8 – 30 mesh, or tabletting | particle size distribution, fines content, dissolution rate |
The process control that matters most to a buyer is the dichloro/trichloro selectivity. Push the chlorination too far and the product drifts toward TCCA, which is a different product with an acidic solution and a different regulatory and handling profile; stop too early and the chlorine content falls below the grade. Residual cyanuric acid and sodium chloride are the main inactive impurities, and both affect the chlorine content per tonne.
5. Dissolution and Dosing
NaDCC dissolves readily, but the rate depends on particle size, water temperature and agitation. Two handling rules apply universally: add the solid to water rather than water to the solid, and never dump granules into a large body of water without pre-dissolving, because undissolved granules settle, create local high concentrations and can damage liners, seals and pool surfaces.
| Format | Dissolution behaviour | Where it fits |
|---|---|---|
| 8 – 30 mesh granular | dissolves in minutes with agitation in a stock tank | stock solutions, batch chlorination, industrial dosing |
| Fines / powder | very fast, but dusty and prone to caking | formulated products rather than direct dosing |
| Tablets | slow, controlled release over days | pools, spas, small water systems, remote sites |
| Effervescent tablets | fast, self-dispersing | field and emergency water treatment, small volumes |
Dosing arithmetic runs from available chlorine to product. One kilogram of a 56% grade delivers 0.56 kg of available chlorine, so a target of 2 mg/L in 10 m3 needs 20 g of chlorine, or 20 / 0.56 = 35.7 g of product. A 5 mg/L shock on 100 m3 needs 500 g of chlorine, or 893 g of product. Stock solutions do not keep: prepare them cool, cover them, and use them within a few days, because a dissolved chlorine solution loses strength far faster than the dry solid.
6. Applications
6.1 Drinking water and emergency water treatment
NaDCC tablets and granules are widely used for batch chlorination of small supplies and for point-of-use treatment where water quality is uncertain. The advantages in this setting are exact dosing from a tablet, no liquid chlorine to transport, and predictable strength. The constraint is regulatory: drinking-water treatment is a regulated application in every market, and approval attaches to the specific product and labelling, not to the CAS number.
6.2 Swimming pools and spas
Granules for shock dosing and tablets for continuous release. Two chemistry points govern pool performance. First, pH control – free chlorine efficiency falls sharply above pH 7.5, and most pool problems described as “chlorine not working” are pH problems. Second, the cyanuric acid balance described in section 8, which is unique to the stabilised chlorine donors.
6.3 Livestock, poultry and aquaculture biosecurity
Drinking-line sanitation, footbaths, surface and equipment disinfection on farms. The practical issues are dosing equipment compatible with hypochlorous acid, organic load in the water (which consumes chlorine before it disinfects), and the need to keep stock solutions fresh. In aquaculture, chlorine must be fully neutralised or allowed to decay before stock is returned to treated water.
6.4 Hospital and environmental disinfection
Surface disinfection, spill control and equipment decontamination where a stabilised solid donor is preferred to liquid hypochlorite for storage reasons. NaDCC solutions are used at higher concentrations for spill control than for routine surfaces, and the chlorine demand of blood and other organic material must be accounted for – a visibly soiled surface consumes chlorine faster than the dose calculation assumes.
6.5 Industrial water systems
Cooling towers, process water, wastewater disinfection and biofilm control. Chlorine is usually applied as a shock or intermittent dose rather than continuously, and the key engineering point is contact time: the residual that matters is the residual after the chlorine demand of the water has been satisfied.
6.6 Food, beverage and institutional cleaning
Surface sanitation and CIP applications where the product and its labelling are approved for food-contact use in the destination market. The two failure modes here are insufficient contact time and interference from organic soil, not insufficient concentration.
7. Comparison with Other Chlorine Donors
| Product | CAS | Available chlorine | Solution pH | Stability in storage | Where it wins | Where it loses |
|---|---|---|---|---|---|---|
| NaDCC | 2893-78-9 | 56 – 64.5% | near neutral | good as a dry solid | general disinfection, dosing safety, tablet formats | slower dissolution than powders; adds cyanuric acid |
| TCCA | 87-90-1 | up to 91.5% | acidic | good as a dry solid | maximum chlorine per tonne of freight; pool tablets | acidic solution, aggressive to equipment, higher hazard classification in some markets |
| Calcium hypochlorite | 7778-54-3 | 65 – 70% | alkaline | loses strength faster; thermal hazard | cheap chlorine, no cyanuric acid | scale formation from hardness, fire risk in storage, alkaline drift |
| Sodium hypochlorite | 7681-52-9 | 10 – 15% | alkaline | degrades monthly, faster when warm | low capital cost, easy continuous dosing | freight on water, short shelf life, hazardous decomposition |
| Chloramine T | 127-65-1 | about 25% | near neutral | good as a solid | milder, slower-releasing, less chlorinous odour | higher cost per unit of chlorine; slower kill |
| Bleaching powder | 7778-54-3 mixtures | 20 – 35% | alkaline | poor – loses strength quickly | low cost in some markets | inconsistent strength, heavy inert load, sludge |
The selection logic is short. If maximum chlorine per tonne of freight and an acidic solution are acceptable, TCCA. If near-neutral dosing, safer handling and tablet or granular flexibility matter more, NaDCC. If cyanuric acid accumulation is a problem – a stabilised pool, a plant that reuses water – then a calcium or sodium hypochlorite source avoids it, at the cost of scale or shelf life. The cyanuric acid question is the one that most often decides between NaDCC and hypochlorite, and it is treated next.
8. The Cyanuric Acid Balance
Both NaDCC and TCCA carry the cyanuric acid skeleton, and that skeleton is released as cyanuric acid when the chlorine is consumed. Cyanuric acid protects free chlorine from UV degradation – which is why these are called stabilised chlorine donors and why they are preferred in outdoor pools – but it accumulates, because the only removal route is dilution or biological degradation in slow sand and similar systems.
| Product | MW | Cyanuric acid released per kg of product | Practical reading |
|---|---|---|---|
| NaDCC (anhydrous basis) | 219.94 | 129.07 / 219.94 = 0.587 kg | every tonne of product adds about 587 kg of cyanuric acid equivalent |
| TCCA | 232.41 | 129.07 / 232.41 = 0.555 kg | similar, slightly lower per kg but higher per unit of chlorine |
| Per kg of available chlorine | – | NaDCC: 0.587 / 0.56 = 1.05 kg; TCCA: 0.555 / 0.915 = 0.61 kg | TCCA adds far less cyanuric acid per unit of chlorine |
That last row is the one worth memorising. Measured per unit of delivered chlorine, TCCA contributes roughly 40% less cyanuric acid than NaDCC, because it carries three N–Cl bonds for the same triazine ring. For a pool on continuous stabilised chlorine, cyanuric acid accumulates until it binds most of the free chlorine into ineffective chlorinated isocyanurate complexes – the “chlorine lock” that makes operators raise the dose and still measure no residual. The remedy is dilution: part-drain and refill, or switch to a non-stabilised source for part of the season. Neither problem is solved by adding more product.
The same arithmetic applies to plants that reuse water. A cooling tower or process loop dosed with a stabilised donor accumulates cyanuric acid in the circulating water; if blowdown is limited for water-saving reasons, the accumulation is faster and a non-stabilised chlorine source may be the better choice.
9. Safety and Water Chemistry Risks
NaDCC is an oxidising solid and its solutions are aggressive. The main risks are well characterised and all of them are controllable:
- Chlorine gas on acid contact. Never store or dose near acids; a spill that mixes the two releases chlorine. Container contamination is the usual route, not a process design error.
- Chloramines with ammonia and amines. Ammonia in the water reacts with chlorine to form chloramines, which are weaker disinfectants and the source of the familiar “swimming pool” odour. In water with significant ammonia nitrogen, the chlorine demand rises before any free residual appears.
- Disinfection by-products. Free chlorine reacts with natural organic matter to form trihalomethanes and other by-products. In drinking water this is regulated; in pools it is managed by controlling precursors and by ventilation of indoor pools.
- Oxidising-solid storage hazards. Contamination with fuels, oils, solvents or reducing agents can cause fire or gas release. Store separately, keep dry, and never return unused product to the drum.
- Corrosion. Hypochlorous acid attacks many metals, particularly at low pH. Use plastic or suitable stainless materials for dosing equipment and check compatibility of seals and gaskets.
- Environmental toxicity. Very toxic to aquatic life. Chlorinated water must be dechlorinated or allowed to decay before discharge; this applies to pool backwash, cooling tower blowdown and disinfection batch dumps.
Personal protection for handling the solid: dust mask, eye protection and gloves. Solutions should be prepared in a ventilated area, never in a confined space, and never in a container that previously held acid or a cleaning product.
10. Purchasing Checklist
| Check | Acceptance criterion | Why |
|---|---|---|
| Available chlorine | 56% min (or 60% for the high grade), method stated | defines value per tonne |
| Moisture | 5 – 10% for the 56% grade | water drives decomposition and chlorine loss; also a hydration indicator |
| pH of solution | 5.5 – 7.0 | confirms the sodium salt and predicts the HOCl fraction in use |
| Particle size | 8 – 30 mesh, with a fines limit | dissolution rate, dust, caking and tabletting behaviour |
| Appearance | white granular, free of discolouration | yellowing or clumping suggests moisture ingress or decomposition |
| Solubility residue | low insoluble matter | residue fouls dosing lines and shows up as sediment in pools and tanks |
| Packaging | moisture-barrier liner, intact seal | the main failure mode in transit is moisture pick-up, not breakage |
| Documentation | COA per batch, SDS, registration status for the intended use | customs, and drinking-water or food-contact approvals where relevant |
Three questions worth asking a supplier: what method is used for available chlorine (iodometric titration is standard), what is the fines content below the stated mesh cut, and whether the batch is anhydrous-dominant or contains a hydrated fraction, since that affects both dissolution and storage behaviour.
11. Packaging and Storage
Standard packaging is 25 kg, 50 kg and 1,000 kg in PE-lined fibre drums, HDPE drums or jumbo bags with a moisture barrier; smaller packs and tablet formats are available. Store in a cool, dry, ventilated place away from sunlight, heat, acids, reducing agents, ammonia, amines, fuels and organic materials. Keep containers closed and dry, do not stack beyond the packaging rating, and rotate stock on a first-in-first-out basis. Under these conditions the product retains its chlorine content for 12 – 24 months; a drum that has been opened repeatedly in a humid warehouse will not.
12. FAQ
Is NaDCC the same as bleaching powder or bleach?
No. Bleach is a sodium hypochlorite solution at 10 – 15% available chlorine; bleaching powder is a calcium-based solid with 20 – 35% and poor storage stability. NaDCC is a stabilised organic chlorine donor at 56 – 64.5%, with the different handling, dosing and cyanuric acid consequences described above.
Why does my pool need more and more chlorine?
Usually cyanuric acid accumulation. The stabiliser that protects chlorine from sunlight also binds it once the concentration gets high, and the only remedy is dilution. Check cyanuric acid before increasing the dose.
Is NaDCC or TCCA better for a pool?
TCCA delivers more chlorine per tonne of freight and less cyanuric acid per unit of chlorine, and it suits continuous tablet feeders. NaDCC gives a near-neutral solution, dissolves faster and is easier on equipment. Many pools use both – TCCA for maintenance, NaDCC for shock.
Can NaDCC be used in drinking water?
Only with a product approved and labelled for that use in the destination market. Emergency and point-of-use treatment are widely practised, but the regulatory basis is product-specific.
How long does a stock solution keep?
Days, not weeks. Dissolved chlorine decays with time, temperature and exposure to light and organic matter. Prepare stock solutions fresh, keep them covered and cool, and assay if a batch is critical.
Does chlorine work at high pH?
Poorly. Above pH 7.5 an increasing share of the free chlorine is present as the much less effective hypochlorite ion. Correct the pH before increasing the dose.
References and Further Reading
- Identity data (formula, molecular weight, InChIKey, canonical SMILES) verified against PubChem CID 517121 (NaDCC) and CID 6909 (TCCA).
- All available-chlorine figures on this page are derived from the equivalence one N–Cl bond ≡ one mole of Cl2 (70.906 g): NaDCC 141.812 / 219.94 = 64.48%; dihydrate 141.812 / 255.97 = 55.40%; TCCA 212.718 / 232.41 = 91.53%.
- The HOCl / OCl- distribution follows from the pKa of hypochlorous acid (about 7.5); the percentages in section 3 are computed from that value.
- Cyanuric acid release is computed from the triazine ring: 129.07 g per mole of donor, divided by the donor molecular weight.
- Regulatory statements are for orientation. Drinking-water, food-contact and biocidal approvals are product- and market-specific and must be confirmed before commercial use.
This article is technical orientation for industrial and municipal buyers. It is not a treatment design, a drinking-water approval or a safety assessment. Work from the SDS of the actual batch and from the regulations of the destination market.
For the specification, grades and packaging of our material see the NaDCC product page. For donor selection see chlorine-releasing disinfectant selection, and for dosing practice see NaDCC dosing and applications. Broader treatment context: the water treatment chemicals hub.