The Short Answer
“Dihydrate” means the sodium dichloroisocyanurate (SDIC, also written NaDCC) you are buying is the crystalline form that carries two molecules of water of crystallization per molecule of active compound: NaC3N3O3Cl2·2H2O. For a purchase decision, three things change and one does not.
What changes: the available chlorine delivered per kilogram of product (the dihydrate tops out at about 55.4% theoretically, versus about 64.5% for the anhydrous form), the mass you must buy, ship, store and feed for a given amount of chlorine, and the identity of the material on the specification sheet — including its CAS number. What does not change: once dissolved, a given number of moles of NaDCC behaves the same way whether it arrived as a hydrate or not. The water of crystallization only adds water; it does not change the solution chemistry per mole of active substance.
The practical consequence is simple. If you compare SDIC prices per kilogram of product rather than per kilogram of available chlorine, the dihydrate will look artificially cheap or expensive, and you may buy the wrong grade for your process.
What the Dihydrate Form Actually Is
The two water molecules in SDIC dihydrate are part of the crystal lattice. They are not free moisture, not a dilution with added water, and not an impurity. The material is a defined stoichiometric compound with a fixed composition.
That distinction matters because buyers sometimes treat “dihydrate” as though it meant “wet” or “lower grade.” It does not. It means the crystal structure includes water, and that water leaves the lattice only when the material is heated or dried with intent. Anhydrous SDIC is produced by driving that water off, which is why the anhydrous grade costs more to make and is usually sold at a higher price per kilogram.
| Property | SDIC dihydrate | SDIC anhydrous |
|---|---|---|
| Molecular formula | NaC3N3O3Cl2·2H2O | NaC3N3O3Cl2 |
| CAS number commonly listed | 51580-86-0 | 2893-78-9 |
| Molar mass (approx.) | 256 g/mol | 220 g/mol |
| Theoretical available chlorine | about 55.4% | about 64.5% |
| Water of crystallization by mass | about 14.1% | none |
| Mass needed for 1 kg of available chlorine (theoretical) | about 1.81 kg | about 1.55 kg |
Always confirm the CAS number, the molecular formula and the declared available chlorine figure on the actual specification and certificate of analysis. Commercial grades are not 100% pure, and the declared figure — not a calculated one — is what your dosing calculations must use.
Why 55.4% Is a Ceiling, Not a Suggestion
Because the theoretical maximum available chlorine for pure SDIC dihydrate is about 55.4%, a specification that claims, for example, 60% or 62% available chlorine cannot describe pure dihydrate. It describes the anhydrous form, or a blend, or an assay reported on some other basis.
This is one of the most common sources of confusion in SDIC purchasing. The market uses the same short name — SDIC, dichlor, NaDCC — for what are chemically two different solids. When you see a quotation sheet, reconcile the declared available chlorine with the declared hydrate form. If they do not agree, ask which material is actually being offered before you compare prices.
How the Dihydrate Form Changes the Cost Comparison
The correct comparison is cost per kilogram of available chlorine, not cost per kilogram of product:
Cost per kg available chlorine = price per kg of product ÷ available chlorine fraction
As an illustration, suppose a dihydrate grade is offered at a hypothetical USD 1.80/kg with 55% declared available chlorine, and an anhydrous grade at a hypothetical USD 2.10/kg with 62% declared available chlorine. The dihydrate works out to 1.80 ÷ 0.55 = USD 3.27 per kg of available chlorine; the anhydrous works out to 2.10 ÷ 0.62 = USD 3.39. On that basis the dihydrate is the better buy per unit of chlorine. Reverse the prices and the conclusion reverses. The numbers here are illustrative only — substitute your own quotations.
Two further costs sit outside that formula and should be added before you decide:
- Freight and packaging per unit of chlorine. About 14% of the dihydrate’s mass is water of crystallization. You pay to transport and store that water, and you need more packaging, pallet space and warehouse volume for the same delivered chlorine.
- Handling and process costs. If your process is built around one grade, switching may require requalification, dosing changes or reformulation, which can outweigh a small difference in raw material price.
What It Means for Dosing and Formulation
If your dosing instruction is written in grams of product, it is tied to a specific grade. Substituting dihydrate for anhydrous at the same mass delivers less chlorine — on theoretical figures, roughly 14% less per gram. To deliver the same chlorine from dihydrate rather than anhydrous, you need about 16% more mass. Recalculate using the declared available chlorine of each grade, not a fixed conversion factor.
For tablets and other fixed-weight formats, the hydration state affects the product in several ways at once:
- A tablet of the same weight and dimensions will not deliver the same chlorine if the raw material changes.
- Compaction behavior differs between a hydrated and an anhydrous crystal, so a formulation validated for one is not automatically valid for the other.
- The dihydrate introduces a defined quantity of bound water into a blend. In dry or moisture-sensitive formulations, that is often the reason anhydrous material is specified instead.
What does not change is the chemistry in the water. Dissolved NaDCC from either form releases the same active species, and the balance between free chlorine species depends on pH, concentration and the cyanuric acid contributed by the molecule — not on whether the solid was hydrated when it was weighed out.
Handling and Storage Implications
Both forms require the same basic precautions: keep them dry, cool, away from acids, ammonia, reducing agents and other incompatible chemicals, and never mix chlorinating products with anything not specifically intended for that purpose. Do not improvise blends.
The specific point about the dihydrate is that its water is held in the lattice and can be released by heat. If your process involves drying, heated blending or elevated storage temperatures, that bound water becomes mobile, which can affect both the assay and the physical quality of the finished product. Where the process is sensitive to water, the anhydrous grade is generally the more appropriate choice, which is also why it is normally the more expensive one.
What to Confirm Before You Order
- The hydrate form and CAS number in writing — not just the trade name “SDIC.”
- Declared available chlorine as a minimum, with the test method, and confirmation that it refers to the product as sold rather than on a dried basis.
- Water content or loss on drying. A properly run loss-on-drying test on the dihydrate should land near the theoretical 14%; a figure near zero indicates anhydrous material.
- Purity versus available chlorine. These are different numbers. Available chlorine tells you the oxidizing capacity; purity tells you how much of the declared compound is present.
- Physical form: powder, granular, compacted or tablet, with particle size or bulk density where your feeder or press requires it.
- Whether you are buying a raw material or a formulated product. A tablet is a formulation, and its behavior depends on excipients, lubricants and compaction, not only on the SDIC grade inside it.
- Suitability for your intended use. Grades intended for one application are not automatically suitable for another. “Food grade” on its own does not establish suitability for drinking-water treatment; that depends on the specific registration, purity profile and applicable regulations for the use you have in mind.
The Buying Decision in One Paragraph
Buy the dihydrate when you want the conventional, widely available commercial form, when your formulation or tablet is built around it, or when it is cheaper per kilogram of available chlorine after freight. Buy the anhydrous form when you need maximum chlorine per unit of mass or volume, when freight and storage volume dominate your cost, or when your process must minimize water in the blend. In every case, compare quotations on a cost-per-available-chlorine basis and reconcile the declared available chlorine with the declared hydrate form.
If you are sourcing SDIC and need help matching a grade to your process, send us the hydrate form and CAS number you require, the minimum available chlorine, the physical form and particle size, the intended application, and your typical order volume. We will confirm what we can supply against those parameters.
