What Is the Difference Between SDIC Assay and Chlorine Strength?
SDIC assay measures the actual percentage of sodium dichloroisocyanurate compound present in a product, while chlorine strength (more precisely called available chlorine) measures the oxidizing and disinfecting capacity of that product expressed as an equivalent weight of elemental chlorine. They are related but distinct values: a higher SDIC assay generally leads to a higher available chlorine reading, yet the two numbers are never identical because they describe different chemical properties.
Confusing these two figures is a common source of error when ordering, formulating, or dosing SDIC-based products. Understanding which number you need—and what each one actually tells you—prevents under-dosing, over-dosing, and miscommunication between suppliers and end users.
Defining SDIC Assay
SDIC assay is a purity specification. It tells you what fraction of a given sample is sodium dichloroisocyanurate (NaDCC) by mass. Manufacturers typically express it as a percentage, such as 56 %, 60 %, or 99 %, depending on whether the material is a technical granule, a formulated tablet, or a refined raw material.
The assay is determined by analytical methods—commonly iodometric titration or high-performance liquid chromatography—that quantify the NaDCC molecule itself. It does not directly measure how much disinfecting power the sample delivers; it only measures how much of the active compound is there.
Two important distinctions affect assay interpretation:
- Anhydrous vs. dihydrate: Anhydrous SDIC (the form most commonly used in pool and water-treatment granules) has a different molecular weight than the dihydrate form. An assay quoted on an anhydrous basis will yield a different available chlorine figure than the same percentage quoted on a dihydrate basis unless the basis is clearly stated.
- Raw material vs. formulated product: A tablet or granule may contain binders, fillers, or stabilizers. The assay of the finished product will be lower than that of the raw NaDCC powder used to make it, even though both are called “SDIC.”
Defining Chlorine Strength (Available Chlorine)
Chlorine strength, in the context of SDIC products, refers to available chlorine—the amount of chlorine that can be released from the compound to perform oxidation or disinfection, expressed as a percentage equivalent to elemental chlorine (Cl₂).
Available chlorine is not a direct measurement of the NaDCC molecule. It is a calculated or titrated value that reflects the oxidizing equivalents the compound can provide. For pure anhydrous sodium dichloroisocyanurate, the theoretical available chlorine is approximately 60 % of the compound’s mass. For the dihydrate form, it is approximately 51 %.
When a product specification sheet lists “chlorine strength” or “available chlorine,” it is telling you the effective disinfecting power per unit mass, not the purity of the NaDCC compound. This is the figure that matters most when you calculate dosing rates for a specific application, because dosing is based on how much oxidizing capacity you need to deliver—not on how many grams of NaDCC you weigh.
How the Two Values Relate
The relationship between SDIC assay and available chlorine is proportional but not one-to-one. You can estimate one from the other if you know the molecular form and the conversion factor, but you cannot treat the two numbers as interchangeable.
| Property | SDIC Assay | Available Chlorine (Chlorine Strength) |
|---|---|---|
| What it measures | Mass fraction of NaDCC compound in the sample | Oxidizing capacity expressed as equivalent Cl₂ |
| Typical unit | Percentage (%) | Percentage (%) |
| For pure anhydrous NaDCC | 100 % | ≈ 60 % |
| For pure dihydrate NaDCC | 100 % | ≈ 51 % |
| Affected by binders/fillers | Yes — assay drops in formulated products | Yes — available chlorine drops proportionally |
| Used primarily for | Quality control and raw-material specification | Dosing calculations and efficacy comparison |
If a technical granule has an SDIC assay of 99 % (anhydrous basis), its available chlorine will be close to 59–60 %. If that same 99 % assay granule is pressed into a tablet with 10 % inert filler, the tablet’s assay might read 89 %, and its available chlorine would drop to roughly 53 %. The relationship holds, but both numbers change because the total composition changed.
Why the Distinction Matters in Practice
Dosing Errors
If you dose based on SDIC assay instead of available chlorine, you will miscalculate the oxidizing power you actually deliver. For example, treating a 56 % assay technical granule as if it were 56 % available chlorine would overdose the system by roughly 6 %, because its actual available chlorine is closer to 33 %. Conversely, using available chlorine to judge raw-material purity ignores the presence of non-NaDCC components that may affect stability or dissolution.
Product Comparison
Two suppliers may both offer “60 % SDIC,” but if one quotes assay and the other quotes available chlorine, the products are not equivalent. A 60 % assay product (anhydrous) delivers about 36 % available chlorine, while a 60 % available chlorine product corresponds to roughly 100 % assay—an entirely different grade. Always verify which metric is being quoted before comparing offers.
Regulatory and Specification Documents
Some regulations or water-treatment standards specify acceptable ranges for available chlorine rather than for NaDCC assay. Others focus on the compound identity and purity. Knowing which parameter a document references prevents the mistake of supplying a product that meets one criterion but not the other.
Common Sources of Confusion
Several practices blur the line between assay and chlorine strength:
- Informal labeling: Marketing materials sometimes use “chlorine content” or “strength” without specifying whether they mean assay or available chlorine.
- Basis confusion: A value stated “as is” for a formulated tablet differs from the same value “on dry basis” for the raw powder. Always check the basis.
- Language overlap: In some markets, “chlorine” is used loosely for both the compound and its oxidizing equivalent. Technical documents should use “available chlorine” or “active chlorine” when referring to disinfecting capacity and “SDIC assay” or “NaDCC purity” when referring to compound content.
When you receive a certificate of analysis or a technical data sheet, look for two separate fields: one that identifies the NaDCC percentage (assay) and one that states available chlorine. If only one number is given, ask the supplier which metric it represents and on what basis it was measured.
Which Number Should You Use?
Use SDIC assay when you are evaluating raw-material quality, verifying that a shipment meets a purity specification, or troubleshooting manufacturing consistency. Use available chlorine (chlorine strength) when you are calculating how much product to add to a water system to achieve a target disinfectant residual or oxidation level.
Neither number alone tells the complete story for a formulated product. A tablet with high available chlorine but low dissolution rate may underperform compared to a granule with the same available chlorine but faster release. But that is a separate consideration from the assay-vs-strength distinction—it does not change the need to know which value you are working with.
A Note on Safety and Compatibility
Regardless of whether you work from assay or available chlorine figures, never mix SDIC with acids, ammonium compounds, or other chlorine-releasing agents outside of controlled, manufacturer-approved formulations. Incompatible mixing can generate hazardous gases. Always follow the safety data sheet for the specific product grade you handle.
If you need to confirm the exact assay or available chlorine value for a particular ENVO CHEMICAL product grade and its intended application, contact our technical team with the product code and the parameter you require—we will provide the verified specification that matches your dosing or quality-control need.
