The three fractions at a glance
Total chlorine is everything the test can oxidize or reduce — free chlorine plus combined chlorine. Free chlorine is the fraction still available to disinfect and oxidize. Combined chlorine is the difference: chlorine that has already reacted with nitrogenous compounds, mostly chloramines. That arithmetic holds in any chlorinated water, including water treated with SDIC (sodium dichloroisocyanurate). What changes with SDIC is the interpretation, because SDIC is a stabilized chlorine donor: it introduces chlorine and cyanuric acid together, and the cyanurate system affects both how much of the free chlorine is instantly active and how a field test responds to it.
| Fraction | What it represents | What it means in SDIC-treated water |
|---|---|---|
| Free chlorine | HOCl and OCl⁻ not yet bound to nitrogen; measured by the DPD free test | Includes the active disinfecting species, but in cyanurate-stabilized water the reading may also include chlorine held on chlorinated isocyanurate species |
| Combined chlorine | Chlorine bound in chloramines and other chlorinated nitrogenous compounds (calculated as total − free) | Comes from contaminants in the water, not from SDIC itself; it is a weaker, slower disinfectant |
| Total chlorine | Free + combined — the full chlorine residual | Confirms a residual exists, but on its own says nothing about how much is still active |
Why SDIC behaves differently from unstabilized chlorine
SDIC is a chlorinated isocyanurate: dry, the chlorine is bound to a cyanuric acid ring, and each molecule carries two available chlorine atoms. Published values commonly put anhydrous SDIC near 60% available chlorine and the dihydrate somewhat lower, but the figure that governs is the one on the certificate of analysis for the specific grade — anhydrous and dihydrate material should not be treated as interchangeable on paper.
When SDIC dissolves, hydrolysis releases hypochlorous acid and cyanuric acid or cyanurate into the water. The cyanurate then reversibly binds a portion of the chlorine. This is the stabilizer effect: chlorine is held in reserve and released as the free active species is consumed, which slows loss from sunlight and keeps a measurable residual for longer.
Three consequences matter when you read a test result:
- The free chlorine reading is not a direct measure of disinfecting strength. The instantly active species is HOCl, and its share of free chlorine depends on pH — HOCl and OCl⁻ sit near a 1:1 split around pH 7.5, with HOCl dominating below it. The cyanurate equilibrium adds a second layer of buffering on top of that.
- Some chlorine bound on cyanurate can register in the free test. DPD responds primarily to HOCl and OCl⁻, but chlorinated isocyanurate species also react to a degree, so a reported free chlorine value in high-cyanuric-acid water can overstate the fraction that is immediately available for disinfection. Read the number together with pH and cyanuric acid, not in isolation.
- Cyanuric acid accumulates. Chlorine is consumed by demand, sunlight, and disinfection; cyanurate largely is not, so it builds up across a season of SDIC dosing. As it rises, the same free chlorine reading corresponds to less active HOCl. This is a shift in the meaning of the number, not a fault in the product.
One thing cyanuric acid is not is combined chlorine. Cyanurate is not an oxidant and does not register as chlorine in a DPD test. Total chlorine minus free chlorine gives combined chlorine; cyanuric acid does not belong in either term.
Where combined chlorine comes from in an SDIC system
SDIC does not produce chloramines by itself in clean water. Combined chlorine forms when free chlorine reacts with ammonia and nitrogenous organic matter already present — urea and creatinine from swimmers, sweat and body oils, contaminated make-up water, certain cleaning or fertilizer residues. Free chlorine converts ammonia through monochloramine, dichloramine, and trichloramine, with the distribution shifting mainly with pH and the chlorine-to-nitrogen ratio.
Two practical points follow:
- Combined chlorine disinfects far more slowly than free chlorine, so a healthy total chlorine number can coexist with weak disinfection if most of the residual is combined.
- The sharp “chlorine” odor and eye or airway irritation associated with heavily used pools come mainly from chloramines, not from free chlorine. Adding more chlorine without removing the nitrogen source can temporarily worsen the odor.
How the three fractions are measured
The standard field approach uses DPD (N,N-diethyl-p-phenylenediamine): one test for free chlorine, a second on the same sample with potassium iodide added for total chlorine, and combined chlorine by subtraction. FAS-DPD titration is preferred where better resolution is needed, because the subtraction method amplifies small errors — when combined chlorine is a small fraction of total, its calculated value carries most of the uncertainty.
Before trusting a combined chlorine value, check these common interferences:
- Other oxidizers. Bromine, ozone, hydrogen peroxide, and potassium monopersulfate can all inflate DPD readings. If any are present, the result is not interpretable as free/combined/total chlorine.
- Monochloramine carryover. At high combined chlorine levels, part of the chloramine can respond in the free chlorine step, compressing the apparent difference between free and total.
- Range and dilution. Readings above the kit’s range require dilution, which changes the sensitivity of the test to everything else in the sample.
- Technique and timing. Read free chlorine first, on a fresh sample, with the same procedure each time. Sample age and sunlight exposure change the result.
Reading the numbers
| Pattern | Usual interpretation | Next check |
|---|---|---|
| Free ≈ total, combined near zero | Little nitrogenous load; the residual is essentially all available | Confirm pH and cyanuric acid, since those set how much of the free chlorine is active HOCl |
| Free low, combined high, total moderate | Chlorine demand from nitrogenous contamination is outpacing the feed | Trace the ammonia source (make-up water, bather load, cleaning residues) before increasing dose |
| Free reads high but disinfection performance is poor | Suspect pH drift or cyanuric acid interference rather than a genuine surplus of HOCl | Measure pH and cyanuric acid; do not simply add more SDIC |
| Total high, persistent odor or irritation | Chloramine accumulation | Identify and remove the nitrogen source; oxidation to breakpoint is a procedure to follow per the water type’s guidance, not a guess |
| Free and total both near zero | Demand has consumed the dose, or contact time and mixing are inadequate | Re-check dose, application point, and contact time against the treated water’s requirements |
Thresholds are water-type specific. Many pool operation guidelines treat combined chlorine above roughly 0.2 mg/L as Cl₂ as a trigger for action, while drinking water, healthcare, and food-contact applications have their own regulatory residual requirements and should never inherit pool targets. Confirm the applicable limit for the exact application before acting on a number.
What to verify before adjusting anything
- Product form and specification. Confirm whether the SDIC in use is anhydrous or dihydrate, its available chlorine content, and whether it is raw material or a formulated blend. Formulated products may contain additional actives that affect test response.
- pH and cyanuric acid. These two parameters determine what the free chlorine reading means. Without them, free-versus-total interpretation is incomplete.
- Test kit status. Check reagent expiry, calibration, and range, and confirm the method is appropriate for the residual levels involved.
- Regulatory target for the water type. The acceptable free chlorine residual, combined chlorine limit, and cyanuric acid ceiling differ by jurisdiction and by use. Apply the one that governs the site.
Safety limits on interpretation
SDIC is a strong oxidizer and must never be mixed with acids, ammonia, or other chemicals; doing so can release dangerous gases. Testing tells you what is in the water, not what is safe to add to it, and no chlorine residual measurement by itself establishes that water is fit to drink. SDIC raw material is not automatically suitable for potable water, and a “food grade” designation does not establish drinking-water approval. Suitability, dose, and contact time must be confirmed for the specific product and the specific application.
Discussing your SDIC water test results
If you want help interpreting free, combined, and total chlorine in an SDIC-treated system, send the SDIC grade and form you are using (anhydrous or dihydrate, raw material or formulated), the current dose and application method, and a set of results including free chlorine, total chlorine, pH, and cyanuric acid, along with the water type being treated. ENVO CHEMICAL can respond against those details.
