What Is Antiklor? Sodium Thiosulfate Explained
Updated
Antiklor is a common trade name for sodium thiosulfate, the reducing agent used aboard ships to neutralize total residual oxidant (TRO), mainly residual chlorine and bromine, in ballast water treated by electrochlorination or chemical injection. Chemically it is sodium thiosulfate pentahydrate, Na2S2O3·5H2O, molar mass 248.18 g/mol, which reacts with the oxidant to form harmless sulfate and chloride before discharge.
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Key takeaways
- Antiklor is a trade name for sodium thiosulfate, the standard reducing agent used to neutralize residual oxidant in ballast water.
- It is usually supplied as the pentahydrate Na2S2O3·5H2O, molar mass 248.18 g/mol; British documents spell it sodium thiosulphate.
- It reduces residual chlorine and bromine to harmless chloride and bromide, oxidising itself to sulfate, across a wide pH range.
- Neutralisation occurs at discharge, with a calibrated TRO analyser trimming the dose to a near-zero overboard residual.
- The neutraliser must match the type-approved system's specification to stay inside its approved active-substance envelope.
- Sea Clean supplies it from Norway in 25 kg bags, 50 kg drums and 1000 kg bulk bags with a two to three year shelf life.
What Does the Name Antiklor Mean?
Antiklor is a marketing and trade name, not a distinct chemical: the word literally signals anti-chlorine, and the product behind it is sodium thiosulfate. Ships may see it under this or similar names on drums and delivery notes, but the active substance and its behaviour are those of sodium thiosulfate.
The compound is most commonly supplied as the pentahydrate, Na2S2O3·5H2O, with a molar mass of 248.18 g/mol, a white crystalline solid. European documentation frequently uses the British spelling sodium thiosulphate, which is the identical substance under a different orthography.
Because Antiklor is simply sodium thiosulfate, the dosing rules, storage requirements and safety precautions for the two names are the same. If a system manual specifies a neutraliser generically, an Antiklor-labelled thiosulfate of the correct grade meets that requirement.
Why Do Ships Need It?
Oxidising ballast water treatment systems, whether electrochlorination units that generate chlorine from seawater or chemical-injection systems that dose an oxidant, leave a residual oxidant in the treated water. That total residual oxidant is what kills organisms, but discharging it untreated would harm the receiving environment and breach discharge limits.
The IMO assesses the discharge chemistry of active-substance systems under the G9 procedure, and approved systems carry a maximum allowable discharge concentration for TRO. To meet it, the ship must reduce the residual to a safe level at the point of discharge, and that is the job of the neutraliser.
Sodium thiosulfate does this by chemically reducing the oxidant to harmless products, allowing the ship to discharge water that is both biologically treated to the D-2 standard and chemically safe. Without an effective neutraliser, an otherwise compliant oxidising BWTS cannot legally discharge.
How Does the Neutralisation Chemistry Work?
Sodium thiosulfate is a reducing agent: it donates electrons to the oxidant, converting residual chlorine or bromine to chloride or bromide while the thiosulfate itself is oxidised to sulfate. The overall result is that reactive, biocidal oxidant becomes inert salts that already occur naturally in seawater.
The reaction is fast and effective across a wide pH range, which is why thiosulfate suits crew-operated dosing rather than tightly controlled laboratory conditions. Dosing follows a practical rule of about 2.5 to 3.0 mg of pentahydrate per mg of Cl2-equivalent TRO, including a 10 to 20 percent safety margin.
Because the end products are sulfate and chloride, correctly dosed thiosulfate does not introduce a new environmental burden. This benign by-product profile is a major reason it is the standard neutraliser for marine ballast water rather than harsher reducing chemicals.
Where Is It Used in the Treatment Process?
Neutralisation happens at discharge, not at uptake. During ballasting the oxidant is generated or dosed to achieve treatment; during deballasting the treated water passes a neutralisation injection point where thiosulfate is added and the residual TRO is measured at an overboard sample point before it leaves the ship.
In an electrochlorination system a side stream is often chlorinated at uptake and the main tank is dosed; at discharge the neutraliser is injected inline. In a chemical-injection system the oxidant and the neutraliser are separate reagents on the skid, and the neutralisation loop is instrumented with a TRO analyser to trim the dose.
In both cases a calibrated TRO analyser closes the loop, adjusting thiosulfate injection to drive the overboard residual to near zero. The neutralisation step is therefore inseparable from the monitoring step, and both must be working for a legal discharge.
How Is It Supplied and Stored?
Sea Clean supplies sodium thiosulfate pentahydrate from Norway in 25 kg bags, 50 kg drums and 1000 kg bulk bags, so operators can match the format to their dosing volume and storage space. The pentahydrate is highly soluble, about 701 g/L at 20 degrees Celsius, so concentrated make-up solutions are easy to prepare.
Stored cool, dry and sealed, the dry reagent keeps a typical shelf life of two to three years. It must be segregated from acids and strong oxidisers, and made-up solution should use fresh water and be batched to near-term use because solution is less stable than the solid.
As an independent, brand-agnostic supplier and service provider, and an authorized agent for Headway Technology, Sea Clean can advise on matching reagent grade, packaging and resupply to a ship's specific BWTS and trading pattern rather than pushing a one-size-fits-all product.
Is Antiklor the Same on Every Ship?
The chemistry is the same, but grade, salt form and packaging can differ. Confirm whether a product is pentahydrate or anhydrous, because that changes the mass you dose per mg of TRO, and confirm the purity grade is suitable for water treatment rather than an industrial off-spec.
Always check that the neutraliser matches what the type-approved system specifies. Using a correct-grade sodium thiosulfate keeps the system inside its approved active-substance envelope; substituting an unspecified chemical can invalidate the approval even if it neutralises chlorine.
If a system's documentation is generic, treat correct-grade sodium thiosulfate pentahydrate as the standard choice and confirm with the maker. Sea Clean can help interpret the type-approval package so the ship stocks a compliant neutraliser under whatever trade name it arrives.
Frequently asked questions
Is Antiklor the same as sodium thiosulfate?
Yes. Antiklor is a trade name and the product behind it is sodium thiosulfate, usually the pentahydrate. The dosing, storage and safety rules for the two names are identical.
Why do ballast water systems need Antiklor?
Oxidising BWTS leave a residual oxidant (TRO) that kills organisms but must not be discharged untreated. Sodium thiosulfate reduces that residual to harmless salts so the water can be legally discharged.
What does sodium thiosulfate turn chlorine into?
It reduces residual chlorine to chloride and residual bromine to bromide, while the thiosulfate is oxidised to sulfate. All three products occur naturally in seawater, so correctly dosed neutralisation adds no new burden.
Can I use any Antiklor on my BWTS?
Use a correct-grade sodium thiosulfate that matches your type-approved system's specification, and confirm whether it is pentahydrate or anhydrous. Substituting an unspecified chemical can invalidate the active-substance approval.
Sources
Related articles
- Neutralisation Systems and Sodium Thiosulfate Dosing in BWTS
- Sodium Thiosulfate Dosing Calculation for Ballast Water
- Sodium Thiosulfate and Electrochlorination BWTS Chemistry
- Electrochlorination BWTS Explained: Electrolysis, TRO Generation and Neutralisation
- What Is a Ballast Water Treatment System? Explained
- How Does BWTS Installation Work? Step by Step
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