How Much Sodium Thiosulfate to Neutralize Chlorine?

The stoichiometric starting point is roughly 3 mg of anhydrous sodium thiosulfate for every 1 mg of hypochlorous acid (free chlorine) in solution, based on a one-to-one molar ratio between the two compounds. In practice, though, that textbook minimum rarely gets the job done fast enough or completely enough, and most real-world applications call for double that dose or more. The right amount depends on your water’s pH, how much chlorine you are dealing with, what form of thiosulfate you are using, and how quickly you need the chlorine gone.

The Basic Ratio and Where It Comes From

Sodium thiosulfate neutralizes chlorine by reacting with it chemically, converting the oxidizing chlorine species into harmless chloride ions. The stoichiometric ratio follows from the molecular weights of the two reactants. Researchers studying dechlorination of wastewater found that the stoichiometric dosage to neutralize 1 mg of residual hypochlorous acid is about 3 mg of sodium thiosulfate at a one-to-one molar ratio, about 4.5 mg at 1.5 times stoichiometry, and about 6 mg at twice stoichiometry.¹

There is an important nuance in how chlorine is measured. Municipal water reports and test kits usually express chlorine concentration as “mg/L Cl₂” or ppm of free chlorine, while the actual chlorine species in your water at typical pH ranges is mostly hypochlorous acid and hypochlorite ion. The molecular weight differences between these forms shift the exact ratio slightly. For most practical purposes, if your test kit reads a free chlorine level in ppm, multiplying that number by roughly 3 gives you the milligrams of anhydrous sodium thiosulfate you need per liter as a bare minimum. Many aquarium guides and water-sampling protocols round this to a simpler rule, something like “one crystal per gallon” or “one drop of stock solution per liter,” which builds in a safety margin.

Why the Minimum Dose Is Rarely Enough

A kinetic study comparing several common dechlorination chemicals found that sodium thiosulfate, along with sodium sulfite, sodium bisulfite, sodium metabisulfite, and ascorbic acid, all need to be dosed above the stoichiometric minimum to achieve complete dechlorination within less than a minute.² At exactly the stoichiometric dose, the reaction proceeds too slowly for most applications. If you are filling a fish tank, sampling water for bacteria testing, or discharging treated water, you typically cannot wait an extended period for the last traces of chlorine to disappear.

Dechlorination research on wastewater showed that at twice the stoichiometric dose, sodium thiosulfate reduced residual chlorine from 50 ppm down to below 1 ppm, and the reaction followed first-order kinetics at that higher dose.¹ First-order kinetics here means the reaction rate depends directly on how much chlorine remains: it is fast at first and slows down as the chlorine concentration drops. The takeaway for anyone dosing in the real world is that a moderate excess speeds things up considerably without causing problems in most situations.

How pH Changes the Picture

The pH of your water has a meaningful effect on how well sodium thiosulfate removes chlorine. In slightly acidic water around pH 6, the same study found the deepest drop in residual chlorine over a three-hour window. At neutral pH (around 7) and alkaline conditions (around 9), the dechlorination still worked but left residual chlorine slightly above 0.5 ppm, while the acidic condition brought it lower.¹

This matters because the form of chlorine in water shifts with pH. Below about pH 7.5, free chlorine exists mostly as hypochlorous acid, which is the more reactive and more toxic form. Above pH 7.5, it shifts increasingly toward hypochlorite ion, which is less reactive. Research into the reaction mechanism between hypochlorous acid and thiosulfate has shown that HOCl reacts through a chlorine-atom transfer pathway, which is faster than the oxygen-atom transfer pathway that the hypochlorite ion favors.³ In plain terms: the chlorine species that dominates in lower-pH water reacts faster with thiosulfate than the species that dominates in higher-pH water. If your water runs alkaline, you may need a slightly higher dose or more time.

Most municipal tap water falls in the 6.5 to 8.5 range. If you are at the lower end, the standard double-stoichiometric dose works well. If your water is on the alkaline side, either bump the dose slightly or give the reaction a few extra minutes before testing for residual chlorine.

Pentahydrate Versus Anhydrous Forms

One of the most common sources of confusion is the difference between anhydrous sodium thiosulfate and the pentahydrate crystal form. When you buy sodium thiosulfate crystals from an aquarium store, a photography supplier, or a chemical vendor, you are almost always getting the pentahydrate version, which has five water molecules attached to each molecule of the salt. Those water molecules add weight without contributing to the dechlorination reaction.

The pentahydrate weighs about 1.56 times as much as the anhydrous form per molecule. If a dosing calculation tells you to use 6 mg of anhydrous sodium thiosulfate per liter, you would actually need roughly 9.4 mg of the pentahydrate crystals to deliver the same amount of active ingredient. This is not a trivial difference. Failing to account for it means underdosing by about 36 percent, which can leave measurable chlorine residual in the water.

If you are working from a pre-made stock solution, the concentration printed on the label should tell you which form was dissolved. Analytical-grade solutions used in laboratories almost always specify whether they are based on the anhydrous or pentahydrate salt. For home use, assume you have the pentahydrate and dose accordingly.

Practical Dosing by Application

The right dose varies with what you are trying to accomplish, because different situations tolerate different levels of residual chlorine and excess thiosulfate.

Aquariums and Fish Tanks

Aquarium keepers typically deal with municipal tap water containing 0.5 to 2 ppm free chlorine. At the high end, neutralizing 2 ppm in a 100-liter tank means removing about 0.2 mg of chlorine per liter, which needs roughly 0.6 mg of anhydrous sodium thiosulfate per liter at stoichiometric minimum, or about 1.2 mg per liter for a comfortable safety margin. In practice, most aquarists use commercial dechlorinators that contain sodium thiosulfate premixed at a concentration designed to be dosed by the capful. If you are making your own stock solution, dissolving about 4 grams of the pentahydrate crystals in a liter of distilled water creates a solution where roughly one milliliter treats 40 liters of tap water with typical chlorine levels.

Be aware that some municipal water systems use chloramine instead of free chlorine. Sodium thiosulfate does break the chlorine-nitrogen bond in chloramine, but the reaction is slower and less complete than with free chlorine. If your water utility uses chloramine, you may need a higher dose and should test residuals afterward, or consider a product specifically formulated for chloramine removal.

Water Sampling and Laboratory Work

When collecting water samples for microbiological testing, the goal is to instantly quench all chlorine so that it does not keep killing bacteria in the sample bottle during transport. Standard practice is to add a pre-measured tablet or a measured volume of thiosulfate solution to the sample container before collection. This is one situation where a generous excess is not only acceptable but preferred, because leaving even a trace of chlorine in the bottle would skew bacterial counts downward. Most sampling protocols specify roughly 100 mg of sodium thiosulfate per liter of sample, far above stoichiometric need, to ensure instant and total quenching regardless of the chlorine level in the source water.

Wastewater Discharge and Industrial Applications

Industrial facilities that discharge chlorinated water to streams or sewers face regulatory limits on residual chlorine. The chlorine levels involved can be much higher than in drinking water, sometimes 10 ppm or more. At these concentrations the double-stoichiometric dose becomes the practical baseline rather than a comfortable excess. A kinetic study of several dechlorination chemicals confirmed that all of them, sodium thiosulfate included, required more than stoichiometric doses to fully remove chlorine within less than a minute.² For wastewater with fluctuating chlorine levels, continuous monitoring and automated feed systems are typical, rather than one-time batch dosing.

Risks of Overdosing

Adding extra sodium thiosulfate is generally safe in the concentrations used for water treatment, but “more is better” has limits. For aquatic life in particular, high concentrations become toxic. A study on common carp found that exposure to sodium thiosulfate at 10 grams per liter caused 80 percent fish mortality over 96 days, with significant damage to gill tissues including hemorrhage and cellular degeneration.⁴ That concentration is thousands of times higher than what you would use for normal dechlorination, so there is a wide margin of safety in typical use. Still, the finding underscores that sodium thiosulfate is not biologically inert at elevated doses.

A more immediate concern for most users is the effect on water chemistry. Research comparing dechlorination agents found that sodium thiosulfate produced the largest pH shift in test waters compared to sodium bisulfite and ascorbic acid.⁵ For a sensitive aquarium or a research application where pH stability matters, this is worth monitoring. In bulk water treatment the shift is usually small enough to be inconsequential, but in small volumes with low buffering capacity, a generous overshoot of sodium thiosulfate can push the pH enough to stress aquatic organisms.

Excess thiosulfate in water can also consume dissolved oxygen, which matters in aquatic systems. In well-aerated tanks this is unlikely to be a problem, but in stagnant or poorly oxygenated water, a large overdose combined with low oxygen levels could create stress for fish or invertebrates.

How Sodium Thiosulfate Compares to Alternatives

Sodium thiosulfate is one of several chemicals used for dechlorination. The most common alternatives are sodium bisulfite, sodium metabisulfite, and ascorbic acid (vitamin C). Each has trade-offs worth knowing about.

Speed is one differentiator. Comparative research found that sodium bisulfite and ascorbic acid both neutralize chlorine faster than sodium thiosulfate under clean-water conditions. However, when organic matter was present in the water, the rate of dechlorination by sodium thiosulfate increased, while the rates for bisulfite and ascorbic acid did not change.⁵ This means thiosulfate may actually perform better in “dirty” water with dissolved organics, like natural surface water or wastewater effluent.

Biological safety is another consideration. The same study found that sodium thiosulfate had the least impact on daphnia (water flea) mortality rates among the three agents tested.⁵ For anyone discharging dechlorinated water into an environment with aquatic life, that lower toxicity is a real advantage. Ascorbic acid has the appeal of being familiar and food-safe, but it was found to be ineffective against monochloramine, making it a poor choice if your water system uses combined chlorine.

Cost and availability tilt toward thiosulfate for most users. It is inexpensive, widely available, and stable in dry crystal form for long periods. Sodium bisulfite works faster but is more irritating to handle and generates sulfur dioxide fumes if it contacts acid. Ascorbic acid degrades relatively quickly in solution and in humid storage conditions.

Using Sodium Thiosulfate in Aquaculture and Hatcheries

Aquaculture operations face a specific version of this dosing question: they need to remove chlorine from large volumes of seawater or freshwater used for disinfection, without leaving residues that harm the organisms they are raising. A study of hatchery water treatment found that mass cultures of the microalga Isochrysis galbana (a common feed species) actually grew better in electrolytically treated seawater that had been neutralized with thiosulfate than in seawater sterilized by autoclaving or UV irradiation.⁶ The likely explanation is that thiosulfate neutralization leaves beneficial trace compounds intact while removing the chlorine, whereas heat sterilization destroys some dissolved organics and nutrients.

For hatcheries and aquaculture facilities, the dosing approach is similar to industrial wastewater treatment: measure residual chlorine after disinfection, calculate the stoichiometric dose, apply at roughly double that amount, then verify with a test kit before introducing organisms. The stakes are higher than in a home aquarium because the volumes are larger and the organisms being cultured may be more sensitive. Larval fish and invertebrates, in particular, can be harmed by chlorine levels far below what adult fish tolerate. Excess thiosulfate at the concentrations used for normal dechlorination does not appear to harm these cultures, and as the hatchery research showed, it may even be beneficial compared to alternative sterilization methods.

Tips for Getting the Dose Right at Home

If you are a home aquarist or a homebrewer who just wants to know what to do, the process is straightforward. First, test your tap water’s chlorine level with an inexpensive drop test kit or test strip. Most tap water falls between 0.5 and 2 ppm of free chlorine. Then use the following approach:

Make a stock solution: Dissolve about 4 grams of sodium thiosulfate pentahydrate crystals in one liter of distilled or dechlorinated water. This gives you a solution of roughly 4,000 ppm.
Dose at about 1 mL per 40 liters: For typical tap water chlorine levels around 1 to 2 ppm, one milliliter of that stock solution is enough for about 40 liters of water, giving a comfortable margin above stoichiometric need.
Test afterward: Wait a minute, then test with your chlorine kit. If you still see residual chlorine, add another half dose and test again.
Adjust for chloramine: If your water utility uses chloramine, double the dose and allow a few extra minutes before testing.

Stored dry, sodium thiosulfate pentahydrate crystals last for years in a sealed container away from heat and moisture. Solutions are less stable and should be used within a few weeks, or kept refrigerated if longer storage is needed. If a stock solution develops a sulfur smell or visible cloudiness, discard it and make a fresh batch.

For anyone working with larger volumes, like filling a pond or a swimming pool after a chlorine shock, the same math scales linearly. Measure total volume, measure chlorine concentration, multiply out to get total milligrams of chlorine, then apply roughly six milligrams of pentahydrate per milligram of chlorine for a reliable double-stoichiometric dose. For a 10,000-liter pond at 2 ppm chlorine, that works out to about 120 grams of crystals dissolved in a bucket of water and distributed evenly.

1
ResearchGate / Journal of Emerging Supply Chain Clean Energy and Process Engineering. Study of Efficiency and Reaction Rates Dechlorination of Nata De Coco Wastewater Using Sodium Thiosulfate
2
Proceedings of the Water Environment Federation. Comparison of reaction kinetics under varying conditions for dechlorination chemicals
3
PubMed. Oxidations at Sulfur Centers by Aqueous Hypochlorous Acid and Hypochlorite: Cl(+) Versus O Atom Transfer
4
Iraqi Journal of Veterinary Sciences. Immunohistochemically and semi-quantities analysis of carp gills exposed to sodium thiosulfate
5
ResearchGate. Comparison of dechlorination rates and water quality impacts for sodium bisulfite, sodium thiosulfate and ascorbic acid
6
Aquaculture. Disinfection of seawater for hatchery aquaculture systems using electrolytic water treatment