Sodium Thiosulfate Pentahydrate vs Anhydrous: Marine Use
Updated
Sodium thiosulfate for ballast water is normally supplied as the pentahydrate, Na2S2O3·5H2O, molar mass 248.18 g/mol, which is about 63 percent thiosulfate by mass because of its five waters of crystallisation. The anhydrous salt (molar mass 158.11 g/mol) is more concentrated per kilogram, so the two are not interchangeable at equal mass; you must match your dosing rule to the exact salt form to avoid over- or under-dosing.
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Key takeaways
- Pentahydrate Na2S2O3·5H2O has a molar mass of 248.18 g/mol and is about 63 percent thiosulfate by mass.
- Anhydrous Na2S2O3 (molar mass 158.11 g/mol) is more concentrated, so about 1.57 kg of pentahydrate equals 1.0 kg anhydrous.
- Common dosing rules are expressed in pentahydrate mass, so pentahydrate is the standard marine form.
- Applying a pentahydrate rule to anhydrous salt overdoses by about 1.57 times; the reverse underdoses and risks an exceedance.
- Pentahydrate is more stable and less hygroscopic than the anhydrous salt, better suited to long shipboard storage.
- Always confirm the salt form on delivery and label make-up tanks accordingly to prevent dosing errors.
What Is the Difference Between the Two Forms?
The pentahydrate, Na2S2O3·5H2O, carries five molecules of water of crystallisation locked into its crystal structure, giving a molar mass of 248.18 g/mol. The anhydrous form, Na2S2O3, has no such water and a molar mass of 158.11 g/mol, so a kilogram of anhydrous salt contains substantially more active thiosulfate than a kilogram of pentahydrate.
Numerically, the pentahydrate is about 63 percent thiosulfate by mass (158.11 divided by 248.18), the remainder being crystal water. This means roughly 1.57 kg of pentahydrate delivers the same thiosulfate as 1.0 kg of anhydrous salt, a difference far too large to ignore in dosing.
Both forms are the same chemical in solution once dissolved; the distinction matters only for the mass you weigh out and for storage behaviour. The British spelling sodium thiosulphate is used for both forms on European documentation.
Which Form Is Used for Ballast Water?
The pentahydrate is the standard marine and general water-treatment form because it is stable, easy to handle and widely available as a crystalline solid. Most dosing rules of thumb, including the common 2.5 to 3.0 mg per mg of Cl2-equivalent TRO, are expressed in terms of pentahydrate mass, so using the pentahydrate keeps calculations straightforward.
Anhydrous sodium thiosulfate exists and is used in some industrial settings, but it is more hygroscopic and less commonly stocked for ballast water. If a ship is ever supplied anhydrous material, the dosing mass must be scaled down by the ratio of the molar masses to avoid a large overdose.
Sea Clean supplies the pentahydrate from Norway in 25 kg bags, 50 kg drums and 1000 kg bulk bags, matching the standard marine expectation and the pentahydrate-based dosing rules used across BWTS makers.
How Does the Salt Form Affect Dosing Math?
Because the pentahydrate is only about 63 percent active thiosulfate, dosing rules built around it already account for the crystal water. If you apply a pentahydrate rule to anhydrous salt, you deliver roughly 1.57 times the intended thiosulfate, a large overdose; applying an anhydrous rule to pentahydrate underdoses by the inverse factor and risks a residual-oxidant exceedance.
To convert, multiply an anhydrous mass by 248.18 divided by 158.11 (about 1.57) to get the equivalent pentahydrate mass, or divide a pentahydrate mass by that factor to get the anhydrous equivalent. The safest practice is to fix one salt form in the ship's dosing procedure and stock only that form.
When preparing stock solution, remember the concentration is expressed as grams of the chosen salt per litre, so a 200 g/L pentahydrate solution is not the same active strength as a 200 g/L anhydrous solution. Label make-up tanks with the salt form to prevent confusion at handover.
How Do Storage and Solubility Compare?
The pentahydrate is a stable crystalline solid that stores well cool and dry for a typical two to three years, and it is highly soluble, about 701 g/L at 20 degrees Celsius, so concentrated stock is easy to prepare. Its main storage concern is caking from absorbed moisture.
Anhydrous sodium thiosulfate is more hygroscopic and will tend to draw in moisture and can partly convert toward the hydrated form on standing, which complicates accurate dosing and storage. This is another reason the pentahydrate is preferred for shipboard use where stores may sit for months.
For either form, keep the reagent sealed, dry and segregated from acids and oxidisers. If a ship holds both forms for any reason, keep them clearly separated and labelled so the wrong salt is never charged into a dosing tank.
What Should You Check on Delivery?
On receipt, confirm the salt form on the label and safety data sheet, because pentahydrate and anhydrous look similar as white solids but dose very differently. A delivery of anhydrous material into a pentahydrate-based system, dosed without adjustment, would substantially overdose.
Check the purity grade is suitable for water treatment and that the packaging matches your order and storage plan. Verify the batch documentation and shelf life so stock can be rotated first-in first-out within its useful period.
Sea Clean provides the reagent with clear documentation and offers independent, brand-agnostic advice on matching the salt form and grade to a ship's type-approved system, so operators do not discover a mismatch at the discharge point.
Which Form Should You Standardise On?
For almost all ballast water applications, standardise on sodium thiosulfate pentahydrate. It is the form assumed by common dosing rules, the most widely available marine grade, more stable in storage than the anhydrous salt, and easy to dissolve into concentrated stock.
Standardising on one form also simplifies training, procedures and procurement, removing a whole class of dosing error rooted in salt-form confusion. Write the pentahydrate into the ship's neutralisation procedure and reorder the same form each time.
If a maker specifically calls for anhydrous material, follow that instruction and adjust the dosing math accordingly, but treat it as the exception. When in doubt, confirm with the system maker or with Sea Clean rather than assuming equivalence at equal mass.
Frequently asked questions
Which sodium thiosulfate form is used for ballast water?
The pentahydrate, Na2S2O3·5H2O, is the standard marine form because it is stable, widely available, and matches common dosing rules. Anhydrous salt is used in some industry but is more hygroscopic and less common onboard.
How much more concentrated is anhydrous thiosulfate?
Anhydrous salt is about 1.57 times more concentrated by mass than the pentahydrate, since the pentahydrate is only about 63 percent thiosulfate. About 1.57 kg of pentahydrate equals 1.0 kg of anhydrous.
Can I swap pentahydrate for anhydrous at the same weight?
No. At equal mass anhydrous delivers about 1.57 times more thiosulfate, so swapping without adjusting the dose causes a large overdose. Scale by the molar-mass ratio or stock only one salt form.
What is the molar mass of sodium thiosulfate pentahydrate?
248.18 g/mol for the pentahydrate Na2S2O3·5H2O, versus 158.11 g/mol for the anhydrous salt. The difference is the five waters of crystallisation in the pentahydrate.
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