The Gas Mask Made of Photo Fixer: How Sodium Thiosulfate Met Chlorine in 1915

On the afternoon of 22 April 1915, near the Belgian town of Ypres, a yellow-green cloud drifted across no-man’s-land toward French and Canadian trenches. Nobody in those trenches owned a gas mask, because until that afternoon there had been no reason to. The chemical that ended up between soldiers and that cloud a few weeks later was not a military invention at all. It was the fixer in every photographer’s darkroom: sodium thiosulfate. This is the story of how it got from the darkroom to the front, why it worked, why it was retired, and why water plants still buy it by the pallet for the same reaction.

The 46-second film: real US Army Signal Corps footage (National Archives, 1918–1921) with three paper-collage illustrations made for it.

What happened at Ypres on 22 April 1915?

On 22 April 1915, during the Second Battle of Ypres, the German army released chlorine gas from cylinders dug into its own front line and let the wind carry it across to the Allied trenches: the first large-scale use of a poison gas as a weapon. Chlorine is heavier than air, so the cloud hugged the ground and poured into trenches and shell holes. It attacks the moist tissue of the eyes, throat and lungs; at the concentrations in that cloud it caused fluid to flood the lungs, and men drowned on dry land.

The historian Tim Cook of the Canadian War Museum, who has read the letters and diaries of the men in that line, describes the effect on the two French divisions that took the full weight of the cloud as “panic and terror as the soldiers feel the gas burning out their lungs.” The French line broke. The Canadian division beside it, which caught the edge of the cloud, held. Within days, troops were being told to urinate on a handkerchief and breathe through it: ammonia in urine does react with chlorine, and it was the only chemistry to hand.

Who was Cluny Macpherson, and what was in his pocket?

Cluny Macpherson was a physician from St. John’s, Newfoundland, born in 1879, who enlisted at the outbreak of the war and became the principal medical officer of the Newfoundland Regiment. When the first gas casualties arrived, the War Office had nothing to issue but cotton pads: the “black veil respirator,” a cotton pad soaked in an absorbent solution and tied over the mouth with black veiling, which reached troops on 20 May 1915. It was better than a handkerchief and not by much.

Macpherson had seen something better. In his own account, quoted in a 2024 CBC feature drawn from The Rooms archive in St. John’s, he had seen a captured German device, a bag pulled over the head, and thought he could improve on it: “I saw that German contraption and I thought I could do something better. And I bought a length of Viyella and some mica and put them in my pocket.” Viyella is a soft wool-and-cotton flannel. Mica is a mineral that splits into clear sheets and does not fog or shatter the way glass does. A hood of flannel with a mica window, soaked in the right solution, would cover the eyes as well as the mouth and could be pulled on in seconds.

He presented the design to the War Office Anti-Gas Department on 10 May 1915, eighteen days after the first attack. Prototypes were tested in a trench filled with chlorine; the story told in Newfoundland is that Macpherson walked through wearing one and reported that he had smelled nothing. The design was adopted as the British Smoke Hood, issued from late May and into June 1915, and Macpherson was put in charge of mass production. It was manufactured until September 1915, and roughly 2.5 million were made before improved versions superseded it.

What was the Hypo helmet actually soaked in?

The soaking solution had three jobs, and each ingredient did one of them. The hood was dipped, wrung out and issued damp, then re-dipped from bottles carried in the line.

The nickname came from the fixer. Photographers had called sodium thiosulfate “hypo” since Sir John Herschel first used it to fix silver images in 1839, under its older and chemically wrong name, sodium hyposulphite. Soldiers called the hood the Hypo helmet, or just the smoke helmet, and the name has stuck in museum catalogues ever since.

How does sodium thiosulfate neutralise chlorine?

Chlorine is dangerous because it wants electrons: it is a strong oxidiser, and it takes those electrons from whatever it touches first, including the lining of a lung. Thiosulfate is a reducing agent: its sulfur atoms sit at a low oxidation state and give electrons up readily. When chlorine gas dissolves into the wet cloth and meets thiosulfate, the chlorine is reduced to chloride, the same ion that is half of table salt, and the thiosulfate is oxidised through to sulfate.

Read the right-hand side of that equation and you can see why the alkali was in the bottle: eight molecules of hydrochloric acid and two of sodium bisulfate for every thiosulfate consumed. Washing soda or bicarbonate in the soak converts that acid to salt and carbon dioxide before it can irritate the airway. The three ingredients were not a recipe someone stumbled on; each one answers a line of the chemistry.

It is worth noticing what the reaction does not do. It does not absorb chlorine the way charcoal absorbs it, by holding it on a surface; it changes chlorine into something else. That is why a thiosulfate hood could be regenerated with a fresh dip, and why the same chemistry works in a river outfall today: thiosulfate does not store chlorine, it converts it.

Why was the Hypo helmet replaced within months?

Because the enemy changed the gas. Thiosulfate is specific to oxidisers; it does nothing against phosgene, which attacks the lungs by a different mechanism and which Germany first used in quantity against British troops on 19 December 1915. The British answer was already in production by then: the P helmet, a similar hood with two glass eyepieces and an exhaust valve, soaked in sodium phenate to absorb phosgene, followed by the PH helmet with hexamine added. By the middle of 1916 the small box respirator, a face mask connected by a hose to a canister of charcoal and chemically treated granules, made hoods obsolete altogether.

None of that makes the Hypo helmet a failure. For the months when chlorine was the gas, it was the difference between a division that broke and a division that held, and it was in two and a half million hands because its active ingredient could be bought by the ton from an industry that had nothing to do with the war. Macpherson went on to train officers in gas defence in Egypt and elsewhere, returned to medicine in St. John’s, and died in 1966. His prototype is held by The Rooms, the provincial museum of Newfoundland and Labrador.

Where does the same reaction run today?

Everywhere chlorine has to be removed from water. Municipal treatment plants chlorinate drinking water and wastewater on purpose, then must take the residual chlorine back out before discharge, because a few tenths of a milligram per litre of free chlorine is lethal to fish and to the invertebrates a river depends on. Sodium thiosulfate and its cousin sodium bisulfite are the two standard dechlorinating agents, and the choice between them is covered in our sodium thiosulfate guide and our engineer’s guide to sodium bisulfite.

• Wastewater and drinking-water plants dose thiosulfate at the outfall to meet a residual-chlorine discharge limit, and to protect membranes and ion-exchange resins downstream of a chlorinated feed.
• Aquaculture and aquariums use it to make chlorinated tap water safe for fish before a water change: the same chemistry, at a teaspoon scale.
• Laboratories add it to sampling bottles so that chlorine does not keep reacting with the sample between the tap and the bench; it is also the titrant in iodometric analysis.
• Pools and spas use it to bring an over-chlorinated pool back to a swimmable level quickly, which is far cheaper than draining.
• Photography and film processing still use it as the fixer, the job it had before 1915.

What to check when you buy sodium thiosulfate for dechlorination

Three things decide whether a bag of sodium thiosulfate does the job you bought it for.

1. Which form it is. The pentahydrate, Na₂S₂O₃·5H₂O, is the common commercial crystal and carries 36 per cent water by mass; the anhydrous salt is lighter per mole of active thiosulfate. Dosing tables are written for one or the other, and using the wrong one is a 57 per cent error in either direction. The form is on the certificate of analysis, which we supply on request.
2. Assay. A technical grade is specified on assay and on a handful of impurities, which is what a dechlorination or pool application needs. Analytical titration needs a reagent grade standardised against a primary standard; if that is your use, say so.
3. Pack size against dose. Thiosulfate solutions slowly oxidise in air and can grow sulfur-metabolising bacteria if made up and left. Buy the pack that matches how fast you use it, and make up solution as you need it rather than by the drum.

Tell us the application and the parameter you are measured against, and we will match the grade to it. For dechlorination, aquaculture, pools and general use, the technical grade below is the one to order.

Common questions

What was the Hypo helmet made of?

A hood of flannel (Macpherson bought Viyella, a wool-and-cotton twill) with a single window of mica, soaked in a solution of sodium thiosulfate, glycerin and an alkali such as washing soda. The wearer pulled it over the head and tucked the skirt into the tunic collar, breathing in and out through the wet cloth.

Why is sodium thiosulfate called hypo?

Because photographers had been calling it that since 1839, when Sir John Herschel used it to fix silver images. The name comes from its older, incorrect chemical name, sodium hyposulphite. The 1915 hood was soaked in the same photographic fixer, so soldiers called it the Hypo helmet.

How does sodium thiosulfate remove chlorine?

By reduction. Chlorine is an oxidiser that takes electrons; thiosulfate is a reducing agent that gives them up. The chlorine becomes chloride, the thiosulfate becomes sulfate, and the acid produced is neutralised by an alkali. One sodium thiosulfate neutralises four chlorine molecules, about 0.56 mg of the anhydrous salt per mg of chlorine on paper.

Who invented the Hypo helmet?

Cluny Macpherson, a physician from St. John’s, Newfoundland, serving as principal medical officer of the Newfoundland Regiment. He presented it to the War Office Anti-Gas Department on 10 May 1915, after seeing a captured German head-bag and deciding he could do better. He was later put in charge of mass-producing it.

Why did the Hypo helmet stop being used?

Germany moved to phosgene in December 1915, and thiosulfate does nothing against phosgene. The P helmet, soaked in sodium phenate, replaced it from November 1915, followed by the PH helmet and, in 1916, the small box respirator with a charcoal canister.

Is sodium thiosulfate still used against chlorine today?

Yes, in water rather than in air. It is one of the two standard agents for dechlorinating treated water before discharge, for making tap water safe for fish, for lowering an over-chlorinated pool, and for stopping chlorine reacting inside laboratory samples. It is not respiratory protection and is not sold as such.

The grade we stock

One product, ten pack sizes: Sodium Thiosulfate Technical Grade, from a 2 lb pack to a pallet of forty 55 lb bags (2,200 lb). Certificate of analysis on request. Tell us the application and we will confirm the form and the dose basis before you order.

Key numbers and sources