Chemistry saving lives: using First World War Hypo helmets to avoid chlorine poisoning

2.1 Brief description of First World War

Several striking photographs from Haber’s (1986) and Freemantle’s (2014, 2015 books were copied using a document camera and projected onto a screen (Figure S3). The First World War (1914–1918) was called the Chemists’ War because of the first use of several chemical munitions, namely, high explosives, poisonous gas, and gas masks (Haber, 1986; Freemantle, 2014; Freemantle, 2015). In addition, military hospitals used antiseptic solutions, anesthetics, and X-ray apparatus to treat the wounded (Freemantle, 2015).

In April 1915, the Imperial German Army released clouds of chlorine over the French and French Algerian armies; this is recognized as the first large-scale poisonous gas attack in history. Many soldiers died from this chlorine attack. In May 1915, the British Army was the first to issue a portable gas mask called the Hypo helmet.

The Hypo helmet was invented by Canadian physician Cluny MacPherson (Haber, 1986). Before the issue of Hypo helmets, Allied soldiers covered their noses and mouths with handkerchiefs and socks soaked with water or urine to protect them from chlorine (Freemantle, 2014, 2015). The Hypo helmet is a fabric hood (Figure 1a) soaked with a mixture of aqueous solutions of sodium thiosulfate and sodium bicarbonate. Although the Hypo helmet could effectively detoxify chlorine, it was eventually replaced by the improved Phenate helmet comprising two eyepieces, an exhaust valve, and a mouthpiece (Figure 1b). By the end of 1916, the Small box respirator comprising a rubber hood, round eyepieces, corrugated tube, and metal canister was introduced (Figure 1c) (Haber, 1986).

Chlorine was used as the poisonous gas for several reasons (Haber, 1986). High concentrations of chlorine gas inhalation can cause pulmonary edema, pulmonary inflammation, respiratory failure, and death (White & Martin, 2010). Chlorine is easy to produce and liquefy using compression and cooling, which made it easy to transport in large quantities to the front line. Because it is more than twice as dense as air (World Health Organization, 1982), it crept across the battlefields and lingered in trenches and shell holes. However, as a weapon, chlorine had a weakness (Haber, 1986). It was difficult to predict where the clouds of chlorine gas would travel to as they were dependent on weather conditions (i.e., wind direction, wind speed, temperature, and humidity) and topography. Moreover, because it has a pungent odor and a characteristic color (World Health Organization, 1982), it was easily noticed and could be avoided by the Allied forces when released.

In addition to chlorine, other chemicals, such as carbonyl dichloride (phosgene) and bis(2-chloroethyl)sulfide (mustard gas), were used as poisonous gases during the First World War (Haber, 1986; Freemantle, 2014, 2015). The use of poisonous gases was so widespread, disposal of dumped and unexploded shells filled with chlorine and other poisonous gases continues, even after more than 100 years after the end of the war (Freemantle, 2018).

2.2 Video demonstration

The following glassware and materials are required to demonstrate chlorine generation and detoxification using the dummy Hypo helmet. Detailed instructions are presented in the Supplementary Material. A photograph and schematic of the reaction apparatus are shown in Figure 2.

1. Reaction apparatus (Supplementary Material Figure S1): a glass tube threaded through a rubber stopper, three L-shaped glass tubes threaded through rubber stoppers, four glass test tubes with side arms, two plastic three-way stopcocks, two empty plastic syringes, and two plastic syringes filled with solid sodium thiosulfate (8 g).

2. Dummy Hypo helmet: glass tube (10 cm in length) filled with a small (4 × 2.5 cm) piece of fabric (50 % cotton; 50 % wool) soaked with a mixture of saturated aqueous solutions of sodium thiosulfate and sodium bicarbonate.

3. Bleaching powder: 0.2 g.

4. Hydrochloric acid: 1 mL (12 mol/L or 4 mol/L).

5. Saturated aqueous solution of sodium thiosulfate: 2 mL.

6. Saturated aqueous solution of sodium bicarbonate: 2 mL.

7. Potassium iodide starch paper: 5 mm square.

8. pH test paper: 5 mm square.

For the video demonstration, we prepared two short videos. One demonstrated the generation and identification of chlorine, and the other demonstrated chlorine detoxification using the dummy Hypo helmet. The video clips have no sound so that instructors can add their commentary to them. For the chlorine generation and identification experiments, 12 mol/L hydrochloric acid was adequate to generate visible chlorine. For the chlorine deactivation experiments, less chlorine was generated by using 4 mol/L hydrochloric acid to ensure detoxification by the dummy Hypo helmet.

The chlorine was generated in a test tube (d) by the reaction of 0.2 g bleaching powder with 1 mL 12 mol/L hydrochloric acid. Note that the chemical reaction makes the test tube (d) hot. This method produces a mixture of chlorine and hydrogen chloride (Scheme C). Chlorine accumulates in the bottom of the test tube, which passes through a glass tube (e) to an empty test tube (f). The empty test tube prevents water vapor from clouding the following test tubes (g and h) that contain potassium iodide starch paper and pH test paper, respectively. Chlorine changes the color of potassium iodide starch paper from white to violet (Figure 3, Scheme D); and hydrogen chloride, which is colorless with a pungent odor (World Health Organization, 1982), changes the color of pH test paper from green to orange (Figure 3). Excess chlorine further decolorizes the two test papers (Figure 3c). An empty syringe (i) is used to pass the generated chlorine through the glass tube (e) and test tubes (f–h). If there is sufficient sodium thiosulfate in the plastic syringe (a), the generated chlorine will not escape. The sodium thiosulfate in the plastic syringe (a) can be reused several times; however, as it gradually deteriorates, it becomes yellowish. The generated chlorine is somewhat difficult to see in the video; hence, instructors must tell the students to watch the video carefully.

Figure 3:

Color changes in potassium iodide starch paper and pH test paper: (a) Before and (b) after exposure to chlorine and hydrogen chloride, and (c) after discoloration by chlorine.

Next, the generated chlorine is detoxified by the dummy Hypo helmet in the glass tube (e). It comprises a rolled piece of fabric soaked with a mixture of saturated aqueous solutions of sodium thiosulfate and sodium bicarbonate. If the amount of the generated chlorine is not high; that is, the concentration of the hydrochloric acid used is low (4 mol/L), the dummy Hypo helmet can deactivate chlorine for a while, and neither the potassium iodide starch paper nor the pH test paper is discolored. Note that the glass tube (e) becomes hot during the neutralization. Chlorine reacts with sodium thiosulfate to generate sodium sulfate, sulfur, and hydrogen chloride (Scheme A). The sulfur produced by the reaction can be observed as yellow staining in a glass tube (Figure 4). Hydrogen chloride is neutralized by sodium bicarbonate to produce harmless materials; sodium chloride, water, and carbon dioxide (Scheme B).

Figure 4:

Dummy Hypo helmet (a) before and (b) after exposure to chlorine.

2.3 Students’ comments

In 2022, approximately one-third of a 90 min chemistry lecture was presented without the experimental videos mentioned in the main text. At the time, participating students told the authors that it would have been better to include a demonstration or video of chlorine generation. In 2023, a lecture was presented that included the experimental videos. In each year, the target students were undergraduates, including second-year environmental science majors and first-year civil engineering majors. The environmental science majors participated in the demonstration lecture as part of a class called General Chemistry. General Chemistry was an elective class; however, it was compulsory for students who wanted to obtain a science teacher’s certificate. On the other hand, the civil engineering majors took the demonstration lecture as part of the Basic Chemistry course, which was required for graduation. All the participants understood the basic properties of chlorine before the lecture, such that chlorine was a gas consisting of diatomic molecules of the halogen group. Many of the participants did not choose chemistry for their university entrance exams nor did they think they were good at chemistry. Students at our university who were not good at chemistry often did not listen to their chemistry lectures enthusiastically. Therefore, the author took to using some of the 90 min lecture time for creative presentation.

After the video demonstration, the students wrote down their impressions of the lecture. During this time, the instructor explained the duality of chlorine. Chlorine, as a poisonous gas, not only killed soldiers but also saved soldiers when used as a cleaning agent in drinking water and a disinfectant in the trenches (McGuire, 2013).

The following are some examples of the students’ impressions:

1. I was very surprised that the munition to protect against poison gas was so simple.

2. The use of poison gas has been and will continue to carry a great risk of retaliation.

3. The cat-and-mouse game … when a gas mask is developed, a more toxic gas is developed, is terrifying.

4. I learned in this lecture that chemicals can kill or help people, depending on how they are used.