Welcome to The Plague Pit – issue number 23.

For this issue, I’m very pleased to welcome back Adrian Tsui with the second of his two articles on chemistry and COVID-19. He has thoughtfully provided a glossary at the end of the article for readers (and site editors) who may be a bit rusty on their chemical terminology.

Adrian is a sixth former at Winchester College, studying Physics, Chemistry and Biology. He has a place at Downing College, Cambridge to read Natural Sciences, starting in 2020. He’s also a dedicated musician.

In this second instalment, we move on to the heavy artillery of disinfectants: bleach. (Note that ‘bleach’ here refers to chlorine bleach, and not peroxide-based bleach, which is more environmentally friendly, but also has its hazards.) Such is its power that it can destroy even the most stubborn of microbes; however, incorrect usage is incredibly dangerous, and may be fatal.

If you find that your local supermarket has run out of bleach due to possible panic buying, fear not. You can make your own bleach just with (a lot of) table salt, a battery and two graphite pencil leads. The set up is shown below:

Please do this in a well-ventilated environment.

Fig. 1. A diagram of the electrolysis of brine.
[+ marks the anode, – the cathode]

To electrolyse brine (see Fig 1), connect the two leads to the battery terminals, WITHOUT THEM TOUCHING, and put them into the brine (a saturated salt solution). Salt (sodium chloride, NaCl) ionises in solution into the ions Na+ and Cl-. Water (H2O) also ionises a little into H+ and OH-. This process is called electrolysis (splitting with electricity).

The reactions at the leads, which are functioning as our electrodes, are as follows:

You will observe some bubbling at the electrodes. If you do, congratulations – it’s working! The gases chlorine (Cl­­₂) and oxygen (O₂) are produced at the anode, and hydrogen gas (H₂) at the cathode, hence the bubbling. Give yourself a pat on the back if you’ve gotten this far without blowing up your kitchen.

Chlorine gas, however, partially dissolves in water. Understanding the various dynamic equilibria taking place (indicated by the double arrows) will help us to explain many things about bleach. Looks complicated, doesn’t it? Don’t scream your head off just yet. As I said, this is a Hitchhiker’s Guide. DON’T PANIC.

(vii)      Cl₂ (g) ⇌ Cl₂ (aq)

(viii)     Cl₂ (aq) + H₂O (l) ⇌ HOCl(aq) + H⁺ (aq) + Cl^– (aq)

(ix)        HOCl (aq) ⇌ OCl^– (aq) + H⁺ (aq)

HOCl (aq), hypochlorous acid, is the active antimicrobial ingredient in bleach. It is a weak acid: a small proportion (around 1 in 5800) of HOCl dissociates into the ions OCl^– and H⁺ (ix). Household bleach is a solution of sodium hypochlorite, NaOCl (really Na⁺ and OCl^– in aqueous solution).

A dynamic equilibrium is a state in which reactants and products transition between each other at equal rates, so there is no net change in concentration. The position of the equilibrium shifts to COUNTERACT any change in the environment. This is Le Chatelier’s Principle. (I will keep referring back to this as LCP.) Two simple examples using (vii), which describes chlorine transitioning between aqueous and gaseous states, are given below.

(1) If I leave the cap on a bottle of bleach open, chlorine gas is allowed to escape. According to LCP, the loss of chlorine gas has to be counteracted by producing more chlorine gas. In other words, the forward reaction must be favoured – the position of equilibrium shifts right. If the bottle remains open for a long time, all of the chlorine will eventually escape.

(2) For chlorine to be able to escape from the binds of water in solution, energy must be put in. In other words, the forward reaction on its own would decrease the temperature of its surroundings, as it siphons energy from its surroundings. If the temperature of the surroundings is increased, the position of the equilibrium shifts towards the RIGHT, favouring the forward reaction, to counteract the increase in temperature. LCP strikes again.

The loss of chlorine gas, as described in the two examples above, will also lead to the depletion of HOCl (see v), the active antimicrobial ingredient, because of LCP. This is why bleach should be stored in a cool place away from sunlight to maximise its lifetime. However, even with the best care, bleach will eventually expire, as chlorine will inevitably escape from the system. Any bleach over a year old should be replaced; even so, you should be able to tell if your bleach has expired if you do not smell anything when you pour it from its container. (It is also recommended that you replace 1:99 bleach dilutions with a fresh batch every 24 hours.)

(viii) describes the transition between chlorine gas and HOCl. Notice the H⁺ on the right: the higher concentration of H⁺, the more acidic (i.e. lower pH) the solution. If H⁺ is removed, then LCP dictates that the equilibrium shifts right to counteract the loss of H⁺. This is why sodium hydroxide, a base (i.e. H⁺ acceptor), is added to remove H⁺ and favour the production of HOCl on the right side of the equilibrium.

As useful as bleach is, it is an incredibly hazardous chemical in the wrong hands. There have been too many bleach-related accidents which could have been prevented by a little chemistry knowledge (and reading the labels). On 27^th December, 2018, a mother had a rather unpleasant task of dealing with a clogged toilet (which was probably because of her child’s toy). She used two bottles of drain cleaner; seeing no improvement, she then poured three litres of bleach down the drain later that evening. The obnoxious cloud of gas filled the house, forcing the whole neighbourhood to evacuate. On 7^th November, 2019, a worker at a Buffalo Wild Wings in Massachusetts cleaned the floor with bleach, oblivious to a spillage of acid-based cleaner. The mixture turned green and started bubbling, producing deadly fumes which killed the manager and injured ten others.

The lethal gas produced in both of these cases is chlorine, Cl₂. On that fateful day of 22^nd April, 1915, the Germans unleashed 170 metric tons of chlorine gas on the French line, marking the start of the Second Battle of Ypres. The French fled in all directions; up to 1400 French troops succumbed. Victims died a slow and painful death by asphyxiation as chlorine rips mercilessly into the respiratory organs:

“It produces a flooding of the lungs – it is an equivalent death to drowning only on dry land. The effects are these – a splitting headache and terrific thirst (to drink water is instant death), a knife edge of pain in the lungs and the coughing up of a greenish froth off the stomach and the lungs, ending finally in insensibility and death. The colour of the skin from white turns a greenish black and yellow, the colour protrudes and the eyes assume a glassy stare. It is a fiendish death to die.”

– Lance Sgt. Elmer Cotton

Thankfully, the use of chlorine in international armed conflicts, along with other chemical and biological weapons, is now prohibited under the Geneva Protocol. However, this does not mean that we are forever safe; the danger now lurks in our own homes. Bleach, which contains sodium hypochlorite (NaOCl) as established before, has the potential to release large amounts of chlorine gas quickly in reactions with seemingly innocuous household chemicals.

Acidic drain cleaners usually contain concentrated acids, such as sulphuric acid (H₂SO₄), hydrochloric acid (HCl) or nitric acid (HNO₃). Acids are proton donors: they give up protons, H⁺, in reactions.

HCl → H⁺ + Cl^–

H₂SO₄ → H⁺ + HSO₄^–

HNO₃ → H⁺ + NO₃^–

Above are three common acids (from top to bottom: hydrochloric acid, sulphuric acid, nitric acid) giving up a proton (the H⁺ on the right). The proton can be accepted by a base in a neutralisation reaction to form water and a salt. (x), for example, describes HCl, the main component of gastric acid, being neutralised by milk of magnesia (magnesium hydroxide), a common antacid.

(x)         2 HCl + Mg(OH)₂ → MgCl₂ + H₂O

 hydrochloric acid + magnesium hydroxide → magnesium chloride + water

Acids break down fats and proteins by acid hydrolysis; sulphuric acid will also dehydrate carbohydrates (like cellulose in toilet paper). As mentioned earlier, the higher the H⁺ concentration, the more acidic the solution. Sulphuric acid dissociates H⁺ ions when dissolved in water. Acidic drain cleaners contain concentrated sulphuric acid, so the H⁺ concentration will be high. Look back at (ii), and think LCP: which way will the equilibrium shift?

If you answered ‘towards the left’, then congratulations. To counteract the addition of H⁺, the equilibrium swings left, favouring the production of Cl₂. The sudden addition of a lot of H⁺ in the form of concentrated acid will therefore lead to the release of large amounts of chlorine gas. This applies not only to drain cleaners, but also vinegar (ethanoic acid, CH₃COOH), a popular household disinfectant (e.g. killing molds). All is not lost, however, if you accidentally mix small quantities of bleach and acid in the bathroom. Simply closing the door and turning on the ventilator will be enough to prevent the accumulation of too much chlorine gas.

Best wishes. And COVER YOUR FACE.

Adrian Tsui

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Glossary

A-D

Acid: a proton donor. Solutions of acids typically have pH <7. A weak acid does not dissociate completely, but a strong acid does. Also see base, dissociation, proton and hydrolysis.

Amphipathic: describes a molecule which has both hydrophilic and hydrophobic regions.

Base: a proton acceptor. Solutions of bases, called alkalis, typically have pH >7. Also see acid, proton, and hydrolysis.

Carbohydrate: a molecule containing carbon, hydrogen and oxygen, general formula C_mH_nO_2n. Examples include glucose, C₆H₁₂O₆; sucrose, C₁₂H₂₂O₁₁; starch and glycogen, both polymers of glucose.

Cracking: the breakdown of longer hydrocarbons into shorter ones. Requires high temperatures. This is how kerosene, petroleum etc are made.

Dissociation: when compounds split into smaller units such as ions. For example, acids undergo dissociation to form protons, and salts undergo dissociation to form cations (positive) and anions (negative). Also see acid and proton.

Dynamic equilibrium: a state in which reactants and products transition between each other at equal rates, so there is no net change in concentration. Obeys Le Chatelier’s Principle. Also see Le Chatelier’s Principle.

E-H

Electrode (in electrolysis): a solid electric conductor used to make contact with the electrolyte. Also see electrolysis.

Electrolysis: the process of passing a direct current through an electrolyte to drive the non-spontaneous chemical breakdown of a compound.

Gene: the basic functional unit of heredity; a sequence of genetic material which codes for a protein.

Hydration: a reaction involving the addition of water.

Hydrocarbon: a molecule made up of hydrogen and carbon only.

Hydrolysis: chemical breakdown of a compound due to reaction with water. Can be catalysed by the presence of acid or base (hence acid/base hydrolysis). Also see acid and base.

L-S

Le Chatelier’s Principle: The position of a dynamic equilibrium shifts to counteract any change in the environment. Also see dynamic equilibrium.

Metabolism: describes all chemical reactions involved in sustaining life in cells and the organism.

Neutralisation: the reaction between an acid and a base. Typically produces a salt and water. Also see acid, base, and salt.

Proton: a hydrogen ion, H⁺. Acids donate protons, while bases accept protons. See acid and base.

Salt (chemistry): an electrically neutral ionic compound. Forms crystals. Not to be confused with table salt.

Table salt: sodium chloride, NaCl. Not to be confused with salt.