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Acids and alkalis are among the most common chemicals in any laboratory, and among the most common causes of lab injuries. Nearly all of those injuries are preventable with a few consistent habits. This guide covers what those habits are and why each one matters.
It’s written for school and college labs, but the principles are the same in research and industry. Always follow your own institution’s rules and risk assessments, which take priority over general advice.
Know the hazard labels
Chemical containers use the GHS (Globally Harmonised System) pictograms. For acids and alkalis, the most common are:
| Pictogram | Meaning | Typical examples |
|---|---|---|
| Corrosion (liquid eating into a hand and a surface) | causes severe skin burns and eye damage; may corrode metals | concentrated HCl, H₂SO₄, HNO₃, NaOH, KOH |
| Exclamation mark | irritant, harmful | dilute acids and alkalis at moderate concentrations, ammonia solution |
| Skull and crossbones | acute toxicity | hydrofluoric acid, some concentrated acids and bases |
| Flame over circle | oxidiser | concentrated nitric acid, perchloric acid |
| Health hazard (person with a star-shaped chest) | serious long-term health effects | some chromium(VI) compounds |
Concentration changes the hazard. 1 mol/dm³ hydrochloric acid is labelled as an irritant in many schools’ guidance, while concentrated (about 12 mol/dm³) hydrochloric acid is corrosive. Always read the label on the bottle you’re actually using.
Personal protective equipment (PPE)
- Eye protection is the single most important item. Safety spectacles are a minimum; goggles or a face shield are needed when working with corrosive concentrations or where splashing is likely. Eye damage from alkalis in particular can be permanent.
- Lab coat, fastened, to protect clothes and skin.
- Gloves when handling corrosive or concentrated solutions. Nitrile gloves resist most dilute acids and alkalis; some concentrated acids need specialist gloves. Gloves are no use if they’re the wrong type or have holes.
- Closed-toe shoes, and long hair tied back.
Working safely
- Use the lowest concentration that does the job. Most school experiments work with 0.1–1 mol/dm³ solutions. Concentrated acids should be handled only by teachers or technicians.
- Work at a tidy bench, with bottles away from the edge and stoppers replaced immediately.
- Carry bottles with two hands, or in a bottle carrier. Never carry them by the neck.
- Pour carefully, with the label facing up (so drips don’t damage it), and over a tray if possible.
- Don’t pipette by mouth. Always use a pipette filler.
- Use a fume cupboard for concentrated acids that give off fumes (concentrated HCl, HNO₃, ammonia solution) and for any reaction that produces toxic gases.
- Never taste or directly smell chemicals. To detect an odour, waft a little towards your nose.
Diluting acids: always add acid to water
Diluting concentrated acids, especially sulfuric acid, releases a large amount of heat. The rule is:
Add acid to water, slowly, with stirring. Never add water to acid.
Why? If you add a small amount of water to a large volume of concentrated acid, the water is heated very quickly by the reaction and can boil instantly, spraying hot concentrated acid out of the container. If you add acid slowly to a large volume of water, the heat is spread through the water, which absorbs it safely.
A memory aid some people use: “Do what you oughta, add acid to water.”
The same principle applies to dissolving solid sodium hydroxide: add the solid slowly to cold water with stirring. It releases a lot of heat and can boil locally.
Storage
- Store acids and alkalis separately, in low cabinets (never above eye level), ideally in trays that contain spills.
- Keep oxidising acids (nitric, perchloric) away from organic materials and other acids.
- Keep sodium hydroxide solutions in polyethylene or rubber-stoppered bottles: they attack glass slowly and can seal glass stoppers in place.
- Keep hydrofluoric acid only in plastic, and only where specifically authorised.
- Label every container clearly with the name, concentration, hazard and date.
- Never store chemicals in food or drink containers.
Incompatible mixtures to avoid
| Don’t mix | Because |
|---|---|
| Acids with bleach (sodium hypochlorite) | releases toxic chlorine gas |
| Ammonia with bleach | forms toxic chloramines |
| Concentrated acids with concentrated alkalis (directly) | violent, heat-producing neutralisation; splashing |
| Nitric acid with organic solvents or metal powders | can react violently or explosively |
| Acids with cyanide or sulfide salts | release highly toxic HCN or H₂S gas |
| Acids with carbonates in a sealed container | CO₂ pressure can burst it |
What to do if something goes wrong
Always tell your teacher or supervisor immediately, however small the incident seems.
Skin splash
- Remove contaminated clothing (carefully, avoiding spreading the chemical).
- Rinse the skin under plenty of cool running water for at least 10–20 minutes.
- Seek medical advice for anything more than a trivial splash of a dilute solution.
Don’t try to “neutralise” an acid on skin with an alkali, or vice versa. The neutralisation reaction releases heat and can make the injury worse. Water is the correct first aid.
Eye splash
- Rinse immediately at an eyewash station or with running water, holding the eyelids open.
- Keep rinsing for at least 15–20 minutes, and longer for alkalis.
- Get urgent medical attention. Eye injuries from alkalis in particular can worsen over hours.
Swallowing
Rinse the mouth with water and get urgent medical help. Don’t induce vomiting.
Inhaling fumes
Move to fresh air and get medical advice if there’s coughing, breathlessness or chest pain.
Spills
- Small dilute spills: wipe up with plenty of water and paper towels, wearing gloves.
- Larger or concentrated spills: clear the area, warn others, and let trained staff deal with it using a spill kit. Acid spills are often neutralised with sodium hydrogencarbonate or a commercial neutraliser, added carefully from the edges inwards (it fizzes); alkali spills with a weak acid such as citric acid.
- Never add water directly to a large spill of concentrated sulfuric acid.
Specific hazards of common reagents
| Reagent | Special hazard |
|---|---|
| Concentrated sulfuric acid | dehydrating agent; chars skin and organic materials; very exothermic dilution |
| Concentrated nitric acid | strong oxidiser; releases toxic brown NO₂ fumes; stains skin yellow |
| Concentrated hydrochloric acid | releases corrosive HCl fumes |
| Sodium and potassium hydroxide | severe eye damage; slippery on skin (turning skin fats into soap); exothermic dissolving |
| Ammonia solution | irritant fumes; can damage eyes |
| Glacial ethanoic acid | corrosive; flammable vapour |
| Hydrofluoric acid | penetrates skin and causes deep, delayed damage and systemic poisoning; see hydrofluoric acid |
Risk assessments
Before any practical, a risk assessment identifies the hazards of each chemical, who could be harmed and how, and what control measures reduce the risk (lower concentrations, smaller volumes, PPE, fume cupboards). Many schools use guidance from national organisations such as CLEAPSS in the UK or equivalent bodies elsewhere. Reading the risk assessment before starting is part of doing the practical properly.
Key takeaways
- Eye protection is essential whenever acids or alkalis are in use.
- Use the lowest concentration that works, and handle concentrated reagents only with proper supervision.
- Always add acid to water, never water to acid.
- Store acids and alkalis separately, labelled, low down, and never mix bleach with acids or ammonia.
- For splashes, rinse with plenty of water for 10–20 minutes (15–20 or more for eyes) and report every incident.
- Learn the chemistry behind the hazards in what is an acid? and what is a base?.
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