Source: Bartlett Group SOPs, University of Michigan Department of Chemistry
Tags: strong oxidants, aqua regia, piranha solution, HNO3, HCl, H2SO4, H2O2, corrosive reagents, fume hood, PPE, spill response
Difficulty: Intermediate | Prerequisites: Acid/Base Bath Safety notes, basic PPE knowledge
Strong oxidants, aqua regia, and piranha solution are among the most hazardous reagents in a chemistry lab. Oxidants can form explosive mixtures with organics; aqua regia dissolves noble metals but produces toxic fumes and must never leave the acid fume hood; piranha solution (concentrated H2SO4 + H2O2) is violently reactive with any organic material and can exceed 100 °C on mixing. All three demand dedicated fume hood work, specific PPE beyond standard lab gear, and strict protocols for preparation, use, and disposal.
Strong oxidising agent (strong oxidant)
A chemical that spontaneously evolves oxygen at room temperature or with slight heating, or that promotes combustion. Examples include peroxides, chlorates, perchlorates, nitrates, and permanganates. Capable of forming explosive mixtures with combustible, organic, or easily oxidised materials.
In simple terms, these chemicals want to give oxygen away, and if they find anything flammable nearby, the result can be fire or explosion.
Aqua regia
A corrosive, fuming yellow liquid made by slowly mixing one volume of concentrated nitric acid (HNO3) with three volumes of concentrated hydrochloric acid (HCl). Dissolves noble metals such as gold, platinum, and silver.
Think of it as the acid mixture strong enough to dissolve gold, which is why its Latin name means "royal water."
Nitrosyl chloride (NOCl)
One of the reactive gas products formed when HNO3 and HCl react in aqua regia. Along with Cl2, it gives aqua regia its yellow colour and fuming character. Both gases are toxic and volatile.
Piranha solution (acid piranha)
A 3:1 mixture of concentrated sulfuric acid (H2SO4) and hydrogen peroxide (H2O2). Used to strip organic residues from substrates. Reaches temperatures above 100 °C upon mixing and is potentially explosive if it contacts organic material.
In simple terms, piranha eats organic matter, and if you give it fuel (acetone, isopropanol, or any organic solvent), it can explode.
Base piranha
A 3:1 mixture of ammonium hydroxide (NH4OH) with hydrogen peroxide (H2O2). Requires heating to 60 °C to activate. Used for similar cleaning purposes as acid piranha.
Calcium gluconate gel
A topical first-aid treatment applied to skin burns from hydrofluoric acid. Also referenced in piranha emergency procedures for acid burns. Must be checked for expiry before each use of HF.
Engineering and work practice controls
All work with strong oxidising agents must be conducted in a fume hood with the sash kept low.
Minimise quantities on hand. Use small amounts to control heat generation.
Evacuated glassware under vacuum with oxidising chemicals must be in a fume hood, glovebox, or otherwise isolated.
Mechanical vacuum pumps need cold traps and, where appropriate, particulate filters. Pump exhaust vents into an exhaust hood.
PPE
ANSI Z.87.1-compliant safety glasses with side shields at all times. Side shields alone are insufficient against splash hazards; full goggles or face protection are needed when splashing is possible.
Nitrile gloves for routine handling; consult EHRS for chemical-resistant glove selection for prolonged contact.
Lab coat, closed-toed shoes, long sleeves.
Storage
Cool, dry location, segregated from all other chemicals.
Never on wooden shelves or in wooden cabinets.
No corks or rubber stoppers.
Never return excess chemical to the original container.
Spill response
Have inert spill-control materials on hand (consult the SDS beforehand). Never use paper towels or combustible absorbents.
Small spills: clean with inert materials, dispose of contaminated waste promptly.
Large spills: do not attempt cleanup. Vacate the lab, call 911, remain nearby to direct safety personnel.
Preparation
Add 1 part HNO3 to 3 parts HCl, pouring slowly inside the acid fume hood.
An orange/red colour and gas bubbles indicate the reaction is progressing.
Prepare only the amount needed. The reaction starts on mixing, and excess increases both safety risk and wasted reagent.
Handling rules
The acid fume hood is dedicated to aqua regia cleaning only.
Glassware must be cleaned with soap and water to remove organics before aqua regia contact.
Never bring organic chemicals into the hood; never remove aqua regia from the hood.
No metal items (stopcock holders, spatulas) in the acid hood.
Never place aqua regia in a closed container. Gas generation will build dangerous pressure.
When not actively working, keep the fume hood sash down and closed.
First aid
Inhalation: move to fresh air immediately; artificial respiration if breathing has stopped.
Ingestion: have the conscious person drink large volumes of water; never induce vomiting; never give fluids to an unconscious person.
Skin: wash promptly with water; remove soaked clothing.
Eyes: flush with water for at least 15 minutes.
For serious injuries, call 911.
Spill response
Spills over 200 mL anywhere, or any spill outside the fume hood: evacuate, call 911.
Spills under 200 mL in the hood with reaction complete (no bubbles): neutralise with acid spill kit, absorb with pig mat.
If reaction is still ongoing: close the sash, wait for completion, then clean.
Disposal
Place in properly labelled glass waste containers inside the acid fume hood.
Never fill waste containers more than halfway. Never fully tighten the cap (gas generation continues).
Only aqua regia in aqua regia waste containers. It is a strong oxidiser and will react with organic chemicals.
Access restrictions
Only permanent Bartlett lab members (graduate students and post-docs) may prepare or use piranha. Undergraduates and rotation students must not handle it and should ask a permanent member for assistance.
PPE (beyond standard)
Full goggles, heavy-duty rubber gloves (not nitrile), acid apron worn over the lab coat.
Preparation and handling
Another lab member must be present and aware.
Use only glass containers (preferably Pyrex). Label clearly. Post a visible warning sign on the fume hood.
Mix in the fume hood with the sash between you and the solution. Verify no organics are present in or near the hood.
Always add the peroxide to the acid (H2O2 to H2SO4), never the reverse. The reaction is immediately exothermic.
Keep H2O2 concentration at the 1:3 ratio. If H2O2 concentration reaches 50% or greater, an explosion can occur.
Substrates go in rinsed with water and dried. Never rinse with acetone before piranha contact.
After cleaning, wash substrates with water into the waste jar.
What can go wrong
Contact with any organic material (acetone, isopropanol, nylon) can cause an explosion.
Adding acids, bases, or water to piranha accelerates the reaction uncontrollably.
A closed container of hot piranha will explode from gas pressure.
Waste disposal
Leave hot piranha in an open, clearly labelled container to cool for several hours or overnight.
Once cool, transfer to a special waste bottle with a vented cap (obtained from the waste management contact).
Piranha waste must never go in regular waste containers.
Emergency procedures
Large exposure: remove victim to safety shower, call 911.
Skin contact: rinse with water for at least 15 minutes.
Eye contact: irrigate for at least 30 minutes; ice pack on eyes until reaching the emergency room.
Inhalation: move to fresh air; seek medical attention for any respiratory symptoms.
"Paper towels are fine for cleaning up an oxidiser spill." Paper towels are combustible. Contact with strong oxidisers can start a fire. Use only inert, non-combustible spill-control materials.
"Aqua regia can be stored in a sealed bottle for later use." Aqua regia continuously generates gas after mixing. A sealed container will pressurise and can rupture. Always keep it open and in the fume hood.
"You can rinse a substrate with acetone before placing it in piranha." Piranha reacts violently with organic compounds, including acetone. Water must always be the last rinse before piranha contact.
"Hot piranha can go straight into a waste bottle." Hot piranha in a sealed container will explode from gas pressure. It must cool in an open, labelled container for several hours before transfer to a vented waste bottle.
⚠️ Know the composition and mixing ratio for aqua regia (1:3 HNO3:HCl) and piranha (1:3 H2O2:H2SO4), and in both cases which component is added to which.
⚠️ The principle that strong oxidisers must be segregated from all other chemicals, stored in cool, dry locations away from combustibles, and never on wooden surfaces.
⚠️ Piranha access restrictions: only permanent lab members, never undergraduates or rotation students.
⚠️ The 200 mL threshold for aqua regia spills: above 200 mL or any spill outside the hood requires evacuation and 911.
⚠️ Piranha waste requires a vented-cap bottle, not standard waste containers.
Aqua regia is prepared by adding ___ part(s) HNO3 to ___ part(s) HCl. (1 part HNO3 to 3 parts HCl.)
True or false: Piranha solution may be stored in a tightly sealed container once it has cooled. (False, it must use a vented-cap waste bottle.)
What colour change in a spent oxidiser spill-absorbent indicates a fire risk? (Trick question: you should never use combustible absorbents at all; use inert materials only.)
True or false: Standard nitrile gloves are sufficient PPE for handling piranha solution. (False, heavy-duty rubber gloves are required.)
The yellow colour and fumes of aqua regia are caused by ______ and ______. (Nitrosyl chloride, NOCl, and chlorine, Cl2.)
Q: Why must aqua regia never be placed in a closed container?
A: The reaction between HNO3 and HCl continuously generates gaseous products (NOCl and Cl2). In a closed container, the pressure builds until the container ruptures or explodes.
Q: What is the correct procedure if more than 200 mL of aqua regia is spilled anywhere in the lab?
A: Call 911 immediately to activate the spill response team. If the spill is outside the fume hood regardless of volume, evacuate the lab and call 911.
Q: Describe the composition and hazards of acid piranha solution. Why is the order of addition critical?
A: Acid piranha is a 3:1 mixture of concentrated H2SO4 to H2O2. The H2O2 is added to the H2SO4 (never the reverse) because the mixing immediately produces a highly exothermic reaction that can exceed 100 °C. At H2O2 concentrations of 50% or greater, an explosion can occur, which is why only the 1:3 ratio is permitted.
Q: Why must a substrate be rinsed with water, not acetone, immediately before going into a piranha bath?
A: Piranha reacts violently with organic compounds. Acetone is organic, so any residual acetone on the substrate could cause a violent reaction or explosion on contact with the piranha solution.
Q: Name three examples of strong oxidising agents commonly found in a chemistry lab.
A: Any three from: hydrogen peroxide, potassium permanganate, chromic acid, sodium perchlorate, ammonium perchlorate, perchloric acid, bromine, calcium hypochlorite, potassium chlorate, among others.
These reagents build on the acid/base handling fundamentals covered in the Acid and Base Bath study notes. The PPE escalation pattern is worth noting: base bath requires neoprene gloves, strong oxidants call for nitrile plus chemical-resistant selection, aqua regia demands goggles plus an apron, and piranha requires the heaviest protection of all (full goggles, heavy rubber gloves, acid apron).
The spill-response principles here (inert absorbents, evacuation thresholds, 911 protocols) carry over directly to the Hydrofluoric Acid SOP and compressed gas emergency procedures.
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