Mixing of incompatible materials (chemicals or wastes) can result in excessive heat, over pressurization, fire or other dangerous situations. If you plan to mix chemicals or wastes in a waste container or group them in a bag, tray or bucket, you must first determine whether any dangerous situations may result.

Consult the Stanford Chemical Safety Database to determine the Storage or Compatibility Group of chemicals intended for mixing. While this tool is intended for use with pure chemicals, diluted materials may exhibit the same characteristics. Additional information is available from The American Chemical Society – Incompatible Chemicals.

Below are some notable situations which have occurred when incompatible materials are mixed:

1. Acids and bases (ex: hydrochloric acid and ammonium hydroxide) generally results in generation of excessive heat, including boiling over. If the mixture boils over, it may result in serious injury.
2. Acids and bleach, azides, cyanides, sulfides, metals, or carbides can result in the generation of toxic fumes. (Acid + Cyanide = HCN (gas)).
3. Nitric or perchloric acid (even when dilute) and any organic material such as ethanol, acetic acid, or oil can result in excessive heat or a fire.
   1. Perchloric acid can react with wood or paper to form cellulose perchlorate which can spontaneously combust.
4. Acetic acid, Acetic Anhydride, and Formic Acid are a special class of chemicals as they are both an acid and organic. In concentrated form, these materials are flammable. They should not be stored or mixed with any mineral acids.
5. Peroxides and organics or metals, such as; hydrogen peroxide and ethanol, aluminum, or copper can result in a fire.
6. Inorganic nitrate salts or bases and organics can form highly unstable compounds which may detonate.
   1. Mixing silver nitrate and ammonia with sodium or potassium hydroxide can form explosive “fulminating silver”.
   2. Mixing silver nitrate and ethanol has resulted in serious fires.
7. Ammonium nitrate or hydroxylamine nitrate and organic material can result in an explosive compound.
8. Potassium permanganate and sulfur has resulted in flash fires.
9. Sodium or potassium chlorate and organics has resulted in explosions.
10. Nitrobenzene and aluminum or tin chloride and organics may result in an explosion.
11. Nitromethane waste must be handled separately.
    1. Never mix nitromethane with bases or amines,
    2. Or with metals or metal compounds.
12. Piranha solution and any organic material or metals can react violently and result in a fire or overpressurization. Piranha solutions in contact with paper products have resulted in trash can fires. Always accumulate Piranha waste in a plastic coated glass bottle with vented cap, and wait until cool before closing. Piranha waste should be handled separate from all other waste streams.
13. Azides and metals can combine to generate shock sensitive salts which can detonate.
14. Activated Platinum group metals on carbon and metal hydrides have caused several lab fires.
15. Chloroform and acetone with a base may react violently.
16. Monomers can self-react and violently generate pressure or fire. Examples are methyl methacrylate, acrylic acid and acrolein.
    1. They are sold with an inhibitor to prevent this reaction. Inhibitors must be monitored to maintain their activity.
17. Monomers and iron, acids or even water can result in violent reactions.
18. Acetic Anhydride and water, glycols or alcohols will result in a violent reaction and possibly a fire.

Links to EH&S Tools
Bleach Fact sheet
Information on peroxide forming compounds
Information on Azide Compounds
Information on Stanford’s Storage Groups
Stanford’s Chemical Waste Poster

Definitions
Group C; Inorganic Bases: Chemicals that are corrosive to metals or skin.
Examples: Sodium Hydroxide, Potassium Hydroxide
Group F; Inorganic Acids: Chemicals that are corrosive to metals or skin.
Examples: Hydrochloric Acid, Hydrofluoric Acid
Group E; Oxidizing Chemicals: Chemicals that will very often react violently with organics.
Examples: Hydrogen Peroxide, Ammonium Persulfate,
Group I: Oxidizing Acids: Inorganic acids that will often react violently with organics
Examples: Nitric Acid, Sulfuric Acid, Chromic Acid, Perchloric Acid
Group G or Group L: Organic chemicals: Chemicals containing carbon, many are flammable
Examples: Alcohols, solvents like Hexane or Dichloromethane, Acetonitrile, Oil