Bases - Hydroxides

Background and Overview of Hazards

Solutions of hydroxides (OH-) in water form strong bases with a high pH. They are highly corrosive to the skin and eyes. Strong bases are insidious in that even a concentrated solution often causes no pain until the corrosive damage is severe.

Concentrated hydroxide solutions evolve heat when mixed with water or acids, which may cause boiling and splattering. 

Alkali metal hydroxides

The hydroxides of alkali metals (Li, Na, K) and calcium in their pure form are water soluble, hygroscopic solids. Dissolving solid hydroxides in water is a highly exothermic process that generates excessive heat. Sodium and potassium hydroxides are strong bases and are extremely destructive to the skin and eyes.

Concentrated hydroxide solutions dissolve aluminum, resulting in the evolution of flammable hydrogen gas. Hydroxides slowly attack glass forming soluble silicates. This reaction causes glass joints and stopcocks to “freeze.” 

Hydroxide solutions rapidly react with carbon dioxide in the air, forming bicarbonates (HCO3-) for alkali hydroxides, or carbonates (CO32-) for calcium hydroxide, respectively, and decreasing the pH of the solution when stored open to air.

Ammonium hydroxide (NH4OH)

Ammonium hydroxide is a solution of ammonia (NH3) in water. Only a small fraction actually exits as ionic NH4+ and OH-. Concentrated ammonium hydroxide contains approximately 29% ammonia and has a pungent odor. With increasing temperature, more ammonia evaporates. Ammonia gas is flammable and has a narrow explosive range in air of 15–25%. The vapors of aqueous ammonia solutions are irritating and toxic.

Tetramethylammonium hydroxide (TMAH)

Tetramethylammonium hydroxide pentahydrate is a solid that is highly toxic if ingested and toxic by skin contact. An aqueous TMAH solution has a high acute oral and dermal toxicity. Skin contact of a solution of >1% TMAH to a small area of the body surface or ingestion can be fatal. 

Safe Handling

  • When handling strong bases, at a minimum wear standard laboratory attire: closed-toe shoes, long pants, a lab coat, safety glasses with side shields or splash goggles, and gloves. 
  • When handling large amounts (>500 mL), or when splashing is likely to occur, wear additional protection to prevent skin contact, especially when using TMAH: A face shield above splash goggles, base-resistant gloves with long cuffs, and a base-resistant apron or smock. 
  • When preparing solutions from solid hydroxides, the container has to be cooled with ice or under cold water. The container should be swirled constantly until the pellets are dissolved to prevent them from getting stuck to the bottom where they can generate excessive heat, possibly leading to container rupture.
  • Always dilute base solutions by adding base to water and not water to base. Adding water to concentrated bases may cause violent boiling of the solution and splashing.
  • Ammonium hydroxide should be handled in a chemical fume hood to avoid breathing ammonia gas.

Emergency Procedures

Accidental Exposure

Skin Contact

Remove contaminated clothing and rinse the affected skin immediately with copious amounts of water for 15 minutes or until pain is relieved. Seek medical attention if the skin is irritated or pain is felt. 

If you were exposed to TMAH, ALWAYS seek medical attention after first aid is performed, even if no pain is felt.

Eye Contact

Use the eye wash to rinse the eye thoroughly for at least 15 minutes, occasionally lifting upper and lower eyelids and rolling the eyeballs. Seek medical attention.

Inhalation

Move into fresh air immediately. Seek medical attention.

Ingestion

Do not induce vomiting. Rinse the mouth with water. Seek medical attention.

Provide the medical team with the Safety Data Sheet SDS for the base.

Spills

Clean spills immediately. Have a spill kit readily available before working with concentrated bases. We recommend solid citric acid, tartaric acid, or sodium bisulfate for neutralizing spills. Commercial spill kits are also available.

Neutralize the spill first by covering it with neutralizer, then sweep up the neutralized material with an absorbent pads or a broom. 

In the case of a large spill of a concentrated base outside the hood, or if the spill cannot be contained, evacuate the area immediately, alerting others nearby. Close the door and keep people from entering. Call 911 immediately.

Storage

Store bases separated from acids and label the cabinet “bases-corrosive.” Use secondary containment for aqueous base solutions and other strong liquid bases. Hydroxides must be separated from oxidizers. 

NEVER store hydroxide solutions in metal containers because of the possibility of hydrogen gas evolution, container leakage, and rupture.

Waste Disposal

Keep base waste separate from other waste streams, and use secondary containment for the waste container. Dispose of waste through the DRS chemical waste program. 

If the waste solution does NOT contain any hazardous metals, consider elementary neutralization for alkali hydroxides, calcium hydroxide, and ammonium hydroxide:

Neutralize with 1N hydrochloric acid. This solution can be prepared by mixing concentrated hydrochloric acid (37% = 12M) 1:12 with water. For preparing 1L of 1N HCl, add 82 mL of concentrated HCl to water.

Neutralization procedure for bases

Use a container large enough for diluting the base and adding the neutralization agent. It should be at least four times the size of the volume of the diluted base to allow safe stirring. A polyethylene bucket can be used to handle large quantities.

  1. Dilute the base to a 10% concentration (by weight) or less. For concentrated bases, simply dilute them 1:10 with water, or pour the base solution onto ice.
  2. Slowly add the neutralization agent while stirring the solution.
  3. Monitor pH changes with a pH meter or pH paper.
  4. Continue to add neutralization reagent until a pH between 6 and 10 is reached.

The neutralized solution can be poured down the drain if it does NOT contain any heavy metals, flammables, or other compounds that are hazardous to humans or the environment. See Disposal of Liquid Non-Hazardos Chemicals for more information.

References

Safety in Academic Chemistry Laboratories. 7th edition 2003. American Chemical Society

Lin, C-C, et al. Tetramethylammonium hydroxide poisoning Clinical Toxicology (2010) 48, 213–217. DOI: 10.3109/15563651003627777

Last Update: 4/21/2015