Mineral Acids

This safety guide provides information about common mineral acids such as hydrochloric, nitric, sulfuric, and phosphoric acid. It does NOT cover the hazards of hydrofluoric or perchloric acid. Refer to the specific DRS safety guides for those two acids.

Background and Overview of Hazards

Mineral acids are water-soluble acids derived from inorganic minerals. They are highly corrosive to the skin and eyes. All concentrated acids react violently with water and bases, evolving heat.

Hydrochloric acid (HCl)

Concentrated hydrochloric acid is a solution of about 38% or 12 M hydrogen chloride (HCl) in water. It has a pungent odor that is corrosive to the respiratory system and eyes. Hydrochloric acid is not classified as an oxidizer or reducer. However, when mixed with an oxidizer such as sodium hypochlorite (bleach) or potassium permanganate, it will evolve toxic chlorine gas. 

As a non-oxidizing acid, HCl attacks and dissolves most non-noble metals, liberating flammable hydrogen gas.

Nitric Acid (HNO3)

Nitric acid is supplied as concentrated acid (68-70%, 16M), fuming strength (>86%), and as anhydrous form (100%). It is a strong oxidizer even when fairly dilute and at room temperature. It oxidizes most organic compounds while it is reduced to nitrous fumes. Anhydrous nitric acid can form highly explosive mixtures with virtually any organic compound. 

The most commonly used 68-70% concentrated nitric acid reacts violently with organic material, resulting in gas evolution and potential pressure buildup, followed by vessel rupture if the container is not vented adequately. Oxidation reactions with some organic solvents can form highly explosive nitrates.

Nitric acid reacts with most metals, liberating either hydrogen gas or nitrogen oxides depending on concentration and metal. It does not dissolve gold or platinum.

Mixing nitric acid with hydrochloric acid (Aqua Regia) will evolve brown fumes consisting of toxic nitrogen oxides. Read the safety guide for Aqua Regia before preparing such a solution.

Nitric acid causes yellow stains on skin.

Sulfuric Acid (H2SO4)

Concentrated sulfuric acid is often supplied as ~98% (18 M). It is a strong oxidizer, hygroscopic, and a strong dehydrating agent. It chars materials made from carbohydrates, such as cotton, paper towels, and wood, by dehydration.

Dilute sulfuric acid reacts with metals like other mineral acids, liberating hydrogen gas. Concentrated sulfuric acid can also dissolve some noble metals such as copper, silver, and mercury liberating sulfur dioxide (SO2). Lead and tungsten do not react with sulfuric acid.

Due to its strong oxidizing and dehydrating power, concentrated sulfuric acid reacts violently with many organic chemicals, resulting in gas evolution.

Phosphoric Acid (H3PO4)

Pure orthophosphoric acid is a water-soluble crystalline solid. The acid, most commonly sold as 85% aqueous solution, is viscous, non-volatile, and non-odorous. Phosphoric acid is a weaker acid and less reactive than the other mineral acids discussed above.

Safe Handling

  • At all times when handling acids, wear at a minimum 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 more likely, wear additional protection to prevent skin contact: A face shield above splash goggles, acid resistant gloves with long cuffs, and an acid-resistant apron or smock.
  • Always handle concentrated acids in a fume hood to avoid breathing in vapors of the acid or evolving gases. If possible, work in secondary containment to contain spills.
  • Always dilute by adding acid to water and not water to acid. Adding water to concentrated acid may cause violent boiling of the solution and splashing.
  • Solutions of 3 M or less can be handled on a bench top, but keep in mind that they are still corrosive. Work in secondary containment when possible.

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. If skin irritation is visible, seek medical attention. 

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 acid.

Spills

Clean acid spills immediately. Have a spill kit readily available before working with any concentrated acid. We recommend solid sodium bicarbonate or calcium carbonate for neutralizing spills. Commercial spill kits are also available.

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

In the case of a large spill of a fuming acid (e.g., concentrated hydrochloric acid) 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 acids in an acid cabinet, preferably in secondary containment. Keep them away from bases and chemicals that will liberate toxic gas if in contact with acid, such as azides, bleach, carbides, cyanides, nitrides, sulfides, and metals.

Nitric acid must be stored separate from organics, including acetic acid.

Waste Disposal

Keep acid waste separate from other waste streams, and use secondary containment for the waste container. Check the waste solution for evolving gases before pouring it into a waste container. Over-pressurization of closed waste containers has led to violent container ruptures in the past. Dispose of waste through the DRS chemical waste program.

If the waste solution does NOT contain any hazardous metals, consider elementary neutralization:

Pour the acid waste into a large quantity of ice (500 grams of ice per 100 mL of acid). Neutralize the mixture with an aqueous basic solution, such as 1M or 10% sodium hydroxide (NaOH) or saturated sodium bicarbonate (NaHCO3) in water until the pH is neutral. The neutralized solution may then be poured down drain. If the neutralized mixture contains heavy metals (e.g., gold, platinum, lead, chromium), the solution should be labeled and disposed of through the chemical waste management system.

    Last Update: 4/16/2015