Faculty of Science

Department of Chemistry

For disposal of other chemicals such as waste silica gel, solids, waste sulfuric acid, waste HF, waste nitric acid, waste hydrochloric acid, waste mixed acids, waste mixed alkalis, waste pump oils, please store them separately with appropriate hazard label and arrange via Lab Supplies for disposal by the waste vendor. Be aware that concentrated acids especially nitric acid, being an oxidizer, can react violently with organic matter and should not be mixed with other organic acids. It can attack plastics over time, therefore, do timely disposal.

If you have small quantities of acids or bases that are free of heavy metals (copper, zinc, silver, etc), you should dilute and neutralize the solution before drain disposal.

Drain disposal of small quantities of acids and bases

Acids and bases must be rendered neutral before being disposed down a lab sink with copious amounts of water. Neutralization should be done in small quantities (i.e., no larger than 50 ml). You should not collect a quantity of the acid/base wastes from different experiments and do a batch neutralization. Do neutralizations in a fume hood as fumes and heat may be generated. PPE must be donned before carrying out the dilution and neutralization. Do your calculations for the dilution and neutralization.

  1. Choose a container with a volume at least twice that of your final volume after neutralization. Slowly add the acid to the cold water to dilute to about 5 %.
  2. For acid neutralization, prepare a 5 % basic solution of sodium carbonate, calcium hydroxide or sodium hydroxide. The use of a carbonate solution allows you to gauge that the acid is neutralized when there are no more bubbles formed. (For base neutralization, add 5 % hydrochloric acid solution.)
  3. Slowly add the base solution to the diluted acid from (1).
  4. Check that the pH is between 6 to 9 before turning on the tap.
  5. Pour the solution down the drain keeping the tap on. Let the tap run for another few minutes after the container has been emptied.

Chromerge, Chromic Acid, and Dichromate Solutions

Chromic acid solution is a mixture of concentrated sulfuric acid and potassium dichromate, Chromerge (chromic acid), or chromium anhydride (chromium trioxide). It is used to clean laboratory glassware because it oxidizes most residues and eats away a very thin layer of the glass surface, leaving a new, clean surface.

Safety Committee recommends that you try substitute such cleaning solutions. Unnecessary hazards related to the use of chromium based sulfuric acid cleaning solutions and reasons for not using it are as follows:

  1. The hexavalent chromium present in the above solutions is considered a human carcinogen.
  2. It is a strong oxidizer that has been known to react violently and explode when combined with oxidizable materials.
  3. The addition of chloride or halogens to chromic acid cleaning solutions can generate the highly toxic and volatile carcinogen, chromyl chloride. The formation of volatile chromyl chloride necessitates that these cleaning solutions be used inside chemical fume hoods.
  4. Used chromic acid cleaning solutions cannot be neutralized and flushed into the sanitary sewer because the chromium metal remains.
  5. There are many non-toxic biodegradable cleaning solutions that can be used instead of chromic acid. See the alternative cleaning solutions listed below.

Most laboratory glassware can be properly cleaned without the use of sulfuric acid based cleaning solutions. Many of the commercial phosphate-based cleaners will do an adequate job of cleaning. 
Suggested alternatives to chromic acid cleaning solutions are

(1) Non hazardous cleaning solutions (safest; try these first)

  • Ultrasonic baths (these work well for many labs)
  • Alconox or similar detergents
  • Pierce RBS-35 or similar detergents
  • Biodegradable surfactants

(2) Strong corrosive solutions (hazardous due their corrosivity)

  • Potassium hydroxide/ethanol solutions (also flammable)
  • Dilute hydrochloric acid

Stop using chromic acid solution unless you have tried the alternatives and found them to be unsatisfactory.

Waste disposal guide for chromic acid cleaning solution:

  1. The amount of solution should be less than 100ml. All work should be done in a fume hood.
  2. Wear nitrile rubber gloves, laboratory coat and eye protection.
  3. Do a 1:1 dilution by slowly adding the solution to a container of water.
  4. Adjust the pH to 1 by the addition of 3M sulfuric acid or sodium carbonate.
  5. With stirring, slowly add solid sodium thiosulfate (~13.5g) until the solution becomes cloudy and blue in color.
  6. Neutralize the solution with sodium carbonate and wait till a blue-gray flocculent precipitate is formed.
  7. Let the mixture stand for 1 week until much of the supernatant can be decanted.
  8. The remaining liquid is allowed to evaporate or the slid filtered through Celite.
  9. The liquid may be washed into the drain with 50 times its volume of water.
  10. The solid residue should be washed with hot water to remove sodium sulfate, then dried, packaged, labeled and lastly sent for disposal.



Drying Agents and Water Reactive Substances

In research laboratories it is common practice to add drying agents to organic solvents to remove trace quantities of water. Caution must be used when choosing a suitable drying agent because many drying agents are extremely water reactive. Some of these reactive materials generate highly flammable gases on contact with water and careless handling could cause a serious fire or explosion. Such chemicals are:

sodium metal, sodium hydride, potassium metal, potassium hydride, calcium metal, calcium hydride, lithium aluminum hydride.

Another reason to avoid using water reactive drying agents is the hazard the agent presents when the material is disposed. If the drying agent has not been removed from the solvent there is a potential for explosion, because all flammable solvent wastes are consolidated into drums. If a water reactive material is accidentally poured into one of these drums, someone could be severely burned or killed by the resulting explosion.

Dehydrite is a drying agent which contains magnesium perchlorate. Magnesium perchlorate is a strong oxidizer which may cause fires or explosions on contact with organic materials.

The safest and most common drying agents are calcium chloride, silica gel, molecular sieves, and calcium sulfate (Drierite). Safety committee highly recommends these materials because of their lower toxicity and stability.

Waste disposal guide for sodium2

  1. For small quantities of sodium only. Wear eye protection, laboratory coat, and nitrile rubber gloves and work in the fume hood.
  2. Place small pieces of solid sodium in a three-necked, round-bottom flask equipped with a stirrer, dropping funnel, condenser, and heating mantle.
  3. Flush the flask with nitrogen. Add 95% ethanol (13 mL per gram of sodium) dropwise at a rate to cause rapid reflux. Stirring is started as soon as enough ethanol has been added to make it feasible.
  4. Stir the mixture and heat under reflux until the sodium is dissolved.
  5. Turn off the heat and add an equal volume of water at a rate that causes no more than mild refluxing.
  6. Cool the solution and neutralize with 6 N sulfuric acid (prepared by cautiously adding 15 mL of concentrated acid to 75 mL of cold water) or hydrochloric acid (prepared by adding concentrated acid to an equal volume of cold water).
  7. Wash into the drain with at least 50 times its volume of water.


Waste disposal guide for Sodium Hydride and Potassium Hydride:

  1. The amount should be less than 10g. All work should be done in a fume hood.
  2. Wear Nitrile rubber gloves, laboratory coat and eye protection. (For KH, face shield is necessary) Mix the hydride with a 1:1:1 mixture by weight of sodium carbonate or calcium carbonate, clay cat litter (bentonite) and sand.
  3. Slowly add butanol (~38 mL/g NaH, ~22 mL/g KH) until the reaction ceases.
  4. Add water very carefully until all the hydride is destroyed.
  5. Let the mixture stand until solids settle.
  6. Decant the liquid into drain with at least 50 times its volume of water.
  7. The solid residue may be discarded with normal procedure.



Waste disposal guide for Calcium Hydride:

  1. The amount should be less than 20g. All work should be done in a fume hood.
  2. Wear Nitrile rubber gloves, laboratory coat and eye protection.
  3. Place the hydride in a 3-necked round-bottom flask equipped with nitrogen inlet dropping funnel and condenser.
  4. Under N2 and with stirring, add methanol (25 mL/g of hydride) dropwise.
  5. When the reaction is complete, slowly add an equal volume of water to the slurry of calcium methoxide.
  6. Wash the solution into the drain with at least 50 times its volume of water.


Waste disposal guide for Lithium Aluminium Hydride:

  1. Wear butyl rubber gloves, laboratory coat, face shield and eye protection.
  2. All work should be done in a fume hood.
    When the amount is less than 10g
  3. Slowly add the hydride to a large excess of butanol in a pail (~24mL/g of hydride).
  4. When the reaction is complete, dilute the mixture with water and allow the solids to settle
  5. Flush the liquid to the drain with at least 50 times its volume of water.
  6. The solid residue may be discarded with normal procedure. 
    When it’s a reaction mixtures of <100mL
  7. The stirred reaction mixture from n grams of LiAlH4 is treated by successive dropwise addition of n mL of water, n mL of 15% NaOH solution and 3n mL of water.
  8. This produces a granular precipitate with is filtered and disposed as normal procedure.
  9. Flush the filtrate down the drain with at least 50 times its volume of water.


If you need further information on how to dispose a specific chemical, check out the following references:

  1. Prudent Practices in the Laboratory: Handling and Disposal of Chemicals (T55.3 Haz.Pr )
  2. Hazardous Laboratory Chemicals Disposal Guide, Second Edition by Margaret-Ann Armour (QA64 Arm)