Thank you for your question regarding the shelf life of chemicals.
The shelf life of a chemical depends on the nature of the chemical, the conditions under which it is stored and how frequently the bottle is opened. Chemicals may deteriorate due to reactivity to air and water, sensitivity to light, air-oxidation to form peroxides, photochemical reactions and volatility. Solutions may also lose water through transpiration (loss of vapour) via the cap and thread, leading to an increase in the concentration of the solution.
Generally, chemicals should be stored in a cool, dry place with no exposure to light, especially direct sunlight. Bottles should be capped tightly after use. The SDS for a particular chemical should be consulted for specific storage and handling requirements (Part 7) and stability and reactivity (Part 10) of the chemical. The month and year of purchase/opening should be recorded on the bottle.
Schools should endeavour to store chemicals in minimum quantities and chemical stock should be assessed periodically. Chemicals should be inspected for absorption of water, any cloudiness or crystals which have developed in a liquid and any change to the colour or form of a substance. Part 9 of the SDS, Physical and Chemical Properties, can be consulted for information on the form and appearance of a chemical if there is any uncertainty about what it should look like. Consideration should be given to whether the chemical is safe to use and whether it is fit for purpose.
A visual inspection of the chemical stock should include an inspection of labels, containers and bottle caps for signs of deterioration. Labels which have become illegible should either be replaced, or the chemical decanted and relabelled, or disposed of. Particular care should be taken with old bottles of corrosive substances which can eat through bottle caps. Spare bottle caps can be purchased through science equipment suppliers.
ASSIST will be developing detailed storage advice for schools in the near future. Please see below for further details on some specific issues to consider in the maintenance of school science chemicals.
Absorption of water
Many substances are hygroscopic, which means that they will absorb water from the air. Examples of hygroscopic substances include: sodium chloride, potassium bromide, ferric chloride, sulphuric acid and glycerol. Some substances are deliquescent which means that they will absorb water from the air to the extent the substance goes into solution. Examples of deliquescent substances include sodium hydroxide, potassium hydroxide, calcium chloride, potassium carbonate and zinc chloride. Absorption of water does not necessarily make the chemical unfit for purpose in a school, as it may still be used for solutions which are used in qualitative observations. However, if a chemical has absorbed so much water that it is very difficult to handle or has formed a solution, then disposal and replacement of the chemical is recommended.
Hygroscopic solids can be protected from moisture in the air by storage in a desiccator along with a desiccant. There is quite a range of substances which can be used as desiccants, however, taking into consideration safety, availability and ease of handling, we recommend silica gel for use in desiccators. Silica gel is available as self-indicating, which means that it will change colour with the absorption of water. Silica gel can be regenerated by drying in an oven at 105–120°C for 2–3 hours.
Storing chemicals in a desiccator
- An evaporating dish or similar open vessel containing a desiccant should be placed in the base of the desiccator. The ceramic plate is placed over the evaporating dish and the bottles of chemicals to be stored are placed on the ceramic plate.
- A small amount of vacuum grease should be applied to the rims of both the lid and base of the desiccator. This creates a barrier to protect against the ingress of water.
- A desiccator should be opened by sliding, not lifting, the lid from the base. The vacuum grease may seize up over time, which makes it very difficult to remove the desiccator lid. In such cases, the grease can be warmed by heating the rim of the desiccator with a heat gun or hairdryer. The old vacuum grease should then be removed with paper towel and fresh grease applied.
- The desiccant used will need to be either regenerated or replaced periodically, depending on the ambient conditions.
- The storage incompatibilities of chemicals should be observed when storing different chemicals together in a desiccator (i.e. incompatible classes of chemicals should not be stored together in the same desiccator).
Absorption of carbon dioxide
Substances such as alkali and alkaline earth metal hydroxides and oxides absorb carbon dioxide from the air, to give the corresponding carbonates as is described by the following exaample equations:
2 NaOH + CO2 --> Na2CO3 + H2O
CaO + CO2 --> CaCO3
Ca(OH)2 + CO2 --> CaCO3 + H2O
Sodium hydroxide pellets are deliquescent and will absorb water and carbon dioxide so that the pellets become fused together and slushy over time. Degradation of sodium hydroxide makes it difficult to handle and the contamination with water and the carbonate means that it cannot be weighed accurately.
Apart from the difficulties in handling degraded sodium hydroxide, the presence of a significant amount of the carbonate may affect the result of an acid–base titration. For such applications it is preferable to use a fresh batch of sodium hydroxide or to purchase a volumetric standard sodium hydroxide solution from a chemical supplier.
Solutions of hydroxides also absorb carbon dioxide from the air. This is most obvious in solutions of calcium hydroxide (limewater) and barium hydroxide, which form insoluble carbonates that precipitate from the solution. Dilute solutions of hydroxides more readily absorb carbon dioxide than concentrated solutions.
Calcium is oxidised on exposure to air to form calcium oxide which can further react with carbon dioxide to form calcium carbonate. While calcium metal has a grey appearance, formation of the degradation products transforms the grey metal to a white, powdery substance. Calcium which has substantially degraded should be disposed of via a hazardous waste disposal company.
Many organic compounds undergo photochemical reactions on exposure to visible or ultraviolet light. For this reason, many chemicals are supplied in amber bottles and should be stored away from direct sunlight. Alkenes can undergo photodegradation to give polymerised products, resulting in a yellow-brown coloured, viscous appearance. Alkenes which have polymerised should be disposed of via a hazardous waste disposal company.
Concentrated nitric acid can form nitrogen oxides on exposure to light, imparting a brown colour to the acid. A slight colour change is generally not a problem for school purposes, however, disposal of the acid should be considered if the colour change is significant.
Some hydrated salts are efflorescent, which means that they can lose their water of crystallisation to the air. The conditions under which this occurs are specific for the individual chemical. For example, sodium thiosulphate pentahydrate will start to lose its water of crystallisation at 33°C in dry air. Copper sulphate pentahydrate will effloresce at 30°C in dry air, with loss of two molecules of water of crystallisation. The deep blue copper sulphate pentahydrate will thus develop a paler blue, powdery appearance on the surface of the crystals. Again, slight degradation of these chemicals is not usually an issue in schools.
Peroxide forming chemicals
Organic substances such as ethers (e.g. diethyl ether), secondary alcohols (e.g. 2-butanol, 2-octanol), tertiary alcohols (e.g. tert-butanol or 2-methyl-2-propanol) and alkenes (e.g. cyclohexene) have the potential to form peroxides on exposure to air and light. If a sufficient concentration of peroxides is generated, there is a risk that the bottle will explode. Peroxides can form at the junction of the lid and the rim of the bottle so that the friction generated when the lid is opened can initiate an explosion. Many chemicals which are susceptible to the formation of peroxides are supplied with added peroxidation inhibitors to prevent peroxide formation. It is possible for all of the inhibitor to be consumed if the substance is very old or sufficiently exposed to air/light. The inhibitor would also be removed through purification of the substance, e.g. by distillation.
Bottles of peroxide forming chemicals should be inspected prior to use. If white crystals are observed around the lid of the bottle, or if a precipitate or a viscous oily layer is present in the liquid within the bottle, then this indicates that dangerously high levels of peroxide are present and the bottle should not be opened. A bottle showing these signs of peroxide formation should be disposed of via a hazardous waste disposal company.
Peroxide forming chemicals can be tested for peroxides using peroxide test strips which can be purchased from chemical suppliers. Chemicals which have the potential to form peroxides should be tested for peroxide concentration periodically and before use.
Potassium metal is commercially available as pieces stored under light paraffin oil. Over time, the potassium becomes oxidised through its reaction with oxygen in the air, forming a surface layer of white or grey potassium oxide, K2O. Over years, the potassium can be oxidised further to the yellow potassium peroxide, K2O2, and with further oxidation, the canary yellow potassium superoxide, KO2, may form. Both the yellow potassium peroxide and the canary yellow superoxide pose an explosion risk in contact with organic materials and may explode violently when handled or cut. Bottles of potassium metal should be checked periodically for signs of peroxidation. If any yellow peroxidised products are observed, then the bottle should not be opened and should be disposed of via a hazardous waste disposal company.
tert-Butanol or 2-methyl-2-propanol has a melting point of about 25°C so that it in cool weather a bottle of tert-butanol can freeze. Observing this for the first time can be alarming as the solid alcohol can be misinterpreted for peroxide formation. The frozen tert-butanol can be melted by placing the bottle in a plastic bag in a warm water bath.
Hydrogen peroxide, H2O2
Hydrogen peroxide is usually supplied with a stabiliser which reduces its rate of decomposition. Nevertheless, its slow decomposition over time is inevitable. Hydrogen peroxide decomposes to oxygen and water according to the equation:
2 H2O2 --> O2 + 2 H2O
At room temperature, it is estimated that stabilised hydrogen peroxide decomposes at about 0.5% per year. Hydrogen peroxide is supplied in bottles with vented caps to avoid the build up of oxygen in the container.
Hydrogen peroxide should be stored upright in the original container in a cool, dry place away from oxidising agents, combustible materials and alkalis. Light catalyses the decomposition of hydrogen peroxide and therefore it should be stored away from light, especially direct sunlight. Hydrogen peroxide should not be stored in a domestic refrigerator which is not spark-proofed, as the build up of oxygen could pose an explosion hazard.
The decomposition of hydrogen peroxide is accelerated with increased temperature; for every 10°C rise in temperature, the rate of decomposition increases 2 to 3 times. Hydrogen peroxide is also very sensitive to contamination. Contamination by metal salts, even at very low concentrations, will result in fast decomposition of hydrogen peroxide to oxygen and water. Contamination with any insoluble solid will also lead to decomposition, to some extent. Hydrogen peroxide is very sensitive to pH, its optimum pH range being pH 3.5 to 4.5. At alkaline pH levels (pH greater than 7), decomposition to oxygen and water is rapid.
The concentration of a hydrogen peroxide solution can be analysed by titration with potassium permanganate (see, for example, https://www.flinnsci.com/hydrogen-peroxide-analysis/vel1253/ [link updated December 2016]).
Bretherick, L., Bretherick’s Handbook of Reactive Chemical Hazards, 4th edition, Butterworths, 1990.
Chem-Supply SDS Copper (II) sulphate pentahydrate, Issue date March 2014, https://www.chemsupply.com.au/documents/CL0681CH2B.pdf
Chem-Supply SDS Hydrogen peroxide 20-50%, Issue date September 2012, https://www.chemsupply.com.au/documents/HA1541CH35.pdf
Chem-Supply SDS Sodium thiosulphate pentahydrate, Issue date August 2014, https://www.chemsupply.com.au/documents/SA0181CH6W.pdf
DeLaHunt, John S.; Lindemann, Theodore G. J. Chem. Health Safety 2007, 14 (2), 21.
Flinn Scientific Inc. Video, Hydrogen Peroxide Analysis, 2016, https://www.flinnsci.com/hydrogen-peroxide-analysis/vel1253/ (updated December 2016)
Grubelnik, Andreas; Meyer, Veronika R.; Bützer, Peter and Schönenberger, Urban W., J. Chem. Educ., 2008, 85 (5) , 634.
ProSciTech Pty Ltd, Laboratory Resource, Silica Gel FAQs, 2013, https://laboratoryresource.com.au/?navaction=getitem&id=155
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Solvay S.A., Solvay Interox Hydrogen peroxide Handling and Storage, 1998, http://www.solvay.us/en/binaries/H2O2_Storage_and_Handling-236781.pdf