Reagent preservation

Although many chemical compounds and elements can be stored indefinitely (ex: sodium chloride, barium sulfate, titanium dioxide, oxalic acid, carbon, gold, etc.) in most if not all storage conditions, there are quite a lot of reagents that tend to undergo chemical changes, regardless of the storage. Even in the best storage conditions, they will degrade over long periods of time. To maintain the long shelf life of them, preservatives as well as other additives are added.

General

Many chemicals, such as ethers or secondary alcohols, tend to undergo autoxidation when in contact with air. Halogenated carbons will too suffer similar effects. Alkaline metals like potassium also build up dangerous peroxides in contact with oxygen.

Other compounds, such as sugar or citric acid will serve as nourishment for mold or other biological organisms. Alcohols, like ethanol, will not turn into their respective carboxylic acids (like acetic acid) if they're concentrated enough.

Some elements like silver will darken in prolonged contact with polluted air, even though silver resists atmospheric oxidation. Many other metals suffer similar effects. Some metals, like magnesium, zinc or lanthanides are relative stable in dry air, but will quickly tarnish if moisture is present.

Note that preservatives have poor efficiency if you do not store your reagents properly.

Reagents and preservatives used

Compound	Preservative	Notes
1,4-Dioxane	Butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), fresh copper, diphenylamine, hydroquinone
Acrolein	Hydroquinone	Does not always work, possibly due to impurities
Barium ferrate	Potassium hydroxide, sodium hydroxide
Benzyl chloride	Propylene oxide	Concentration 0.25-1%
Chloroacetone	Calcium carbonate	Aprox. 1%
Chloroform	Ethanol, methanol, pentene (amylene)	Accepted values of 1% for ethanol; 100 ppm for amylene
Citric acid	Biocides, like benzalkonium chloride	Aqueous solutions and wet citric acid are prone to mold; solid or anhydrous citric acid less so
Dichloromethane	Ethanol, methanol, cyclohexane, cyclohexene, amylene	Accepted values of 100 ppm for amylene, cyclohexane, cyclohexene
Diethyl ether	Butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), fresh copper, diphenylamine, ethanol, hydroquinone	Accepted values 1-2% for ethanol
Diisopropyl ether	Butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), fresh copper, diphenylamine, hydroquinone	The amount of preservative added must be greater compared to other ethers
Formaldehyde	Methanol	For aqueous formaldehyde (Formalin) the concentration is 10-20%; Inhibts oxidation and polymerization
Hydrogen peroxide	Phosphoric acid, EDTA
Isopropanol	Butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), fresh copper, diphenylamine, hydroquinone	Only required if kept for more than 1-2 years in contact with air
Lead(II) azide	Dextrin	Generally used in Pb(N3)2 aqueous solutions and during production to inhibit the growth of large crystals
Lead(IV) acetate	Anhydrous (glacial) acetic acid, acetic anhydride
Methyl iodide	Copper or silver wire/powder
Natural rubber latex	Ammonia	0.2 % m/m (short term storage) to 0.7 % m/m (long term storage)
Pentaerythritol tetranitrate	Urea
Potassium ferrate	Potassium hydroxide
Schweizer's reagent	Excess ammonia
sec-Butanol	Butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), fresh copper, diphenylamine, hydroquinone	Only required if kept for more than 1-2 years in contact with air
Smokeless powder	Diphenylamine
Sodium bisulfite	Sulfur dioxide
Sodium ferrate	Sodium hydroxide
Sulfur trioxide	Antimony pentafluoride, Antimony pentachloride	Concentration 0.5-1%; antimony pentafluoride gives a better performance than the pentachloride
Tetraamine copper(II) complex	Excess ammonia
Tetraaminecopper(II) persulphate	Excess ammonia
Tetrahydrofuran	Butylated hydroxytoluene (BHT), Butylated hydroxyanisole (BHA), fresh copper, diphenylamine, hydroquinone	Accepted values of 100-300 ppm for BHT

Storage atmosphere

Some materials that react with air are best or can only be safely stored under air-free atmosphere. While noble gasses like argon are commonly used, being relative cheap, other gasses like nitrogen, sulfur hexafluoride, most saturated perfluorocarbons can also be used. Carbon dioxide or gaseous saturated hydrocarbons (methane, ethane, propane, butane) can also be used in some cases. Some compounds like sulfur dioxide or ammonia are being used as preservatives themselves.

Compound	Noble gasses	Nitrogen	Sulfur hexafluoride	Perfluorocarbons	Carbon dioxide	Hydrocarbons	Notes
Alkali metals	Compatible	Compatible, save for lithium	Compatible	Incompatible	Incompatible	Compatible	Lithium is incompatible with SF6 at high temperatures
Alkaline metal hydroxides	Compatible	Compatible	Compatible	Compatible	Incompatible	Compatible
Alkaline earth metals	Compatible	Compatible	Compatible	Compatible	Incompatible	Compatible	Calcium and strontium slowly react with nitrogen
Alkali earth metal hydroxides	Compatible	Compatible	Compatible	Compatible	Incompatible	Compatible
Bisulfites	Compatible	Compatible	Compatible	Compatible	Compatible	Compatible	Only compatible with carbon dioxide in dry conditions
Chlorocarbons	Compatible	Compatible	Compatible	Compatible	Compatible	Compatible
Ethers	Compatible	Compatible	Compatible	Compatible	Compatible	Compatible
Sulfites	Compatible	Compatible	Compatible	Compatible	Compatible	Compatible

Safety

Reagents treated with preservatives must be inspected periodically for any signs of degradation, in the event said preservatives failed to do their job.

References

Relevant Sciencemadness threads

• Adding and stabilisers to/from ethers and halohydrocarbons