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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.
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
|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)|
|Potassium ferrate||Potassium hydroxide|
|Schweizer's reagent||Excess ammonia|
|sec-Butanol||Only required if kept for more than 1-2 years in contact with air|
|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|
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|
Reagents treated with preservatives must be inspected periodically for any signs of degradation, in the event said preservatives failed to do their job.
- Polymer Latices: Science and technology Volume 2: Types of latices, D.C. Blackley, p. 28