How to prevent and mitigate the deterioration of chemical reagents?
Product Manager
Sandra Forbes
Whether chemical reagents will deteriorate during storage depends on two factors: internal factors, which are determined by the chemical structure of the reagent itself, and external factors, which are determined by the environmental conditions in which the reagent is stored.
To ensure proper storage, one must understand the relationship between the structure and properties of the reagent, and secondly, create an external environment suitable for reagent storage.
1. Reasons for deterioration of chemical reagents
The environment mainly refers to the storage temperature, light and medium. The medium generally refers to air and mixed impurities. In addition to oxygen, carbon dioxide, and water vapor in the air, the storage room often contains various volatile reagents that have diffused into the air, such as hydrogen chloride, nitric acid, hydrogen sulfide, sulfur dioxide, bromine, iodine, ethylene, formaldehyde, and other volatile gases. In addition, there are also dust particles that have mixed with the air, including inorganic and organic substances as well as various microorganisms.
Chemical reagents will gradually change and deteriorate under certain temperature, light, and medium conditions, which involves both physical and chemical changes. The former causes loss of chemical reagents, while the latter can completely deteriorate and render the reagents ineffective. The reasons that trigger and promote changes in chemical reagents can be roughly summarized as follows:
a. Volatilization
Volatilization is the most common cause of reagent loss, concentration changes, and specification degradation caused by volatile reagents. The general characteristics of volatile reagents are: small molecular weight and low boiling point. Common inorganic reagents include concentrated hydrochloric acid, concentrated nitric acid, and fuming sulfuric acid. Organic reagents include liquid substances with a small number of carbon atoms, such as methanol, ethanol, petroleum ether, and gasoline.
b. Sublimation
A class of reagents with sublimation properties are generally molecular crystals with low sublimation heat. In the laboratory, there are generally two types of reagents that sublimate at room temperature, such as iodine or naphthalene, and reagents that sublimate under heated conditions, such as sulfur and mercuric chloride. The sublimation of such reagents mainly causes loss and pollutes the air.
c. Deliquescence and dilution
There are only a few chemical reagents that can deliquesce, most of which are easily soluble compounds. Generally, the radius of anions is much smaller than that of cations, but there are also cases where the radii of anions and cations are similar but the cations carry more charge. After such reagents absorb water to form a saturated solution on the surface, if the resulting water vapor pressure is less than the partial pressure of water vapor in the air, deliquescence will continue until all the solution is formed. Such as: sodium hydroxide, soda lime, etc. Organic substances that are easily affected by moisture include sodium acetate, ammonium acetate, etc.
Dilution refers to the phenomenon that the concentration of a reagent solution decreases due to absorption of moisture from the air. The cause of this is similar to that of deliquescence, which is due to the external water vapor partial pressure being greater than the water vapor partial pressure of the reagent. Common reagents that are prone to dilution include concentrated sulfuric acid, orthophosphoric acid, and ethylene glycol.
d. Weathering
The cause of weathering is opposite to deliquescence, which is due to the higher partial pressure of water vapor in crystalline hydrates than in the air. The drier the air, the faster the weathering rate.
Common weathering agents in the laboratory include: Na2CO3·10H2O, Na2SO4·10H2O, CuSO4·5H2O, MgSO4·7H2O, ZnSO4·7H2O, etc. Weathering occurs without affecting the chemical properties of the reagent, but it changes the appearance of the reagent and reduces its quality.
e. Concentration and crystallization
The reason for concentration and crystallization is that under dry conditions, the water vapor pressure of the reagent solution is higher than that of the outside air, causing the evaporation of water in the solution and resulting in concentration and crystallization. This phenomenon is common for various solid solutes, especially for solutions with high concentrations. Although concentration and crystallization have little effect on the properties of the reagent in the bottle, they can also change its concentration, specifications, and appearance. For some organic compounds with low melting points, crystallization may occur when the external temperature drops significantly. A notable example is glacial acetic acid.
f. Hydrolysis
Most of the easily hydrolyzed salt reagents have covalent bonds. All salts formed by strong acids and weak bases, or weak acids and weak bases, will undergo varying degrees of hydrolysis when exposed to water. Some metal halides in the laboratory are easily hydrolyzed, such as TiCl4, AlCl3, FeCl3, SnCl2, etc. They are cation compounds with high charge and small radius or non-inert gas-type cation compounds. Such reagents can deteriorate due to easy hydrolysis of water-absorbing substances, and easily hydrolyzed organic compounds are compounds containing acyl groups, such as esters, acid halides, and other substances.
g. Decomposition
Decomposition reactions are also the cause of initiating and promoting reagent loss and deterioration. The decomposition rate of reagents is usually closely related to the ambient temperature. The decomposition rate accelerates at high temperatures, and binary compounds that are usually easily decomposed have lower bond energies. The lower the bond energy, the easier it is to decompose, for example, iodine compounds are more easily decomposed than bromides. Some decomposition reactions are also related to the water content of the reagent, such as ammonium bisulfate. The higher the water content, the faster the decomposition rate at higher temperatures. However, oxygen-containing acid salts, such as nitrates and permanganates, decompose only when heated.
h. Oxidation and reduction
Generally, the reagents that are easily oxidized have low standard electrode potentials and are reducing agents, often with the word "low" or "sub" in their names. There are also some lively metal and non-metal elements, peroxides and some organic reagents, such as ferrous sulfate, ferrous ammonium sulfate, sulfurous acid, anhydrous sodium sulfite, zinc powder, reduced iron powder, acetaldehyde, etc. The more reducing the reagent is, the more likely it is to be deteriorated by oxidation. The oxidation is caused by oxygen in the air and oxidizing impurities.
Reagents with high standard electrode potentials are generally stable in the air when they are in solid form, such as potassium permanganate and potassium dichromate. However, when present in solution, it is susceptible to deterioration due to interaction with some reducing impurities in the air, such as H2S, SO2, and other solutions such as KMnO4 and K3Fe(CN)6.
i. Non-oxidation-reduction reaction
The deterioration of some reagents does not necessarily cause changes in the valence state of the elements, that is, non-oxidation-reduction reactions can also cause them to fail. The most common examples in the laboratory are: quicklime becomes slaked lime by absorbing water, and further absorbs carbon dioxide and becomes ineffective; Sodium hydroxide and potassium hydroxide solids also contain impurities due to absorption of carbon dioxide, and if exposed to air for a long time, they will completely convert into carbonates; In addition, magnesium oxide, barium oxide, and barium hydroxide should also be prevented from reacting with carbon dioxide in the air.
j. Polymerization and condensation
Organic substances containing unsaturated double bonds or triple bonds in their molecular structure are prone to polymerization, such as formaldehyde solution, which often polymerizes to form white trioxymethylene, and potassium cyanide reagent, which is also prone to polymerization. The solution of oxoacid salts formed by high-charged and small-radius central ions can precipitate polyacid salt precipitates due to condensation, such as ammonium molybdate solution, which can condense to precipitate ammonium tetramolybdate. Trioxymethylene can also release formaldehyde gas after heating treatment, but some reagents, once undergoing polymerization or condensation reactions, are often irreversible, resulting in deterioration and failure.
k. Photochemical reaction
Light, as a form of energy, can also cause certain reagents to react and deteriorate. An example is the decomposition of silver salts triggered by light energy. Light can also trigger oxidation reactions, such as benzaldehyde being easily oxidized by air to benzoic acid under light exposure, and aniline changing from colorless to brown. In addition, mercuric iodide, mercuric sulfate, potassium ferrocyanide, etc. are also prone to photochemical reactions.
l. Mildew
The so-called mildew refers to the phenomenon of mold breeding and reproduction in chemical reagents. There are countless bacteria and microorganisms in the dust in the air, which can reproduce under certain temperature conditions. The carbohydrates, esters, protein reagents, and organic reagents containing nitrogen, sulfur, and phosphorus in the laboratory are good nutrients and breeding grounds for the proliferation of fungi. The above reagents can be mildewed as long as they are not tightly sealed and have some contact with air.
Chemical reagents undergo various changes due to the above reasons, which can usually be detected by color, shape, smell, and quantitative increase or decrease. However, some changes require experimental methods to distinguish, such as whether anhydrous alcohol contains water, which can only be determined by using specialized reagent standards and testing methods. Therefore, the scientific storage of various reagents is a meticulous work, which has a lot of knowledge and articles to be written.
2. How to prevent and mitigate the deterioration of chemical reagents?
In order to ensure the normal development of chemical teaching, scientific research and chemical production, reduce reagent loss, and alleviate reagent deterioration, the following methods and measures can usually be taken:
a. Sealing
This is the most common and universal method. The material and sealing degree of the reagent bottle should be determined according to the nature of the reagent. For example, "three acids" and liquid bromine with strong corrosivity can be stored in reagent bottles with ground glass or screw-cap glass bottles with plastic liners. Hydrofluoric acid should be stored in silver or plastic containers, etc.
Sealing is applicable to all chemical reagents that are volatile, sublimable, deliquescent, diluted, weathered, hydrolyzed, and subject to oxidation-reduction and mildew. For reagents that are highly susceptible to decomposition and produce gases, incomplete sealing is generally not recommended, as leaving some space may cause the container to break. In addition to general sealing, wax sealing can be added, or self-made nitro-tincture can be used for sealing, such as aluminum trichloride and phosphorus pentoxide.
b. Isolation
Reagents that can react with air and water, such as very reactive metals and non-metals, should be stored in relatively stable liquids or inert gases that are isolated from the reagents. Potassium and sodium should be immersed in engine oil, while yellow phosphorus should be immersed in water for storage. This isolation method is also known as liquid seal method, the former is called oil seal, and the latter is called water seal. Water storage can also reduce the loss of certain volatile reagents. For example, adding a thin layer of water to the reagent containing liquid bromine and carbon disulfide can greatly reduce the loss of volatilization and air pollution.
There are many types of inorganic and organic reagents in the laboratory, with different properties, and they should be stored in a reasonable manner. Organic and inorganic substances should be separated, as well as common drugs and dangerous substances. Oxidants and flammable substances should be separated, as well as reducing agents, volatile acids and bases. By separating these aspects, one can avoid adverse effects between drugs, and secondly, even if an accident occurs, it can prevent drug interactions and create greater hidden dangers.
c. Avoid light
Typically, dark brown reagent bottles with good light-shielding properties are used. Place the reagent in a dark or shaded dedicated reagent cabinet. It is also possible to wrap the reagent bottle in black thick paper of photographic paper, such as the storage of concentrated nitric acid, potassium iodide, sodium iodide, and mercuric chloride.
d. Low temperature
Ordinary volatile reagents are often stored in a cool and dark place, such as concentrated nitric acid, concentrated hydrochloric acid, ammonia water, etc. Some special biochemical reagents should be stored in water tanks or refrigerators, such as enzyme reagents.
e. Ventilation
Although containers holding chemical reagents are typically sealed, incidents of leakage are inevitable. In high-temperature summer weather, explosive mixed gases are more likely to form. Therefore, the storage room must be well-ventilated, with dedicated exhaust fans installed and regularly turned on to promote air circulation.
f. Timely
This is based on the characteristics of certain reagents, especially those that are highly susceptible to deterioration and failure. Appropriate measures should be taken to ensure timely preparation, use, and disposal.
For example, it is best to prepare and use hydrogen sulfide solution, chlorine water, bromine water, and iodine water that are highly prone to oxidation in a timely manner; After preparing the silver nitrate solution, ammonia water, and acetaldehyde solution for the silver mirror reaction, they should be used promptly to avoid affecting the results; After the ferrous sulfate solution is prepared, some reduced iron powder should be added to prevent it from being oxidized; After using the solution of starch, sucrose, and protein, the reagent bottle should be cleaned in time to prevent mildew.
When preparing the above reagents, in addition to paying attention to the timing, the amount of preparation should also be determined according to the need, so as to avoid waste caused by excess.
Aladdinsci: https://www.aladdinsci.com