In chemistry, a base can be defined as a substance that dissociates in an aqueous solution to form Hydroxide ions (OH− ). A weak base is a base that doesn’t completely dissociate into its constituent ions when dissolved in a solution. Some parts of the weak base dissociate into hydroxide ions, while some remain undissociated inside the aqueous solution.
The conjugate acid of a weak base is always a weak acid, and the conjugate base of a weak acid is always a weak base.
How to identify a weak base?
Typically, the pH of a weak base ranges between 7 and 11. The concentration of OH− is often used as an alternative to pH to measure the relative H+ / OH− concentration in solution. The higher hydroxide means a lower hydrogen concentration, therefore, a greater pH.
Plus, weak bases have a lower base dissociation constant (Kb). It is analogous to the acid dissociation constant and represents the base’s relative strength. As the bases get weaker, their Kb values become smaller.
In this article, we have used pKb values to represent the basicity of a compound. It is simply the negative base-10 logarithm of the Kb of a solution.
Now that you have a general idea of pH and Kb values, let’s take a look at some of the most common examples of weak base and why they are important.
Basicity (pKb): 8.05
Hydroxylamine is an oxygenated derivative of ammonia. Pure hydroxylamine is a colorless, crystalline solid that is soluble in water. Its inorganic salts are powerful reducing agents used to synthesize various polymers and as constituents of photographic developers. And they can act as antioxidants for fatty acids.
Hydroxylamine is also an intermediate in the biological nitrification by microbes such as bacteria. In the semiconductor industry, it is often used to remove photoresists after lithography.
Researchers are studying its nitrate salt called hydroxylammonium nitrate as a rocket propellant. Findings show that its solid form can be used to make solid propellants, and its water solution can be used to prepare monopropellants.
Since hydroxylamine poses a dangerous fire hazard when exposed to intense heat, its potential uses must be evaluated carefully. The nature of the explosive hazard is still not well understood.
Basicity (pKb): 4.20
Trimethylamine is a colorless tertiary amine in which every hydrogen atom is substituted by a methyl group. The central nitrogen atom is attached to three methyl groups in a trigonal pyramidal geometry.
Trimethylamine is gas at room temperature but is highly soluble in water. It is generally sold as a 40% solution in water. At lower concentrations, it smells like a rotting fish. At higher concentrations, it smells like ammonia. Short-term inhalation of high concentrations or long-term inhalation of low concentrations can cause severe health problems.
The compound is an excellent nucleophile, which means it can form bonds with electrophiles by donating an electron pair. It has several industrial applications — it is primarily used to synthesize plant growth regulators, dye leveling agents, tetramethylammonium hydroxide, and choline.
Basicity (pKb): 8.1
Hydrazine is a compound containing two single-bonded nitrogen atoms and four peripheral hydrogen atoms. Its aqueous solution (more than 37% concentration) is colorless, corrosive, and toxic by ingestion and skin absorption.
When hydrazine is mixed with water, it forms a monohydrate that is denser (1.032 g/cm3) than the anhydrous substance.
N2H4 + H2O –> [N2H5]+ + OH–
Like ammonia, hydrazine has basic (alkali) chemical properties. It’s a highly reactive base and reducing agent used in a wide range of medical and industrial applications.
About 100,00 metric tonnes of hydrazine is produced globally every year. It is primarily used as a foaming agent to produce polymer foams.
It is also used as a rocket fuel propellant — As the fuel burns, hydrazine decomposes into ammonia, nitrogen, and hydrogen gases, producing extensive amounts of heat energy. Hydrogen and nitrogen gases are forced out of the rocket through a nozzle to create thrust.
10. Aluminum hydroxide
Basicity (pKb): 7.5
In its pure form, aluminum hydroxide is white granules or powder with a density of about 2.4 g/mL. Although it is not soluble in water, it quickly dissolves in strong bases and acids.
Aluminum hydroxide has two unique properties:
- Amphoterism: This means it can react either as a base or acid. For instance, the OH in aluminum hydroxide can act as a weak base when mixed with a strong acid like hydrochloric acid. On the other hand, when it reacts with a strong base like sodium hydroxide, the OH in aluminum hydroxide acts as a weak acid.
- Polymorphism: This means the compound can exist in multiple forms. Aluminum hydroxide is known to have four different forms — bayerite, gibbsite, doyleite, and nordstrandite.
Aluminum hydroxide is commonly used as an added ingredient in the antacid medication. It has been proven effective at treating heartburn, acid indigestion, and upset stomach. It is also used in the manufacturing industry and as a fire retardant filler for polymer applications.
9. Zinc Hydroxide
Basicity (pKb): 8.15
Zinc Hydroxide appears as a white solid and has a density of 3.05 g/cm3. It occurs naturally in three rare minerals, namely ashoverite, sweetite, and wulfingite. It is also synthesized in the lab by mixing sodium hydroxide solution with a solution of any zinc salt.
Like hydroxides of other metals, such as chromium, tin, beryllium, lead, and aluminum, zinc hydroxide is amphoteric. This means it can react both as an acid and as a base. It dissolves readily in a solution of a strong base (like sodium hydroxide) and also in a solution of strong acid (like hydrochloric acid).
Zinc Hydroxide is mostly used as an absorbing agent in surgical dressings. It is also used as an intermediate for manufacturing pesticides and pigments.
8. Sodium bicarbonate
Basicity (pKb): 3.67
Sodium bicarbonate is a white solid crystal, which often appears as a fine powder. It has a alkaline taste, which is similar to that of sodium carbonate (washing soda). Its natural mineral form is Nahcolite, which occurs in association with halite, burkeite, borax, thenardite, and thermonatrite.
NaHCO3 + H2O –> Na+ + H2CO3 + OH–
The compound hydrolyzes in water yielding a sodium cation and a hydroxide ion. The presence of hydroxide ions makes the compound slightly basic.
It is primarily used in baking as a leavening agent. When mixed with water, it acts as an antacid, which is quite effective in treating acid indigestion and heartburn.
Sodium bicarbonate reacts quickly with acids, producing carbon dioxide gas. That is why it is widely used to neutralize acid spills and undesirable acid solutions in chemical labs.
Basicity (pKb): 8.75
The structure of pyridine is quite similar to benzene, with one methine group replaced by a nitrogen atom. The presence of nitrogen (and its lone pair) in the benzene ring makes pyridine a unique compound in chemistry.
Pyridine is a significantly weaker base than alkylamines and typical aliphatic tertiary amines. It is a water-soluble and highly flammable liquid with an unpleasant “fish-like” odor. Although pure pyridine has no color, the impure or older samples can appear yellow.
This base is primarily used to dissolve other compounds and make various products, including dyes, paints, insecticides, medicines, food flavorings, vitamins, adhesives, and rubber products. It is also found in many natural materials in the environment.
Basicity (pKb): 3.35
Ethylamine is a corrosive primary amine. It is a weak base, which means it does not ionize completely in an aqueous solution to form ethyl-ammonium cations and hydroxide anions.
An equilibrium is established between the non-ionized ethylamine molecules and the two ions that form due to its ionization.
Like other primary amines, Ethylamine is an excellent solvent for lithium metal. Those solutions are used for Reducing unsaturated organic substances such as alkynes and naphthalenes.
Furthermore, Ethylamine creates toxic nitrogen oxides during combustion. It is usually kept in a closed container; however, prolonged exposure to high temperatures may cause it to rupture violently.
Basicity (pKb): 3.45
Propylamine belongs to the class of organic compounds called monoalkylamines. These compounds contain a primary aliphatic amine group.
Propylamine is soluble in water and less dense than water. It’s a colorless volatile liquid, and its vapors are heavier than air. During combustion, it creates toxic nitrogen oxides.
Propylamine is typically found in a lower concentration within a few different foods like green bell peppers, orange bell peppers, and in the highest concentration within in red bell peppers and yellow bell peppers. Researchers have also detected it in wild celeries and common grapes.
In the lab, Propylamine hydrochloride is prepared by mixing ammonium chloride with 1-propanol at high pressure and temperature using a Lewis acid catalyst like ferric chloride.
Propylamine is mostly used for synthesizing and analyzing other chemicals.
4. Ammonium Hydroxide
Basicity (pKb): 4.75
Although ammonium hydroxide is a naturally occurring compound, it is also manufactured by humans for a number of reasons. It is made by dissolving ammonia into the water. The resulting liquid is colorless and has a pungent smell.
The solution contains significant quantities of water and ammonia and smaller quantities of ammonium ion and hydroxide ion. Since ammonium hydroxide is not completely dissociated into ions, it’s a weak base. Its pH ranges between 7 and 10.
This compound has several applications. In food, for example, ammonium hydroxide is used in concentrations of 0.7%, whereas households cleaners are made with 5-10% concentrations. It is also utilized in the conventional qualitative inorganic analysis as a complexant and base.
Basicity (pKb): 3.34
Methylamine is a usual aliphatic primary amine, in which the NH2 is connected to a carbon chain. All aliphatic primary amines, including this one, are stronger bases than ammonia.
As you can see in the formula, the major difference between ammonia and methylamine is the presence of the CH3 group in the latter. Since the alkyl group pushes electrons away from itself, a small negative charge builds up on the nitrogen atom, making the lone pair even more attractive towards hydrogen ions.
And because ammonia doesn’t have any electron-donating group, it is a weaker base than methylamine (in which nitrogen is more negative, so it picks up H+more readily).
Like ammonia, methylamine is a colorless gas or a liquid with a pungent odor. It can easily catch fire. Under prolonged exposure to high temperatures, methylamine containers may rupture violently. That’s why it must be handled with care.
Methylamine is mostly sold as a solution in tetrahydrofuran, ethanol, methanol, or as an anhydrous gas in pressurized containers. It is widely used for making pesticides, surfactants, pharmaceuticals, paint removers, and rubber chemicals.
The US Drug Enforcement Administration classifies methylamine as a List 1 chemical due to its illicit production of methamphetamine.
Basicity (pKb): 9.13
Aniline is the simplest aromatic amine that appears as a yellow-brownish oily liquid with a musty fishy odor. It contains an amine attached to a benzene ring.
More specifically, a lone pair over the Nitrogen atom in the NH2 group is in conjugation with the Pi electron of the benzene ring. Therefore, aniline cannot easily lose the electron pair, which makes it a weak base.
It is highly soluble in alcohol and ether and slightly soluble in water. When it reacts with strong acids, it forms anilinium ions.
Aniline is toxic in nature. It is rapidly absorbed by the skin, lungs, and gastrointestinal tract of experimental animals. However, it is widely used for synthesizing chemicals, especially agricultural chemicals, photographic chemicals, and dyes.
Basicity (pKb): 4.75
Ammonia has a great ability to form hydrogen bonds. When it is dissolved in water, it gains hydrogen ions from the water, producing ammonium and hydroxide ions.
NH3(aq) + H2O <–> NH4+(aq) + OH–(aq)
This reaction runs both ways, which means it is reversible. At any time during the reaction, nearly 99% of the substance still remains as ammonia molecules, and only 1% actually produces hydroxide ions. Since only a small part of ammonia is converted into hydroxide ions in solution, it is a weak base.
Ammonia is a building-block chemical used for manufacturing a wide range of products. Approximately 90% of the ammonia produced globally is used in fertilizer to enhance food production for billions of people. The rest is used in rubber and paper industries, wastewater treatment, and household cleaning products.
Frequently Asked Questions
Is Barium hydroxide a weak base?
No. It’s a strong base like Potassium hydroxide and Sodium hydroxide. It dissolves very well in water to give barium ions (Ba2+) and hydroxyl ions (OH–).
Furthermore, the solubility of Barium hydroxide is greater than Magnesium hydroxide, Beryllium hydroxide, and Calcium hydroxide. This is why it is a stronger base than other Group 2 metal hydroxides.
What are some examples of strong bases?
Strong bases are usually created from the hydroxides of alkali earth metals and alkali metals. Sodium hydroxide, Barium hydroxide, and Lithium hydroxide are some of the most common examples of strong bases. They can deprotonate weak acids.
There also exist superbases that are even stronger. They are Group 1 sales of carbanions, amides, and hydrides. Their conjugate acid is extremely weak. Sodium amide, Butyl lithium, and Ethoxide ion are some examples of superbases. They are often used in organic chemistry as reagents.
Is water an acid or base?
Water, in its pure form, is neither acidic nor basic — it is natural, having a pH level of 7.
However, impure water can be slightly acidic or basic in nature, depending on the components (like sand, heavy metals, or other chemicals) present in the water. When there are more negatively charged hydroxyl than positively charged hydroniums, the water becomes basic. Similarly, when there are more hydroniums than hydroxyls, the water becomes acidic.