- Structure
- Crystal and its ions
- Morphology
- Properties
- Physical appearance
- Molar mass
- Melting point
- Density
- pH
- Water solubility
- K
- Refractive index
- Stability
- Obtaining
- Applications
- Food processing
- Sewage disinfectant
- Paper industry
- Gas absorber
- Personal care
- Construction
- Risks and side effects
- References
The calcium hydroxide is an inorganic compound whose chemical formula is Ca (OH) 2. It is a white powder that has been in use for thousands of years, during which time it has earned several traditional names or nicknames; among them, we can mention slaked, dead, chemical, hydrated or fine lime.
In nature it is available in a rare mineral called portlandite, of the same color. Due to this, Ca (OH) 2 is not obtained directly from this mineral, but from a heat treatment, followed by hydration, of the limestone. Lime, CaO, is obtained from this, which is subsequently quenched or hydrated to produce Ca (OH) 2.
A solid sample of calcium hydroxide. Source: Chemicalinterest
Ca (OH) 2 is a relatively weak base in water, since it can hardly dissolve in hot water; but its solubility increases in cold water, because its hydration is exothermic. However, its basicity continues to be a reason to be careful with it when handling it, as it can cause burns to any part of the body.
It has been used as a pH regulator for different materials or foods, as well as being a good source of calcium in regards to its mass. It has applications in the paper industry, in the disinfection of sewage, in depilatory products, in foodstuffs made from corn flour.
However, its most important use has been as a construction material, since lime hydrates when mixed with the other ingredients in plaster or mortar. In these hardened mixtures, the Ca (OH) 2 absorbs carbon dioxide from the air to consolidate the sand crystals together with those formed from calcium carbonate.
Currently, research is still being carried out with the aim of developing better construction materials that have Ca (OH) 2 directly in their composition as nanoparticles.
Structure
Crystal and its ions
Ions of calcium hydroxide. Source: Claudio Pistilli
In the upper image we have the ions that make up calcium hydroxide. Its very formula Ca (OH) 2 indicates that for each Ca 2+ cation there are two OH anions - which interact with it through electrostatic attraction. The result is that both ions end up establishing a crystal with a hexagonal structure.
In such hexagonal crystals of Ca (OH) 2 the ions are very close to each other, which gives the appearance of being a polymeric structure; although there is no formal Ca-O covalent bond, still given the notable difference in electronegativity between the two elements.
Structure of calcium hydroxide
The structure generates octahedra CaO 6, that is, Ca 2+ interacts with six OH - (Ca 2+ -OH -).
A series of these octahedra make up a layer of the crystal, which can interact with another by means of hydrogen bonds that keep them intermolecularly cohesive; however, this interaction vanishes at a temperature of 580 ° C, when Ca (OH) 2 is dehydrated to CaO.
On the side of high pressures, there is not much information in this regard, although studies have shown that at a pressure of 6 GPa the hexagonal crystal undergoes a transition from the hexagonal to the monoclinic phase; and with it, the deformation of the CaO 6 octahedra and their layers.
Morphology
Ca (OH) 2 crystals are hexagonal, but that is not an impediment for them to adopt any morphology. Some of these structures (such as strands, flakes or rocks) are more porous than others, robust or flat, which directly influences their final applications.
Thus, it is not the same to use crystals from the mineral portlandite than to synthesize them so that they consist of nanoparticles where a few rigorous parameters are followed; such as the degree of hydration, the concentration of CaO used, and the time the crystal is allowed to grow.
Properties
Physical appearance
White, odorless, powdery solid with a bitter taste.
Molar mass
74.093 g / mol
Melting point
580 ° C. At this temperature it decomposes releasing water, so it never reaches vaporization:
Ca (OH) 2 => CaO + H 2 O
Density
2,211 g / cm 3
pH
A saturated aqueous solution thereof has a pH of 12.4 at 25 ° C.
Water solubility
The solubility of Ca (OH) 2 in water decreases with an increase in temperature. For example, at 0 ° C its solubility is 1.89 g / L; while at 20ºC and 100ºC, these are 1.73 g / L and 0.66 g / L, respectively.
This indicates a thermodynamic fact: the hydration of Ca (OH) 2 is exothermic, so obeying Le Chatelier's principle the equation would be:
Ca (OH) 2 <=> Ca 2+ + 2OH - + Q
Where Q is the heat released. The hotter the water, the more equilibrium will tend to the left; that is, less Ca (OH) 2 will dissolve. It is for this reason that in cold water it dissolves much more than in boiling water.
On the other hand, said solubility increases if the pH becomes acidic, due to the neutralization of the OH - ions and the displacement of the previous balance to the right. Even more heat is released during this process than in neutral water. Besides acidic aqueous solutions, Ca (OH) 2 is also soluble in glycerol.
K
5.5 · 10 -6. This value is considered small and is consistent with the low solubility of Ca (OH) 2 in water (same balance as above).
Refractive index
1,574
Stability
Ca (OH) 2 remains stable as long as it is not exposed to CO 2 from the air, as it absorbs it and forms calcium carbonate, CaCO 3. Therefore, it begins to become impurified in a solid mixture of Ca (OH) 2 -CaCO 3 crystals, where there are CO 3 2- anions competing with OH - to interact with Ca 2+:
Ca (OH) 2 + CO 2 => CaCO 3 + H 2 O
In fact, this is the reason why concentrated Ca (OH) 2 solutions turn milky, as a suspension of CaCO 3 particles appears.
Obtaining
Ca (OH) 2 is obtained commercially by reacting lime, CaO, with a two to three-fold excess of water:
CaO + H 2 O => Ca (OH) 2
However, carbonization of Ca (OH) 2 can occur in the process, just as explained above.
Other methods to obtain it consist of using soluble calcium salts, such as CaCl 2 or Ca (NO 3) 2, and basifying them with NaOH, so that Ca (OH) 2 precipitates. By controlling parameters such as water volumes, temperature, pH, solvent, degree of carbonization, maturation time, etc., nanoparticles with different morphologies can be synthesized.
It can also be prepared by selecting natural and renewable raw materials, or waste from an industry, that are rich in calcium, which when heated and its ashes will consist of lime; and from here, again, Ca (OH) 2 can be prepared by hydrating these ashes without the need to waste limestone, CaCO 3.
For example, agave bagasse has been used for this purpose, assigning added value to waste from the tequila industries.
Applications
Food processing
Pickles are first soaked in calcium hydroxide to make them crispier. Source: Pixabay.
Calcium hydroxide is present in many foods in some of its preparation stages. For example, pickles, such as gherkins, are dipped in an aqueous solution of the same to make them more crisp when they are packed in vinegar. This is because the proteins on its surface absorb calcium from the environment.
The same occurs with corn grains before transforming them into flour, as it helps them release vitamin B 3 (niacin) and facilitates their grinding. The calcium it provides is also used to add nutritional value to certain juices.
Ca (OH) 2 can also replace baking powder in some bread recipes, and clarify the sugary solutions obtained from sugar cane and beets.
Sewage disinfectant
The clarifying action of Ca (OH) 2 is due to the fact that it acts as a flocculating agent; that is, it increases the size of the suspended particles until they form flocs, which later settle or can be filtered.
This property has been used to disinfect sewage, destabilizing its unpleasant colloids to the view (and smell) of the spectators.
Paper industry
Ca (OH) 2 is used in the Kraft process to regenerate the NaOH used to treat wood.
Gas absorber
Ca (OH) 2 is used to remove CO 2 from closed spaces or in environments where its presence is counterproductive.
Personal care
Ca (OH) 2 is tacitly found in formulations for depilatory creams, as its basicity helps to weaken the keratin of the hairs, and thus, it is easier to remove them.
Construction
Calcium hydroxide is formed part of the structures of old construction sites such as the pyramids of Egypt. Source: Pexels.
Ca (OH) 2 has been present since time immemorial, integrating the masses of plaster and mortar used in the construction of Egyptian architectural works such as the pyramids; also buildings, mausoleums, walls, stairs, floors, supports, and even to rebuild dental cement.
Its fortifying action is due to the fact that when “breathing” the CO 2, the resulting crystals of CaCO 3 end up integrating the sands and the other components of such mixtures to a better degree.
Risks and side effects
Ca (OH) 2 is not a strongly basic solid compared to other hydroxides, although it is more so than Mg (OH) 2. Even so, despite not being reactive or flammable, its basicity is still aggressive enough to cause minor burns.
Therefore, it must be handled with respect, as it is capable of irritating the eyes, tongue and lungs, as well as triggering other illnesses such as: loss of vision, severe alkalization of the blood, skin rashes, vomiting and sore throat.
References
- Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
- Wikipedia. (2019). Calcium hydroxide. Recovered from: en.wikipedia.org
- Chávez Guerrero et al. (2016). Synthesis and characterization of calcium hydroxide obtained from agave bagasse and investigation of its antibacterial activity. Recovered from: scielo.org.mx
- Riko Iizuka, Takehiko Yagi, Kazuki Komatsu, Hirotada Gotou, Taku Tsuchiya, Keiji Kusaba, Hiroyuki Kagi. (2013). Crystal structure of the high-pressure phase of calcium hydroxide, portlandite: In situ powder and single-crystal X-ray diffraction study. American Mineralogist; 98 (8-9): 1421–1428. doi: doi.org/10.2138/am.2013.4386
- Hans Lohninger. (June 05, 2019). Calcium Hydroxide. Chemistry LibreTexts. Recovered from: chem.libretexts.org
- Aniruddha S. et al. (2015). Synthesis of Nano Calcium Hydroxide in Aqueous Medium. The American Ceramic Society. doi.org/10.1111/jace.14023
- Carly Vandergriendt. (April 12, 2018). How Is Calcium Hydroxide Used in Food, and Is It Safe? Recovered from: healthline.com
- Brian Clegg. (May 26, 2015). Calcium hydroxide. Recovered from: chemistryworld.com