Calcium carbonate

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Calcium carbonate

Names
IUPAC name Calcium carbonate
Other names Aragonite; calcite; chalk; lime; limestone; marble; oyster; pearl
Identifiers
CAS Number	471-34-1
3D model (JSmol)	Interactive image Interactive image
ChEBI	CHEBI:3311
ChEMBL	ChEMBL1200539
ChemSpider	9708
DrugBank	DB06724
ECHA InfoCard	100.006.765
EC Number	207-439-9
E number	E170 (colours)
KEGG	D00932
PubChem CID	10112
RTECS number	FF9335000
UNII	H0G9379FGK
CompTox Dashboard (EPA)	DTXSID3036238

Properties
Chemical formula	CaCO3
Molar mass	100.0869 g/mol
Appearance	Fine white powder; chalky taste
Odor	odorless
Density	2.711 g/cm3 (calcite) 2.83 g/cm3 (aragonite)
Melting point	1,339 °C (2,442 °F; 1,612 K) (calcite) 825 °C (1,517 °F; 1,098 K) (aragonite)
Boiling point	decomposes
Solubility in water	0.013 g/L (25 °C)
Solubility product (Ksp)	3.3×10−9
Solubility in dilute acids	soluble
Acidity (pKa)	9.0
Magnetic susceptibility (χ)	−3.82×10−5 cm3/mol
Refractive index (nD)	1.59
Structure
Crystal structure	Trigonal
Space group	32/m
Thermochemistry
Std molar entropy (So298)	93 J·mol−1·K−1
Std enthalpy of formation (ΔfH⦵298)	−1207 kJ·mol−1
Pharmacology
ATC code	A02AC01 (WHO) A12AA04 (WHO)
Hazards
Safety data sheet	ICSC 1193
NFPA 704 (fire diamond)	0 0 0
Lethal dose or concentration (LD, LC):
LD50 (median dose)	6450 mg/kg (oral, rat)
NIOSH (US health exposure limits):
PEL (Permissible)	TWA 15 mg/m3 (total) TWA 5 mg/m3 (resp)
Related compounds
Other anions	Calcium bicarbonate
Other cations	Beryllium carbonate Magnesium carbonate Strontium carbonate Barium carbonate Radium carbonate
Related compounds	Calcium sulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Crystal structure of calcite

Calcium carbonate is a chemical compound with the formula CaCO₃. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is the main component of eggshells, snail shells, seashells and pearls. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale. It has medical use as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause hypercalcemia and digestive issues.

Chemistry

Calcium carbonate shares the typical properties of other carbonates. Notably it

• reacts with acids, releasing carbon dioxide (technically speaking, carbonic acid, but that disintegrates quickly to CO₂ and H₂O):

CaCO₃(s) + 2 H⁺(aq) → Ca²⁺(aq) + CO₂(g) + H₂O(l)

• releases carbon dioxide upon heating, called a thermal decomposition reaction, or calcination (to above 840 °C in the case of CaCO₃), to form calcium oxide, commonly called quicklime, with reaction enthalpy 178 kJ/mol:

CaCO₃(s) → CaO(s) + CO₂(g)

Calcium carbonate reacts with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.

CaCO₃(s) + CO₂(g) + H₂O(l) → Ca(HCO₃)₂(aq)

This reaction is important in the erosion of carbonate rock, forming caverns, and leads to hard water in many regions.

An unusual form of calcium carbonate is the hexahydrate, ikaite, CaCO₃·6H₂O. Ikaite is stable only below 8 °C.

Preparation

The vast majority of calcium carbonate used in industry is extracted by mining or quarrying. Pure calcium carbonate (such as for food or pharmaceutical use), can be produced from a pure quarried source (usually marble).

Alternatively, calcium carbonate is prepared from calcium oxide. Water is added to give calcium hydroxide then carbon dioxide is passed through this solution to precipitate the desired calcium carbonate, referred to in the industry as precipitated calcium carbonate (PCC):

CaO + H₂O → Ca(OH)₂

Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O

Structure

The thermodynamically stable form of CaCO₃ under normal conditions is hexagonal β-CaCO₃ (the mineral calcite). Other forms can be prepared, the denser (2.83 g/cm³) orthorhombic λ-CaCO₃ (the mineral aragonite) and hexagonal μ-CaCO₃, occurring as the mineral vaterite. The aragonite form can be prepared by precipitation at temperatures above 85 °C; the vaterite form can be prepared by precipitation at 60 °C. Calcite contains calcium atoms coordinated by six oxygen atoms; in aragonite they are coordinated by nine oxygen atoms. The vaterite structure is not fully understood. Magnesium carbonate (MgCO₃) has the calcite structure, whereas strontium carbonate and barium carbonate (SrCO₃ and BaCO₃) adopt the aragonite structure, reflecting their larger ionic radii.

Occurrence

Calcite is the most stable polymorph of calcium carbonate. It is transparent to opaque. A transparent variety called Iceland spar (shown here) was used to create polarized light in the 19th century.

Geological sources

Calcite, aragonite and vaterite are pure calcium carbonate minerals. Industrially important source rocks which are predominantly calcium carbonate include limestone, chalk, marble and travertine.

Biological sources

Calcium carbonate chunks from clamshell

Eggshells, snail shells and most seashells are predominantly calcium carbonate and can be used as industrial sources of that chemical. Oyster shells have enjoyed recent recognition as a source of dietary calcium, but are also a practical industrial source. Dark green vegetables such as broccoli and kale contain dietarily significant amounts of calcium carbonate, but they are not practical as an industrial source.

Extraterrestrial

Beyond Earth, strong evidence suggests the presence of calcium carbonate on Mars. Signs of calcium carbonate have been detected at more than one location (notably at Gusev and Huygens craters). This provides some evidence for the past presence of liquid water.

Geology

Surface precipitation of CaCO3 as tufa in Rubaksa, Ethiopia

Carbonate is found frequently in geologic settings and constitutes an enormous carbon reservoir. Calcium carbonate occurs as aragonite, calcite and dolomite as significant constituents of the calcium cycle. The carbonate minerals form the rock types: limestone, chalk, marble, travertine, tufa, and others.

In warm, clear tropical waters corals are more abundant than towards the poles where the waters are cold. Calcium carbonate contributors, including plankton (such as coccoliths and planktic foraminifera), coralline algae, sponges, brachiopods, echinoderms, bryozoa and mollusks, are typically found in shallow water environments where sunlight and filterable food are more abundant. Cold-water carbonates do exist at higher latitudes but have a very slow growth rate. The calcification processes are changed by ocean acidification.

Where the oceanic crust is subducted under a continental plate sediments will be carried down to warmer zones in the asthenosphere and lithosphere. Under these conditions calcium carbonate decomposes to produce carbon dioxide which, along with other gases, give rise to explosive volcanic eruptions.

Carbonate compensation depth

The carbonate compensation depth (CCD) is the point in the ocean where the rate of precipitation of calcium carbonate is balanced by the rate of dissolution due to the conditions present. Deep in the ocean, the temperature drops and pressure increases. Calcium carbonate is unusual in that its solubility increases with decreasing temperature. Increasing pressure also increases the solubility of calcium carbonate. The carbonate compensation depth can range from 4,000 to 6,000 meters below sea level.

Role in taphonomy

Calcium carbonate can preserve fossils through permineralization. Most of the vertebrate fossils of the Two Medicine Formation—a geologic formation known for its duck-billed dinosaur eggs—are preserved by CaCO₃ permineralization. This type of preservation conserves high levels of detail, even down to the microscopic level. However, it also leaves specimens vulnerable to weathering when exposed to the surface.

Trilobite populations were once thought to have composed the majority of aquatic life during the Cambrian, due to the fact that their calcium carbonate-rich shells were more easily preserved than those of other species, which had purely chitinous shells.

Uses

Construction

The main use of calcium carbonate is in the construction industry, either as a building material, or limestone aggregate for road building, as an ingredient of cement, or as the starting material for the preparation of builders' lime by burning in a kiln. However, because of weathering mainly caused by acid rain, calcium carbonate (in limestone form) is no longer used for building purposes on its own, but only as a raw primary substance for building materials.

Calcium carbonate is also used in the purification of iron from iron ore in a blast furnace. The carbonate is calcined in situ to give calcium oxide, which forms a slag with various impurities present, and separates from the purified iron.

In the oil industry, calcium carbonate is added to drilling fluids as a formation-bridging and filtercake-sealing agent; it is also a weighting material which increases the density of drilling fluids to control the downhole pressure. Calcium carbonate is added to swimming pools, as a pH corrector for maintaining alkalinity and offsetting the acidic properties of the disinfectant agent.

It is also used as a raw material in the refining of sugar from sugar beet; it is calcined in a kiln with anthracite to produce calcium oxide and carbon dioxide. This burnt lime is then slaked in fresh water to produce a calcium hydroxide suspension for the precipitation of impurities in raw juice during carbonatation.

Calcium carbonate in the form of chalk has traditionally been a major component of blackboard chalk. However, modern manufactured chalk is mostly gypsum, hydrated calcium sulfate CaSO₄·2H₂O. Calcium carbonate is a main source for growing biorock. Precipitated calcium carbonate (PCC), pre-dispersed in slurry form, is a common filler material for latex gloves with the aim of achieving maximum saving in material and production costs.

Fine ground calcium carbonate (GCC) is an essential ingredient in the microporous film used in diapers and some building films, as the pores are nucleated around the calcium carbonate particles during the manufacture of the film by biaxial stretching. GCC and PCC are used as a filler in paper because they are cheaper than wood fiber. In terms of market volume, GCC are the most important types of fillers currently used. Printing and writing paper can contain 10–20% calcium carbonate. In North America, calcium carbonate has begun to replace kaolin in the production of glossy paper. Europe has been practicing this as alkaline papermaking or acid-free papermaking for some decades. PCC used for paper filling and paper coatings is precipitated and prepared in a variety of shapes and sizes having characteristic narrow particle size distributions and equivalent spherical diameters of 0.4 to 3 micrometers.

Calcium carbonate is widely used as an extender in paints, in particular matte emulsion paint where typically 30% by weight of the paint is either chalk or marble. It is also a popular filler in plastics. Some typical examples include around 15 to 20% loading of chalk in unplasticized polyvinyl chloride (uPVC) drainpipes, 5% to 15% loading of stearate-coated chalk or marble in uPVC window profile. PVC cables can use calcium carbonate at loadings of up to 70 phr (parts per hundred parts of resin) to improve mechanical properties (tensile strength and elongation) and electrical properties (volume resistivity). Polypropylene compounds are often filled with calcium carbonate to increase rigidity, a requirement that becomes important at high usage temperatures. Here the percentage is often 20–40%. It also routinely used as a filler in thermosetting resins (sheet and bulk molding compounds) and has also been mixed with ABS, and other ingredients, to form some types of compression molded "clay" poker chips. Precipitated calcium carbonate, made by dropping calcium oxide into water, is used by itself or with additives as a white paint, known as whitewashing.

Calcium carbonate is added to a wide range of trade and do it yourself adhesives, sealants, and decorating fillers. Ceramic tile adhesives typically contain 70% to 80% limestone. Decorating crack fillers contain similar levels of marble or dolomite. It is also mixed with putty in setting stained glass windows, and as a resist to prevent glass from sticking to kiln shelves when firing glazes and paints at high temperature.

In ceramic glaze applications, calcium carbonate is known as whiting, and is a common ingredient for many glazes in its white powdered form. When a glaze containing this material is fired in a kiln, the whiting acts as a flux material in the glaze. Ground calcium carbonate is an abrasive (both as scouring powder and as an ingredient of household scouring creams), in particular in its calcite form, which has the relatively low hardness level of 3 on the Mohs scale, and will therefore not scratch glass and most other ceramics, enamel, bronze, iron, and steel, and have a moderate effect on softer metals like aluminium and copper. A paste made from calcium carbonate and deionized water can be used to clean tarnish on silver.

Health and diet

500-milligram calcium supplements made from calcium carbonate

Calcium carbonate is widely used medicinally as an inexpensive dietary calcium supplement for gastric antacid (such as Tums). It may be used as a phosphate binder for the treatment of hyperphosphatemia (primarily in patients with chronic kidney failure). It is used in the pharmaceutical industry as an inert filler for tablets and other pharmaceuticals.

Calcium carbonate is used in the production of calcium oxide as well as toothpaste and has seen a resurgence as a food preservative and color retainer, when used in or with products such as organic apples.

Calcium carbonate is used therapeutically as phosphate binder in patients on maintenance haemodialysis. It is the most common form of phosphate binder prescribed, particularly in non-dialysis chronic kidney disease. Calcium carbonate is the most commonly used phosphate binder, but clinicians are increasingly prescribing the more expensive, non-calcium-based phosphate binders, particularly sevelamer.

Excess calcium from supplements, fortified food, and high-calcium diets can cause milk-alkali syndrome, which has serious toxicity and can be fatal. In 1915, Bertram Sippy introduced the "Sippy regimen" of hourly ingestion of milk and cream, and the gradual addition of eggs and cooked cereal, for 10 days, combined with alkaline powders, which provided symptomatic relief for peptic ulcer disease. Over the next several decades, the Sippy regimen resulted in kidney failure, alkalosis, and hypercalcaemia, mostly in men with peptic ulcer disease. These adverse effects were reversed when the regimen stopped, but it was fatal in some patients with protracted vomiting. Milk-alkali syndrome declined in men after effective treatments for peptic ulcer disease arose. Since the 1990s it has been most frequently reported in women taking calcium supplements above the recommended range of 1.2 to 1.5 grams daily, for prevention and treatment of osteoporosis, and is exacerbated by dehydration. Calcium has been added to over-the-counter products, which contributes to inadvertent excessive intake. Excessive calcium intake can lead to hypercalcemia, complications of which include vomiting, abdominal pain and altered mental status.

As a food additive it is designated E170, and it has an INS number of 170. Used as an acidity regulator, anticaking agent, stabilizer or color it is approved for usage in the EU, USA and Australia and New Zealand. It is "added by law to all UK milled bread flour except wholemeal". It is used in some soy milk and almond milk products as a source of dietary calcium; at least one study suggests that calcium carbonate might be as bioavailable as the calcium in cow's milk. Calcium carbonate is also used as a firming agent in many canned and bottled vegetable products.

Several calcium supplement formulations have been documented to contain the chemical element lead, posing a public health concern. Lead is commonly found in natural sources of calcium.

Agriculture and aquaculture

Agricultural lime, powdered chalk or limestone, is used as a cheap method for neutralising acidic soil, making it suitable for planting, also used in aquaculture industry for pH regulation of pond soil before initiating culture.

Household cleaning

Calcium carbonate is a key ingredient in many household cleaning powders like Comet and is used as a scrubbing agent.

Pollution mitigation

In 1989, a researcher, Ken Simmons, introduced CaCO₃ into the Whetstone Brook in Massachusetts. His hope was that the calcium carbonate would counter the acid in the stream from acid rain and save the trout that had ceased to spawn. Although his experiment was a success, it did increase the amount of aluminium ions in the area of the brook that was not treated with the limestone. This shows that CaCO₃ can be added to neutralize the effects of acid rain in river ecosystems. Currently calcium carbonate is used to neutralize acidic conditions in both soil and water. Since the 1970s, such liming has been practiced on a large scale in Sweden to mitigate acidification and several thousand lakes and streams are limed repeatedly.

Calcium carbonate is also used in flue gas desulfurisation applications eliminating harmful SO₂ and NO₂ emissions from coal and other fossil fuels burnt in large fossil fuel power stations.

Calcination equilibrium

Calcination of limestone using charcoal fires to produce quicklime has been practiced since antiquity by cultures all over the world. The temperature at which limestone yields calcium oxide is usually given as 825 °C, but stating an absolute threshold is misleading. Calcium carbonate exists in equilibrium with calcium oxide and carbon dioxide at any temperature. At each temperature there is a partial pressure of carbon dioxide that is in equilibrium with calcium carbonate. At room temperature the equilibrium overwhelmingly favors calcium carbonate, because the equilibrium CO₂ pressure is only a tiny fraction of the partial CO₂ pressure in air, which is about 0.035 kPa.

At temperatures above 550 °C the equilibrium CO₂ pressure begins to exceed the CO₂ pressure in air. So above 550 °C, calcium carbonate begins to outgas CO₂ into air. However, in a charcoal fired kiln, the concentration of CO₂ will be much higher than it is in air. Indeed, if all the oxygen in the kiln is consumed in the fire, then the partial pressure of CO₂ in the kiln can be as high as 20 kPa.

The table shows that this partial pressure is not achieved until the temperature is nearly 800 °C. For the outgassing of CO₂ from calcium carbonate to happen at an economically useful rate, the equilibrium pressure must significantly exceed the ambient pressure of CO₂. And for it to happen rapidly, the equilibrium pressure must exceed total atmospheric pressure of 101 kPa, which happens at 898 °C.

Equilibrium pressure of CO₂ over CaCO₃ (P) versus temperature (T).

P (kPa)	0.055	0.13	0.31	1.80	5.9	9.3	14	24	34	51	72	80	91	101	179	901	3961
T (°C)	550	587	605	680	727	748	777	800	830	852	871	881	891	898	937	1082	1241

Solubility

With varying CO₂ pressure

Travertine calcium carbonate deposits from a hot spring

Calcium carbonate is poorly soluble in pure water (47 mg/L at normal atmospheric CO₂ partial pressure as shown below).

The equilibrium of its solution is given by the equation (with dissolved calcium carbonate on the right):

CaCO3 ⇌ Ca2+ + CO2− 3

Ksp = 3.7×10−9 to 8.7×10−9 at 25 °C

where the solubility product for [Ca²⁺][CO²⁻
₃] is given as anywhere from K_sp = 3.7×10⁻⁹ to K_sp = 8.7×10⁻⁹ at 25 °C, depending upon the data source. What the equation means is that the product of molar concentration of calcium ions (moles of dissolved Ca²⁺ per liter of solution) with the molar concentration of dissolved CO²⁻
₃ cannot exceed the value of K_sp. This seemingly simple solubility equation, however, must be taken along with the more complicated equilibrium of carbon dioxide with water (see carbonic acid). Some of the CO²⁻
₃ combines with H⁺ in the solution according to

HCO− 3 ⇌ H+ + CO2− 3

Ka2 = 5.61×10−11 at 25 °C

HCO⁻
₃ is known as the bicarbonate ion. Calcium bicarbonate is many times more soluble in water than calcium carbonate—indeed it exists only in solution.

Some of the HCO⁻
₃ combines with H⁺ in solution according to

H2CO3 ⇌ H+ + HCO− 3

Ka1 = 2.5×10−4 at 25 °C

Some of the H₂CO₃ breaks up into water and dissolved carbon dioxide according to

H2O + CO2(aq) ⇌ H2CO3

Kh = 1.70×10−3 at 25 °C

And dissolved carbon dioxide is in equilibrium with atmospheric carbon dioxide according to

${\displaystyle {\frac {P_{{\ce {CO2}}}}{[{\ce {CO2}}]}}\ =\ k_{{\ce {H}}}}$

where kH = 29.76 atm/(mol/L) at 25 °C (Henry constant), PCO2 being the CO2 partial pressure.

For ambient air, P_CO2 is around 3.5×10⁻⁴ atmospheres (or equivalently 35 Pa). The last equation above fixes the concentration of dissolved CO₂ as a function of P_CO2, independent of the concentration of dissolved CaCO₃. At atmospheric partial pressure of CO₂, dissolved CO₂ concentration is 1.2×10⁻⁵ moles per liter. The equation before that fixes the concentration of H₂CO₃ as a function of CO₂ concentration. For [CO₂] = 1.2×10⁻⁵, it results in [H₂CO₃] = 2.0×10⁻⁸ moles per liter. When [H₂CO₃] is known, the remaining three equations together with

Calcium ion solubility as a function of CO₂ partial pressure at 25 °C (K_sp = 4.47×10⁻⁹)

PCO2 (atm)	pH	[Ca2+] (mol/L)
10−12	12.0	5.19×10−3
10−10	11.3	1.12×10−3
10−8	10.7	2.55×10−4
10−6	9.83	1.20×10−4
10−4	8.62	3.16×10−4
3.5×10−4	8.27	4.70×10−4
10−3	7.96	6.62×10−4
10−2	7.30	1.42×10−3
10−1	6.63	3.05×10−3
1	5.96	6.58×10−3
10	5.30	1.42×10−2

H2O ⇌ H+ + OH−

K = 10−14 at 25 °C

(which is true for all aqueous solutions), and the fact that the solution must be electrically neutral,

[Ca²⁺] + 2 [H⁺] = [HCO⁻
₃] + [CO²⁻
₃] + [OH⁻]

make it possible to solve simultaneously for the remaining five unknown concentrations (note that the above form of the neutrality equation is valid only if calcium carbonate has been put in contact with pure water or with a neutral pH solution; in the case where the initial water solvent pH is not neutral, the equation is modified).

The adjacent table shows the result for [Ca²⁺] and [H⁺] (in the form of pH) as a function of ambient partial pressure of CO₂ (K_sp = 4.47×10⁻⁹ has been taken for the calculation).

• At atmospheric levels of ambient CO₂ the table indicates that the solution will be slightly alkaline with a maximum CaCO₃ solubility of 47 mg/L.
• As ambient CO₂ partial pressure is reduced below atmospheric levels, the solution becomes more and more alkaline. At extremely low P_CO2, dissolved CO₂, bicarbonate ion, and carbonate ion largely evaporate from the solution, leaving a highly alkaline solution of calcium hydroxide, which is more soluble than CaCO₃. Note that for P_CO2 = 10⁻¹² atm, the [Ca²⁺][OH⁻]² product is still below the solubility product of Ca(OH)₂ (8×10⁻⁶). For still lower CO₂ pressure, Ca(OH)₂ precipitation will occur before CaCO₃ precipitation.
• As ambient CO₂ partial pressure increases to levels above atmospheric, pH drops, and much of the carbonate ion is converted to bicarbonate ion, which results in higher solubility of Ca²⁺.

The effect of the latter is especially evident in day-to-day life of people who have hard water. Water in aquifers underground can be exposed to levels of CO₂ much higher than atmospheric. As such water percolates through calcium carbonate rock, the CaCO₃ dissolves according to the second trend. When that same water then emerges from the tap, in time it comes into equilibrium with CO₂ levels in the air by outgassing its excess CO₂. The calcium carbonate becomes less soluble as a result, and the excess precipitates as lime scale. This same process is responsible for the formation of stalactites and stalagmites in limestone caves.

Two hydrated phases of calcium carbonate, monohydrocalcite CaCO₃·H₂O and ikaite CaCO₃·6H₂O, may precipitate from water at ambient conditions and persist as metastable phases.

With varying pH, temperature and salinity: CaCO₃ scaling in swimming pools

In contrast to the open equilibrium scenario above, many swimming pools are managed by addition of sodium bicarbonate (NaHCO₃) to about 2 mM as a buffer, then control of pH through use of HCl, NaHSO₄, Na₂CO₃, NaOH or chlorine formulations that are acidic or basic. In this situation, dissolved inorganic carbon (total inorganic carbon) is far from equilibrium with atmospheric CO₂. Progress towards equilibrium through outgassing of CO₂ is slowed by

1. the slow reaction

   H₂CO₃ ⇌ CO₂(aq) + H₂O;

2. limited aeration in a deep water column; and
3. periodic replenishment of bicarbonate to maintain buffer capacity (often estimated through measurement of ‘total alkalinity’).

In this situation, the dissociation constants for the much faster reactions

H₂CO₃ ⇌ H⁺ + HCO⁻
₃ ⇌ 2 H⁺ + CO²⁻
₃

allow the prediction of concentrations of each dissolved inorganic carbon species in solution, from the added concentration of HCO⁻
₃ (which constitutes more than 90% of Bjerrum plot species from pH 7 to pH 8 at 25 °C in fresh water). Addition of HCO⁻
₃ will increase CO²⁻
₃ concentration at any pH. Rearranging the equations given above, we can see that [Ca²⁺] = K_sp/[CO²⁻
₃], and [CO²⁻
₃] = K_a2 [HCO⁻
₃]/[H⁺]. Therefore, when HCO⁻
₃ concentration is known, the maximum concentration of Ca²⁺ ions before scaling through CaCO₃ precipitation can be predicted from the formula:

${\displaystyle [{\ce {Ca^2+}}]_{\max }={\frac {K_{\mathrm {sp} }}{K_{\mathrm {a} 2}}}\times {\frac {[{\ce {H+}}]}{[{\ce {HCO3-}}]}}}$

The solubility product for CaCO₃ (K_sp) and the dissociation constants for the dissolved inorganic carbon species (including K_a2) are all substantially affected by temperature and salinity, with the overall effect that [Ca²⁺]_max increases from freshwater to saltwater, and decreases with rising temperature, pH, or added bicarbonate level, as illustrated in the accompanying graphs.

The trends are illustrative for pool management, but whether scaling occurs also depends on other factors including interactions with Mg²⁺, B(OH)⁻
₄ and other ions in the pool, as well as supersaturation effects. Scaling is commonly observed in electrolytic chlorine generators, where there is a high pH near the cathode surface and scale deposition further increases temperature. This is one reason that some pool operators prefer borate over bicarbonate as the primary pH buffer, and avoid the use of pool chemicals containing calcium.

Solubility in a strong or weak acid solution

Solutions of strong (HCl), moderately strong (sulfamic) or weak (acetic, citric, sorbic, lactic, phosphoric) acids are commercially available. They are commonly used as descaling agents to remove limescale deposits. The maximum amount of CaCO₃ that can be "dissolved" by one liter of an acid solution can be calculated using the above equilibrium equations.

• In the case of a strong monoacid with decreasing acid concentration [A] = [A⁻], we obtain (with CaCO₃ molar mass = 100 g/mol):

[A] (mol/L)	1	10−1	10−2	10−3	10−4	10−5	10−6	10−7	10−10
Initial pH	0.00	1.00	2.00	3.00	4.00	5.00	6.00	6.79	7.00
Final pH	6.75	7.25	7.75	8.14	8.25	8.26	8.26	8.26	8.27
Dissolved CaCO3 (g/L of acid)	50.0	5.00	0.514	0.0849	0.0504	0.0474	0.0471	0.0470	0.0470

where the initial state is the acid solution with no Ca²⁺ (not taking into account possible CO₂ dissolution) and the final state is the solution with saturated Ca²⁺. For strong acid concentrations, all species have a negligible concentration in the final state with respect to Ca²⁺ and A⁻ so that the neutrality equation reduces approximately to 2[Ca²⁺] = [A⁻] yielding [Ca²⁺] ≈ 1/2 [A⁻]. When the concentration decreases, [HCO⁻
₃] becomes non-negligible so that the preceding expression is no longer valid. For vanishing acid concentrations, one can recover the final pH and the solubility of CaCO₃ in pure water.

• In the case of a weak monoacid (here we take acetic acid with pK_a = 4.76) with decreasing total acid concentration [A] = [A⁻] + [AH], we obtain:

[A] (mol/L)	1	10−1	10−2	10−3	10−4	10−5	10−6	10−7	10−10
Initial pH	2.38	2.88	3.39	3.91	4.47	5.15	6.02	6.79	7.00
Final pH	6.75	7.25	7.75	8.14	8.25	8.26	8.26	8.26	8.27
Dissolved CaCO3 (g/L of acid)	49.5	4.99	0.513	0.0848	0.0504	0.0474	0.0471	0.0470	0.0470

For the same total acid concentration, the initial pH of the weak acid is less acid than the one of the strong acid; however, the maximum amount of CaCO₃ which can be dissolved is approximately the same. This is because in the final state, the pH is larger than the pK_a, so that the weak acid is almost completely dissociated, yielding in the end as many H⁺ ions as the strong acid to "dissolve" the calcium carbonate.

• The calculation in the case of phosphoric acid (which is the most widely used for domestic applications) is more complicated since the concentrations of the four dissociation states corresponding to this acid must be calculated together with [HCO⁻
  ₃], [CO²⁻
  ₃], [Ca²⁺], [H⁺] and [OH⁻]. The system may be reduced to a seventh degree equation for [H⁺] the numerical solution of which gives

[A] (mol/L)	1	10−1	10−2	10−3	10−4	10−5	10−6	10−7	10−10
Initial pH	1.08	1.62	2.25	3.05	4.01	5.00	5.97	6.74	7.00
Final pH	6.71	7.17	7.63	8.06	8.24	8.26	8.26	8.26	8.27
Dissolved CaCO3 (g/L of acid)	62.0	7.39	0.874	0.123	0.0536	0.0477	0.0471	0.0471	0.0470

where [A] = [H₃PO₄] + [H
₂PO⁻
₄] + [HPO²⁻
₄] + [PO³⁻
₄] is the total acid concentration. Thus phosphoric acid is more efficient than a monoacid since at the final almost neutral pH, the second dissociated state concentration [HPO²⁻
₄] is not negligible (see phosphoric acid).

Acetylene

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Acetylene

Acetylene

Names
Preferred IUPAC name Acetylene
Systematic IUPAC name Ethyne
Identifiers
CAS Number	74-86-2
3D model (JSmol)	Interactive image
Beilstein Reference	906677
ChEBI	CHEBI:27518
ChEMBL	ChEMBL116336
ChemSpider	6086
ECHA InfoCard	100.000.743
EC Number	200-816-9
Gmelin Reference	210
KEGG	C01548
PubChem CID	6326
RTECS number	AO9600000
UNII	OC7TV75O83
UN number	1001 (dissolved) 3138 (in mixture with ethylene and propylene)
CompTox Dashboard (EPA)	DTXSID6026379

Properties
Chemical formula	C2H2
Molar mass	26.038 g·mol−1
Appearance	Colorless gas
Odor	Odorless
Density	1.1772 g/L = 1.1772 kg/m3 (0 °C, 101.3 kPa)
Melting point	−80.8 °C (−113.4 °F; 192.3 K) Triple point at 1.27 atm
Sublimation conditions	−84 °C; −119 °F; 189 K (1 atm)
Solubility in water	slightly soluble
Vapor pressure	44.2 atm (20 °C)
Acidity (pKa)	25
Conjugate acid	Ethynium
Magnetic susceptibility (χ)	−12.5×10−6 cm3/mol
Structure
Molecular shape	Linear
Thermochemistry
Std molar entropy (So298)	201 J/(mol·K)
Std enthalpy of formation (ΔfH⦵298)	+226.88 kJ/mol
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H220, H336
Precautionary statements	P202, P210, P233, P261, P271, P304, P312, P340, P377, P381, P403, P405, P501
NFPA 704 (fire diamond)	1 4 3
Autoignition temperature	300 °C (572 °F; 573 K)
NIOSH (US health exposure limits):
PEL (Permissible)	none
REL (Recommended)	C 2500 ppm (2662 mg/m3)
IDLH (Immediate danger)	N.D.
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Acetylene (systematic name: ethyne) is the chemical compound with the formula C₂H₂. It is a hydrocarbon and the simplest alkyne. This colorless gas (lower hydrocarbons are generally gaseous in nature) is widely used as a fuel and a chemical building block. It is unstable in its pure form and thus is usually handled as a solution. Pure acetylene is odorless, but commercial grades usually have a marked odor due to impurities such as divinyl sulfide and phosphine.

As an alkyne, acetylene is unsaturated because its two carbon atoms are bonded together in a triple bond. The carbon–carbon triple bond places all four atoms in the same straight line, with CCH bond angles of 180°.

Discovery

Acetylene was discovered in 1836 by Edmund Davy, who identified it as a "new carburet of hydrogen". It was an accidental discovery while attempting to isolate potassium metal. By heating potassium carbonate with carbon at very high temperatures, he produced a residue of what is now known as potassium carbide, (K₂C₂), which reacted with water to release the new gas. It was rediscovered in 1860 by French chemist Marcellin Berthelot, who coined the name acétylène. Berthelot's empirical formula for acetylene (C₄H₂), as well as the alternative name "quadricarbure d'hydrogène" (hydrogen quadricarbide), were incorrect because chemists at that time used the wrong atomic mass for carbon (6 instead of 12). Berthelot was able to prepare this gas by passing vapours of organic compounds (methanol, ethanol, etc.) through a red hot tube and collecting the effluent. He also found that acetylene was formed by sparking electricity through mixed cyanogen and hydrogen gases. Berthelot later obtained acetylene directly by passing hydrogen between the poles of a carbon arc.

Preparation

Since the 1950s, acetylene has mainly been manufactured by the partial combustion of methane. It is a recovered side product in production of ethylene by cracking of hydrocarbons. Approximately 400,000 tonnes were produced by this method in 1983. Its presence in ethylene is usually undesirable because of its explosive character and its ability to poison Ziegler–Natta catalysts. It is selectively hydrogenated into ethylene, usually using Pd–Ag catalysts.

Acetylene factory with annual capacity of 90,000 tons, commissioned in 2020 by BASF.

Until the 1950s, when oil supplanted coal as the chief source of reduced carbon, acetylene (and the aromatic fraction from coal tar) was the main source of organic chemicals in the chemical industry. It was prepared by the hydrolysis of calcium carbide, a reaction discovered by Friedrich Wöhler in 1862 and still familiar to students:

CaC₂ + 2H₂O → Ca(OH)₂ + C₂H_2↑

Calcium carbide production requires extremely high temperatures, ~2000 °C, necessitating the use of an electric arc furnace. In the US, this process was an important part of the late-19th century revolution in chemistry enabled by the massive hydroelectric power project at Niagara Falls.

Bonding

In terms of valence bond theory, in each carbon atom the 2s orbital hybridizes with one 2p orbital thus forming an sp hybrid. The other two 2p orbitals remain unhybridized. The two ends of the two sp hybrid orbital overlap to form a strong σ valence bond between the carbons, while on each of the other two ends hydrogen atoms attach also by σ bonds. The two unchanged 2p orbitals form a pair of weaker π bonds.

Since acetylene is a linear symmetrical molecule, it possesses the D_∞h point group.

Physical properties

Changes of state

At atmospheric pressure, acetylene cannot exist as a liquid and does not have a melting point. The triple point on the phase diagram corresponds to the melting point (−80.8 °C) at the minimal pressure at which liquid acetylene can exist (1.27 atm). At temperatures below the triple point, solid acetylene can change directly to the vapour (gas) by sublimation. The sublimation point at atmospheric pressure is −84.0 °C.

Other

At room temperature, the solubility of acetylene in acetone is 27.9 g per kg. For the same amount of dimethylformamide (DMF), the solubility is 51 g. At 20.26 bar, the solubility increases to 689.0 and 628.0 g for acetone and DMF, respectively. These solvents are used in pressurized gas cylinders.

Applications

Welding

Approximately 20% of acetylene is supplied by the industrial gases industry for oxyacetylene gas welding and cutting due to the high temperature of the flame. Combustion of acetylene with oxygen produces a flame of over 3,600 K (3,330 °C; 6,020 °F), releasing 11.8 kJ/g. Oxyacetylene is the hottest burning common fuel gas. Acetylene is the third-hottest natural chemical flame after dicyanoacetylene's 5,260 K (4,990 °C; 9,010 °F) and cyanogen at 4,798 K (4,525 °C; 8,177 °F). Oxy-acetylene welding was a popular welding process in previous decades. The development and advantages of arc-based welding processes have made oxy-fuel welding nearly extinct for many applications. Acetylene usage for welding has dropped significantly. On the other hand, oxy-acetylene welding equipment is quite versatile – not only because the torch is preferred for some sorts of iron or steel welding (as in certain artistic applications), but also because it lends itself easily to brazing, braze-welding, metal heating (for annealing or tempering, bending or forming), the loosening of corroded nuts and bolts, and other applications. Bell Canada cable-repair technicians still use portable acetylene-fuelled torch kits as a soldering tool for sealing lead sleeve splices in manholes and in some aerial locations. Oxyacetylene welding may also be used in areas where electricity is not readily accessible. Oxyacetylene cutting is used in many metal fabrication shops. For use in welding and cutting, the working pressures must be controlled by a regulator, since above 15 psi (100 kPa), if subjected to a shockwave (caused, for example, by a flashback), acetylene decomposes explosively into hydrogen and carbon.

Acetylene fuel container/burner as used in the island of Bali

Portable lighting

Calcium carbide was used to generate acetylene used in the lamps for portable or remote applications. It was used for miners and cavers before the widespread use of incandescent lighting; or many years later low-power/high-lumen LED lighting; and is still used by mining industries in some nations without workplace safety laws. Carbide lamps were also used extensively as headlights in early motor vehicles and as an early light source for lighthouses.

Plastics and acrylic acid derivatives

Except in China, use of acetylene as a chemical feedstock has declined by 70% from 1965 to 2007 owing to cost and environmental considerations. Acetylene can be semihydrogenated to ethylene, providing a feedstock for a variety of polyethylene plastics. Another major application of acetylene, especially in China is its conversion to acrylic acid derivatives. These derivatives form products such as acrylic fibers, glasses, paints, resins, and polymers.

Niche applications

In 1881, the Russian chemist Mikhail Kucherov described the hydration of acetylene to acetaldehyde using catalysts such as mercury(II) bromide. Before the advent of the Wacker process, this reaction was conducted on an industrial scale.

The polymerization of acetylene with Ziegler–Natta catalysts produces polyacetylene films. Polyacetylene, a chain of CH centres with alternating single and double bonds, was one of the first discovered organic semiconductors. Its reaction with iodine produces a highly electrically conducting material. Although such materials are not useful, these discoveries led to the developments of organic semiconductors, as recognized by the Nobel Prize in Chemistry in 2000 to Alan J. Heeger, Alan G MacDiarmid, and Hideki Shirakawa.

In the early 20th century acetylene was widely used for illumination, including street lighting in some towns. Most early automobiles used carbide lamps before the adoption of electric headlights.

In the 1920s, pure acetylene was experimentally used as an inhalation anesthetic.

Acetylene is sometimes used for carburization (that is, hardening) of steel when the object is too large to fit into a furnace.

Acetylene is used to volatilize carbon in radiocarbon dating. The carbonaceous material in an archeological sample is treated with lithium metal in a small specialized research furnace to form lithium carbide (also known as lithium acetylide). The carbide can then be reacted with water, as usual, to form acetylene gas to feed into a mass spectrometer to measure the isotopic ratio of carbon-14 to carbon-12.

Natural occurrence

The energy richness of the C≡C triple bond and the rather high solubility of acetylene in water make it a suitable substrate for bacteria, provided an adequate source is available. A number of bacteria living on acetylene have been identified. The enzyme acetylene hydratase catalyzes the hydration of acetylene to give acetaldehyde:

C₂H₂ + H₂O → CH₃CHO

Acetylene is a moderately common chemical in the universe, often associated with the atmospheres of gas giants. One curious discovery of acetylene is on Enceladus, a moon of Saturn. Natural acetylene is believed to form from catalytic decomposition of long-chain hydrocarbons at temperatures of 1,700 K (1,430 °C; 2,600 °F) and above. Since such temperatures are highly unlikely on such a small distant body, this discovery is potentially suggestive of catalytic reactions within that moon, making it a promising site to search for prebiotic chemistry.

Reactions

Vinylation: hydration, hydrohalogenation, and related reactions

In vinylation reactions, H-X compounds add across the triple bond. Alcohols and phenols add to acetylene to give vinyl ethers. Thiols give vinyl thioethers. Similarly, vinylpyrrolidone and vinylcarbazole are produced industrially by vinylation of 2-pyrrolidone and carbazole.

The hydration of acetylene is a vinylation reaction, but the resulting vinyl alcohol isomerizes to acetaldehyde. The reaction is catalyzed by mercury salts. This reaction once was the dominant technology for acetaldehyde production, but it has been displaced by the Wacker process, which affords acetaldehyde by oxidation of ethylene, a cheaper feedstock. A similar situation applies to the conversion of acetylene to the valuable vinyl chloride by hydrochlorination vs the oxychlorination of ethylene.

Addition to formaldehyde

Acetylene adds to ketones and aldehydes in the presence of base catalysts. With carbonyl groups to give α-ethynyl alcohols in ethynylation reactions: Formaldehyde gives sequentially propargyl alcohol and butynediol. 1,4-Butynediol is produced industrially in this way from formaldehyde and acetylene.

Carbonylation

Walter Reppe discovered that in the presence of catalysts, acetylene react to give a wide range of industrially significant chemicals.

With carbon monoxide, acetylene reacts to give acrylic acid, or acrylic esters, which can be used to produce acrylic glass:

Organometallic chemistry

Acetylene and its derivatives (2-butyne, diphenylacetylene, etc.) form complexes with transition metals. Its bonding to the metal is somewhat similar to that of ethylene complexes. These complexes are intermediates in many catalytic reactions such as alkyne trimerisation to benzene, tetramerization to cyclooctatetraene, and carbonylation to hydroquinone:

Fe(CO)₅ + 4 C₂H₂ + 2 H₂O → 2 C₆H₄(OH)₂ + FeCO₃ at basic conditions( 50-80 °C, 20-25 atm).

In the presence of certain transition metals, alkynes undergo alkyne metathesis.

Metal acetylides, species of the formula L_nM-C₂R, are also common. Copper(I) acetylide and silver acetylide can be formed in aqueous solutions with ease due to a poor solubility equilibrium.

Acid-base reactions

Main article: Acetylide § Preparation

Acetylene has a pK_a of 25, acetylene can be deprotonated by a superbase to form an acetylide:

HC≡CH + RM → RH + HC≡CM

Various organometallic and inorganic reagents are effective.

Safety and handling

Acetylene is not especially toxic, but when generated from calcium carbide, it can contain toxic impurities such as traces of phosphine and arsine, which give it a distinct garlic-like smell. It is also highly flammable, as are most light hydrocarbons, hence its use in welding. Its most singular hazard is associated with its intrinsic instability, especially when it is pressurized: under certain conditions acetylene can react in an exothermic addition-type reaction to form a number of products, typically benzene and/or vinylacetylene, possibly in addition to carbon and hydrogen. Consequently, acetylene, if initiated by intense heat or a shockwave, can decompose explosively if the absolute pressure of the gas exceeds about 200 kilopascals (29 psi). Most regulators and pressure gauges on equipment report gauge pressure, and the safe limit for acetylene therefore is 101 kPa_gage, or 15 psig. It is therefore supplied and stored dissolved in acetone or dimethylformamide (DMF), contained in a gas cylinder with a porous filling (Agamassan), which renders it safe to transport and use, given proper handling. Acetylene cylinders should be used in the upright position to avoid withdrawing acetone during use.

Information on safe storage of acetylene in upright cylinders is provided by the OSHA, Compressed Gas Association, United States Mine Safety and Health Administration (MSHA), EIGA, and other agencies.

Copper catalyses the decomposition of acetylene, and as a result acetylene should not be transported in copper pipes.

Cylinders should be stored in an area segregated from oxidizers to avoid exacerbated reaction in case of fire/leakage. Acetylene cylinders should not be stored in confined spaces, enclosed vehicles, garages, and buildings, to avoid unintended leakage leading to explosive atmosphere. In the US, National Electric Code (NEC) requires consideration for hazardous areas including those where acetylene may be released during accidents or leaks. Consideration may include electrical classification and use of listed Group A electrical components in USA. Further information on determining the areas requiring special consideration is in NFPA 497. In Europe, ATEX also requires consideration for hazardous areas where flammable gases may be released during accidents or leaks.