Guide
Calcium hydroxide data sheet for carbide lime: Ca(OH)₂ properties solubility, pH, decomposition, hazards and storage
Calcium hydroxide, Ca(OH)₂, is the main solid in carbide lime: molar mass 74.09 g/mol, density about 2.2–2.24 g/cm³, solubility only 1.73 g/l at 20 °C and falling as water warms. A saturated solution has a pH of 12.4 at 25 °C. It loses water to become quicklime at about 580 °C.
- Formula and molar mass
- Ca
- Solubility in water
- About 0.2 % at 0 °C and 1.73 g/l
- pH of saturated solution
- 12.4 at 25 °C; carbide lime slurry measured at 12.6
- Thermal decomposition
- Loses water at about 580 °C
- GHS hazard
- H318 causes serious eye damage
Calcium hydroxide, Ca(OH)₂, is slaked lime, and it is what every carbide-to-water acetylene generator makes alongside its gas. By the reference calculation compiled by S. A. Miller (1965), each kilogram of 300 l/kg carbide leaves about 1.17 kg of dry residue, of which about 1.11 kg is calcium hydroxide. This data sheet gives the physical and chemical properties of Ca(OH)₂ from NIOSH, the Hazardous Substances Data Bank and the ECHA notifications, then sets out how carbide lime differs from the commercial hydrated lime sold to builders, and how to handle and store it. Uses are covered under uses of carbide lime and disposal under carbide lime sludge handling.
Calcium hydroxide properties
| Property | Value | Source |
|---|---|---|
| Formula | Ca(OH)₂ | — |
| Common names | Slaked lime, hydrated lime, carbide lime (by-product); mineral portlandite | — |
| CAS number | 1305-62-0 | NIOSH Pocket Guide |
| Molar mass | 74.09 g/mol (NIOSH gives 74.1) | NIOSH Pocket Guide |
| Appearance | White powder; carbide lime is grey because of fine carbon | Cardoso et al., Powder Technology 2009 |
| Density, solid | 2.2–2.24 g/cm³ | HSDB, ICSC, NIOSH (2.24) |
| Bulk density, dry powder | 450–640 kg/m³ | HSDB via PubChem |
| Solubility in water, 0 °C | about 0.2 % | NIOSH Pocket Guide |
| Solubility in water, 20 °C | 1.73 g/l; 0.160 g per 100 g water | HSDB via PubChem |
| Solubility in boiling water | very slight | HSDB via PubChem |
| pH, saturated solution | 12.4 at 25 °C | HSDB via PubChem |
| Thermal decomposition | loses water at about 580 °C, giving CaO | NIOSH Pocket Guide; ICSC 0408 |
| Refractive index | 1.574 and 1.545 | HSDB via PubChem |
| Reaction with CO₂ | Ca(OH)₂ + CO₂ → CaCO₃ + H₂O | — |
| Incompatible with | Maleic anhydride, phosphorus, nitromethane, nitroethane and other nitroparaffins; attacks some metals | NIOSH Pocket Guide |
| NIOSH REL | 5 mg/m³ TWA | NIOSH Pocket Guide |
| OSHA PEL | 15 mg/m³ total dust; 5 mg/m³ respirable | NIOSH Pocket Guide |
Three of these properties drive how carbide lime behaves on a plant.
Low and falling solubility
Only about 1.7 grams dissolve in a litre of water at 20 °C, and less in hot water, because Ca(OH)₂ is one of the few common solids with retrograde solubility. In a generator running near 70 °C almost all the lime is suspended solid, which is why the slurry settles in a pit and why the clear liquor above it is still saturated lime water.
High pH
A saturated solution sits at pH 12.4 at 25 °C. Carbide lime slurry measured on plants is at 12.6–12.8 (EIGA Doc 143) and 12.2–12.9 (Ramasamy, Agamuthu and Ibrahim, 2002). That puts it at the pH 12.5 threshold that Schedule II Class C2 of the Hazardous and Other Wastes Rules 2016 uses to define a corrosive aqueous waste, which is discussed under carbide lime sludge handling.
Carbonation
Calcium hydroxide takes carbon dioxide from the air and turns into calcium carbonate. That is how lime mortar hardens, but for a by-product being sold as lime it is a loss: the longer carbide lime sits exposed, the more carbonate it contains.
Rules that matter
The best side-by-side measurement is by Cardoso, Fernandes, Pileggi, Cincotto and John, Powder Technology 195 (2009) 143–149:
| Property | Commercial hydrated lime | Carbide lime |
|---|---|---|
| Ca(OH)₂, thermogravimetric | 86.1 % | 88.4 % |
| CaCO₃ | 8.9 % | 7.3 % |
| Graphitic carbon | not reported | about 1.8 % |
| MgO | 0.31 % | 0.06 % |
| Fe₂O₃ + Al₂O₃ | 0.68 % | 0.68 % |
| Loss on ignition | 26.9 % | 26.4 % |
| Density | 2.35 g/cm³ | 2.21 g/cm³ |
| BET specific surface | 18.0 m²/g | 11.3 m²/g |
| Median particle size, D50 | about 10 µm | about 20 µm |
Beyond that analysis, carbide lime differs in four ways that matter in practice:
Carbide lime compared with commercial hydrated lime
- Carbon The roughly 1.8 % graphite from the coke is inert but visible as grey specks, which rules it out for whitewash and finishing plaster.
- Sulphide and other traces An unpublished British Oxygen Co. analysis of a typical sludge, dated 1944 and compiled by S. A. Miller (1965), found 96.3 % Ca(OH)₂ and 0.08 % calcium sulphide, with about 0.12 % sulphide reported in 1961 where the filtrate was recycled to the generator, and traces of thiocyanate. EIGA Doc 143 adds metals, chlorides, sulphates and ammonium, and stresses that the impurities vary with the carbide source.
- Moisture Carbide lime is delivered wet. EIGA Doc 143 gives about 70–75 % water for generator slurry, 60–65 % for settled sludge and 50 % or less for filter-pressed paste, against under 2 % free moisture for bagged hydrated lime.
- Dissolved acetylene Fresh sludge carries dissolved acetylene, about 1.0 % of the plant's make at a 60 °C generator and 0.5–0.6 % at 75 °C (Hölemann and co-workers, 1959), and the liquor typically about 500 mg/l of acetylene, 150 mg/l of H₂S and 0.8 mg/l of PH₃ (Strizhevskii, 1961). Commercial hydrated lime has none. See acetylene dissolved in carbide sludge.
Hazards
| Hazard | Classification or figure | Source |
|---|---|---|
| Eye damage | H318, causes serious eye damage, notified by 76.9 % of companies; 22.4 % notify H314 | ECHA C&L Inventory via PubChem |
| Skin irritation | H315, notified by 45.3 % | ECHA C&L Inventory via PubChem |
| Respiratory irritation | H335, notified by 38.9 %; dust exposure limit 5 mg/m³ (NIOSH REL) | ECHA; NIOSH |
| Symptoms | Eye, skin and upper respiratory irritation; eye and skin burns; cough | NIOSH Pocket Guide |
| Reaction with acids | Heat; with carbide lime, release of H₂S and PH₃ | EIGA Doc 226 §10.4 |
| Residual acetylene | Flammable atmosphere in enclosed sludge spaces | EIGA Doc 226 §6.3 and §9.2 |
The signal word is Danger and the pictograms GHS05 (corrosive) and GHS07 (irritant) on the aggregated notifications. The working rules follow: goggles rather than safety glasses wherever slurry can splash, alkali-resistant gloves and boots, an eyewash at every loading and discharge point, and washing lime off skin before it dries. Carbide itself and its own fire hazard are covered in calcium carbide safety data.
Rules that matter
Storage and handling
- Keep it wet or keep it sealed Slurry and paste stay pumpable or diggable and do not dust; dried powder must be bagged and sealed to stop dusting and carbonation.
- Store in a watertight, open or ventilated area EIGA Doc 226 §9.2 asks for lime storage that is watertight, to protect soil and groundwater, and open to the atmosphere, to prevent acetylene building up.
- Keep acids away EIGA Doc 226 §10.4 limits acid neutralisation in a lime pit to small acid quantities in large lime volumes, to avoid H₂S and phosphine emissions and foaming. Tell any buyer who will neutralise acid with it.
- Do not use lime water as cooling water or let it drain uncontrolled; EIGA Doc 226 §5.3 rules out both because of its impurities and pH.
- Do not put carbide dust straight into the pit Air-slake or wash it first (EIGA Doc 226 §6.5); dry carbide in wet lime releases acetylene at once.
- Site the drying pits correctly PESO's safety-distance table for acetylene plants (Gas Cylinders Rules 2016, Form E) places lime sludge drying pits at least 9 m from the boundary wall and the acetylene generation unit.
All other plant residues, from spent purifier mass to old cylinders, are covered in acetylene plant waste streams.
Rules that matter
What this means for you
- Treat carbide lime as a strong alkali, around pH 12.5: protect eyes first.
- Expect it to be 88 % or more Ca(OH)₂ on a dry basis, grey, wet and variable with the carbide source.
- Store it watertight but open to the air, away from acids, and never in a closed shed.
- Get a NABL-accredited analysis of your own material, including pH, before deciding how to sell or dispose of it.
Calcium hydroxide data sheet — frequently asked questions
What is the pH of calcium hydroxide solution?
12.4 at 25 °C for a saturated solution, according to the Hazardous Substances Data Bank. Carbide lime slurry from acetylene generators has been measured slightly higher, at 12.6–12.8 by EIGA and 12.2–12.9 across three plants by Ramasamy and co-workers in 2002, because it also carries dissolved impurities. Either way it is strongly alkaline and damages eyes.
Why does calcium hydroxide dissolve less in hot water?
Calcium hydroxide has retrograde solubility: dissolving it releases heat, so by Le Chatelier's principle less dissolves as the water gets warmer. NIOSH gives about 0.2 % at 0 °C and HSDB 1.73 g/l at 20 °C, and HSDB describes it as only very slightly soluble in boiling water. A hot generator therefore holds most of its lime as suspended solid.
At what temperature does calcium hydroxide turn into quicklime?
It loses its water and becomes calcium oxide at about 580 °C, the figure given by the NIOSH Pocket Guide and the International Chemical Safety Card. The reaction, Ca(OH)₂ → CaO + H₂O, is the reverse of slaking. Turning dried carbide lime back into quicklime is possible, but it takes fuel and the product carries the carbide's impurities.
Is calcium hydroxide corrosive?
To eyes, yes. In the ECHA C&L Inventory 76.9 % of notifiers classify it as H318, causes serious eye damage, and 22.4 % as H314, causes severe skin burns. Most also list skin and respiratory irritation. A splash of lime slurry in the eye is an emergency: rinse at once with plenty of clean water and get medical help.
Why is carbide lime grey when calcium hydroxide is white?
Because it carries about 1.8 % fine graphitic carbon from the coke used to make the carbide, as measured by Cardoso and co-workers in 2009. The carbon is chemically inert and the lime underneath is at least as pure as builders' hydrated lime, at 88.4 % Ca(OH)₂ against 86.1 %, but the grey colour rules carbide lime out of whitewash and finishing plaster.
How should calcium hydroxide be stored?
Covered and away from acids and air. Exposed lime absorbs carbon dioxide and turns into calcium carbonate, so pit-aged carbide lime loses quality. Acid releases hydrogen sulphide and phosphine from carbide lime and generates heat. Wet carbide lime must also be stored in the open or in a ventilated, watertight area because it can still give off acetylene.
References
- European Chemicals Agency — C&L Inventory, calcium dihydroxide (EC 215-137-3)
- European Industrial Gases Association. (n.d.). Best Available Techniques for Acetylene Production (EIGA Doc 226). https://www.eiga.eu/uploads/documents/DOC226.pdf
- European Industrial Gases Association. (n.d.). Guide to Carbide Lime Applications (EIGA Doc 143). https://www.eiga.eu/uploads/documents/DOC143.pdf
- NIOSH Pocket Guide to Chemical Hazards — Calcium hydroxide https://www.cdc.gov/niosh/npg/npgd0092.html
- PubChem — Calcium hydroxide (HSDB, ICSC and ECHA C&L Inventory data)
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