Lime water

Lime water

Lime water , also known as sintered lime water or just sintered water , is the common name for an aqueous calcium hydroxide solution. Lime water is alkaline and slightly caustic.

Manufacturing

If poorly soluble calcium hydroxide is suspended in water and the clear supernatant is filtered off, lime water is obtained.

A calcium hydroxide suspension is also obtained by stirring calcium oxide into water, from which lime water is obtained by filtration:

${\displaystyle \mathrm {CaO+H_{2}O\longrightarrow Ca(OH)_{2}} }$

Lime water is a by-product of the manufacture of sump lime. Here the solution covers the quicklime (slaked lime) that has been slaked with water , while it matures into slaked lime in the ripening basin.

Use for analysis

Carbon dioxide can be detected with clear lime water : When air containing CO ₂ is passed through, a precipitate of calcium carbonate forms . If carbon dioxide is passed through for a longer period, the calcium carbonate dissolves again with the formation of calcium hydrogen carbonate .

According to the following reaction equation, unstable carbonic acid (H ₂ CO ₃ ) is initially formed in the first step of the process . This or the carbonate ions derived from it react with the calcium hydroxide contained to form calcium carbonate , which precipitates as a solid and causes the suspension to be cloudy:

${\displaystyle \mathrm {CO_{2}+H_{2}O\rightleftharpoons H_{2}CO_{3}} }$

${\displaystyle \mathrm {H_{2}CO_{3}+Ca(OH)_{2}\longrightarrow CaCO_{3}+2\;H_{2}O} }$

If lime water is left open in the air for a longer period of time, the solution becomes unusable because it absorbs CO ₂ from the atmosphere with the formation of calcium carbonate.

${\displaystyle \mathrm {Ca(OH)_{2}+CO_{2}\longrightarrow CaCO_{3}\downarrow +\ H_{2}O} }$

This reaction also takes place in the lime water sample for the detection of carbon dioxide in a sample gas.

Similar to lime water, barite water reacts with carbon dioxide.

Use in monument preservation

Attempts have been made in the past to consolidate morbid natural stones or plaster mortar with both lime water and barite water : The solution was introduced into the capillary system in the hope that the precipitation products would stabilize the grain structure.

The effectiveness of such measures has been scientifically refuted: If there is any increase in strength , this is due to a rearrangement of calcium hydroxide still present in the mortar . In experiments, this effect could also be triggered by a simple soaking with water.

literature

• Manfred Koller: The fairy tale of consolidation with "Kalkwasser" . In: Restauratoren Blätter 17, 20 Years of Stone Conservation 1976–1996. Vienna, 1996, p. 17.
• Eric F. Hansen et al .: A Review of Selected Inorganic Consolidants and Protective Treatments for Porous Calcareous Material. In: Reviews in Conservation 4 (2003) pp. 13-26.
• Hans Ettl and Eberhard Wendler: Structural plaster consolidation with lime water? - Limits and alternatives. In: Contributions to the Preservation of Art and Cultural Property , No. 1, 2005, pp. 129–133.
• Eric Ferguson Doehne and Clifford A. Price: Stone conservation. An overview of current research. 2nd. Los Angeles 2010 (Getty Conservation Institute, Research in conservation).