Calcium hydroxide is an inorganic compound widely used in biochemical and molecular biology laboratories as a mild base and precipitating agent. Owing to its controlled solubility and relatively high biocompatibility, it is particularly suitable for laboratory procedures such as calcium phosphate–mediated transfection, protein purification processes, and polysaccharide solubilization. These properties make calcium hydroxide a valuable reagent in a variety of biochemical protocols and experimental workflows.
Chemical Properties
Calcium hydroxide (Ca(OH)2, molecular weight 74.09 g/mol; CAS 1305-62-0) is a white, odorless powder characterized by a hexagonal brucite-like layered crystal structure with a density of approximately 2.21 g/cm³ and a melting point of about 580 °C. In this structure, Ca²⁺ ions are octahedrally coordinated by six hydroxide (OH⁻) groups interconnected through hydrogen bonds. The compound is only slightly soluble in water (approximately 1.73 g/L at 20 °C), and its solubility decreases as temperature increases, in accordance with Le Chatelier’s principle. A saturated aqueous solution, commonly referred to as “limewater,” reaches an alkaline pH of around 12.4 due to the equilibrium dissociation of calcium hydroxide into Ca²⁺ and OH⁻ ions. In the presence of carbon dioxide, calcium hydroxide reacts to form insoluble calcium carbonate (CaCO₃), a reaction frequently used for analytical detection and titration endpoints.
Biochemical Applications
In molecular biology, calcium hydroxide is commonly used in calcium phosphate transfection systems, typically at concentrations ranging from 10 to 50 mM. Under optimized conditions—often near pH 6.95 and with an approximate Ca²⁺ to DNA ratio of 2:1—it facilitates the co-precipitation of plasmid DNA into mammalian cells by forming DNA-containing hydroxyapatite crystals that can be internalized by cells. In protein chemistry workflows, saturated calcium hydroxide suspensions may be used to selectively precipitate caseins and other phosphoproteins at alkaline pH values between 11 and 12. Additionally, in carbohydrate and polysaccharide research, solutions of approximately 0.1 M Ca(OH)2 are applied to disperse alginate gels or to solubilize chitosan, enabling further enzymatic or biochemical analyses. In histological preparation, the mild alkaline properties of calcium hydroxide can also be exploited for the decalcification of bone and cartilage tissues prior to paraffin embedding, often preserving antigenic structures more effectively than strong mineral acids.
