Physicochemical Properties of MgCl2

Magnesium chloride (MgCl2) is an ionic compound consisting of one magnesium cation (Mg²⁺) and two chloride anions (Cl⁻). The anhydrous form has a molecular weight of 95.21 g/mol and appears as a white, hygroscopic crystalline solid. The hexahydrate form (MgCl2·6H2O, MW 203.30 g/mol) is the most commonly encountered hydrate in laboratory settings and presents as colourless or white deliquescent flakes.

MgCl2 is highly soluble in water and dissociates completely in aqueous solution, releasing free Mg²⁺ ions. This ready availability of divalent magnesium ions underpins the compound's broad utility as a biochemical reagent. The compound has a melting point of approximately 714 °C (anhydrous) and a density of 2.32 g/cm³. Aqueous solutions are mildly acidic due to partial hydrolysis of the magnesium ion.

Key Laboratory Applications

PCR and nucleic acid amplification: Mg²⁺ is an indispensable cofactor for thermostable DNA polymerases such as Taq polymerase. The concentration of MgCl2 in PCR buffer directly influences polymerase activity, primer annealing specificity, and overall amplification efficiency. Typical PCR reactions use 1.5–2.5 mM MgCl2, though optimisation is often required for specific primer–template combinations.

Enzyme cofactor: Many enzymes, including kinases, phosphatases, ATPases, and restriction endonucleases, require Mg²⁺ as a structural or catalytic cofactor. MgCl2 is routinely added to reaction buffers for these enzymatic assays at concentrations ranging from 1 to 10 mM.

Cell culture and competent cell preparation: MgCl2 solutions are used in the preparation of chemically competent bacterial cells (e.g., in combination with CaCl2) and in washing buffers to maintain ionic strength and membrane integrity.

Protein biochemistry: Mg²⁺ stabilises nucleotide–protein complexes (e.g., ATP·Mg²⁺), supports correct protein folding in certain systems, and is included in lysis and assay buffers for kinase and phosphatase activity measurements.

Grades, Purity, and Selection Guide

Selecting the appropriate grade of MgCl2 is critical to experimental reproducibility. The following grades are commonly available:

• Molecular biology grade: Low in DNase, RNase, and protease contamination; recommended for PCR, RT-PCR, and other nucleic acid applications.
• ACS reagent grade: Meets American Chemical Society purity specifications; suitable for analytical chemistry and general biochemical use.
• Anhydrous vs. hexahydrate: The anhydrous form is preferred when precise molar concentrations are required and water of crystallisation would interfere. The hexahydrate is more stable under ambient conditions and easier to weigh accurately.

Researchers should confirm the grade specification on the certificate of analysis (CoA) and verify that heavy metal and endotoxin levels are within acceptable limits for sensitive biological assays.

Storage, Handling, and Safety

Storage: Anhydrous MgCl2 is highly hygroscopic and must be stored in tightly sealed containers in a cool, dry environment, away from moisture. The hexahydrate form should similarly be kept sealed to prevent deliquescence. Both forms are stable at room temperature when properly sealed. Stock solutions (e.g., 1 M in ultrapure water) can be autoclaved and stored at room temperature or 4 °C for extended periods.

Handling: MgCl2 is considered a low-hazard compound. Standard laboratory personal protective equipment (PPE) — including gloves, lab coat, and safety glasses — should be worn when handling powders or concentrated solutions to avoid skin and eye irritation. Dust inhalation should be minimised.

Safety classification: MgCl2 is not classified as hazardous under standard GHS criteria at typical laboratory concentrations. Always consult the current Safety Data Sheet (SDS) supplied with the product before use.

Disposal: Dilute aqueous solutions may generally be disposed of via the drain in accordance with local regulations. Consult your institution's environmental health and safety guidelines for large quantities.