The term “citric acid Sigma” refers to citric acid products from Sigma-Aldrich, now part of MilliporeSigma. Citric acid is a weak organic acid found in citrus fruits, and the “Sigma” designation specifies that the chemical meets the company’s quality control standards. These standards apply to its various product grades intended for research, pharmaceutical, and manufacturing use.
Product Specifications and Grades
Sigma-Aldrich provides citric acid in two primary forms: anhydrous and monohydrate. The anhydrous form, with the chemical formula HOC(COOH)(CH₂COOH)₂, has a molecular weight of 192.12 g/mol. The monohydrate version incorporates a single water molecule into its crystal structure, giving it a formula of HOC(COOH)(CH₂COOH)₂ · H₂O and a higher molecular weight of 210.14 g/mol. This difference is important for accurate measurements when preparing solutions of specific concentrations.
The company offers several grades of citric acid, each defined by its purity and intended use. The ACS Reagent grade, for instance, meets or exceeds the purity standards set by the American Chemical Society, with a typical purity of ≥99.5%. This makes it suitable for many general laboratory procedures and analytical testing.
For biological research, BioXtra grade citric acid is processed to ensure low levels of endotoxins for cell culture applications. The United States Pharmacopeia (USP) grade is appropriate for use in pharmaceutical formulations and food-related applications. ReagentPlus® is another common grade with a purity of ≥99.5%, used for various research applications.
Laboratory and Research Applications
In a laboratory, a frequent use for citric acid is preparing buffer solutions. A citrate buffer maintains a stable pH in the acidic to neutral range, typically between 3.0 and 6.2. This is useful for biochemical assays and enzyme reactions that require specific pH conditions for accurate results.
Another application is its function as a chelating agent, binding to metal cations like calcium (Ca²⁺) and magnesium (Mg²⁺). This action is useful in molecular biology to protect DNA and RNA from degradation by nucleases, which require these metal ions to function. By sequestering the ions, citric acid inactivates these enzymes.
This chelating property is also applied in its use as an anticoagulant for blood samples. Formulated as sodium citrate, it prevents the blood clotting cascade by binding to the calcium ions required for the process, keeping blood liquid for diagnostic testing. Researchers also use citric acid to adjust the pH of various solutions and media.
Preparation of Common Solutions
To prepare a 1 Molar (1 M) solution of citric acid, the molecular weight of the specific form—anhydrous or monohydrate—must be used. For example, to make one liter of a 1 M solution, a scientist would weigh 192.12 grams of the anhydrous form. For the monohydrate form, 210.14 grams would be needed for the same concentration.
The procedure begins by weighing the required amount of citric acid powder. The solid is then transferred to a beaker containing a portion of deionized water, typically about 70-80% of the desired final volume. The mixture is stirred until all the citric acid has completely dissolved.
Once the solid is fully dissolved, the solution is transferred into a volumetric flask of the correct final volume. More deionized water is added to bring the bottom of the meniscus to the calibration mark. The flask is then capped and inverted several times to ensure the solution is homogeneous.
Safety and Handling Protocols
Handling citric acid is guided by its Safety Data Sheet (SDS). While it is a weak acid, it can cause serious eye and skin irritation upon direct contact. Personal Protective Equipment (PPE) is required, including safety glasses or goggles and nitrile gloves to protect from exposure.
Citric acid should be stored in a tightly sealed container in a cool, dry, and well-ventilated area. It is incompatible with strong bases, metals, and oxidizing agents and should be stored away from these substances to prevent reactions.
In case of accidental contact, standard first aid procedures should be followed. If the chemical gets in the eyes, they should be rinsed with water for several minutes, and for skin contact, the area should be washed with plenty of water. Disposal must comply with local, state, and federal regulations, and institutional guidelines should always be consulted.