Sodium 4-Aminosalicylate Dihydrate: Lattice Water Compensation
Hygroscopic Equilibrium and Lattice Water Compensation in Aqueous Mobile Phase Preparation
When preparing reference standards of Sodium 4-Aminosalicylate Dihydrate (CAS 6018-19-5), the dihydrate form introduces a critical variable: lattice water. Unlike anhydrous salts, the dihydrate contains two water molecules per formula unit, which can shift under ambient humidity. In a typical QC lab, a freshly opened container may exhibit a water content of 14.5–15.2% by Karl Fischer titration, but after repeated opening, surface hydration can drift. This is not a purity defect—it is a hygroscopic equilibrium phenomenon. For chromatographers, the practical implication is that weighing 100.0 mg of the dihydrate does not equate to 100.0 mg of the anhydrous active moiety. Instead, the effective mass of 4-Amino-2-hydroxybenzoic acid sodium salt dihydrate must be corrected using the batch-specific water content from the Certificate of Analysis (COA).
From field experience, a non-standard parameter worth monitoring is the crystallization behavior at sub-ambient temperatures. If a mobile phase containing this compound is stored at 2–8°C, needle-like crystals may form over 48 hours, not due to insolubility but because the dihydrate lattice reorganizes into a lower-hydrate species. This can clog HPLC frits and alter retention times. Pre-filtering cold mobile phases through a 0.22 µm membrane is a simple mitigation. For robust method transfer, always specify the hydration state and the correction protocol. Our Sodium 4-Aminosalicylate Dihydrate is supplied with a detailed COA that includes water content by KF, enabling precise compensation.
Gravimetric Correction Factors for Accurate Molarity Calculations in Reference Standards
Calculating the molarity of a Sodium 4-Aminosalicylate Dihydrate standard solution requires a gravimetric correction factor (GCF). The molecular weight of the dihydrate is 211.15 g/mol, while the anhydrous Sodium 4-Aminosalicylate is 175.12 g/mol. If the COA reports a water content of 14.8%, the actual dihydrate purity is 99.5%, but the effective anhydrous content is only 84.7%. Thus, to prepare a 1.000 mg/mL stock solution (as anhydrous), you must weigh 1.180 mg of the dihydrate per mL. The formula is:
Corrected mass = (Target anhydrous mass) × (MW_dihydrate / MW_anhydrous) / (1 - water content)
For a 100 mL volumetric flask targeting 100.0 mg anhydrous, weigh 118.0 mg of the dihydrate. This correction is essential when the standard is used to assay 4-Aminosalicylic acid sodium salt dihydrate in pharmaceutical formulations. In our experience, labs that skip this step see a systematic bias of +15–18% in impurity profiling. Always cross-check the COA for the exact water content; typical industrial purity specifications allow a range of 14.0–16.0% water. For reference, see our article on Industrial Purity Specifications Sodium 4-Aminosalicylate Dihydrate.
Solvent Incompatibility and Precipitation Risks with Organic Modifiers
In reversed-phase HPLC, Sodium 4-Aminosalicylate Dihydrate is highly soluble in water (>200 mg/mL), but its solubility drops sharply in organic solvents. Methanol and acetonitrile, common organic modifiers, can induce precipitation if the aqueous fraction falls below 70%. This is particularly problematic during gradient elution when the organic phase exceeds 30%. A non-standard observation is that trace metal ions (e.g., Fe³⁺ from stainless steel tubing) can catalyze the formation of a colored complex, leading to ghost peaks. Chelating the mobile phase with 0.1 mM EDTA eliminates this issue. For isocratic methods, a mobile phase of phosphate buffer pH 6.8 with ≤25% methanol provides stable baselines. When developing a method, always perform a solubility screen: mix the standard solution with the intended mobile phase at the highest organic concentration and check for turbidity after 24 hours. This simple test prevents column damage and ensures reproducible quantitation of Sodium p-aminosalicylate dihydrate.
Degassing Protocols to Prevent Oxidative Discoloration in Chromatographic Analysis
Dissolved oxygen in mobile phases can oxidize Sodium 4-Aminosalicylate Dihydrate, leading to a yellow-brown discoloration and the appearance of degradation peaks. The amino and hydroxyl groups on the aromatic ring are susceptible to oxidative coupling, especially at pH >7. In our labs, we observed that a 0.1 mg/mL standard solution stored at room temperature without degassing showed a 5% loss of main peak area after 8 hours, with a new peak at RRT 1.3. Helium sparging or vacuum filtration through a 0.45 µm nylon membrane effectively removes oxygen. For long-term stability, add 0.1% sodium metabisulfite as an antioxidant, but validate that it does not interfere with detection at 265 nm. A practical tip: prepare working standards daily and store them in amber volumetric flasks at 4°C. This protocol is critical when using Sodium 4-amino-2-hydroxybenzoate dihydrate as a system suitability standard. For more on handling reactive intermediates, see Sodium 4-Aminosalicylate Dihydrate: Exothermic Coupling Control.
Bulk Packaging and COA Parameters for Industrial QC Applications
For industrial QC labs, sourcing Sodium 4-Aminosalicylate Dihydrate in bulk requires attention to packaging and COA parameters. Our product is available in 25 kg fiber drums with double PE liners, or 210L steel drums for larger quantities. The COA includes:
| Parameter | Specification | Typical Value |
|---|---|---|
| Assay (HPLC, anhydrous) | ≥98.0% | 99.2% |
| Water content (KF) | 14.0–16.0% | 14.8% |
| Appearance | White to off-white powder | White powder |
| Heavy metals (as Pb) | ≤10 ppm | <5 ppm |
| Residual solvents | Meets USP | None detected |
Note that the water content is not an impurity but a stoichiometric component; however, batch-to-batch variability can affect standard preparation. Always request the batch-specific COA before use. For logistics, the product is classified as non-hazardous, but protect from moisture and light during transport. The dihydrate form is stable for 24 months when stored at 15–25°C in unopened containers. For procurement, consider the total cost of ownership: a higher initial purity reduces the need for correction and re-testing.
Frequently Asked Questions
What is sodium aminosalicylate used for?
Sodium aminosalicylate, specifically the dihydrate form, is primarily used as a pharmaceutical intermediate in the synthesis of anti-tuberculosis agents. In analytical chemistry, it serves as a reference standard for HPLC assay and impurity profiling due to its well-characterized UV absorption and stability when properly handled.
What is the CAS number of sodium aminosalicylate dihydrate?
The CAS number for sodium aminosalicylate dihydrate is 6018-19-5. This identifier is essential for regulatory documentation, procurement, and ensuring you receive the correct hydration state, as the anhydrous form has a different CAS number.
How do I adjust molarity calculations for the dihydrate form?
To adjust for the dihydrate, use the gravimetric correction factor: multiply the target anhydrous mass by the ratio of molecular weights (211.15/175.12) and divide by (1 - water content from COA). For example, if water content is 14.8%, weigh 1.180 g of dihydrate per gram of anhydrous needed.
What mobile phase conditions are compatible with sodium 4-aminosalicylate dihydrate?
Aqueous mobile phases with pH 6.0–7.0 and ≤30% organic modifier (methanol or acetonitrile) are generally compatible. Avoid high organic concentrations to prevent precipitation. Adding 0.1 mM EDTA can chelate metal ions and improve peak symmetry.
How stable are working standards under ambient lab conditions?
Working standards of sodium 4-aminosalicylate dihydrate in water are stable for 8–12 hours at room temperature if protected from light and oxygen. For longer stability, store at 2–8°C in amber glassware and degas the solvent. Daily preparation is recommended for critical assays.
Sourcing and Technical Support
Ensuring accurate chromatographic results starts with a reliable supply of high-purity Sodium 4-Aminosalicylate Dihydrate. Our product is manufactured under strict quality control, with batch-specific COAs that detail water content, assay, and impurity profile. We offer flexible packaging from 25 kg to bulk IBCs, with secure logistics to maintain dihydrate integrity. Partner with a verified manufacturer. Connect with our procurement specialists to lock in your supply agreements.