Zinc chloride anhydrous pharma grade API, CAS 7646-85-7, molecular formula ZnCl₂, relative molecular mass 136.30 g/mol, is supplied as a white to off-white crystalline powder under product designation ZC-PH-250. The material is intended for tablet, capsule, granule, oral liquid, and injectable manufacturing, where compendial purity, low particulate burden, and control of elemental impurities are release parameters. The anhydrous grade contains 47.97% w/w elemental zinc. It differs from oral-grade zinc sulfate heptahydrate, which contains 22.74% w/w zinc, and zinc gluconate, which contains 14.35% w/w zinc; zinc oxide has a higher zinc mass fraction at 80.34%, but its practical insolubility in water makes it unsuitable for clear injectable solutions without acid solubilization. The chloride counterion in ZnCl₂ provides high aqueous solubility and an acidic solution environment.

The product is manufactured under an API quality system aligned with ICH Q7 for active pharmaceutical ingredients. It is packaged in double low-density polyethylene liners inside sealed high-density polyethylene drums under nitrogen. Because the material is deliquescent, the package is specified to prevent water sorption during storage and re-testing. The storage condition is a dry, chemically isolated area; the product should not be stored near ammonia, volatile amines, alkali metal hydroxides, or other acid-reactive materials. The compound has a density of 2.907 g/cm³ at 25 °C, a melting point near 290 °C, and a boiling point near 732 °C. Zinc chloride dissolves in water, ethanol, glycerol, and acetone; dissolution in water is exothermic and yields an acidic solution, with pH and precipitation behavior dependent on concentration and temperature.

What distinguishes anhydrous pharma-grade zinc chloride from technical-grade zinc chloride?

Technical-grade zinc chloride is used in galvanizing, soldering flux, wood preservation, and chemical synthesis. Its specification does not control lead, arsenic, cadmium, mercury, or other elemental impurities to pharmaceutical daily-permitted-exposure levels, and it does not carry a low-endotoxin or low-particulate guarantee. Pharma-grade ZnCl₂ is manufactured in dedicated equipment and tested for compendial attributes such as assay, clarity and color of solution, pH, sulfate, insoluble matter, and elemental impurities. In tablet and capsule manufacturing, the most important difference is the absence of insoluble oxychloride or oxide haze in the pharma-grade product; technical-grade material can produce a cloudy solution that reacts with phosphate or carbonate buffers and causes visible particles in liquid and injectable forms. For parenteral use, the pharma grade is also controlled for bacterial endotoxins; the specification is assigned from the intended maximum adult dose and the finished injection monograph, rather than by a single fixed API limit.

The following comparative matrix is based on atomic masses and general solubility behavior of common zinc sources.

In solid oral dosage manufacturing, ZnCl₂ anhydrous is usually incorporated at elemental zinc doses of 5–50 mg per unit; the required API mass is obtained by dividing the elemental zinc claim by 0.4797. The product is not freely flowing at normal room humidity, and it will form a wet surface film if exposed to ambient air for extended periods. Direct compression is therefore conducted in dehumidified rooms with relative humidity controlled below the deliquescence point. The API is first premixed with a portion of anhydrous filler and passed through a 0.710 mm screen to break loose agglomerates. In capsule filling, dosator or tamping-pin machines require consistent powder flow; even small amounts of sorbed water increase fill weight variability and can produce soft, deformed capsules. Aqueous wet granulation of ZnCl₂ is generally avoided because the compound dissolves in the granulating fluid and recrystallizes on drying, leading to non-uniform distribution and surface oxychloride formation. Dry granulation by roller compaction is preferred; roll force, roll gap, and screen speed are formulation-dependent, and published data for ZnCl₂-specific compaction behavior is limited, so a factorial design is used to balance granule particle size distribution and residual moisture.

Tablet and capsule formulations containing ZnCl₂ should be evaluated for chloride-induced corrosion on uncoated steel tooling. Polished stainless steel is adequate for short campaign runs, but longer campaigns benefit from nickel-plated or hard chrome tooling and dry air purge on the press. Magnesium stearate can be used as a lubricant, but blending time should be short because deliquescent ZnCl₂ can increase the risk of agglomerate formation under high shear. A glidant such as colloidal anhydrous silica at 0.25–0.5% w/w is often required for low-dose blends.

Compendial impurity control and the oxychloride dissolution boundary

Zinc chloride dissolves in water to form hydrated zinc ions, chloride ions, and species derived from hydrolysis. At pH values above approximately 5.0–6.0, hydrous zinc hydroxide or zinc oxychloride tends to precipitate; the exact boundary depends on zinc concentration, temperature, ionic strength, and the presence of complexing agents. Published data for concentrated pharma-grade ZnCl₂ solutions in complex formulation vehicles is limited; therefore, pH adjustment studies must be run on the actual formula to define the safe operating window for clarity. Compendial release testing is used to detect the impurities that matter in this system: sulfate can form insoluble barium or calcium salts in parenteral admixtures, insoluble matter contributes to visible particles, and lead and other heavy metals are controlled to ICH Q3D (R2) limits according to route-specific permitted daily exposure. The typical release assay for the compendial product is 97.0–100.5% ZnCl₂ on the anhydrous basis, using complexometric titration; identification is confirmed by zinc and chloride chemical tests.

For oral liquid formulations, ZnCl₂ is dissolved in purified water and pH-adjusted with hydrochloric acid or sodium hydroxide. The anion is chloride, so the solution has a saline rather than a sulfate or gluconate taste; taste masking with citric acid or fruit flavor is generally required for oral palatability. Because ZnCl₂ solutions hydrolyze, the dissolution step is performed with the acid added first or simultaneously to keep the pH below the precipitation boundary. The amount of free zinc ion available for absorption can be affected by buffers such as citrate, acetate, or amino acid-based systems; formulation development should characterize bioavailable zinc rather than total zinc alone. The product is typically used as the zinc source in zinc chloride injection; the USP monograph for Zinc Chloride Injection describes a sterile nonpyrogenic solution for intravenous use after dilution. A common presentation contains 1 mg/mL of elemental zinc, equivalent to approximately 2.09 mg/mL of anhydrous zinc chloride. Sterile filtration through a 0.22 μm polyethersulfone or PVDF membrane is performed prior to filling; nylon membranes may be less suitable at the low pH of concentrated acidic solutions.

Injectable-grade material must meet particulate matter limits for subvisible particles and be controlled for bacterial endotoxins. The API is not sterile by default; terminal sterilization or aseptic filtration of the finished solution is required. Compatibility with other trace element salts is influenced by the chloride content: the compounding sequence in multi-trace element admixtures is designed to avoid local precipitation of less soluble salts and to maintain the final pH below the least soluble cation’s precipitation point. The product should not be mixed with phosphate buffers or carbonate-buffered diluents before pH is stabilized, because zinc phosphate or zinc carbonate precipitates are highly insoluble and can block infusion filters. Release of a batch for parenteral use requires a bacterial endotoxin result below the limit calculated for the maximum zinc dose in the finished formulation.

When aqueous wet granulation is replaced by dry granulation for deliquescent ZnCl₂ blends

If a solid oral dosage form contains ZnCl₂ in a formula with microcrystalline cellulose, povidone, or starch, aqueous wet granulation can partially dissolve the API during spray addition. The dissolved zinc chloride migrates to the granule surface during drying, where it can form a hard outer layer and create content-uniformity and hardness problems. Dry granulation by roller compaction avoids this failure mode. The ZnCl₂-containing pre-mix is compacted between counter-rotating rolls at a formulation-dependent roll force and then milled through an oscillating granulator. The resulting granules retain the anhydrous character of the API and show less batch-to-batch moisture variability than wet-granulated material. However, deliquescent powder may still adhere to the rolls and side seals when ambient humidity is elevated; cooled rolls and dry nitrogen purge through the compaction chamber are used to suppress this failure. For products in which roller compaction cannot achieve the required granule hardness, a non-aqueous granulation solvent may be considered, but solvent selection must account for the high solubility of ZnCl₂ in polar solvents and the need to control residual solvent according to ICH Q3C.

When ZnCl₂ is processed as a granule for pediatric or geriatric oral powders, the particle size distribution should be selected to avoid separation during dosing. The API is often mixed with a directly compressible diluent and dry-granulated to a target granule size of 200–500 μm; the exact range depends on the final sachet or bottle-scoop delivery system. The material should not be subjected to prolonged high-shear mixing with hygroscopic ingredients because the resulting surface water can generate localized chloride-rich brine and cause equipment corrosion or poor flow.

The analytical certificate for each batch records the assay on the anhydrous basis, water content by Karl Fischer titration, and the elemental impurity profile because batch-to-batch variation in ZnCl₂ is most often related to water sorption and surface oxide formation rather than to total zinc content. The raw material is screened at incoming inspection for caking and liner integrity; any container found with surface wetting or deliquescence is isolated and sampled from multiple locations because caking can create heterogeneous water distribution. Zinc chloride is not interchangeable with zinc sulfate or zinc gluconate on a salt weight basis in a registered formulation. The zinc mass fraction of ZnCl₂ is approximately 2.11 times that of zinc sulfate heptahydrate and 3.34 times that of zinc gluconate. A formulation containing 25 mg elemental zinc as ZnCl₂ requires 52.1 mg of anhydrous ZnCl₂, whereas the same zinc content requires 109.9 mg of zinc sulfate heptahydrate or 174.2 mg of zinc gluconate. This salt weight difference changes tablet core size, capsule fill, and the choice of excipients. Zinc chloride solutions have a lower pH than equimolar zinc sulfate or zinc gluconate solutions, so injection formulas require careful pH adjustment and may have different compatibility with rubber stoppers, glass, and co-ingredients.

The API is re-packaged only in low-humidity environments, typically below 20% RH. Secondary packaging includes tamper-evident HDPE drums with a desiccant sachet where required, and incoming containers are inspected for liner damage, caking, and surface wetting. In long-term storage studies, the main degradation pathway is moisture uptake rather than chemical degradation; water content increases with open-container exposure and leads to a reduction in assay on the anhydrous basis because the material forms hydrates. Therefore, the product should be used promptly after opening or stored in a dry nitrogen-blanketed system. The product should not be stored near volatile bases, alkali metal hydroxides, or strong reducing agents. For injectable applications, the bacterial endotoxin level is maintained through controlled raw-material selection and manufacturing hygiene; the final product is only released after the specified endotoxin and particulate test results are available. Cross-contamination control is provided by a dedicated mineral and trace-element processing area, separate from organic API and hormone manufacture.