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Select magnesium chloride by its intended use, chemical form, impurity limits, and applicable market requirements. A technical-grade flake suited to dust control is not automatically suitable for food production, while a high assay alone does not establish pharmaceutical suitability. Start with the finished product or process requirement, then compare offers on the same chemical and moisture basis.

The most consequential distinction is often hidden in a short product description: magnesium chloride can be supplied as anhydrous material, hydrated crystals or flakes, or an aqueous solution. These forms contain different proportions of magnesium chloride, behave differently during storage, and cannot be compared accurately by price per tonne alone.

Compare Grades by Intended Use

Grade names describe a product's intended quality level, but the names themselves are not a substitute for a specification. Two offers labeled “industrial grade” can differ substantially in soluble impurities, insoluble residue, particle size, and consistency between batches. Similarly, a label such as “food grade” needs support from a suitable specification and documentation for the intended market.

These categories overlap less than their names suggest. Laboratory-grade material, for example, is not automatically qualified for a medicinal product: analytical purity and pharmaceutical manufacturing documentation address different requirements. Conversely, a technical grade with an appropriate contaminant profile can be entirely adequate for an industrial process that does not need a pharmaceutical specification.

Understand the Form Before Comparing Assay

Hexahydrate, anhydrous material, and solution

Magnesium chloride hexahydrate contains six molecules of water of crystallization for each formula unit of magnesium chloride. Pure hexahydrate therefore contains about 46.8% magnesium chloride by mass. That is a chemical conversion, not a guarantee of commercial product purity. A certificate reporting an assay as magnesium chloride hexahydrate is reporting on a different basis from one reporting anhydrous magnesium chloride content.

For example, a hypothetical product assayed at 98% as the hexahydrate corresponds to approximately 45.9% magnesium chloride on an anhydrous-equivalent basis, before considering how the test and specification treat other components. Comparing that number directly with 98% anhydrous magnesium chloride would create a false impression that the two products contain the same amount of active salt.

Anhydrous magnesium chloride offers a higher theoretical concentration of magnesium chloride per unit of material. It also attracts moisture, so its received condition and handling environment matter. Hydrated flakes or crystals introduce more water per unit of active salt but can suit processes in which that water is acceptable. A liquid solution avoids an on-site dissolving step, while requiring storage and transport capacity for the water already present.

For a solution, concentration should state both the unit and the basis, such as mass percentage of magnesium chloride in the supplied liquid. Density and temperature are useful when equipment meters by volume, because a tank reading in liters cannot be converted reliably to kilograms of active salt without an appropriate density value. Confirm whether concentration is specified at dispatch, on receipt, or by batch analysis.

Moisture is not always an impurity

Water of crystallization is part of the hexahydrate's chemical form. Free surface moisture, by contrast, can affect flow, packaging condition, and dosing accuracy. A single “moisture” figure is difficult to interpret unless the test method explains what it measures. For hydrated material, loss on drying under particular conditions can include water released from the crystal structure; it should not automatically be treated as free water.

Where solids must feed through a hopper, ask for particle-size distribution, caking expectations, and storage conditions as well as assay. A product that meets chemical limits can still bridge in a feeder or dissolve unevenly if its physical form differs from the one used during process qualification.

Match Impurities to the Application

Assay gives an overall measure of the declared substance; it does not identify every component in the remainder. The impurities that matter most vary with the end use and the source of the material. A meaningful specification sets limits for relevant substances and states the test methods, rather than relying on a purity claim alone.

• Calcium and sulfate: These can affect formulations and contribute to deposits when solution chemistry favors precipitation. Assess them alongside water quality, concentration, temperature, and other ingredients.
• Insoluble matter: Residue matters in clear solutions, fine nozzles, filters, and processes where sediment contaminates the finished product. A low value on a certificate is useful only when the method and reporting limit suit the application.
• Iron and other metals: Trace contaminants can influence color, product stability, or suitability for sensitive uses. Set individual limits where a particular metal matters; a general appearance test does not establish its concentration.
• Other salts: Sodium and potassium compounds can change formulation balance or increase unwanted salt loading, even when the magnesium chloride assay meets the stated minimum.

For a process that already contains calcium, sulfate, or alkaline ingredients, a clear incoming magnesium chloride solution does not prove the final mixture will remain clear. Test the complete formulation at its working concentration and temperature, using the intended order of addition. A precipitate formed during blending has a different cause from insoluble particles already present in the raw material.

Application-Specific Selection

Dust control and deicing

For road treatment, the effective concentration and delivered form influence application equipment and dosage. Liquid can be applied directly with suitable spraying equipment; flakes require equipment and operating conditions appropriate to a solid. Compare products on the amount of magnesium chloride delivered to the treated surface, not just on the weight of solution or flakes purchased.

Chloride exposure can contribute to corrosion of metals and affect nearby vegetation, soil, or water. Local conditions, application rates, drainage, and equipment materials all matter. A product suitable for one site should not be assumed suitable for a sensitive bridge, landscaped area, or location with restrictions on runoff. Review the product's safety information and the site's applicable requirements before use.

Construction materials

In magnesium-based cement systems, magnesium chloride solution can participate in reactions with magnesium oxide. The appropriate concentration is tied to the formulation, the reactivity of the oxide, mixing water, curing conditions, and performance requirements. Substituting a different hydrate or solution concentration without recalculating the formulation changes both the salt and water balance.

Excess soluble chloride and moisture exposure also deserve attention where metal components or moisture-sensitive finishes are present. Assess the complete assembly and intended service environment; a raw-material assay alone cannot establish durability or compatibility with reinforcement and fasteners.

Food, feed, and pharmaceutical uses

For food applications, the intended function and destination determine which specifications and permissions apply. Magnesium chloride used as a coagulant, for example, must be evaluated in the context of the finished food and the applicable jurisdiction. Review identity, assay, contaminant limits, and traceability against the requirements relevant to that use.

Feed and pharmaceutical applications require their own assessment. A document stating that material meets a general chemical purity limit does not establish that it meets a feed specification or a particular pharmacopoeial monograph. Where a monograph or customer standard is specified, compare each required test and limit with the offered material and confirm the documented manufacturing and change-control requirements.

Read the Certificate Against the Purchase Specification

A certificate of analysis should identify the product form, batch, test results, units, and applicable test methods or references. Compare actual batch results with the agreed limits. A specification sheet describes the acceptance criteria for a product; a batch certificate reports the results for a particular lot. They serve different purposes.

Pay attention to reporting conventions. “Not detected” has limited value without a detection or reporting limit, particularly when a contaminant limit is low. Likewise, an assay reported on a dried or anhydrous basis needs conversion before it is compared with a value reported as received. If a result is calculated from another measurement, the calculation basis should be clear.

When a formulation is sensitive to trace contaminants, establish whether the certificate reports measured results for each batch or relies on periodic testing. The sampling method also matters: a small sample from the top of a bag or tank may not represent a shipment that has segregated, settled, or absorbed moisture.

Storage, Compatibility, and Delivered Cost

Magnesium chloride is hygroscopic, so packaging integrity matters from filling through storage. Exposed solid can absorb moisture, cake, or become wet, altering its flow and potentially the concentration delivered by a volumetric feeder. Specify packaging that suits the transport route, expected humidity, storage period, and discharge method. Reseal partially used packages and protect them from water ingress.

Liquid shipments require compatible tanks, pumps, seals, and transfer lines. Consider corrosion resistance in the actual chloride concentration and operating temperature, including areas where liquid can collect or evaporate. Material compatibility should be evaluated for the complete system rather than inferred from the product name alone.

Separate identity and hazard controls also matter when several chemicals are handled at the same site. Chromium(VI) oxide CAS#1333-82-0, used in chromium electroplating, is a different substance with strong oxidizing and serious health hazards; its storage and handling requirements must not be inferred from those of magnesium chloride. Apply the current safety data sheet and site compatibility procedures to each material.

To compare commercial offers, calculate the delivered cost per unit of magnesium chloride on a consistent basis. For solids, use the stated anhydrous-equivalent content; for liquid, use the concentration by mass and include the cost of transporting and storing the solution. Add relevant handling, dissolution, filtration, and disposal costs where they differ. A lower price per tonne of supplied product does not necessarily mean a lower cost per tonne of usable salt.

Before changing a qualified source or form, agree on a representative sample and evaluate it in the actual process. Record the initial specification, trial conditions, and acceptance criteria so that a successful laboratory dissolution is not mistaken for proof of full-scale feeding, formulation stability, or finished-product performance.