Compare inorganic salt quotations by calculating how much of the same defined chemical quantity each delivery contains. Establish the supplied hydration state, the chemical formula used to express assay, and whether that assay applies to the material as received or after a specified correction. Then apply the supported conversion to delivered net weight. A 99% dry-basis assay does not mean that every delivered kilogram contains 0.99 kg of anhydrous salt. Crystal water and the assay calculation may already account for the same mass difference; applying both corrections can understate the purchased quantity.

Define the Chemical Quantity Before Comparing Prices

“Inorganic salt content” can refer to the named hydrate, its anhydrous equivalent, a metal element or a particular ion. Those quantities require different calculations.

For example, magnesium sulfate heptahydrate is MgSO₄·7H₂O, while anhydrous magnesium sulfate is MgSO₄. Manufacturer references list their respective molar masses as 246.47 and 120.37 g/mol. The seven water molecules form part of the hydrate’s formula mass.

For the ideal pure heptahydrate:

Anhydrous MgSO₄ equivalent per kg hydrate = 120.37 ÷ 246.47 ≈ 0.4884 kg

Thus, pure magnesium sulfate heptahydrate contains approximately 48.84% anhydrous MgSO₄ equivalent by mass. This is a stoichiometric calculation, not a measured assay or a purity limit.

The consequence appears directly in aqueous preparation. Equal masses of pure heptahydrate and pure anhydrous salt supply different amounts of MgSO₄. If a charge calculation treats both as 100% anhydrous MgSO₄, the hydrate delivers fewer formula units than intended.

Choose the comparison quantity according to the purchasing task:

An anhydrous-equivalent calculation does not establish that the delivered solid is physically anhydrous. It expresses quantity on a common chemical basis.

This article concerns commercial quantity reconciliation for defined salts, using magnesium sulfate as the worked example. It does not establish interchangeability between industrial, food, pharmaceutical or electronic grades.

Read the Assay as a Calculation Statement

A certificate stating “Assay: 99.5%” is incomplete for quantity reconciliation unless it identifies both the chemical reporting form and the mass basis.

Ask two separate questions:

What is the result expressed as?

Examples include MgSO₄, MgSO₄·7H₂O or Mg.

What mass is in the denominator?

Examples include the original sample, a dried residue or a sample mass corrected using a specified water determination.

These distinctions occur in actual specifications. A Merck magnesium sulfate heptahydrate specification lists a complexometric assay calculated on dried substance alongside a separate loss-on-drying requirement. Their coexistence illustrates why a high dried-basis assay should not be read as the anhydrous salt fraction of the delivered material. The specification is evidence of that reporting convention, not a universal requirement for commercial magnesium sulfate.

Before calculating purchased quantity, obtain the assay method’s expression of results. A product name, a percentage and the words “dry basis” may still leave the conversion unresolved.

Use One Supported Conversion Route

The correct route depends on what the reported assay already includes.

These routes are alternatives. They should not be stacked automatically.

If an as-received result already reports MgSO₄ equivalent, multiplying again by the MgSO₄-to-heptahydrate molar-mass ratio removes crystal-water mass a second time.

Likewise, when a supported dried-basis calculation already converts the original sample to anhydrous salt equivalent, an additional hydrate factor would double-count the correction.

If the supplier cannot explain which route generated the result, hold the quantity comparison. Request the calculation basis rather than choosing whichever interpretation produces the lowest price.

Apply a Dry-Basis Correction Only When Its Basis Is Established

For a defined procedure in which the residue assay is expressed as anhydrous salt equivalent, the mass balance is:

As-received anhydrous-equivalent fraction = dried-basis assay fraction × residue fraction

If the residue fraction is established by the applicable drying-loss result:

Residue fraction = 1 − drying-loss fraction

This relationship requires an appropriate analytical basis. Confirm that:

• The assay and drying result represent the same lot and material condition.
• The reported assay describes the defined residue or equivalent corrected mass.
• The drying procedure does not invalidate the quantified analyte.
• The calculation has not already been applied to the reported result.

A generic moisture value from another procedure cannot automatically replace the correction specified by the assay method.

A Hypothetical Delivered-Quantity Calculation

Assume a hypothetical magnesium sulfate heptahydrate lot has:

• Delivered net weight: 20,000 kg.
• Dried-basis assay: 99.5%, expressed as MgSO₄.
• Applicable drying loss: 51.0%.
• Agreed delivered cost: US$8,000.

For this example, assume the method establishes that the 49.0% residue fraction and dried-basis assay form a valid conversion to as-received MgSO₄ equivalent.

As-received MgSO₄-equivalent fraction = 0.995 × 0.490 = 0.48755

Delivered MgSO₄ equivalent = 20,000 × 0.48755 = 9,751 kg

Delivered cost per kg MgSO₄ equivalent = US0.820

The figures are illustrative, not market prices or actual batch results.

Multiplying delivered weight by 99.5% alone would incorrectly produce 19,900 kg of MgSO₄ equivalent. Applying a hydrate molar-mass factor after the supported dry-basis conversion would introduce a second, unjustified correction.

The calculated 48.755% equivalent content is close to the ideal heptahydrate’s theoretical value, but that proximity does not independently prove hydrate identity or exclude impurities.

Do Not Treat Drying Loss as Free Moisture

The published preview of USP General Chapter 〈731〉 defines loss on drying as the volatile matter removed under specified conditions. It does not define every drying loss as free water. This definition is useful here without making a pharmaceutical procedure a requirement for industrial salt purchasing.

For a hydrate, distinguish the water incorporated in the specified formula from additional water associated with the delivered material. A substantial drying loss can be consistent with a named hydrate; its meaning depends on the material and procedure.

Do not subtract theoretical crystal water and a measured drying loss together unless the method explicitly establishes that they represent separate quantities.

ChemicalCell’s guide to Karl Fischer and loss-on-drying results addresses the separate analytical question of whether two reported moisture results measure comparable attributes.

Connect the Analysis to the Delivered Net Weight

The calculation is only as representative as the sample and quantity records supporting it.

For bagged or drummed salts, use product net weight rather than gross package weight. For bulk deliveries, agree whether dispatch weighing, receiving weighing or another measurement establishes commercial quantity.

For a salt susceptible to moisture exchange, require a sample-handling record that identifies the packaging condition and exposure before analysis. If the tested sample has changed condition, its assay correction may no longer describe the delivered material.

Where supplier and receiving results disagree, compare the chemical reporting form, drying or water procedure, sample history and calculation before attributing the difference to the shipment.

A batch adjustment should follow agreed sampling, retest and settlement rules. A receiving result should not silently replace the contractual basis.

Decide Whether the Quotations Are Ready to Compare

Request each document for the decision it supports. The specification defines the required form and acceptance basis. The certificate of analysis provides lot-specific results. The method or calculation statement explains their conversion. Weight records establish delivered quantity.

ChemicalCell’s Inorganic Salts directory supports browsing related material families. Confirm the exact hydrate, grade and specification for each candidate.

Close the Quantity Basis Before Selecting the Lower Price

For an aqueous preparation, comparing equivalent salt quantities can expose differences hidden by price per delivered tonne. It does not establish equal dissolution behavior, impurity suitability or approval for the intended use.

Keep the physical form as a separate acceptance requirement where the process depends on a particular hydrate. A cheaper anhydrous-equivalent quantity does not override that requirement. Proposed form changes should enter controlled trials at matched salt equivalents under the intended preparation conditions before commercial approval.

Obtain one reconciled quantity statement for each quotation: supplied formula and hydration state, assay reporting form, as-received conversion, representative lot results, delivered net weight and total cost to the same destination. Rank the offers after those quantities reconcile, with unresolved conversions kept on hold.