Sodium Metabisulfite COA: Separate Salt Assay from SO2 Equivalent

A sodium metabisulfite percentage is incomplete until the report says what it is expressed as. A result calculated as Na₂S₂O₅ and a result expressed as SO₂ equivalent represent different chemical-equivalent masses, even when the sample-mass basis is the same. Comparing their printed percentages directly can give a purchasing team the wrong conclusion about an offer.

Use the MOS sodium metabisulfite product page to identify the material being discussed, then qualify the actual offered grade and lot documents. This guide explains how to read the reporting basis; it does not certify a grade, supply a batch result or authorize an application.

Read the analyte before the percentage

Start with the complete result label, not the largest number on the certificate. Sodium metabisulfite is Na₂S₂O₅, CAS 7681-57-4. The FAO/WHO JECFA specification, prepared in 1999, identifies that formula and reports an iodine-based assay in Na₂S₂O₅ equivalents. That named historical method reference is useful for interpreting a result; it is not evidence that a MOS shipment complies with that specification or with a current end-use requirement.

Also distinguish a specification minimum, a typical value and a measured lot result. They may appear in neighboring columns but answer different questions. The general chemical COA review guide covers identity and document traceability; the additional issue here is the chemical basis of the assay itself.

Convert a reporting basis, not a performance result

The stoichiometric SO₂ equivalent of Na₂S₂O₅ uses two SO₂ units per Na₂S₂O₅ formula unit. Using the cited JECFA formula weight for the salt and the PubChem SO₂ reference, the mass ratio is 2 × M(SO₂) / M(Na₂S₂O₅), approximately 0.674. This is equivalent-mass arithmetic, not a statement that a bag contains that fraction as free SO₂ gas.

On the same sample-mass basis, and only when the stated methods support this equivalence:

SO₂-equivalent fraction ≈ 0.674 × Na₂S₂O₅-equivalent assay fraction.

For an expressly hypothetical report of 96.0% as Na₂S₂O₅, the corresponding calculation is approximately 64.7% as SO₂ equivalent. Conversely, a hypothetical 65.0% as SO₂ corresponds arithmetically to about 96.4% as Na₂S₂O₅ equivalent. These are examples, not MOS specifications or test results, and the rounded calculations do not establish whether either material is better.

The conversion is conditional. An as-received result cannot be compared directly with a dry-basis result. A missing moisture/basis record cannot be replaced by an assumed correction, and an unspecified loss-on-drying value should not simply be treated as water. Have the laboratory or quality team resolve incompatible methods or reporting conventions before normalizing the numbers.

A matching assay does not qualify a lot

An equivalent assay answers the question defined by its method. It does not independently establish iron, arsenic, lead, insoluble matter, identity, or suitability for a particular formulation. The cited JECFA document itself separates assay from other purity tests. Its limits must not be copied into an offered MOS grade without a relevant, approved specification and evidence.

Two lots with similar reported assay may therefore differ in an acceptance property that matters to the buyer. Conversely, two certificates using different equivalent bases may look very different while describing broadly similar equivalent content. Neither observation permits acceptance without the agreed lot identity, test scope and limits.

Compare documents before comparing unit prices

First determine which offers meet the independently agreed grade, impurity and application requirements. Only then can an assay-equivalent comparison be useful commercially. For eligible offers expressed on the same validated basis, dividing the offered price per unit mass by the agreed assay fraction gives a cost per unit of that reporting equivalent.

Keep the denominator explicit: cost per unit of Na₂S₂O₅ equivalent is not the same figure as cost per unit of SO₂ equivalent. Neither is automatically the cost of achieving a process outcome. Freight, packaging, testing and handling may also differ; equivalent-assay arithmetic does not authorize substitution or establish an application dose.

If the reporting basis remains unresolved, retain the original quoted mass and price rather than manufacture a normalized ranking. The useful purchasing decision is to compare qualified offers on a common, documented basis, not to reward the highest-looking percentage.

Record a specification that can actually be checked

A concise enquiry should identify sodium metabisulfite and the intended application, state the assay analyte and reporting basis, name the proposed test method, and separate the assay requirement from independently controlled impurities. Specify whether the acceptance limits and results are as received or on another defined basis, and agree the sampling/lot-document requirements with the quality team.

Send that brief with quantity, proposed packaging and destination through the existing sodium metabisulfite enquiry page. Request the current offered-grade specification, specimen COA and SDS, followed by the actual lot evidence at the agreed stage. A catalogue identity is the starting point for that discussion, not a substitute for it.