Citric acid
Citric acid identity, hydrate and assay bases, source-specific physical properties, and limits of chelation and compatibility claims.
Tags: chemicals, organic-acids, citrates
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Identity and supplied forms
Merck 100241 anhydrous and 100244 monohydrate both specify 99.5–100.5% on an anhydrous assay basis. Their Karl Fischer water limits differ, ≤0.5% and 7.5–8.8%, respectively. The hydrate still has a distinct CAS number and formula mass. Citric acid is 2-hydroxypropane-1,2,3-tricarboxylic acid. Merck anhydrous product 100241 Merck monohydrate product 100244
| Form | CAS number | Formula | Molar mass, g/mol |
|---|---|---|---|
| Anhydrous | 77-92-9 | C₆H₈O₇ | 192.12 |
| Monohydrate | 5949-29-1 | C₆H₈O₇·H₂O | 210.14 |
The values above are those on the linked Merck product pages. The dot denotes crystal water, rather than an aqueous solution concentration. Jungbunzlauer describes the monohydrate as containing one water molecule per formula unit and as efflorescent in dry air. Both forms occur as colourless crystals or white crystalline powder. Jungbunzlauer monohydrate.
Commercial citric acid can originate from microbial fermentation of a carbohydrate substrate. Jungbunzlauer lists acidification, buffering, and chelation among its functions. These are chemical roles, not a definition of a formulated manufacturing product. Jungbunzlauer anhydrous.
Grade, assay, and water basis
The examples below are supplier specifications, not measured results for a particular lot.
| Named product | Assay basis and specification | Separate water specification |
|---|---|---|
| Merck 100241, anhydrous powder, EMPROVE ESSENTIAL | Alkalimetric assay, calculated on anhydrous substance, 99.5% to 100.5% | Karl Fischer, at most 0.5% |
| Merck 100244, monohydrate, EMSURE ACS, ISO, Reag. Ph Eur | Acidimetric assay, calculated on anhydrous substance, 99.5% to 100.5% | Karl Fischer, 7.5% to 8.8% |
| Avantor J.T.Baker 0115-05, monohydrate, granular, multi-compendial, revision 0 | C₆H₈O₇ assay on anhydrous basis, 99.5% to 100.5% in its USP, EP/BP, and JP rows | 7.5% to 9.0% in those respective rows |
Sources for the complete specification tables are Merck 100241, Merck 100244, and Avantor 0115-05, pages 1 and 2.
Interpretive synthesis. An assay on an anhydrous basis describes the result after correction for water. It does not mean that the supplied monohydrate contains that mass percentage of water-free acid. An upper assay limit above 100% is an analytical specification, not evidence of more than 100% chemical mass fraction. The cited tables do not turn titrimetric assay into a complete impurity analysis. Their separate impurity and water tests remain distinct.
The two monohydrate water intervals differ and remain product-specific. A grade designation does not establish a current lot's values. Merck's certificate-of-analysis lookup is lot-specific. No lot certificate was used here. Avantor's specification also refers elemental impurity information to separate documents, which are outside this reference's coverage. The full compendial methods and current paid standards were not reviewed. Supplier conformance statements are reported as supplier claims.
Derived mass relationships
The following are stoichiometric calculations from the cited formula masses, not measured product composition or a preparation recipe.
- Ideal monohydrate contains
192.12 / 210.14 = 0.91425, about 91.425% water-free citric acid by mass. - Its remaining crystal-water fraction is about 8.575% by mass. Crystal water is part of the ideal hydrate formula, not an impurity.
- For the same number of citric acid formula units, the ideal monohydrate mass is
210.14 / 192.12 = 1.09380times the ideal anhydrous mass.
Those identities assume pure, stoichiometric forms. They do not establish assay or water content for a supplied lot. A reported concentration also needs its basis. Mass percent, grams per litre of solution, and grams per quantity of solvent describe different quantities.
Physical properties and conflicting reports
| Source and form | Reported property | Applicability limit |
|---|---|---|
| Merck 100241, anhydrous | Density 1.67 g/cm³ at 20 °C. Bulk density 560 kg/m³. Melting entry 153 °C with decomposition. Solubility entry 1330 g/L. | The page supplies no temperature or denominator definition for that solubility entry, and no bulk-density method. |
| Merck 100244, monohydrate | Density 1.54 g/cm³ at 20 °C. Bulk density 800 to 1000 kg/m³. Melting entry 135 to 152 °C. Solubility entry 880 g/L. | The page supplies no temperature or denominator definition for that solubility entry, and no thermal measurement method. |
| ILO/WHO ICSC 0855, anhydrous, March 1998 | Water solubility 59 g/100 mL at 20 °C. Melting point 153 °C. Decomposition at 175 °C, with chemical-danger text specifying above 175 °C. | The card does not give a solubility measurement method or distinguish solvent volume from final solution volume. |
Sources are Merck 100241, Merck 100244, and ICSC 0855.
Interpretive synthesis. Solid density, bulk powder density, and aqueous solution density are different properties. The solubility entries above do not establish a common saturation curve. Their unresolved concentration bases prevent a reliable numerical reconciliation. The thermal entries likewise do not establish one universal melting or decomposition threshold. No dehydration temperature, phase-transition curve, or heating-rate correction is established here.
Acidity, citrate, and chelation
Derived chemical notation. The three carboxylic acid groups give the successive acid-base forms H₃Cit, H₂Cit⁻, HCit²⁻, and Cit³⁻. This notation describes protonation states, not four supplied solid grades. Cit³⁻ is the fully deprotonated citrate ion. A citrate salt also contains counterions, so its formula mass differs from either acid form.
IUPAC defines chelation as binding through at least two separate sites of one ligand to a single central atom. Jungbunzlauer identifies citric acid as a chelating agent. IUPAC Gold Book, chelation, version 5.0.0, 2025, Jungbunzlauer anhydrous.
Chemical synthesis. Acid dissociation and metal binding are different equilibria. Quantitative speciation requires the temperature, activity or concentration convention, ionic medium, and relevant acid and metal-complex constants. No numerical acid-dissociation or formation constants are established in this reference. A chelation function alone supplies no universal metal-removal rate, corrosion resistance, or surface-finish result.
ICSC 0855 reports reactions with oxidants and bases and attack on metals. Those broad hazard statements do not identify corrosion rates or establish compatibility with a particular alloy, coating, polymer, or formulated product. ICSC 0855.
Hazard and source scope
The G-Biosciences RC-039 anhydrous ACS-grade SDS, US English, version 12.0, revised December 17, 2025, classifies eye irritation as category 2 and single-exposure respiratory irritation as category 3. It uses H319 and H335. Its section 3 lists citric acid at 100%. That SDS composition declaration is not a lot assay. The cover identifies RC-039, while section 1 gives product code 210C. G-Biosciences SDS, cover and sections 1 to 3.
The same SDS calls the solid nonflammable in section 9, while ICSC 0855 describes it as combustible and identifies a dust-explosion hazard. Neither supplies a shared test basis that resolves those descriptions. The SDS's closing statement limits it to health, safety, and environmental information rather than a guarantee of product properties. These documents do not establish worldwide classification or legal compliance. G-Biosciences SDS, sections 9 and 16, ICSC 0855.
The Merck and Jungbunzlauer web pages were read on October 30, 2026 PDT. They expose no revision date for the cited property and specification tables. The Avantor PDF bears revision 0 but no legible document date. Its manufacturer identity remains Avantor even though Thermo Fisher hosts the file. These sources describe named products or general substance hazards. They do not establish a current lot's composition, a customer's capability, or a qualified manufacturing process.
Related manufacturing context is Surface-finishing processes.