Methylene blue is one name that sits on several chemical-identity records, and those records display different molecular weights. PubChem's record for the chloride salt displays 319.9, its record for the bare cation displays 284.4, and its record for the trihydrate displays 373.9. None is an error. Each is the calculated weight of a different entity, and reading the records correctly means matching each number to its entity before comparing anything. The same discipline applies to identifiers, because one record can carry several registry-format numbers without explaining them, and to origin statements, because a made in USA claim answers a marketing question under a federal standard, not a chemistry question.
The scope is the record, not the material. Basic identity, handling, and storage belong to the existing methylene blue guide; questions about a specific analytical report belong to the certificate reading guide. Nothing here describes any supplier's material, lot, or sample; every value below is copied from a cited record or shown as a placeholder.
What does a chemical-identity record describe?
A chemical-identity record is a curator's description of one defined chemical entity, not of a sample, a container, or a lot. PubChem's record for methylene blue, compound identifier 6099, carries a molecular formula, a molecular weight displayed to one decimal place, a systematic name, and a connectivity string, and each field describes the chloride salt entity as PubChem has curated it. Three parts of a record decide what its values mean. The fields are the statements. The issuer is the organization that curates them, here PubChem at the National Library of Medicine. The scope is the entity named in the record's title. A weight on a record is calculated from the recorded formula; it is not a measurement, and it says nothing about what any material contains.
A chemical-identity record describes an entity as curated by its issuer, never a sample. Its fields, including formula, displayed weight, systematic name, and identifiers, are statements about that defined entity within the record's stated scope. When two records show different values under one name, the useful question is not which record is right but which entity each record describes.
What formula and molecular weight does the methylene blue salt record show?
The PubChem record with compound identifier 6099 is titled Methylene Blue and describes the chloride salt. Read on September 21, 2026, it displays the molecular formula C16H18ClN3S and a molecular weight of 319.9. Its systematic name is [7-(dimethylamino)phenothiazin-3-ylidene]-dimethylazanium chloride, and its connectivity string ends with the chloride written as a separate ion, [Cl-]. The landing page title carries the same name, formula, and identifier. Each value belongs to the recorded entity, not to any material. The weight is displayed to one decimal place. That precision is the record's formatting, not a statement about analytical accuracy; no source read for this page explains why PubChem formats it that way, so this page describes the display without explaining it. Copy the value as displayed, with its precision, the record, and the date read.
One name returns different weights on different pages because different pages have bound the name to different entities: near 284.4 means the bare cation, near 319.9 the chloride salt, near 373.9 the trihydrate. Pages also differ in display precision, so a value with three decimals may be the same formula at a finer display; round to the coarser precision before comparing, and any remaining difference is about entities or formulas, not rounding. A weight without a named entity is incomplete, and the entity comes from the record's title, formula, and name, never from the search term.
Why do three records show three different molecular weights for methylene blue?
Three PubChem records carry the methylene blue name or a close form of it, and each describes a different entity. Compound identifier 4139 is the methylthioninium cation, formula C16H18N3S+, displayed weight 284.4; the plus sign records the positive charge. Compound identifier 6099 is the chloride salt, formula C16H18ClN3S, displayed weight 319.9. Compound identifier 104827 is titled Methylene Blue Trihydrate, formula C16H24ClN3O3S, displayed weight 373.9. The cation's systematic name is the salt's name with the word chloride removed. The trihydrate's systematic name joins the azanium cation, chloride, and trihydrate with semicolons, which marks components held together in one entity rather than bonded into one molecule. All three weights are calculated from their formulas and displayed to one decimal. A document that gives one of these numbers has chosen one of these entities, whether it says so or not.
PubChem shows 284.4, 319.9, and 373.9 under the methylene blue name because three records describe three entities: the methylthioninium cation on CID 4139, the chloride salt on CID 6099, and the trihydrate on CID 104827. Identify which entity a document means before comparing its weight to any of these values. A weight compared across entities is a category mismatch, not a discrepancy.
| Entity | PubChem CID | Formula | Displayed weight | How the record marks the entity |
|---|---|---|---|---|
| Methylthioninium cation | 4139 | C16H18N3S+ | 284.4 | Name lacks the word chloride; plus sign records charge |
| Chloride salt | 6099 | C16H18ClN3S | 319.9 | Name ends in chloride; connectivity ends with a separate chloride ion |
| Trihydrate | 104827 | C16H24ClN3O3S | 373.9 | Name joins cation, chloride, and trihydrate with semicolons |
The three values are mutually consistent, and a reader can check that independently. PubChem's record for water, compound identifier 962, displays H2O and a weight of 18.015, so three waters add 54.045; added to the salt's 319.9, that gives 373.945, which displays as 373.9 and matches the trihydrate record. The formulas agree: the trihydrate exceeds the salt by H6O3, three H2O, and the salt exceeds the cation by one Cl. Every step is editorial arithmetic on displayed values, not a PubChem statement.
Consistency check (editorial; inputs are displayed record values) salt, CID 6099 319.9 + water, CID 962, x 3 + 54.045 (18.015 x 3) = 373.945 -> 373.9, matches trihydrate, CID 104827 salt minus cation, CID 4139 35.5 (one Cl)
One limit outranks the arithmetic. The trihydrate record states a stoichiometry, three water units per formula unit, not the water content of any physical material; only a measurement reported for that material can state that, and none is read here. The record says what trihydrate means, not what is in a jar.
Why are CAS numbers, CIDs, and dye names three separate namespaces?
Three kinds of identifier appear on methylene blue records, and they come from three different issuers. A compound identifier, or CID, is PubChem's own database key; 6099, 4139, and 104827 are CIDs and mean nothing outside PubChem. A CAS registry number is assigned by a chemical registration body, and PubChem only lists it among deposited synonyms. A dye name such as C.I. Basic Blue 9 comes from a dye-classification namespace that is neither of the first two. The salt record's synonym list shows why. It contains six strings in registry-number format, 61-73-4, 105504-42-5, 12262-49-6, 152071-32-4, 97130-83-1, and 150645-86-6, plus two EINECS-style numbers, the string CAS-61-73-4, and dye-class names including C.I. Basic Blue 9 trihydrate and TETRAMETHYLTHIONINE CHLORIDE TRIHYDRATE. PubChem does not explain what each registry-format string covers, and neither does this page.
Reconcile two records by writing each identifier next to its issuer, never as a bare number. A PubChem CID is a database key, a CAS registry number belongs to the registration body that assigned it, and a dye name such as C.I. Basic Blue 9 is a third namespace. One record can carry several registry-format strings without explanation, so a matching number proves only that the same string appears in both places.
A string naming a trihydrate sits on the record for the chloride salt. That is not an error; a deposited synonym is a string a depositor associated with the record, and it does not make the record a statement about solid form. This page therefore maps none of the six registry-format strings to a form and prints no registry number for the trihydrate record, because no source read here verifies one. The working rule is issuer with value: not 61-73-4 alone, but CAS registry number 61-73-4, as listed among PubChem synonyms for CID 6099, read September 21, 2026.
How does the phenothiazine ring system appear in methylene blue's chemical name?
The parent ring system of methylene blue has its own record, and reading it beside the salt record shows how a class name relates to a substituted derivative. The NIST Chemistry WebBook, Standard Reference Database 69, lists phenothiazine with the formula C12H9NS, the CAS registry number 92-84-2, and a molecular weight of 199.272, with an InChIKey and spectral sections. That record describes the parent ring, not methylene blue. The salt record embeds the ring in its systematic name, [7-(dimethylamino)phenothiazin-3-ylidene]-dimethylazanium chloride: phenothiazin is the core, position 7 carries a dimethylamino group, position 3 carries the dimethylazanium group with the ylidene ending, and chloride is the separate counterion the connectivity string writes as [Cl-]. The cation has four more carbons and two more nitrogens than the parent ring, matching the two dimethyl-substituted nitrogen groups at positions 3 and 7.
[7-(dimethylamino)phenothiazin-3-ylidene]-dimethylazanium chloride 7-(dimethylamino) substituent at ring position 7 phenothiazin ring core; NIST parent record C12H9NS, CAS 92-84-2 3-ylidene the next group attaches at ring position 3 dimethylazanium charged nitrogen group at position 3 chloride separate counterion; connectivity ends with [Cl-]
Display precision is an issuer's choice: NIST shows 199.272 with three decimals and PubChem shows 319.9 with one, so a worksheet records each value at its own precision. A class name is not an identity: phenothiazine names a ring system shared by many compounds, so a document that calls a material only a phenothiazine compound has named a class, not an entity. The dye-class names on the salt record place the compound in a dye namespace; as chemistry, a dye colors a substrate and a redox indicator changes color with oxidation state, and this page names those classifications only as chemistry and interprets no spectrum.
Which sources can document methylene blue's origin history?
An origin-history claim states when, where, and by whom a compound was first made or described. As documentation it is a historical claim, and only a record class built to carry history can carry it: a dated primary document, or a dated corporate or institutional archive that cites one. Chemical-identity records carry no history. PubChem's fields are formula, weight, names, and identifiers; the NIST record's fields are formula, registry number, weight, and spectra. None states a first-synthesis year, a chemist, or a company, and this page states none either. No source of the right class was available for this page, so the history is recorded as unresolved rather than filled in from memory or a tertiary summary. A claim needs a dated source of the right class before it leaves the unresolved column.
- Chemical-identity records such as PubChem and NIST: cannot. Never cite one for a date, a person, or a company.
- The registry entry behind a CAS number: cannot. It dates the registration at most, not the compound.
- A dated primary document, such as a period publication, patent, or archive item: can. Record the document, its date, and where it was read.
- A corporate or institutional heritage page: can, as that organization's own account. Record the URL and retrieval date; an unretrievable page is not a source.
- A tertiary summary: cannot serve as evidence. Follow its pointer to the primary source and log the summary only as the lead.
Logging an unresolved claim takes three lines: the claim as heard, the class of source it needs, and the status. Teaching example: claim, methylene blue was first synthesized in <year> by <person> at <company>; needs, a dated primary document or an archive that cites one; status, unresolved, no qualifying source read.
What does a made in USA statement claim under the FTC standard?
The Federal Trade Commission's guidance on complying with the made in USA standard sets one core test: an unqualified made in USA claim requires that the product be all or virtually all made in the United States. The FTC weighs the share of manufacturing cost that is domestic, how far removed any foreign content is from the finished product, and how important that content is to the product. A qualified claim states the extent, amount, or type of domestic content. A marketer making either claim needs a reasonable basis, meaning competent and reliable evidence, and carries an ongoing duty to review it. A brand name or U.S. address alone is not automatically an origin claim, while symbols and images can imply one. The 2021 Labeling Rule codified the standard for labels, including online and catalog contexts, and the FTC does not pre-approve claims.
A made in USA statement is a marketing claim about a product's domestic content under the FTC standard. It is not a chemical property, and no identity record can confirm or deny it. The marketer must hold substantiation for it, and the standard is separate from the customs marking rules for imported articles. A reader who sees the statement has not seen that substantiation, so any product's eligibility to make the claim stays unresolved.
Two distinctions matter. The claim is about a product, not a company; a company headquartered in the United States has said nothing about where a product it sells was made. And the claim rests on the marketer's evidence, which a reader does not see. A worksheet entry therefore copies the exact words, notes where they appeared and whether they were qualified, describes any flag or symbol that implied origin, and records that substantiation was not reviewed.
How does country-of-origin marking for imported articles work under CBP rules?
Customs marking is a different regime from advertising claims. U.S. Customs and Border Protection's informed compliance publication on marking of country of origin explains that under 19 U.S.C. 1304 and the regulations it cites, an imported article must be marked so that the ultimate purchaser in the United States can learn its country of manufacture, production, or growth. The ultimate purchaser is generally the last person in the United States who receives the article in the form in which it was imported. When an article is worked on in a second country, origin shifts only through substantial transformation, meaning a new article with a different name, character, and use. Containers have their own marking rules and exceptions. The publication describes the regime; it certifies no article's origin, and origin determinations belong to the agency.
| Aspect | CBP marking under 19 U.S.C. 1304 | FTC made in USA claim |
|---|---|---|
| What it governs | The marking an imported article must carry | A domestic-origin claim a marketer chooses to make |
| Who it informs | The ultimate purchaser in the United States | Anyone who sees the label, advertisement, or catalog |
| Core test | Country of manufacture, production, or growth; substantial transformation across countries | All or virtually all, or a truthful qualified claim |
| Who decides | The agency makes origin determinations | The marketer substantiates; the FTC does not pre-approve |
In a worksheet, a marking is an observation and a claim is an assertion, on different lines. Record a marking as the exact text, the object it appeared on, and whether that object was the article or its container.
A record-reading worksheet for methylene blue documentation
The worksheet below fixes the reading habits from the preceding sections into eight fields. It is a template for reading any methylene blue record, whether from a database, a supplier document, or a literature table. Every field accepts the value not specified, and writing not specified is the correct output whenever a document is silent. The fields are the record name as written, the issuer, the entity, the molecular formula, the displayed weight with its precision, each identifier with its issuer, the retrieval date, and the unresolved fields. The entity field takes cation, chloride salt, trihydrate, or not specified. Nothing in the worksheet asks for a sample value, lot number, supplier, or price, because none of those belongs to a record.
| Field | What to record | Teaching example entry |
|---|---|---|
| Record name as written | Exact title or name string, unedited | Methylene Blue (from a record title) |
| Issuer | Who curates the record | <issuer> |
| Entity | Cation, chloride salt, trihydrate, or not specified | not specified |
| Molecular formula | Exactly as displayed, including any charge sign | <formula as displayed> |
| Displayed weight and precision | The value as displayed and the decimals shown | <value>, <n> decimals |
| Identifiers with issuer | Each identifier beside its namespace and issuer | CAS registry number <string>, as listed among <issuer> synonyms |
| Retrieval date | The date read, because curated records change | <date read> |
| Unresolved fields | Fields left at not specified, plus claims awaiting a qualifying source | entity; origin history |
| Field | CID 4139 | CID 6099 | CID 104827 |
|---|---|---|---|
| Record name as written | methylthioninium (retrieval name) | Methylene Blue | Methylene Blue Trihydrate |
| Entity | cation | chloride salt | trihydrate |
| Molecular formula | C16H18N3S+ | C16H18ClN3S | C16H24ClN3O3S |
| Displayed weight and precision | 284.4, one decimal | 319.9, one decimal | 373.9, one decimal |
| Identifiers with issuer | CID 4139, PubChem | CID 6099, PubChem; six registry-format synonyms, unexplained | CID 104827, PubChem; no registry number verified here |
| Unresolved fields | none in scope | meaning of each registry-format synonym | any registry number; water content of any material |
When a certificate or label disagrees with the comparison table, the worksheet records both values and stops. Reading the document's own methods and results belongs to the certificate reading guide; locating a lot document is where quality documentation is listed when it exists.
Frequently asked questions
Why does methylene blue have more than one CAS number?
PubChem's synonym list for the chloride salt record, CID 6099, carries six strings in registry-number format and explains none of them. Registry numbers come from the registration body, not PubChem, which only lists them as deposited synonyms. Record each number with the record it came from, and never assume two numbers are interchangeable. A document showing a number outside the six is a finding to record, not an error; only that document's issuer can say which entry it intended.
Is 319.9 or 373.9 the correct molecular weight for methylene blue?
Both are correct for their entity. 319.9 is the chloride salt's displayed weight on PubChem CID 6099; 373.9 is the trihydrate's on CID 104827. A document that gives a weight must also name the entity. When it does not, the weight can suggest one: near 373.9 points to the trihydrate stoichiometry, near 319.9 to the salt, near 284.4 to the cation. That inference is editorial and stays unresolved until the document confirms it, and neither value says how much water any sample holds.
What does the chloride mean in methylthioninium chloride?
Methylthioninium is the cation, on PubChem CID 4139 with the formula C16H18N3S+ and a displayed weight of 284.4. Chloride is the counterion that balances the charge. The salt record, CID 6099, ends its systematic name with the word chloride, adds Cl to the formula, ends its connectivity string with a separate chloride ion, and displays 319.9. A document that says methylthioninium without chloride may mean the cation alone; its weight should show which.
Can a database record tell me where or when a compound originated?
No. Identity records carry formula, weight, names, and identifiers; the NIST record adds a registry number and spectra. None carries a synthesis year, a chemist, a company, or a country. An origin history needs a dated primary document or an archive that cites one. A product's origin is a different question: a made in USA claim rests on the marketer's substantiation under the FTC standard, and an imported article's marking follows customs rules. Every origin claim stays unresolved until a source of the right class is read.
Does a certificate's molecular weight have to match the database record?
It has to match the record for the same entity, compared at the coarser of the two precisions. A certificate stating 373.9 beside a record showing 319.9 is not a contradiction if the certificate describes the trihydrate. Identify the entity on each document first, then round the finer value to the coarser display and compare. If a mismatch remains, record both values and move to certificate interpretation, which belongs to the site's guide to reading a certificate of analysis.
Sources and scope notes
Formula, weight, name, and identifier values are copied from PubChem records for compound identifiers 6099, 4139, 104827, and 962, the PubChem synonym list for CID 6099, and the NIST Chemistry WebBook record for phenothiazine, each read September 21, 2026, UTC. Records reflect curation dates, not the retrieval date; all sums and differences are editorial checks on displayed values.
The origin sections paraphrase the FTC guidance on the made in USA standard and the CBP publication on country-of-origin marking, both read September 21, 2026. Neither agency pre-approves or certifies any claim or marking, and nothing here evaluates any product's eligibility. No synthesis history is stated, because no source of the right class was read.
Research-only scope: this page describes how to read chemical-identity records and origin statements for laboratory research documentation. It makes no statement about the origin, purity, form, water content, testing, or origin-claim eligibility of any Glow material or any other material, and it is not a purchasing or use guide.
- PubChem PUG REST, CID 6099 (Methylene Blue, chloride salt): formula, weight, name, connectivity
- PubChem landing page, CID 6099
- PubChem PUG REST, CID 4139 (methylthioninium cation)
- PubChem PUG REST, CID 104827 (Methylene Blue Trihydrate)
- PubChem landing page, CID 104827
- PubChem PUG REST, CID 962 (water): formula and weight for the hydrate check
- PubChem PUG REST, synonyms for CID 6099 Deposited strings; the record explains none of them.
- NIST Chemistry WebBook, SRD 69, phenothiazine Parent ring record, not methylene blue.
- U.S. FTC, Complying with the Made in USA Standard A marketing-claim standard, not a chemical property.
- U.S. CBP, Marking of Country of Origin on U.S. Imports Describes the marking regime; certifies no origin.