Erythrosin

Silver bromide is blind to green. Three or four drops of a dilute red dye make it see, and the price is that every subsequent operation moves to a deep red safelight. Erythrosin is the substance that buys green sensitivity, and it is also the food colour whose American authorisation was revoked in January 2025 — both of which belong on this page.

In photography

What a sensitising dye does. Silver bromide and iodobromide are intrinsically sensitive only to blue and violet. A dye adsorbed on the crystal surface absorbs a green photon and passes the energy into the crystal, so that green light can form a latent image. The material becomes orthochromatic: sensitive to blue and green, blind to red. That is the definition the whole of an ortho emulsion’s behaviour follows from, and this course’s Part V orthochromatic project is where the dose and the make are set out.

Its spectral shape, and its flaw. Wall’s 1924 account is precise about both: “Erythrosine is the dye generally used”, and it “gives a strong yellow, orange and green sensitiveness, but a marked want of sensitiveness in the blue-green”. That gap is the signature of the eosine dyes; Wall notes in the same passage that the newer dimethylstyrylpyridine methiodide gives even sensitiveness from bright blue to 560 nm “without the usual gap in the blue-green, characteristic of the eosine dyes”. Erythrosin is the dye that works, not the dye that works best; it survives here because its handling can be defended.

Two routes, and the trade between them. Wall states it once: “Ordinary, or non-colour sensitive plates, may be orthochromatised by bathing in solutions of dyes. As a rule, such plates have a higher colour sensitivity than those coated with an emulsion to which the dye is added during the mixing, but they do not keep quite as well.” His bath is “a 1:5000 aqueous solution with the addition of 0.5 per cent of ammonia” — 200 mg of dye per litre — which is a bath concentration and cannot be set beside an in-emulsion dose as though the two were the same quantity. This course takes the in-emulsion route, and the ammonia in Wall’s bath is one reason: Part V excludes it.

How much, and where this page stops. Two published in-emulsion doses, one from the emulsion literature of 1941 and one from the course’s Tier 2 practitioner seventy-two years later, agree within a factor of about 1.7 at roughly 100 to 170 mg of dye per mole of silver — a few milligrams for a make carrying 5 g of silver nitrate, dispensed as three or four drops of a 2 per cent stock. Those readings were made against the original pages in the course’s Part V evidence review, which the Part V pages cite with their locators. The mirrored copies of those two sources in this course’s research corpus do not include the pages in question, so this page gives the range and its provenance rather than quoting either directly. Where the optimum sits for a particular emulsion no source establishes: that is the student’s measurement, not a number to copy.

⚠Erythrosin, the FDA and Red No. 3 — both halves, in the FDA's own words

A reader who looks this substance up will meet a headline before they meet a spectrum, so the course puts it here rather than burying it. On 15 January 2025 the U.S. Food and Drug Administration announced that it “is revoking the authorization for the use of FD&C Red No. 3 as a matter of law, based on the Delaney Clause of the Federal Food, Drug, and Cosmetic Act”. The petition that prompted it cited “two studies that showed cancer in laboratory male rats exposed to high levels of FD&C Red No. 3 due to a rat specific hormonal mechanism”.

The same notice, in the same paragraph, states the regulator’s own reading of what that means: “The way that FD&C Red No. 3 causes cancer in male rats does not occur in humans. Relevant exposure levels to FD&C Red No. 3 for humans are typically much lower than those that cause the effects shown in male rats. Studies in other animals and in humans did not show these effects; claims that the use of FD&C Red No. 3 in food and in ingested drugs puts people at risk are not supported by the available scientific information.”

Read it for what it is. The Delaney Clause prohibits authorisation of a colour additive found to induce cancer in animals, without a weighing of dose or mechanism; the revocation is therefore a legal outcome under a zero-tolerance rule, and it concerns eating the dye, not weighing a gram of it once. Manufacturers have until 15 January 2027 for food and 18 January 2028 for ingested drugs to reformulate, and the notice records that other countries still permit certain uses. Neither half of this may be dropped: a course that omitted the revocation would be hiding a headline, and a course that omitted the mechanism would leave a reader with the wrong conclusion.

Properties

A red powder or granules; tetraiodofluorescein, C₂₀H₈I₄O₅, relative molecular mass 835.89. Four iodine atoms account for 507 of that 836.

Two CAS numbers, and the page names which is which. PubChem’s CID 3259 is titled Erythrosine and carries both 15905-32-5, the free acid, whose structure it depicts and whose formula and mass are given above, and 16423-68-0, the disodium salt, EC 240-046-0. The ECHA notifications attached to the record are those of the disodium salt, and the disodium salt is what is sold as the dye and what the photographic literature means. Its formula is C₂₀H₆I₄Na₂O₅ and its mass 879.9, about 5 per cent heavier. This page carries the free acid’s identity because that is the identity of the record it is cross-checked against; buy on CAS 16423-68-0, label the stock with that number, and remember that a dose in one form is not a dose in the other.

Solubility is the same problem in another guise. The record’s only figure is HMDB’s 0.7 mg/mL — plausible for the sparingly soluble free acid, irreconcilable with the routine practice of making a 2 per cent stock, 20 mg/mL, in half water and half ethanol. The course reads the discrepancy as the difference between the depicted acid and the salt actually sold, says so, and records that it holds no measured solubility for the disodium salt at all.

Handling

Erythrosin is not classified under GHS, and the form of words matters. The aggregated ECHA entry for EC 240-046-0 rests on 39 reports across 3 notifications; 37 of the 39 report that the substance does not meet the criteria. But PubChem’s own note records that only 5.1 per cent of companies provided GHS information, and the same entry records 2 notifications, from 2 of the 39 reports, that did lodge hazard statement codes — codes this course has not read. The honest description is unclassified on a thin evidence base. It is not “harmless”, and this course does not use that word of it.

Nor is there a workplace exposure limit: EH40 lists nothing under erythrosine, erythrosin or tetraiodofluorescein, and the NIOSH Pocket Guide nothing either. EH40’s paragraph 6 states that absence from the list does not indicate a substance is safe, and under COSHH exposure must then be reduced as far as is reasonably practicable with no number to work to. Dust discipline therefore applies regardless of the absent classification, which is the whole of the reasoning behind the PPE here.

Why Level B, and on what conditions. The exposure that exists is a single weighing of about a gram of a fine red powder to make 50 mL of stock; thereafter the substance is dispensed in drops and reaches the emulsion in milligrams. Level B of the course rubric names fine powders that must not be inhaled, and an unquantified dust with no limit is exactly the case its controls were written for: goggles, gloves, a mask as a precaution, work over a tray in still air, and a labelled stock. The course permits erythrosin at Level B in the Part V orthochromatic project only, bought in the smallest available quantity and made up in one operation. Panchromatic sensitisation — pinacyanol chloride and its relatives — remains Level D study only, because the corpus holds no hazard record for any of those dyes and they demand total darkness.

The operation that would take it out of Level B is weighing the powder more than once. A course that dosed the dye by weighing each time would be repeating the only hazardous step in the sequence, and that is precisely what the single-stock condition exists to prevent.

!The real cost is a safelight, not a hazard

The dye’s expensive consequence is optical. An orthochromatic material is green-sensitive, so it cannot be handled under the safelight the course’s earlier, blue-sensitive projects use. ILFORD’s current ORTHO Plus sheet states the rule for a manufactured ortho film — “the blue and green sensitivity enables the film to be handled in deep red safelight” — and Kodak’s darkroom-illumination handbook gives the filters: 1A, light red, for slow orthochromatic materials and high-resolution plates; filter 2, dark red, for fast orthochromatic materials, at 15 or 25 watts. Deep red, not darkness. That is a filter change and a fog test, both of which this course already owns, and it is the best thing the project has to teach: change the dye, and the light you may work by changes with it.

Waste

Milligrams of dye, in wash water that already carries silver, nitrate and halide. It gets no separate disposal instruction; it joins the silver-bearing stream, which this course bottles, labels and takes to a licensed household hazardous waste route, as ILFORD directs domestic users in the United Kingdom. The item needing a decision of its own is the leftover 2 per cent stock: bottle it and take it with the rest, both because it is half ethanol by volume and because “unclassified on a thin evidence base” is not a licence to pour something down a drain. Check your local regulations; they decide.