6-Nitrobenzimidazole

In 1932 two chemists at the National Bureau of Standards made this compound themselves rather than test the photographic product it had become, because the patents were live. Burt Carroll and Donald Hubbard synthesised nitrobenzimidazole, recrystallised it from hot water and from alcohol, and measured what it did to a silver bromide grain.

In photography

Its one job. HSDB and Haz-Map both give the same use in the same words: an antifogging agent in photographic developers. Kodak’s 1928 primer, written a few years earlier, still knew only the inorganic restrainers, “bromides and iodides of sodium or potassium”, added “to compensate for any chemical fog produced by the developer, or inherent in the emulsion”. 6-Nitrobenzimidazole was among the first organic compounds to take that job over, and Carroll and Hubbard record it being studied as a direct “substitute for soluble bromide in development”.

⚗The chemistry

Their measurements are the clearest account of an organic antifoggant the course holds. They prepared the silver compound by precipitating it from dilute aqueous solutions of nitrobenzimidazole and silver nitrate, washed it thoroughly, and used a silver electrode to measure the silver ion concentration over a saturated solution of it at 30 °C. At pH 7.1 the compound proved more insoluble than silver bromide. At pH 3.5 the silver ion concentration over the same saturated solution rose to 6.4 × 10⁻⁶ N, because “the silver is readily displaced by hydrogen”: acid takes the antifoggant off the silver and hands the grain back.

C₇H₅N₃O₂ + Ag⁺ → AgC₇H₄N₃O₂ + H⁺

Silver nitrobenzimidazolate, and the proton whose return reverses it

The strength of the effect follows. In emulsions near neutrality the additive lowers the silver ion concentration a little, “but experiments on afterripening with the addition of nitrobenzimidazol indicated that, mol for mol, it produces about ten times the effect of soluble bromide in delaying afterripening”. Carroll and Hubbard were careful about what that did not prove: “Something more than the decrease in silver ion concentration must be involved, and another method of attack would be necessary to decide what this might be.” They point at adsorption — “the insolubility of the silver compound of nitrobenzimidazol indicates that this material should be strongly adsorbed by the silver bromide, most probably by the silver ions” — and leave it there.

The cost, and it is a real one. The same paper divided experimental emulsions after digestion and sensitised the halves with erythrosin, pinaflavol and pinacyanol, with and without the additive. The antifoggant “selectively depressed the sensitivity conferred by the dyes, the effect being greater for the exposures through the filter than for the white-light exposures of the unsensitised portions”, in the order erythrosin, pinaflavol, pinacyanol. Their explanation is the adsorption argument again: a compound that binds to silver ions competes with the basic dyes for the same sites, so erythrosin, the acid dye of the three, suffers least. An organic antifoggant in a spectrally sensitised emulsion therefore costs colour sensitivity, and costs it unevenly across the spectrum — an effect of the same order as soluble bromide of equal restraining power, but distributed differently among the dyes.

Where it went. Modern practice uses benzotriazole for this job, and the course has found no current manufacturer formula calling for 6-nitrobenzimidazole. It is still sold and it appears in older published formulas; this page exists so that a reader who meets it in one knows what it is.

Properties

An off-white to pale yellow fluffy powder — Carroll and Hubbard’s recrystallised material formed “characteristic groups of flesh-colored needles” — melting at 207 to 209 °C, which their 209 °C matches. There is no hydrate.

Its solubility is the practical problem. CAMEO gives less than 0.1 mg/mL in water at 61.7 °F and calls it insoluble; HSDB gives more than 250 mg/L without stating a temperature. Either way it is far less soluble in water than potassium bromide, and a working solution has to be made another way: HSDB records it as very soluble in alcohol and soluble in acid, slightly soluble in ether and benzene, insoluble in chloroform. Dissolve it in alcohol, or in dilute acid or alkali, and dose the developer from that stock.

Its pH behaviour is reported here as Carroll and Hubbard measured it rather than as a dissociation constant, because the IUPAC compilation the course uses carries no row for this compound: the silver salt gives up its silver to acid, so the compound is at its most effective in an alkaline developer and progressively less so as the pH falls.

Handling

The aggregated ECHA notifications classify it Warning, with the exclamation-mark and health-hazard pictograms. The aggregation is small: 49 company reports across 7 notifications. H315, H319 and H335 — skin irritation, serious eye irritation, respiratory irritation — each appear in 83.7 per cent of them, and H351, suspected of causing cancer, in 10.2 per cent, that is five reports. CAMEO is blunter than the majority view, stating that the chemical “is harmful by inhalation, ingestion, or skin absorption”, asking for Tyvek-type clothing and a NIOSH-approved half-face respirator with a combination cartridge when weighing and diluting, and citing suspected carcinogen status in its first-aid text. EH40 lists no workplace exposure limit for it, and its paragraph 6 says absence from that list does not indicate that a substance is without hazard.

Why Level C. The course rubric puts at Level C “reagents classified as acutely toxic, carcinogenic, mutagenic, reproductive toxins or sensitisers at the concentrations used, where a fume cupboard or specialist disposal is the recognised control”. A minority suspected-carcinogen classification is still a carcinogenicity classification; the control CAMEO gives for weighing and diluting is respiratory protection, which a home darkroom cannot properly supply or fit-test; and the waste is not a domestic-drain stream. Level B would have been the reading had the irritant statements been all there was. They are not, and the course does not take the optimistic reading of a five-report minority when benzotriazole, with no such classification, does the same job.

⚠Safety

This course gives no home procedure for 6-nitrobenzimidazole. If you meet it in a supervised laboratory: weigh and dilute inside engineered extraction, wear splash goggles and chemical-resistant gloves changed the moment they are contaminated, and cover the forearms, because CAMEO records harm by skin absorption as well as by inhalation and ingestion. Keep it from strong oxidisers and strong bases, which CAMEO names, and from heat, which it records as producing carbon monoxide, carbon dioxide and oxides of nitrogen. Read the supplier’s safety data sheet before the jar is opened: with only seven notifications behind the aggregated classification, the sheet for the product you have is a more reliable guide than the aggregate.

Waste

At working strength the compound leaves in the developer; the concentrated stock and any unused solid are a different stream. Kodak’s J-300 guidance sends an amateur’s developers, stop baths, fixers after silver recovery and wash water to the sewer where a household has one, and lists solvents and other prohibited materials among what must not go. The minority carcinogenicity classification and H412, harmful to aquatic life with long lasting effects, put this substance outside that route in this course. Collect it separately, label the bottle with the substance and the date, and use a licensed route: for a domestic user in the United Kingdom the household waste and recycling centre’s chemical cupboard ILFORD points to, and at any scale a licensed waste contractor. Check your local regulations, which govern.

History

Carroll and Hubbard’s 1932 paper is also the best short account of how organic antifoggants entered photography. “A number of patents have recently been secured on organic materials which are described as preservatives for emulsions,” they write; one of them, nitrobenzimidazol, “has also been studied as a substitute for soluble bromide in development. Since its use is patented, any practical investigation by this laboratory is unjustified, but the mechanism of its action is of considerable scientific interest.” Their citation for that development work is Trivelli and Jenson in the Journal of the Franklin Institute, volume 210, page 287, 1930 — a paper the course has not read.

Two things are worth taking from that paragraph. The first is that a national standards laboratory declined to test a patented product’s practical performance and confined itself to mechanism, a piece of research etiquette that shaped what got published. The second is the date: by 1930 the organic antifoggant was already a commercial article — nineteen years before Kodak Limited’s 1949 handbook offered “K.A.F. ‘Kodak’ Anti-Fog Powder”, “for addition to developing solutions when a tendency to chemical fog occurs”, without saying what was in it.