Hydrogen peroxide

Almost every chemical in this encyclopaedia arrives from a photographic supplier. This one arrives from a chemist’s shop, in a brown bottle, at a strength printed on the label in two different systems — and the distance between that bottle and the substance the safety literature describes is the whole of what a reader needs to understand about it.

In photography

It finishes a cyanotype. A washed cyanotype is not yet the colour it will be: the shadow tones have been partly reduced to colourless Prussian white during exposure, and they regain density by aerial oxidation over hours or a day. A bath of dilute peroxide does that at once. Ware’s instruction is precise — 0.3 per cent w/v, for no more than half a minute, before the final wash — and so is the way he tells you to make it: dilute 50 cc of the commonly available 6 per cent solution, the so-called “20 volume”, to one litre of water.

The suppliers’ sheets agree on the strength while reaching it differently. Bostick & Sullivan mix 100 mL of 3 per cent into 900 mL of tap water and agitate for 60 to 90 seconds, noting that small gas bubbles form on the print and that it is washed for a further 8 to 10 minutes afterwards. The Photographers’ Formulary’s kit uses 50 mL of 3 per cent in 500 mL. All three land on the same 0.3 per cent.

!It changes when you see the print, not what the print is

Ware says twice, in two different publications, that the peroxide bath makes no ultimate difference to the print densities in the final outcome. Bostick & Sullivan call the step “not necessary” and say it lets the artist see the final image sooner. The Formulary offers it as a way of restoring a faded print.

So the honest description of this reagent is that it buys impatience. It is genuinely useful for one thing that is not impatience: if a print is going onto a densitometer, waiting twenty-four hours for air to finish the job is not an option, and Ware suggests adding the peroxide to the wash bath for exactly that case.

Ware’s chemistry of the step is worth having, because it explains a mistake. The oxidation of Prussian white back to Prussian blue consumes protons and generates hydroxide ions, and hydroxide hydrolyses Prussian blue. Left to build up, the bath that restores the colour starts destroying it, which is why Ware says such re-oxidations are best carried out under acid conditions and why half a minute is a limit rather than a suggestion.

It was a hypo eliminator, once. Abney’s 1885 instruction for a plate about to be intensified is that after thorough washing the film may be treated with peroxide of hydrogen in water — a drachm of “what is called a 20-volume solution” to five ounces of water — soaked for half an hour and washed again. The idea is straightforward: oxidise the residual thiosulfate into something that washes out more readily. Wall’s 1912 dictionary reviews the whole family of eliminators and gives the verdict the course still holds: there is no doubt that plain water used freely is the best hypo eliminator, and a dozen changes of five minutes each will remove all traces. The modern washing aid is a different chemistry again, and the peroxide route is history.

☠What this bath replaced

Ware records that a bath of 5 to 10 per cent potassium or ammonium dichromate was commonly used for this re-oxidation in the past, and that its high toxicity now discourages the practice. The course’s chromium policy makes that a rule rather than a discouragement: chromium(VI) is not used at any level. Ware’s own instruction is blunter than a policy — under no circumstances should you use a dichromate bath for this reoxidation. Peroxide does the same job from a chemist’s shop.

Properties

H₂O₂, relative molecular mass 34.015, CAS 7722-84-1, miscible with water in all proportions. Two properties do all the work on this page.

The first is that it is unstable by design: it decomposes to water and oxygen, faster when warm, in light, in contact with metals, or in the presence of many organic solvents. That is why the bottle is brown, why the Formulary says not to cap it tightly, and why a solution loses potency on the shelf. The gas bubbles Bostick & Sullivan tell you to expect on a print are the same decomposition happening where you can see it.

The second is that its strength is quoted in two systems and they are not interchangeable at a glance. A “20 volume” solution is the 6 per cent one, on Ware’s own equivalence; the older name counts oxygen released rather than mass dissolved, which is why the numbers look so different. A formula written in volumes and a bottle labelled in per cent is one of the easier ways to be out by a factor of ten, and the course writes per cent w/v throughout.

Handling

Everything about the classification of this substance depends on what is in the bottle, and the record makes that explicit: the harmonised European entry is filed under the name “hydrogen peroxide solution …%”, with the concentration left as a blank to be filled in. Its statements — may cause fire or explosion, severe skin burns and eye damage, harmful if swallowed or inhaled — describe concentrated material. So do the aggregated notifications, in which 99.6 per cent of nearly two thousand company reports give H314.

The course therefore states what it can and refuses what it cannot. It cannot tell you the classification of the 3 per cent bottle in your bathroom cabinet from this record. What it can tell you is that HSE’s EH40 sets a workplace exposure limit of 1 ppm, 1.4 mg/m³, over eight hours and 2 ppm over fifteen minutes, and that this limit applies to the vapour regardless of the strength it came from.

⚠Why Level A, and the one operation that is not

Against the rubric, the operation this course actually performs — a 0.3 per cent w/v bath, at room temperature, in an open tray, for thirty seconds — is Level A. There is no vapour worth extracting, no exotherm, no corrosivity at that dilution, and the controls are the gloves and eye protection already on the bench.

The one operation that takes it out of Level A is opening a strong bottle. The same substance is sold far more concentrated than the 3 and 6 per cent Ware and the kit sheets use, and the classification climbs with the concentration: CAMEO keeps separate datasheets for solutions above 20 per cent and above 60 per cent, records that contact with most common metals and their compounds may cause violent decomposition, and warns that containers may rupture violently under prolonged heat or fire. Buy the weakest solution that will do the job, dilute it the moment it arrives, and never store a concentrate beside anything combustible.

Waste

A used oxidation bath is mostly water with a decomposing solute, and the decomposition products are water and oxygen. What decides its handling is not the peroxide but what the print gave it: iron and hexacyanoferrate from a cyanotype, which belong in that process’s collected waste rather than in a drain. Kodak’s J-300 guidance covers an amateur’s ordinary streams and excludes prohibited materials; ILFORD’s advice to domestic users in the United Kingdom is to bottle used chemistry, label it and take it to a household waste and recycling centre. Check your local regulations; they govern.

History

The blueprint trade wanted this reaction and did not want this bottle. Ware records that the preferred oxidant, marketed under a proprietary name as a blueprint “developer”, was urea peroxide — a solid, which travels better than a solution and has none of the toxic disadvantages of dichromate. He also records the reservation that has kept solid organic peroxides out of this course: they present an explosion hazard in storage. So the trade’s answer, the Victorian answer and the modern answer are three different compounds doing one job — dichromate, urea peroxide, and the weakest peroxide bottle on a chemist’s shelf — and the reasons for each substitution were safety rather than performance.