Rapid Fixer

Rapid Fixer is the modern fast-fixing alternative to traditional sodium-thiosulfate fixers like Kodak F-5. The "rapid" character comes from substituting ammonium thiosulfate for sodium thiosulfate as the fixing agent — ammonium thiosulfate is roughly 2-3× faster at converting undeveloped silver halide to soluble silver thiosulfate complexes, reducing typical fixing time from F-5's 5-10 minutes to 1-2 minutes for most films and 2-3 minutes for fiber-base paper.^[1]

Rapid Fixer is the default fixer for routine 2026 darkroom work — the speed advantage compounds across long printing sessions (a 30-print session that takes 90 minutes in F-5 takes 60 minutes in rapid fixer), and modern emulsions are designed assuming rapid-fixer workflow timing. Most commercial photo-chemistry suppliers (Kodak, Ilford, Photographer's Formulary, Sprint) sell branded rapid fixers; the most-used commercial products are Kodafix (Kodak), Ilford Rapid Fixer (Ilford), and TF-4 (Photographer's Formulary, alkaline variant — see separate recipe).

The trade-off vs F-5: rapid fixer is non-hardening in most commercial formulations (the hardening variant exists but is less common). For modern darkroom workflows with controlled handling and drying, hardening isn't necessary — and the absence of a hardening agent means rapid fixer washes out faster (15-20 min for fiber prints vs F-5's 30+ min) and is less likely to leave residual chemistry that affects print permanence.

Rapid fixer is supplied as a liquid concentrate (typically Part A only, sometimes Part A + Part B for hardening variants) that mixes 1:4 to 1:7 with water for film fixing or 1:9 for paper fixing. Concentrate keeps 1+ year sealed; opened concentrate keeps 6+ months in tightly-sealed full bottles.

When to choose Rapid Fixer over F-5

Rapid Fixer is most often weighed against F-5 (the slower hardening alternative) and TF-4 (alkaline variant):

• vs F-5: Rapid Fixer is 2-3× faster, non-hardening (for most commercial products), washes out faster. Choose Rapid Fixer for routine workflow speed; F-5 only when hardening is specifically needed (rough handling, intensive matting/framing).
• vs TF-4 Alkaline: TF-4 is the alkaline rapid-fixer variant — even faster wash than standard rapid fixer (5-10 min vs 15-20 min for fiber). Choose TF-4 for archival workflows where minimum-wash-time matters; standard Rapid Fixer for routine work where the time difference doesn't justify the slightly higher TF-4 cost.
• vs F-24 Non-Hardening Fixer: F-24 is sodium-thiosulfate based; slower than rapid fixer but cheaper to mix. Choose F-24 when hand-mixing makes economic sense for high-volume work; Rapid Fixer for the convenience of the liquid concentrate.

The ammonium-thiosulfate chemistry

Rapid fixer's speed advantage comes from the ammonium ion's effect on thiosulfate kinetics:

• Ammonium thiosulfate ((NH₄)₂S₂O₃) — the active fixer; reacts faster with silver halide than sodium thiosulfate due to the ammonium ion's contribution to silver-complex stability
• Sodium sulfite — preservative; protects thiosulfate from acid decomposition (releases sulfur if exposed to acid without sulfite buffering)
• Acetic acid (in acidic variants) — pH adjuster; brings working solution to ~pH 5
• Boric acid (sometimes) — buffer for stability
• No hardener in most commercial formulations — modern emulsions handle non-hardening fixing without surface damage

The faster fixing kinetics also produce a faster silver-bromide load in the working bath — rapid fixer reaches exhaustion sooner on a per-volume basis than sodium-thiosulfate fixers. Watch for clearing-time drift as the bath ages.

Working-solution and capacity workflow

Standard reference times in Rapid Fixer working solution at 20°C:

Material	Dilution	Time
35mm/120 film	1:4 (Kodafix), 1:5 (Ilford)	2-4 minutes (use 2× clearing time)
Sheet film 4x5	1:4	3-5 minutes
Fiber-base paper	1:9	2 minutes per bath × 2 baths (4 min total for archival workflow)
RC paper	1:9	30-60 seconds total (single bath sufficient)

Capacity per liter (working solution):

• ~30 8x10 fiber prints (slightly higher than F-5)
• ~6-8 rolls of 35mm or 120 film

Two-bath fixing is recommended for fiber prints regardless of fixer choice — fresh fixer in second bath, cycled to first when first is exhausted. This gives consistent fixing safety margin without depending on bath-monitoring.

Practical notes

• Kodafix vs Ilford Rapid Fixer vs Photographer's Formulary: All three are commercially equivalent for standard non-hardening rapid fixer. Kodafix offers a hardening variant (mix with Part B); Ilford is non-hardening only; Photographer's Formulary sells both variants. Choose by availability and price.
• Always use a stop bath before rapid fixer — rapid fixer is less sensitive to developer carry-over than F-5 but a citric or acetic stop bath still extends fixer life.
• Working solution keeps 6 months sealed; partial bottles oxidize within ~3 months. Discard if cloudy, sulfurous-smelling, or shows precipitate.
• PPE: Standard fixer-handling — nitrile gloves and eye protection. Ammonium thiosulfate has stronger ammonia smell than sodium thiosulfate; ventilation matters more in long printing sessions.
• Wash with HCA or Perma Wash for fiber prints to reduce wash time and ensure archival levels.

Related recipes

• [[recipe-kodak-fixer-f-5|Kodak Fixer (F-5)]] — slower hardening alternative; the comparison standard
• [[recipe-tf-4-alkaline-fixer|TF-4 Alkaline Fixer]] — alkaline rapid-fixer variant; faster wash
• [[recipe-f-24-non-hardening|F-24 Non-Hardening Fixer]] — sodium-thiosulfate non-hardening alternative
• [[recipe-citric-acid-stop-bath|Citric Acid Stop Bath]] — required pre-fix step
• [[recipe-hypo-clearing-agent|Hypo Clearing Agent]] / [[recipe-perma-wash|Perma Wash]] — wash-aid step
• [[recipe-krst|Kodak Rapid Selenium Toner]] — archival toner that follows the fixer + wash chain