Kodak Fixer (F-5)

Kodak Fixer F-5 is the canonical sodium-thiosulfate hardening fixer formula in B&W photography — published by Kodak in the 1930s and continuously referenced as the standard against which every other B&W fixer is compared. The formula is one of the most-cited recipes in the entire darkroom canon: 240 g sodium thiosulfate (the thiosulfate "hypo"), 15 g sodium sulfite, 13 ml glacial acetic acid, 7.5 g boric acid, and 15 g potassium alum per liter of working solution.^[1]

F-5 is a hardening fixer — the potassium alum component cross-links gelatin in the emulsion, producing a tougher, more abrasion-resistant surface on the developed negative or print. This was essential for the rough handling and air-drying conditions of mid-20th-century darkrooms but matters less in modern darkroom practice with careful handling and controlled drying. The hardening property is the main reason photographers choose F-5 today: when prints will be intensively handled (matting, framing, archival processing involving multiple bath transfers), F-5's hardening surface is more durable than non-hardening alternatives.

The trade-off: hardening fixers wash out more slowly than non-hardening fixers — the cross-linked gelatin holds residual fixer chemistry more tenaciously. F-5 typically requires 30+ minutes of running-water wash for fiber prints to achieve archival residual-thiosulfate levels, vs 15-20 minutes for non-hardening alternatives. Use of a wash-aid (Hypo Clearing Agent, Perma Wash) is more important with F-5 than with non-hardening fixers.

F-5 is mixed from individual chemicals (Kodak no longer sells pre-mixed F-5 powder — it was discontinued from packaged form in the early 2000s) and used full-strength as a working solution. Capacity ~25 8x10 prints per liter or ~5-6 rolls of 35mm/120 film per liter before exhaustion.

When to choose F-5 over Rapid Fixer or TF-4

F-5 is most often weighed against Rapid Fixer (ammonium thiosulfate, faster) and TF-4 (alkaline non-hardening, archival-optimized):

• vs Rapid Fixer: Rapid fixer is 2-3× faster (1-2 min for film vs F-5's 5-10 min). Choose Rapid Fixer for routine workflow speed; F-5 when hardening matters or when traditional film/paper combinations are sensitive to ammonium ion (rare but documented for some chlorobromide papers).
• vs TF-4 Alkaline Fixer: TF-4 is non-hardening and alkaline — washes out in ~5-10 min vs F-5's 30+ min. Choose TF-4 for archival workflows where minimum-wash-time matters; F-5 when hardening is the priority.
• vs F-24 Non-Hardening: F-24 is the same Kodak family without the alum hardener. Choose F-24 when you want F-5's chemistry without the hardening; F-5 when you specifically need the hardening surface.

The thiosulfate fixing chemistry

F-5's chemistry combines five distinct chemical roles:^[2]

• Sodium thiosulfate (240 g/L) — the active fixer; reacts with undeveloped silver halide to form soluble silver thiosulfate complexes that wash out of the emulsion
• Sodium sulfite (15 g/L) — preservative; protects the thiosulfate from acid decomposition (sodium thiosulfate releases sulfur gas if exposed to acid without sulfite protection)
• Glacial acetic acid (13 ml/L) — pH adjuster; brings the bath to ~pH 5, which activates the alum hardener and provides additional protection against developer carry-over
• Boric acid (7.5 g/L) — buffer; keeps the pH stable as developer carry-over (which is alkaline) accumulates in the bath over many prints
• Potassium alum (15 g/L) — the hardener; provides aluminum ions that cross-link gelatin chains in the emulsion

The complete chemistry is well-balanced: F-5 maintains its working pH and capacity even after substantial use because the boric acid + sulfite buffering system absorbs the alkaline carry-over from developer trays. This is why F-5 capacity is high relative to simpler "plain hypo" fixers.

Working-solution and capacity workflow

Standard reference times in F-5 working solution at 20°C:

Material	Time
35mm/120 film (most modern emulsions)	5-10 minutes (use 2× clearing time as the rule of thumb)
Fiber-base paper	5-10 minutes total — 2 baths of 5 min each is the archival standard
RC paper	2-3 minutes total — 2 baths of 1-1.5 min each

Capacity per liter (when to discard or use a second bath):

• ~25 8x10 fiber prints (then activity drops too low for safe complete fixing)
• ~5-6 rolls of 35mm or 120 film
• For archival paper work, the two-bath fixing approach (fresh fixer in second bath, cycled to first bath when first is exhausted) gives much higher safety margin

Detection of exhaustion: F-5 develops a slight yellow-brown tint as it ages (silver bromide load); test with a clip of fresh film — if clearing takes longer than 90 seconds vs the original 60 seconds, the bath is approaching exhaustion.

Practical notes

• Mix from individual chemicals: Kodak doesn't sell pre-mixed F-5 packets in 2026. Source from Photographer's Formulary, Bostick & Sullivan, or Photo's Spot. Hand-mixing is straightforward but requires the full chemical inventory.
• Mix in published order: dissolve thiosulfate in 750 ml water at 50°C first, then add sulfite, then acetic acid (slowly — generates heat and may release sulfur dioxide briefly), then boric acid, finally potassium alum. Top up to 1 liter.
• Working solution keeps 6 months in tightly-sealed full bottles; discard sooner if cloudy or sulfurous-smelling (sulfur precipitation indicates bath failure — discard).
• Always use a stop bath before F-5 — direct transfer from alkaline developer to F-5 produces yellow staining on prints. Citric acid stop bath is sufficient; acetic-acid stop bath also works.
• PPE: Standard fixer-handling — nitrile gloves and eye protection. Acetic acid is mildly irritating during mixing; potassium alum is a known sensitizer for cumulative exposure.
• Wash thoroughly — F-5's hardening property means longer wash times. Use a wash aid (HCA or Perma Wash) for fiber prints to reduce wash time and ensure archival residual-thiosulfate levels.

Related recipes

• [[recipe-rapid-fixer|Rapid Fixer]] — ammonium-thiosulfate alternative; 2-3× faster fixing time
• [[recipe-f-24-non-hardening|F-24 Non-Hardening Fixer]] — same Kodak family without alum hardener
• [[recipe-tf-4-alkaline-fixer|TF-4 Alkaline Fixer]] — Photographer's Formulary archival alternative; non-hardening, alkaline, fast wash
• [[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 that follows F-5 in the workflow
• [[recipe-krst|Kodak Rapid Selenium Toner]] — archival toner that follows the fixer + wash chain