Sodium thiosulfate is an inorganic compound with the chemical formula Na2S2O3, most commonly encountered in photographic use as its pentahydrate form Na2S2O3·5H2O - large, transparent, colourless crystalline lumps or granules that dissolve readily in water to produce a clear, neutral to mildly alkaline solution with a faint sulphurous odour. It is the most fundamental and historically important fixing agent in photographic chemistry, universally known in photographic circles by its traditional name of hypo - a contraction of hyposulfite, the incorrect but historically entrenched earlier name for the thiosulfate compound - and has been the standard fixing agent for silver halide photographic materials since its introduction to photography by Sir John Herschel in 1839, just months after the public announcement of the photographic processes of Daguerre and Talbot.
The fixing action of sodium thiosulfate is based on its ability to form stable, water soluble coordination complexes with silver ions, enabling it to dissolve and remove the residual unexposed and undeveloped silver halide crystals remaining in the photographic emulsion after development. During the fixing process, the thiosulfate ions in solution react with the silver halide - most commonly silver bromide in modern films and papers, though silver chloride and silver iodide containing emulsions are also fixed by the same mechanism - to form a series of soluble silver thiosulfate complexes of progressively higher thiosulfate to silver ratios as the fixation proceeds. These soluble complexes are freely diffusible through the swollen gelatin emulsion and pass out into the surrounding fixing solution, from which they are removed by subsequent washing with water to leave a clear, stable, light insensitive image of pure metallic silver embedded in the gelatin emulsion.
The chemistry of sodium thiosulfate fixation is more complex than a simple dissolution reaction, involving the sequential formation of at least three distinct silver thiosulfate complexes of different stoichiometry and solubility as the ratio of thiosulfate to silver increases during the fixing process. The initial complex formed at low thiosulfate concentrations or short fixing times is relatively insoluble and prone to decomposition, potentially causing yellowing and staining of the fixed material during subsequent washing and storage if fixation is incomplete. As fixing proceeds and the thiosulfate concentration relative to the dissolved silver increases, more soluble and stable higher order complexes are formed that wash out of the emulsion more readily and leave fewer residual silver compounds that could cause image deterioration over time. This progression from less to more soluble complexes is the chemical basis for the well established practice of fixing photographic materials for the minimum time necessary to achieve complete clearing of the emulsion - visible as the disappearance of the white or milky opacity caused by undissolved silver halide - followed by a further equivalent period in fresh fixer to ensure the conversion of the initial insoluble complexes to the more stable and washable higher order forms.
Sodium thiosulfate fixer is used in two principal modes in photographic processing. As a plain or alkaline fixer - typically a simple aqueous solution of sodium thiosulfate at a concentration of around 200 to 300 grams per litre, sometimes with the addition of sodium sulfite as a preservative - it provides effective fixation of both films and papers under a wide range of processing conditions and is compatible with all standard developing and stop bath sequences. As an acid hardening fixer - sodium thiosulfate combined with an acidifying agent such as sodium metabisulfite or acetic acid and a hardening agent such as potassium alum - it provides the additional benefits of emulsion hardening and acidification described in the entries for those individual components, making it better suited to high temperature processing environments and situations where maximum physical protection of the emulsion during handling and washing is required.
The working life and capacity of a sodium thiosulfate fixing bath diminishes progressively with use as the thiosulfate is consumed in the fixing reaction and as dissolved silver accumulates in the solution from the silver halide removed from successive films and papers. As the silver concentration in the fixer increases, the equilibrium between silver thiosulfate complex formation and decomposition shifts in a direction that reduces the fixing efficiency and increases the proportion of the less stable, less soluble initial complexes formed during fixation, ultimately compromising the completeness and reliability of fixation and the washability of the fixed silver complexes. Regular testing of fixer capacity using fixing time tests or silver estimation methods, and timely replacement or replenishment of exhausted fixer, is therefore essential for consistent, archivally sound photographic processing.
The thorough washing of fixed photographic materials to remove all residual thiosulfate and silver thiosulfate complexes from the emulsion is one of the most critical factors in determining the archival permanence of processed photographs. Residual thiosulfate remaining in the emulsion after inadequate washing undergoes slow decomposition to produce sulfur and silver sulfide, causing yellow-brown staining and image deterioration over time. Residual silver thiosulfate complexes similarly decompose to deposit colloidal silver and silver sulfide in the emulsion, producing the yellow staining and silvery surface sheen characteristic of inadequately fixed or washed photographic materials. Hypo clearing agents - solutions of sodium sulfite or other compounds that accelerate the diffusion and removal of thiosulfate from the emulsion - are widely used between the fixing and washing stages to reduce the washing time required to achieve adequate hypo elimination and to ensure more complete removal of residual fixing chemicals from the emulsion.
Sodium thiosulfate is of low to moderate toxicity and is generally considered relatively safe to handle compared to many other photographic processing chemicals, being used in medical applications as an antidote for cyanide poisoning and as a treatment for certain dermatological conditions. Standard precautions including the avoidance of skin and eye contact, good ventilation to prevent the accumulation of sulphurous vapours from the solution, and appropriate disposal of spent fixer - which contains significant concentrations of dissolved silver that should be recovered rather than disposed of through drainage - are adequate for its safe use in photographic processing applications.