What is the purpose of using Sodium Sulfite in film developing? Can it be used in a Rotary processor? And how much should be added to a liter of developer before dilution? And does this affect the life of the developer? Thanks Ken

-- Kenneth Hedden (Chaselucy@aol.com), April 27, 2001

Answers

Ken:

Most film developers already have the appropriate amount of SS in the formula making it unnecessary to add any at time of use. Developers such as Microdol X or D76 that already contain large amounts of SS have nothing to gain by adding more to the working solution. However, developers like FG7 that contain only small amounts of SS can have it added to the working solution to shorten dev times and increase the solvent action. Some photogs also add small amounts of SS to Rodinal to soften the graininess a bit. Usually though, there isn't much to be gained by adding SS to an already formulated film developer.

-- Ken Burns (kenburns@twave.net), April 27, 2001.

I have found Edwal FG7 to benefit greatly from the addition of SS, when processing 35mm films. A gain in film speed and increased edge definition is very noticeable with Kodak's HIE infrared, HP5+ and TXP. The highlights print much easier and with the infrared there is less of the halo effect where highlights are immediately adjacent to shadow or much darker areas (edge definition). I have never used SS with any other developer... t

-- tom meyer (twm@mindspring.com), April 28, 2001.

someone sent me this thru the usenet: Xosni wrote me that he is looking for improved sharpness. I am posting an answer here as well as to him via e-mail because I think it may be of general interest. The effect of sulfite in a developer is subject to some misundersanding. In fact, sulfite has a rather complex releationship with the developing agents and the development process, some of the confusion comes from this complexity. Sulfite does two major things: one, it acts to protect the developing agent from oxidation; two, it acts as a solvent for the undeveloped silver halide. The absorption of oxygen by the sulfite has two effects. One is that it protects the developing agents from oxidation by the air. Developing agents are reducers, their purpose is to absorb oxygen from the exposed silver halide grains and convert them into metallic silver. Oxygen from the air, and oxygen dissolved in the solution competes with the oxygen in the developing silver grains for the reducer and will quickly ruin it if its not protected by sulfite or some other oxygen absorber. Beside protecting the reducing agent from atmospheric oxygen the sulfite acts to regenerate the reaction products of development. It is here that the amount of sulfite can affect the apparent sharpness of the image. When a develping agent (reducing agent) develops exposed silver it produces both the silver and some other substances as a result of the chemical reaction. These substances can either retard or accelerate development, depending on the developing agent. Metol reaction products tend to restrain development, those from Hydroquinone tend to accelerate it. This effect takes place in a very small area around the developing silver halide crystals. The reaction products can move within the emulsion only by diffusion. Those which move toward the outside can be removed by agitation but those which move sideways in the emulsion have an effect on the development nearby. At the border of a high density and low density area the effect of these reaction products is to modify the rate of devlopment in a very narrow area near the border. This results in a line on each side of the division in densities. The direction of the density of the line, and its degree, depend on the type of reducing agent or combination of agents, on the amount of emulsion swelling, and on the amount of sulfite. Sulfite tends to keep these effect from happening. It turns out that the human eye interprets high contrast at a deviding line between dark and bright as sharpness. In film this is called acutance. It is different from resolution. It has been demonstrated that a high actance image looks sharper than one with lower actance (or edge contrast) but higher resolution. Its an optical illusion. A tight, sharp grain pattern also gives the illusion of sharpness. Low sulfite developers tend to produce stronger edge effects and thus, greater acutance. Since the effect is fixed in scale it is less visible as the silze of the format gets larger. Sulfite also acts in a couple of other ways. It is a salt and tends to prevent emulsion swelling. This can have a effect on grain since the harder gelatin makes it more difficult for nearby grains of silver to migrate toward each other causing clumping. It is groups of grains which we see as film grain. The individual grains are submicroscopic and are investigated using an electron or proton microscope. The solvent action of sulfite also has an effect on grain, but not by dissolving away a part of the developed grain (although very large amounts of sulfite can do this a little). The sulfite etches the surface of the halide grains. Moderate amounts of sulfite etch enough halide to expose sensitivity specks benieth the surface, making them developable, and increasing effective film speed. Too much sulfite, or the use of more powerful solvents, like thiocyanate, can etch right past some of these specks, destroying part of the latent image and lowering the speed. The sulfite also affects the morphology, or shape, of the developing silver grains. Very low sulfite developers, or experimental ones without sulfite, develop rounded silver crystals, about the same shape as the undeveloped crystals. This is also true of solution physical development. Higher sulfite developers result in "filimentary" silver. Electron micrographs of these grains look like tufts of steel wool. The filimentary silver tends to have greater covering power than the sharp crystals so the effect is fewer holes for light to get through when many grains are in an area of the film, as in dense areas. The effect of the very small silver grains is somewhat statistical in nature, remember that _visible_ grain is the result of many submicroscopic grains. The solvent effect of the sulfite has the effect of bluring out the effect of graininess. This has no effect on sharpness because it is on too small a scale. Sulfite also encourages physical devleopment. The halide dissolved by the sulfite can be deposited on growing silver crystals by the devloper, increasing their covering power. Filimentary crystals are typical of chemical development, sharp edged crystals of physical development. Nearly all practical developers result in a combination of the two types. Excessive physical development can have the effect of bluring edges and can reduce actual resolution. It takes a lot of physical devlopment to get this effect, even strongly solvent developers like Kodak Microdol-X full strength do not affect resolution when compared to a lower sulfite deeveloper, like Rodinal. Because of the effect on the latent image high sulfite developers which are not very active will loose some film speed when used at full strength but gain it back when diluted. They will also get grainier because the anti-swelling effect of the sulfite is lost. Low sulfite developers may be the opposite, they will become somewhat less grainy and may loose some film speed as they are diluted. Rodinal is an example. Rodinal is very high pH since it uses potassium hydroxide as the accelerator. Hydroxide has little buffering power so the pH becomes lower as the dilution incrases reducing emulsion swelling and grain. Probably Rodinal with sulfite added acts more like a high sulfite developer. In any case, it is not the solvent power of sulfite which results in some high sulfite developers making somewhat unsharp images but rather the reduction of edge effects. Since these are due to the antioxidant property of the sulfite any other anti-oxidant will have similar effects. Boy, this has turned out to be a long answer to a short question:-)" "

-- xosni (xosni@gega.net), April 29, 2001.

Well put Xosni!

-- Scott Walton (scotlynn@shore.net), April 30, 2001.

I'm afraid that long message has Richard Knoppow's name all over it!

-- Wayne (wsteffen@skypoint.com), May 05, 2001.