Sodium Sulfite

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

Ken:

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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.

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

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:-)"

"

I have to disagree with a couple of the assertions in Ahmad's intersting post above. First,

Sodium Sulfite doesn't inhibit emulsion swelling, it promotes emulsion swelling. Sodium

Sulfate inhibits swelling, which is why it an ingredient in tropical developers, but it's not

the same chemical. Swelling has an effect on grain clumping, and sulfite inhibits edge

effects in MQ developers because it facilitates the reactivation of metol by hydroquinone,

but I don't think emulsion swelling and edge effects are significantly linked. If anything, I

would expect emulsion swelling to somewhat favor edge effects, since development

byproducts from areas of high development activity and unused developer from areas of

low activity would migrate more easily within the emulsion layer. At any rate, the high pH

of Rodinal promotes emulsion swelling regardless of its sulfite content.

Also: "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."

This is incorrect; ascorbate is an antioxidant and ascorbate developers promote edge

effects because the byproduct development is acidic and inhibits development in adjacent

areas. The reputation of sulfite for reducing edge effects is specific to MQ developers like

D76, as far as I know, and does have something to do with sulfite's antioxidant effect in

that context. However, I believe that physical development does generally result in lower

resolution on the film, particularly in highlights areas, so developers with enough sulfite to

promote physical development aren't generally used when you want maximum definition.

It is sulfite's solvent action which provides the silver in solution for physical development

to occur, so in that sense I think it is the solvent power of sulfite which softens details in

images. However, it's not like the detail is being dissolved by the sulfite, which is what

many people seem to think.

To address Ken's original question: There's generally no advantage to adding additional

sulfite to a developer and if you do the effect can be hard to predict. Some developers will

become more active, some less, some will produce finer grain, some coarser grain. As for

shelf life, developers like straight D76 which are designed for reuse already contain tons

of sulfite, whereas it's pointless to try to save a one-shot developer for reuse by adding

sulfite. I wouldn't bother with it; it's not even worth thinking about unless you're

interested in formulating your own developers.