developer preservatives - what dictates quantity?

[el wacho]

hello everyone,

what dictates the amount of preservative in a film or paper developer? it is a broad question but i'm hoping to understand the role of preservatives such as sodium sulfite and sodium metabisulfite better.

for example, in d-72, is the role of the sodium sulfite purely to act as an oxygen scavenger or is it contributing to the alkalinity the way it does in d-23?

how does sodium metabisulfite act as a preservative?


thanks in advance.

 

[Anon Ymous]

Sodium sulfite levels will also affect the granularity of the produced image. Higher levels of sulfite will give smoother grain. That's why D76/Xtol diluted will give somewhat bigger grain, but also more sharpness. In the case of D72, alkalinity is also affected by the use of sodium carbonate.

EDIT: Also keep in mind that metabisulfite solutions are acidic, so what preservative you'll use depends on the application.

 

[Ian Grant]

Metabisulphite is used as an antioxidant particularly in two part powder developers, it gives of Sulphur Dioxide which preserves the developing agents.

In solution it's often used to lower the pH, kike in D25 compared to D23, or in simple acid fixers.

Sulphites are silver solvents which is why they give fine grain.

Ian

 

[kozesluk]

may i please ask a question here?

in modern concentrated developers the potassium sulfite is used as a preservative. does it act as a silver solvent too?
potassium sulfite (which i don't have) can be substituted by sodium sulfite, considering sodium s. oxidizes more rapidly by air oxygen and final solution will not last as long as using potassium s. right?
substitution ratio should be derived from their relative molar weights (1g potassium s. for aprox. 0,8g sodium s.)?

 

[dancqu]

What Dictates Developer Preservative Quantity?

Formulas dictate. The needed longevity dictates.
Where the preservative answers the need for an
alkaline solution certain minimums must be met.
In the case of sodium sulfite silver solvency may
dictate widely differing amounts.

At working strength my one-shot developers
use very little preservative. Dan

 

[Ian Grant]

Concentrates use Potassium salts because of their higher solubility.

So Ilford, Agfa, Kodak liquid developers often use Potassium Sulphite, and Potassium Carbonate, although the carbonate is often partially replaced by a much smaller amount of Potassium Hydroxide.

In more recent years Ilford & Kodak have been using compounds like Diethanolamine Bisulphite in instead as developers with extremely long shelf lives can be manufactured. (Ilfotec HC, HC110, T-max Dev etc).

Ia

 

[Photo Engineer]

This question is so complex that there is no simple answer. It would literally take a chapter in a textbook to cover this subject, perhaps more. Sorry.

PE

 

[Jordan]

In developers, sulfite scavenges oxygen, provides alkalinity, promotes physical development, promotes super-additivity with hydroquinone (IIRC), swells the emulsion a bit (IIRC), and probably other functions that I'm not aware of. All of these processes happen at different rates and are pH-dependent as well. It's a complicated problem.

 

[Photo Engineer]

Well, Jordan, you are spot on. The problem becomes even more complicated when there are other preservatives besides Sulfite that can be used. This is a huge field.

PE

 

[Alan Johnson]

For a relatively simple case, D-76H:
Metol...............2.5g
Sodium Sulfite..100g
Borax...............2g
Water..............1L
I believe the concentration of the sodium sulfite is not decided by its preservative action,nor by the pH which is adjusted by borax addition,nor by superadditivity as only 5g/L sulfite is required to remove metol oxidation products,but by the fact that 100g/L sulfite is needed to give the required solvent action.Thus it is the required solvent action that decides the amount of preservative in this case,if I am right.

 

[Photo Engineer]

Alan;

Sulfite reacts more rapidly with HQ oxidation products than with Metol oxidation products. Otherwise, you are correct.

PE

 

[Alan Johnson]

Geoffrey Crawley's FX-55 developer working solution contains only 2.5g/L sodium sulfite and 1.2g/L sodium metabisulfite.I wrote to him asking what the sulfite does.He replied (Amateur Photographer Dec 13 2008) :
"The sodium sulfite plays the same role as in any developer,acting as a preservative and giving partial regeneration of the agents during processing.Its slight solvent action on the emulsion's silver halides helps start development by giving access to the internal latent image centres.In excess-more than 60g per working litre-it begins to erode the surface latent image reducing film speed,though solvent action refines image grain."
So that probably indicates what happens in other acutance developers that have a low sulfite content compared to D-76H mentioned earlier.

 

[Ray Rogers]

Geoffrey Crawley's FX-55 developer working solution contains only 2.5g/L sodium sulfite and 1.2g/L sodium metabisulfite.I wrote to him asking what the sulfite does.

No developing agent ?

I once found a gold toner that contained no gold.
I wonder if they are part of a new process?

 

[Alan Johnson]

There's a debate about phenidone ascorbate developers.
Put the sulfite in,leave the sulfite out
Do the hokey cokey and turn around
That's what it's all about.