Dumb question about “pinch” of Sulfite mixing convention

[Michael R 1974]

Metol doesn’t dissolve in Sodium Sulfite solutions, which is why the Sulfite goes in after the Metol when scratch-mixing. But a common convention is to dissolve a small amount (“pinch”) of the Sulfite before the Metol, which is supposed to scavenge dissolved Oxygen and therefore result in better preservation of the Metol while it is being dissolved.

Haist says nothing about the pinch of Sulfite, using two examples, D-76 and D-72.

So, why does the pinch of Sulfite (in say 750ml water) not cause any precipitation of Metol? The two possible explanations I can come up with are:

1)The concentration of the Sulfite solution must be significantly higher to cause precipitation of Metol

2)The entire pinch of Sodium Sulfite combines with dissolved Oxygen in the water to form Sodium Sulfate. My understanding (again from Haist) is this reaction is one of the ways in which Sodium Sulfite acts as a preservative in developers. And I’m assuming that Sulfate in solution doesn’t cause precipitation of Metol. That’s just a guess though.

Answers?

[Alan Johnson]

From "Photographic Theory and Practice-Part 4:Monochrome Processing" LP Clerc,DA Spencer,RE Jacobson 1971 p496:
"Metol......is soluble in water 4.5 percent at 15 C........... (from a concentrated solution) the free base is precipitated when sodium sulphite is added."

So,IMO,it could be that more concentrated sulfite solution forms the free base on the surface of any added metol particles that stops them from dissolving.

IDK if that is the correct explanation, never come across this before.

[Bruce Osgood]

Snip

So,IMO,it could be that more concentrated sulfite solution forms the free base on the surface of any added metol particles that stops them from dissolving.

End Snip

This has been my understand as well.

[Rudeofus]

But we can assume that, once Metol is in solution, its free base ions float around freely, yet they don't precipitate when more Sulfite is added.

Could it be that Metol dissolves better when its free base part can fully ionize? In other words it may not so much be an issue with Sulfite, but more with Sulfite's high pH. It may equally hard to dissolve Metol in Carbonate. The reason that the issue is usually associated with Sulfite may come from the fact that unlike other alkalis Sulfite is needed to preserve the Metol.

When I get to it, I'll try to dissolve some Metol in 30 g/l Sodium Metabisulfite and in 30 g/l Sodium Carbonate. My guess is it will dissolve in the Metabisulfite but not in the Carbonate.

[Alan Johnson]

More of the quote from LP Clerc:
"On adding carbonate to a concentrated solution of metol,the free base is precipitated,as it is also when sodium sulphite is added.The free base melts at 85C. It is soluble in alcohol,ether and acetone and in strongly alkaline aqueous solutions."
As you mention, that looks like ionization makes the free base soluble.

[Gerald C Koch]

Metol is soluble in concentrated sulfite solutions. People do it every day when they mix up Kodak single package D-76. Remember in this case everything goes in at the same time. Metol just dissolves more slowly and requires more stirring. Kodak also recommends warm (125F) water. Without the small addition of sulfite the amount of metol that is oxidized is insignificant. When making up D-23, I have without thinking dissolved the sulfite first and can say that with heat and a magnetic stirrer to do the work that Metol will dissolve eventually. So the reason is mainly speed of dissolution.

Developers which also contain hydroquinone in addition to Metol can precipitate the addition product of these two developing agents. I routinely mix up a concentrated developer which forms a slurry with the addition of the hydroquinone. The adduct dissulves when the alkali is then added.

[Michael R 1974]

Gerald - interesting. Why does Haist say it doesn't work? In fact, why does everyone say it doesn't work? Could this be yet another myth busted?

I assumed in the case of Kodak's single packages that all their secret additives, coatings etc somehow either made Metol soluble at the same time as Sulfite, or prevented the Sulfite from going into solution before the Metol.

[DREW WILEY]

About all I know is that once I forgot to add the pinch of sulfite first and the metol wouldn't dissolve at
all! Packaged D76 contains a sequestering agent.

[Gerald C Koch]

Hi Michael,

Perhaps it is more a matter of patience than anything else.

When in college one of the professors had a hobby to trace errors in text books back to the original source. In one case the original error had been "plagerized" from text book to text book for over 50 years! For over a hundred years the conventional wisdom was that the inert gases did not form compounds with other elements. Yet not too long ago a graduate student mixed xenon and fluorine gases together and produced a solid compound xenon tetrafluoride. This led to several other xenon compounds like xenic acid.

[Gerald C Koch]

About all I know is that once I forgot to add the pinch of sulfite first and the metol wouldn't dissolve at
all! Packaged D76 contains a sequestering agent.

Metol will dissolve in plain water the solubility is 50g/l according to the MSDS.

Some one noticed that oxygen in the water caused a discoloration. True, but the amount of oxidation is insignificant in the case of developers. But since then people have been adding a pinch of sulfite for none other than cosmetic purposes.

The sequestering agent is to prevent calcium sulfite from precipitating when tap water is used.