Experiments with Metol and ascorbic acid.

[Ryuji]

Originally Posted by gainer

Another point is that the curve from Mees & James shows that the effect of either ascorbic acid or sodium sulfite increases as mole fraction rises to about 0.05, then levels off with further increase. Thus 10 g/l sulfite is well above that point while 1.9 g/l even of full strength ascorbic acid is far below. If you were to bring the ascorbic acid up to 8.8 g/l (as an ascorbate, of course) you might see the activity increase without sulfite. However. I don't think you have quite enough pH without the sulfite. It should be about 9.

I have a feeling that it is the molar concentration of either the ascorbic acid or the sulfite that counts, not the ratio of either to the Metol, as long as the pH is right.

You have focused on reducing properties of the two agents, but you might want to know that ascorbate is a much much stronger reducing agent than sulfite. The kind of reactions they undergo are also different.

One point that's also omitted from your discussion is the emulsion type. For the purpose of this discussion, what you want to know is where the latent image centers are made. The most recent technology allows emulsions that make latent image on the surface of the grain, where developer is directly accessible, especially when exposure is relatively slow (which means anything but very brief flash exposure in modern standard). However, in old technology emulsions, latent image centers can be made in the grain or on the surface. Especially when the grain is exposed with very brief exposure of light (which means 1/60 or faster in some long exposure type emulsions), or high energy radiations, more latent image centers form inside the grain. Also, direct positive emulsions have latent image (intentionally created fog centers, rather) in the interior. These internal latent images are not directly accessible to your developers and therefore the sensitometric curves can differ from those obtained with developers that can access internal latent images.

I have made several different emulsions, some of which form latent image in the interior, and others near the corners of the grains outside, and I do see quite noticeable differences with the kind of factors you are looking at, including the sulfite concentration.

[gainer]

I was intrigued by the fact that sulfite and ascorbate showed almost identical curves in the excerpt that was shown in "The Theory.... " while hydroquinone showed no effect. Most simple analyses of MQ developers omit mention of the importance of sulfite to the "regeneration" of Metol by hydroquinone. The operation of ascorbate is certainly different in the long run from that of sulfite, and the regeneration of metol by hydroquinone appears not to take place without either sulfite or ascorbate. Ascorbate and Metol without sulfite, with the proper pH, can make a developer that is as active as Metol and hydroquinone with sulfite, which would lead one at least to guess that ascorbate is both developing agent and antioxidant.

IIRC, the antioxidant and the reducing properties of ascorbate use different parts of the molecule. The body does remarkable things with it to allow it to pass into the brain through the blood supply. We can use either ascorbic or isoascorbic acid in our developers, but the body needs ascorbic acid for vitamin C, from what I have read.

[monkeykoder]

Sorry to post in such an old thread but I have to say with my limited knowledge I would hesitate quite strongly to heat anything flammable in a microwave. If you have spent a considerable amount of money on the microwave you MIGHT be safer but as far as I know now microwave heats even remotely evenly. Just say you're microwaving a burrito (at least it is a relatively safe bet) if you stick the burrito towards the center of the microwave you'll usually notice (at least in cheaper models) that the endpoints of the burrito are MUCH cooler than the center. Even given that for PG this is a (relatively) safe method (assuming there is no metal in there and that the vapor will never reach the radiation source which if I'm not mistaken does spark) if you're microwave is failing usually this leads to even greater temperature gradients which may or may not cause it to get above the autoignition temperature (this is of course a college student approaching this from the microwave burrito perspective... Your mileage may vary).

[gainer]

There is a variety of plastic wrap that is advertised as "Microwave safe". Cover the vessel you are heating with this type of wrap so that the edges are sealed but the center is loose enough to contact the fluid being heated. Before ANY vapor can escape the vessel to be ignited, the wrap will have to rise from contact with the fluid and baloon up. This action ought to be easily visible throuh the window. Heating will instantly stop when the microwave is turned off. (No more energy will be supplied to the fluid.) Uneven heating now plays no part in the process. If the wrap swells or even just barely leaves contact with the fluid surface, the vapor pressure is at least equal to atmospheric and the fluid is boiling. Until that point is reached, there is no possibility of the vapor being ignited by spark or open flame.