Experiments with Metol and ascorbic acid.
The factors affecting developer activity are temperature, pH,
concentration, products of aerial oxidation, products of silver halide
reduction, and the orientations of the heavenly bodies.
According to some experimental results reported in "The Theory of the
Photographic Process" as little as 0.05 moles (6.3 grams) of sodium sulfite in
a liter of Metol developer will double the rate of development that is
obtained when no sulfite is present, pH being held at 8.7 in both cases.
It is explained that oxidation products of Metol restrain development, and
that sulfite counteracts these products by forming the sulfonate.
Ascorbic acid will do the same degree of acceleration without sulfite, also at pH 8.7. In this case the oxidized Metol is reduced back to its original form. The ascorbate is itself oxidized and pH is decreased in the process. The decrease in amount of active Metol in the one case or the decrease of pH in the other may not cause much difference in capacity and storage longevity between a Metol-sulfite and a metol-ascorbate developer. However, it would seem that a well buffered working solution might tip the balance in favor of the ascorbate.
I will explore first a 0.05 molar concentration of Metol, which is
approximately that of D-23, together with a 0.05 molar concentration of
ascorbic acid. It remains to formulate the alkali needed to make the pH less
than that at which an ascorbate becomes a developer and great enough to ensure activity of Metol.
I calculate that 0.1 moles of sodium hydroxide will neutralize the 0.05
moles of sulfuric acid attached to the Metol base plus the 0.05 moles of
ascorbic acid, leaving 0.05 moles each of sodium sulfate and 4-(methyl-
amino)phenol and 0.05 moles of sodium ascorbate. The net pH thus far is still
not alkaline enough to develop film in a reasonable time, nor does it have the
necessary buffering action to keep the pH from going lower yet. A few grams of borax should bring the pH up to about 9.2 but even 20 grams will have little
more effect on pH and should give a cushion against reduction of pH during
The resulting recipe is:
Ascorbic acid.....8.8 g
Sodium hydroxide..4 g
Water to 1 liter.
The concentration of Metol is about 13% greater than in D-23 and the
concentration of sodium sulfite is nil.
The pH of this mixture, which I measure with test strips to be about 9, is
below that at which the ascorbate is a developer of any consequence. The
initial activity of the mixture is about that which could have been obtained
with sulfite in place of the ascorbate, so it does not seem that synergism
between Metol and ascorbate is the explanation.
The ratio of borax to caustic in this formula is quite close to the
effective ratio in sodium metaborate. 14.5 grams of sodium hydroxide and 69
grams of borax in a liter make a solution that is often used as a substitute
for 10% sodium metaborate. 276 ml of such a solution, or 27.6 grams of Kodalk if you prefer, will contain the equivalent of 4 grams of sodium hydroxide and about 20 grams of borax. It may be easier to get sodium metaborate than sodium hydroxide through the UPS. The recipe then becomes:
Ascorbic acid......8.8 g
Sodium metaborate..27.6 g or 276 ml of 10% sodium metaborate solution.
Water to 1 liter.
All ingredients are quickly dissolved in room temperature water. HP5+
developed for 8 minutes at 68 F gives normal contrast, showing that the
solution could be diluted. In fact, diluting with an equal part of water
increased development time by only 25%, probably because the pH of this
solution does not change much with dilution. 125 ml diluted with 125 ml of
water did a 36 exposure roll of HP5+ to normal contrast in 10 minutes, which
means of course that a liter of the above formula will do 16 standard rolls
without reuse. As a matter of interest, twice the above recipe can be squeezed into one liter.
By the way, don't be confused by the fact that there exist 4 mol and 8 mol
metaborates. A gram of one has the same number of atoms of sodium, boron,
oxygen and hydrogen as a gram of the other. At 53.6 C, without losing any
water, Na2B2O4.8H2O becomes NaBO2.4H2O which is stable to 105 C. In other words, the distinction between 4 mol and 8 mol sodium metaborate is academic, not practical when we specify solution strength in terms of weight per unit volume. You may see this for yourself at www.borax.com.
I am attaching a scan of a 10x print from FP4+ developed 8 minutes, 68 F. This is not of very high resolution, but may serve to illustrate gradations. I have also attached a higher resolution scan of a small part of the same print. If you print the high res. scan to 6x9 inches, you will see about a 50X magnification of that part. I have no idea how these will show up, but you can imagine that the originals are better.
As it turned out, the attachments showed up at the end of the thread on superadditivity started by PE. My fault.
Last edited by gainer; 06-03-2007 at 10:23 AM. Click to view previous post history.
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.
Originally Posted by gainer
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.
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.
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).
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.
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