Superadditive amidol developers?

[grainyvision]

I see Amidol often used as a standalone developing agent, but only very rarely with other developing agents. Gainer had documented that a proportionally tiny amount of amidol (0.1g vs 10g) and ascorbic acid was capable of developing a negative on film. If there is some way to reduce or otherwise neutralize oxidized amidol, then there is also some hope of forming a superadditive pair AND being capable of reliably using lower quality stocks of amidol. I personally have some Amidol I purchased in 2019 and promptly forgot about, and is now a murkey dark grey, and produces deep brown solutions. It produces a bad color, but does work well alone for developing prints. The typical formulas called for will stain everything it touches. However, I tried making a formula using amidol and ascorbic acid, and there was no staining despite the solution still remaining a deep yellow-brown. There was no staining of trays or glass like I had seen before. However, this developer could produce a reasonable amount of density at pH 6.5 on some fogged RC paper. (just tested under daylight). With more bicarbonate added to bring pH to 7, it could produce a reasonable max density for printing with. I tested this and found the developer to produce a nice gradation but was quite slow, requiring likely 3-5m for standard modern FB papers of today, and producing a speed decrease with 2m of development. Spiking the solution with a very small amount of potassium carbonate very quickly brought the pH to around 9. At this pH paper would readily produce a reasonable max black in 2m. I've not done any real formal testing other than "it works" though. The concentrate listed below is a dark yellow-brown. When diluted 100ml to make 1L of working solution, the developer is initially a pale yellow-brown. Leaving this solution out overnight shifts the color to "dead ascorbate" bright orange and the solution will not produce any density at all even with greatly extended time. The concentrate was left open to air overnight and used to make a working solution 12 hours later. This solution worked well with no issues. No observable color change could been seen in either the concentrate nor working solution.

GVXB1 prototype #1 -- amidol-ascorbic acid print developer
Usage 100ml to make 1L of working solution.
* 200ml water
* 50ml propylene glycol (used for solubility reasons, unsure if this is actually needed)
* 25g ascorbic acid
* 5g amidol
* 5ml 1% benzotriazole solution (mine is dissolved in glycol)
* 26.5g sodium bicarbonate
* 1g potassium bromide
* 5g sodium sulfite (this is very difficult to dissolve and likely unneeded)
* pH ~7
* 0.4g potassium carbonate (if wanting to "spike" for faster development times, likely more stable without this)
* Top to 300ml with water
* pH ~9
* Appearance: Deep yellow-brown

Initial tests with images on Ilford Warmtone FB with spiking produced very warm tones with good black tones, similar to what I've always heard Amidol developers described to give. Without spiking produced a similar image but with less contrast and less speed (under exposed slightly). I did not try extending development beyond 2m to see if that would solve the issue.

Tests with another prototype formula (attempted monobath using a patent based formula) produced streaking, fog, low dmax, and poor tonality for paper, but may have been reasonable for film. This was using an amidol-catechol formula, it died quite quickly in the tray also, but was of much higher pH ~10.

I've seen one other person messing around with amidol as a superadditive developer. This was Jay DeFehr (a name that I at least recognize) and republished here: http://real-photographs.co.uk/formulae/print-developers/an-interesting-amidol-recipe/ However, this was a suspension rather than actually dissolving the amidol, so I'm unsure if it is a fair comparison to this.

Anyway, has anyone messed with formulating a superadditive amidol developers or seen any other work in this direction?

Print example:

 

[Alan Johnson]

Ascorbate has a reduction potential below many developing agents if you check out their Poubaix diagrams.
Amidol is not known in this context.
Le Clerc "Photography Theory and Practice" vol 4 $581 says it has been suggested that amidol developers can be stabilized by the addition of 1-2 g/L metol.
This seems to imply that metol may regenerate amidol rather than the reverse case which is normal for superadditivity.

 

[grainyvision]

Ascorbate has a reduction potential below many developing agents if you check out their Poubaix diagrams.
Amidol is not known in this context.
Le Clerc "Photography Theory and Practice" vol 4 $581 says it has been suggested that amidol developers can be stabilized by the addition of 1-2 g/L metol.
This seems to imply that metol may regenerate amidol rather than the reverse case which is normal for superadditivity.

Ascorbic acid is inactive at such low ph and a relatively small amount of amidol can be used at a relatively high ph and remain stable for over 6 hours, I believe that even if not documented, this may be a super additive pair wherein the amidol functions similar to phenidone but with lower activation pH and overall much weaker superadditivity.

Amidol in this seems to be a very greatly compensating agent which behaves in a peculiar way. More testing with the proposed developer above reveals that it produces very warm tones but with somewhat weak blacks, or rather very brown blacks which contain ample shadow detail but highlights and mid tones are also rendered differently. Mid tones look a little bit bunched up compared to reference, more dim and less contrasty than would be expected. While highlights retain excellent detail and a moderately high contrast appearance. There is speed loss of about 1/2 stop compared to a PQ developer but also a decrease in contrast of 1 grade... the results are a soft developer that is likely suitable for portrait work and working with long scale negatives. The negative I was testing with was short scale landscape and likely not a great show of what this may be capable of. I need to mix a new prototype though, as I believe more amidol is needed to make a reasonable print developer and likely also a lower ph. Also there are solubility issues with this concentrate. I believe the sulfite fell out of solution. Activity seems unchanged

edit: one possibility considering reduction potential is that ascorbate reacts with the oxidized/radical of amidol to form a new developing agent with new properties. This could be similar to how sulfite reacts with oxidized hydroquinone to form hydroquinone monosulfonate. This may explain the weak activity when the solution is a close to neutral ph like 7.5. Amidol in such conditions should readily produce a dense max density. There is also a color change when mixing the concentrate from a nearly black-brown to suddenly changing to yellow-brown after adding about 10g of bicarbonate and lifting ph to around 5.

If this is a new agent, then it would be especially interesting to figure out a way to isolate it, though I imagine that is far beyond my chemistry skill level

 

[Alan Johnson]

I would guess that Amidol [2,4 diaminophenol] would be more like p-phenylenediamine and would be adsorbed on the silver bromide grains. It might give an oxidation product that partially blocks the surface but can be regenerated with a secondary reducing agent. This may occur also with Halcyon ppd-ascorbate developer.
Amidol might be superadditive with metol or catechol which are used in some formulas to preserve it.
Also ,of course the ascorbate may be superadditive.

 

[grainyvision]

I would guess that Amidol [2,4 diaminophenol] would be more like p-phenylenediamine and would be adsorbed on the silver bromide grains. It might give an oxidation product that partially blocks the surface but can be regenerated with a secondary reducing agent. This may occur also with Halcyon ppd-ascorbate developer.
Amidol might be superadditive with metol or catechol which are used in some formulas to preserve it.
Also ,of course the ascorbate may be superadditive.

That may also be the case. I've seen a reference that the oxidation products of amidol is a very powerful retainer. I've tried adding phenidone to a similar formulation and had no change in activity, whereas phenidone would normally increase activity at this ph

 

[ruilourosa]

There are formulas for first developers in reversal processes that use amidol alone.
Amidol is messy to use...

 

[grainyvision]

There are formulas for first developers in reversal processes that use amidol alone.
Amidol is messy to use...

I find Amidol without ascorbic acid is excessively messy and unpredictable, but the formula above is surprisingly consistent even if far from perfect still. It also is not messy unless you leave a solution out and allow droplets to dry or oxidize, which will turn a deep purple and does stain. The solution as is though does not stain trays etc. I've tried a few other amidol developers and all stain trays even when being diligent about cleaning up

 

[ruilourosa]

Your formulation procedures should go simpler...
Ss is probably needed...
Carbonate bicarbonate buffer is off... And you could try a self buffered alcali like borax or metaborate first..
Ascorbate amidol ratio should be tested..
No need of glycol...

Next: why amidol?? Just adding toxicity... And messiness... No increased density Over other developers...
Specially on Open trays and my handsy attitude towards prints... I prefer to eliminate everything that can affect my health...

 

[Rudeofus]

If Amidol and AA were truly superadditive, then with 25g of AA in the mix there should be no trace of "compensating" in the results section. Any developer sufficiently powerful and concentrated should reproduce the paper's characteristic curve, period. You state, that staining is strongly reduced and that the developer is usable for six whole hours, so the AA most definitely does something useful here, but the development work may still be mostly done by the Amidol. If your observations about "compensating" and "low contrast in the mid tones" prove correct, then the AA may be able to reduce higher oxidized products of Amidol, but not single oxidized Amidol. If this case the AA may act more as oxidized developer scavenger than as secondary developer.

Either way, very interesting experiments! I have been looking for a near neutral developer for a while, since self coated emulsions are extremely weak and high pH will likely further weaken it. I have already thought about Amidol developers, but with the AA the concept may work even better.

 

[Lachlan Young]

If this case the AA may act more as oxidized developer scavenger than as secondary developer.

This seems a much more realistic scenario to me, especially as the Amidol 'stain' seems likely to be a dye coupler (or similar) formed via oxidation. DTPA to eliminate Fenton, AA, and Amidol might be the best combination, but whether it really has genuine benefits over a carefully balanced PQ/ PA developer is an open question. It is striking to me that at around the point that the properties of HQMS salts start to get properly understood, Glycin, Amidol & several of the other more unusual developing agents quietly & quickly fall by the wayside - though obviously, correlation ≠ causation.

 

[Alan Johnson]

In Jay DeFehr's Halcyon [ppd/ascorbate] the quantities of the two agents are much more nearly equal than the 1:5 ratio here.

 

[grainyvision]

If Amidol and AA were truly superadditive, then with 25g of AA in the mix there should be no trace of "compensating" in the results section. Any developer sufficiently powerful and concentrated should reproduce the paper's characteristic curve, period. You state, that staining is strongly reduced and that the developer is usable for six whole hours, so the AA most definitely does something useful here, but the development work may still be mostly done by the Amidol. If your observations about "compensating" and "low contrast in the mid tones" prove correct, then the AA may be able to reduce higher oxidized products of Amidol, but not single oxidized Amidol. If this case the AA may act more as oxidized developer scavenger than as secondary developer.

Either way, very interesting experiments! I have been looking for a near neutral developer for a while, since self coated emulsions are extremely weak and high pH will likely further weaken it. I have already thought about Amidol developers, but with the AA the concept may work even better.

I believe the compensating action may be due to the oxidation product of both ascorbic acid and amidol both being very powerful restrainers. Even superadditive pairs can easily produce compensating action when the effect is relatively mild, as can be seen by my recent formulation with glycin and phenidone.

Here is a new formulation, I posted it on a different group so just copy-pasting. It is greatly simplified. The amount of sulfite I believe is essential to how superadditive the mixture is. The much lower sulfite (most did not dissolve) in the previous formulation produced less activity even at higher pH and stronger concentration. Copied post:

Today I'm showing a unique superadditive amidol + ascorbic acid print developer which functions at pH 7. It creates a very warm tonation with warm black tones and the ability to modify shadow contrast by increasing or decreasing dilution. By using this contrast control, it is possible to quite easily print from long scale, contrasty negatives, without compromising the depth of blacks too greatly, nor darkening highlights too much. This aspect needs more testing though.
It is surprisingly clean working, requiring no restrainer, only a moderate amount of amidol, and makes use of ascorbic acid and sulfite which appears to both preserve and create a superadditive effect with amidol. Because of how it preserves the Amidol, this also means it comes with the benefit of likely being able to make use of lesser grades or otherwise expired amidol. The ascorbic acid and sulfite combination appears to clean up the amidol, removing it's easily staining properties and will noticeably lighten the color of the solution.

GVXB1, prototype #2
To make 100ml of concentrate

• 70ml room temp or only slightly warm distilled water
• 20g ascorbic acid (this may not all dissolve)
• 3g amidol
• 13g potassium bicarbonate (add VERY slowly, will bubble a lot)
• 0.7g potassium carbonate
• 5.3g sodium sulfite (do NOT add early! Otherwise it will result in release of sulfur dioxide gas)
• top to 100ml with water
• Final pH, 6. Appears a dark but still translucent brownish orange
• Use 50 to 100ml of concentrate added to 1L of water to make working solution. Develop for 2-4m. MUST use an acid stop bath
• Concentrate is expected to last weeks to months. Working solution remains active over 24 hours, but changes after 12 hours
  
  Attached example prints:
• Ship 1 and ship 2 w/ folded corner, printed from very thick pushed FP4+ negative, grade 2.5, Ilford MGV RC. Regular print is 25+500 GVXB1. Folded corner print is comparison against a very warmtone PQ developer (note shadow differences)
• Subway 1, Fomabrom FB, 25+500 GVXB1. "Torture test" 35mm HP5+@3200 negative which is long density scale. Grade 2
• Subway 2, Fomabrom FB, identical exposure, 50+500 GVXB1 (notice deeper blacks and shadow contrast)

 

[Loose Gravel]

As a side note and not to distract from your conversation ...

I've not used Amidol for paper, but I have recipes for a couple of pyro amidol developers. Pyrodol (my own formula) shows promise, but it is not as good as Wimberley's WD2H+. This developer builds contrast in the shadows (FP4+) better than anything I've found. Really makes the low end jump.

 

[DREW WILEY]

Sodium carbonate definitely accelerates amidol; but if any acidic ingredient is present, they just cancel each other out to a proportionate degree. For example, because the tray life of amidol with added sodium carbonate is short, people would add citric acid to improve the tray life, but that just offset the utility of adding sodium carbonate in the first place. My own amidol paper formula uses only amidol, sodium sulfite, a pinch of benzotriazole, and a little citric acid as the antioxidant.

But I don't understand why you even want to take amidol down this path. It frankly doesn't work well in combination with most other developing agents. And if you want a split of deep blue black tones in shadows and warm upper tones in a warmtone paper like MGWT, there are far more reliable ways to do that.

As far as that unrelated film developer tweak Loose Gravel just mentioned, putting amidol into pyro sometimes works, but can potentially become a negative-ruining crap shoot, difficult to predict. Bt what exactly did you mean by, Build low end contrast? - more density at the bottom, and hence more of a toe, or a steeper toe with better shadow gradation instead? I'd like to see a comparison densitometer plot.

 

[grainyvision]

Sodium carbonate definitely accelerates amidol; but if any acidic ingredient is present, they just cancel each other out to a proportionate degree. For example, because the tray life of amidol with added sodium carbonate is short, people would add citric acid to improve the tray life, but that just offset the utility of adding sodium carbonate in the first place. My own amidol paper formula uses only amidol, sodium sulfite, a pinch of benzotriazole, and a little citric acid as the antioxidant.

But I don't understand why you even want to take amidol down this path. It frankly doesn't work well in combination with most other developing agents. And if you want a split of deep blue black tones in shadows and warm upper tones in a warmtone paper like MGWT, there are far more reliable ways to do that.

As far as that unrelated film developer tweak Loose Gravel just mentioned, putting amidol into pyro sometimes works, but can potentially become a negative-ruining crap shoot, difficult to predict. Bt what exactly did you mean by, Build low end contrast? - more density at the bottom, and hence more of a toe, or a steeper toe with better shadow gradation instead? I'd like to see a comparison densitometer plot.

Honestly it seems like amidol has not been greatly well researched in combination with other agents at least from a public view. There likely is a reason that there was never anything commercialized during amidol's hey-day, but the lack of data about this is quite frustrating. Hence, why I was trying it to see what would happen. I wasn't looking for anything in particular with this combination, only seeing what would happen since the amidol stock I have is annoyingly expired and stains everything terribly. I personally am curious how this would react with film, but also do not at all trust it with actual pictures I care about, so it'll likely be a print developer only experiment.

 

[ruilourosa]

The magic bullet chase...

 

[DREW WILEY]

One issue with amidol is that it can be difficult to acquire consistent quality powder. In the past decade or so a large amount of poor quality Chinese amidol has flooded the market, which leaves an orange stain hard to wash out, a problem allegedly due to nickel vat contamination, but hard to say the exact reason. When there was once an excellent lab supplier locally, who mainly dealt with pharmaceutical and biotech labs, but also sold darkroom chemicals, he told me that photographic amidol was actually of a higher grade than medical amidol; and he was a phD. So too were his counter staff. They really knew their stuff. In the past few years, I've gotten high quality Euro amidol from Artcraft in NYC.

But given this fact, it's not hard to understand why people tended to view odd mixtures of amidol almost like voodoo; because it was hard to predict the outcome. Used more simply in paper developer formulas, if the quality of the amidol is good, it is easy to tame. Bromide papers in particular responded wonderfully to it if you were seeking a cold tone, but so could chloride contact printing papers like Azo. But the nature of this particular thread is to stretch those relatively predictable characteristics to the snapping point by introduced other variables. Have at it ! - I'll stick to what I know works instead.

 

[grainyvision]

One issue with amidol is that it can be difficult to acquire consistent quality powder. In the past decade or so a large amount of poor quality Chinese amidol has flooded the market, which leaves an orange stain hard to wash out, a problem allegedly due to nickel vat contamination, but hard to say the exact reason. When there was once an excellent lab supplier locally, who mainly dealt with pharmaceutical and biotech labs, but also sold darkroom chemicals, he told me that photographic amidol was actually of a higher grade than medical amidol; and he was a phD. So too were his counter staff. They really knew their stuff. In the past few years, I've gotten high quality Euro amidol from Artcraft in NYC.

But given this fact, it's not hard to understand why people tended to view odd mixtures of amidol almost like voodoo; because it was hard to predict the outcome. Used more simply in paper developer formulas, if the quality of the amidol is good, it is easy to tame. Bromide papers in particular responded wonderfully to it if you were seeking a cold tone, but so could chloride contact printing papers like Azo. But the nature of this particular thread is to stretch those relatively predictable characteristics to the snapping point by introduced other variables. Have at it ! - I'll stick to what I know works instead.

Amidol is definitely not a favorite agent of mine. Only a curiosity since I had some which I specifically got to testing with self made and some bought chloride papers. Emulsion making is definitely not my favorite thing to mess with though, and chloride papers are relatively rare now and expensive, so I expect to not be able to use the 100g of amidol I have solely for that purpose, so why not see what else it can do and especially if the annoying easily staining property could be resolved. My grade of amidol is English from Artcraft. it stains everything it touches other than photographic paper but does not produce the infamous pink stain

 

[DREW WILEY]

Amidol has always stained "other things". That's what rubber gloves are for. You don't want to end up with black fingernails like Brett Weston and some Punk Rock star with black nail polish did. But it's not supposed to stain prints, and doesn't if uncontaminated. I once used quite a bit of it with classic bromide graded papers; and it did a wonderful job, especially if a cold tone was sought. A few current VC papers respond well to it, but not many. I have about a full pound of high quality amidol left; and well-sealed, the powder should keep fine for a long time until I find another paper which especially benefits from it.

Although I had 3 years of organic chemistry in college, that was long ago, and I've discovered that it is a lot more fun to forget all of that and just be an alchemist myself, dipping owl feathers and snail shells into the developer time to time, just to see what will happen.

 

[grainyvision]

An approach (1963) by Polaroid researchers is disclosed in U.S. 3,091,530 wherein they propose substituted amidol derivatives retaining its desirable properties but with greater stability and that do not stain (which is critical in the proposed application). Unfortunately there do not appear to be other ways around its staining propensity. There are some older reported ways of increasing solution life, but this does not directly address staining, and this is all based on relatively ancient research.

I believe that the staining property of amidol is not the actual amidol, but rather whatever junk that it readily oxidizes into. Ascorbic acid + sulfite seems to prevent the formation of this staining oxidation product, as long as the solution is not left out to the air for so long that all of the ascorbate oxidizes (which takes... >2 days of the solution sitting in a tray by my measurement so far). What is produced by this reaction is unknown, and it is unknown if is a useful developing agent itself... All I know is that the combo I have mixed doesn't have the annnoying staining properties. The concentrate is not greatly stable however, producing small cubic black-grey crystals which I assume are some form of oxidized amidol derivative. This is soluble in water but not in the concentrate (likely less soluble than amidol) and only begins to occur about 2 days after the concentrate has been standing. It would be very interesting to isolate whatever this is and run some tests with it, but as you say, amidol is quite annoying to work with, especially if purposefully oxidizing it

 

[DREW WILEY]

For paper developer, I mix up the sodium sulfite, benzatriazole restrainer, and citric acid preservative in advance, and add the amidol powder right before each daily session. And that's about its tray life - a one day session of 8 hrs at most. The one potential disadvantage of my own formula is that, being acidic, you need to use a plain water stop step, which doesn't work fast and consistently enough for some papers like MG Classic or Cooltone.

As far as the SLIGHT film speed boost to PMK development goes, the addition of amidol is more risk than it's worth. There are other tweaks of pyrogallol which do that, though I didn't find them worth the extra fuss or money either. I'll save my amidol for paper use only.

 

[Rudeofus]

I believe the compensating action may be due to the oxidation product of both ascorbic acid and amidol both being very powerful restrainers. Even superadditive pairs can easily produce compensating action when the effect is relatively mild, as can be seen by my recent formulation with glycin and phenidone.

Ascorbic Acid does not produce desensitizing oxidation products, but it does decrease in pH as it gets oxidized. Is there a chance you can measure pH before and after a dev run? Judging from the chemical structure there is only one possible explanation for oxidized Amidol to cause a restraining effect: if it keeps sticking to silver just like PPD does. However, if Amidol would do this, then Ascorbic Acid would inhibit just this effect, that's how all these MQ/PQ/PQ/... developers build up contrast, and that effect is even more visible with all these PPD+secondary dev type formulas.

I could think of two different effects/experiments:

1. pH drop during development. This could be easily avoided by switching buffers from carbonate/bicarbonate to Borax/Boric Acid or Hydrogenphosphate/Dihydrogenphosphate, both of which do quite well at pH 6-7.
2. Silver developing capacity of developer in terms of "moles of silver developed per mol of Amidol". If that number is greatly increased by the addition of 20 g/l Ascorbate, then superadditivity is likely a factor in this boost. If capacity stays more or less the same as without ascorbate, then the ascorbate does good things but isn't superadditive.

BTW the two buffers I suggested are beneficial for yet another reason: if mixing the developer causes "great amount of bubbling", then this is most likely Carbon Dioxide lost in the mixing process. Depending on temperature and mixing procedure different amounts of Carbon Dioxide will escape, which means: you don't get a predictable composition. Using Borax/Boric Acid or Hydrogenphosphate/Dihydrogenphosphate buffers could completely avoid this issue.

 

[Rudeofus]

This seems a much more realistic scenario to me, especially as the Amidol 'stain' seems likely to be a dye coupler (or similar) formed via oxidation. DTPA to eliminate Fenton, AA, and Amidol might be the best combination, but whether it really has genuine benefits over a carefully balanced PQ/ PA developer is an open question. It is striking to me that at around the point that the properties of HQMS salts start to get properly understood, Glycin, Amidol & several of the other more unusual developing agents quietly & quickly fall by the wayside - though obviously, correlation ≠ causation.

In terms of cost or ease of use it will hardly beat PQ developers. Yes, P + HQMS are great for film, but this combo is hardly cost effective for printing. I guess the hope was to preserve some of the attractive properties of Amidol (image tone, low pH), while avoiding its annoying properties (staining, short shelf life). Yes, DTPA or ATMP would be great addition.

 

[grainyvision]

Ascorbic Acid does not produce desensitizing oxidation products, but it does decrease in pH as it gets oxidized. Is there a chance you can measure pH before and after a dev run? Judging from the chemical structure there is only one possible explanation for oxidized Amidol to cause a restraining effect: if it keeps sticking to silver just like PPD does. However, if Amidol would do this, then Ascorbic Acid would inhibit just this effect, that's how all these MQ/PQ/PQ/... developers build up contrast, and that effect is even more visible with all these PPD+secondary dev type formulas.

I could think of two different effects/experiments:

1. pH drop during development. This could be easily avoided by switching buffers from carbonate/bicarbonate to Borax/Boric Acid or Hydrogenphosphate/Dihydrogenphosphate, both of which do quite well at pH 6-7.
2. Silver developing capacity of developer in terms of "moles of silver developed per mol of Amidol". If that number is greatly increased by the addition of 20 g/l Ascorbate, then superadditivity is likely a factor in this boost. If capacity stays more or less the same as without ascorbate, then the ascorbate does good things but isn't superadditive.

BTW the two buffers I suggested are beneficial for yet another reason: if mixing the developer causes "great amount of bubbling", then this is most likely Carbon Dioxide lost in the mixing process. Depending on temperature and mixing procedure different amounts of Carbon Dioxide will escape, which means: you don't get a predictable composition. Using Borax/Boric Acid or Hydrogenphosphate/Dihydrogenphosphate buffers could completely avoid this issue.

There is no measurable pH drop before and after development. In fact, I suspect the pH may somehow increase very slightly. I'm using pH test strips though so it's within measurement of error. This may be due to buffers or some such in photographic papers rather than any development effect.

The bicarbonate is used to neutralize the ascorbic acid and amidol rather than strictly to build a buffer. Hydroxide would be better for this, I just didn't want to bother with the amount of tedium and precision needed for that. I could potentially even use citric acid/citrate, since I believe at pH 6 this developer would still work well. With how amidol works though, buffers seem to oddly be undesirable. Amidol in acid developers form the image from the bottom-up due to reacting with gelatin which raises local pH slightly. If there is a strong buffer system in place, this local pH effect would be greatly reduced.

I used ascorbic acid primarily because I had it. I got some sodium ascorbate today that may be better to have to avoid worrying about neutralization. Using ascorbic acid and sodium ascorbate together would likely allow for proper pH without any alkali at all other than sulfite

 

[Rudeofus]

There is no measurable pH drop before and after development. In fact, I suspect the pH may somehow increase very slightly. I'm using pH test strips though so it's within measurement of error. This may be due to buffers or some such in photographic papers rather than any development effect.

The carbonate/bicarbonate/Carbon Dioxide system would buffer quite well at pH around 6 or 7, so you may have more buffering than you intended. Some buffering is even quite useful, because you do want to set a well defined pH in your developer to obtain consistent results. This is particularly important for a developer, which does more than just "develop everything which looks developable".

The bicarbonate is used to neutralize the ascorbic acid and amidol rather than strictly to build a buffer. Hydroxide would be better for this, I just didn't want to bother with the amount of tedium and precision needed for that. I could potentially even use citric acid/citrate, since I believe at pH 6 this developer would still work well.

Bicarbonate does more than just "neutralize the acid", it does form a carbonate/bicarbonate/Carbon Dioxide buffer system, the Carbon Dioxide part of which you already observed. Citric Acid and Sodium Citrate also form a buffer, but one most effective in the pH range between 3.0 and 6.2. Look at Sigma's Buffer Resource Page for more useful buffers. Monobasic and Dibasic Phosphates give you pH range from 5.7 all the way up to 8.0, which covers the whole range you wanted to look at.

With the combination Sodium Ascorbate and Ascorbic Acid, together with mono and dibasic phosphates you can adjust both pH and buffer strength as you like it: first adjust Sodium Ascorbate and Ascorbic Acid until your pH is roughly where you want it. Then use Sigma's page to tell you the ratio between monobasic and dibasic phosphate to match that pH, then mix the calculated amounts of monobasic and dibasic phosphate into your developer. Higher total amount of phosphate means higher buffer strength and vice versa.

Do note, that Sulfite also buffers at pH 7, so you may even get away without an explicit buffer.