Progress on XTOL-concentrate

[Photo Engineer]

NP, just clarifying my comment combined with yours.

PE

 

[albada]

A question for chemists and chemist-wannabees

The question is:
What is the pH of equal weights of borax and boric acid dissolved in distilled water? I used 15 g/L of both.

To help with pH measurements, I decided to create a reference-buffer near the pH of the developer I'm working on, and noticed that equal weights of borax and boric acid were close to what I wanted according to the Henderson-Hasselbalch equation. I calculated a pH of 8.28 (at 30C), but measured 8.05, so I was far off. I used a pKa of 9.139 for boric acid at 30C. Furthermore, I know that borax splits into boric acid and its conjugate base upon dissolution, so I added the concentration of borax to that of boric acid. Here's my math:

g = grams of powder per litre
b = 381.37 = mw of Borax
a = 61.83 = mw of boric acid
B = g / b
A = g * (1/b + 1/a)
pH = pKa + log(B/A) = pKa + log(1/(b*(1/b + 1/a)) = pKa + log(1/(1+b/a)) = pKa - log(1 + b/a)
Plugging in the numbers: 9.139 - log(1 + 381.37/61.83) = 8.28
​

But boric acid is polyprotic, and according to wikipedia, has pKa2=12.74 and pKa3=13.8. I don't know how to account for those. Would they cause my calculation to be in error by 0.23?

Also, before adding the boric acid, I measured the pH of the borax solution as 9.164 (at 30C). It should be 9.14, so my (home-made) pH meter was off by .024, which is accurate enough. So the measurement of 8.05 should be close, so I really am .23 off. Any clue why?

Thanks,

Mark Overton

 

[Kirk Keyes]

Home made pH meter? Sounds pretty good for home made.

 

[Kirk Keyes]

Do you have a temp sensor to correct your pH measurements?

 

[Kirk Keyes]

Also, buffers, for whatever reason, often are off a little bit from calculated values. You can made fine adjustments to them. For borate buffers, use boric acid to make it more acidic, and sodium hydroxide to raise the pH.

 

[albada]

Home made pH meter? Sounds pretty good for home made.

I'm using the electrode from a cheap Hanna pH-meter that broke, and a Fluke voltmeter shows the mV output, from which pH can be easily calculated using stored calibration-values. But I had to build a unity-gain box to buffer the weak output of the electrode. Pictures of it are about halfway down this page: (there was a url link here which no longer exists)

Mark Overton

 

[albada]

I measure temperature with a thermometer and compensate manually. Commercial buffers have tables of pH-values based on temperature printed on them, so I know what pH to expect from them.

Also, buffers, for whatever reason, often are off a little bit from calculated values. You can made fine adjustments to them. For borate buffers, use boric acid to make it more acidic, and sodium hydroxide to raise the pH.

That's good to know. Although if I can hit +/-0.03, that should be good enough. Do you think commercial buffers are that accurate? Anyway, my problem is that my home-mixed buffer was over 0.20 off from what I calculated, so I clearly made a mistake in the theory somewhere.

Mark Overton

 

[albada]

I have never heard anyone other than yourself say that Phenidone produces more fog than Dimezone-S.

I finally collected some test-strips I'd developed over the last few weeks, and measured base+fog (B+F) levels for Phenidone at various pH-levels using TMY2. All test-strips were developed to approximately the same contrast as XTOL. Here's the graph:



The graph is jumpy and crude, partly because I changed pH-meters partway through. But it shows a trend: As pH rises, Phenidone's B+F rises. For comparison, XTOL uses Dimezone-S, and its B+F is .25 at pH 8.3 (with TMY2). Phenidone only gets close to XTOL's level when pH is no more than around 8.10.

Caveat: All of the Phenidone developers in that graph use ascorbic acid and sodium metaborate. Other chemistries might cause Phenidone to behave differently.

Mark Overton

 

[Photo Engineer]

Mark;

Another good reason to use Dimezone-S, unless there is an antifoggant in XTOL!

PE

 

[albada]

Mark;
Another good reason to use Dimezone-S, unless there is an antifoggant in XTOL!
PE

If only the crystallization issue could be solved. A concentrate with a little TEA and mostly PG crystallized. I'm thinking of reversing that by using mostly TEA and minimal PG to thin it some. A little citric acid will bring the pH back down. With so little PG in it, perhaps it won't crystallize.

BTW, I forgot to mention that Phenidone gets slightly grainier as its fog-level rises. The difference is small enough that most people might not notice, but it's perceivable on close exam. That makes it more of a challenge for Phenidone to match the image-quality of Dimezone-S.

Mark Overton

 

[Photo Engineer]

Coarse grains are less developable. As fog increases, the coarse grains are developed and thus the grain increases. This is a "normal" trait of fog formation.

PE

 

[albada]

A question for chemists and those who create developers:
Why did Ryuji Suzuki and others use Salicylic acid and TEA to chelate iron, when DTPA will work?

Here's some interesting info about DTPA: www.dow.com/scripts/litorder.asp?filepath=/113-01343.pdf
Notice that Fe (iron) is included in the list of metals chelated. Iron is the curse of ascorbate developers because it eventually destroys all ascorbate via the Fenton reaction. Hence the motive to chelate iron. DTPA is used in XTOL (1 g/L, according to the patent), so I'm wondering why others don't use it.

Mark Overton

 

[Gerald C Koch]

For the very practical reason that both salicylic acid and TEA are easier to obtain than DTPA. I bought salicylic acid from my local compounding pharmacy.

 

[Photo Engineer]

Iron III salts + EDTA is claimed to cause more rapid oxidation of Ascorbic Acid than other mixtures such as Salicylic Acid.

PE

 

[Gerald C Koch]

The number of chelating agents that have been proven not to catalyse the Fenton reaction is rather small. Ryuji found very little data on the internet.

 

[Photo Engineer]

Way back when, he said he was going to get a patent on his special chelating agent, but I have seen nothing in the literature yet.

PE

 

[albada]

Way back when, he said he was going to get a patent on his special chelating agent, but I have seen nothing in the literature yet. PE

He's been silent for months. I suspect he lost interest.

For nearly a year, I thought that (1) DTPA would not chelate iron, and (2) Kodak avoided the problem by using iron-free chemicals that we ordinary folks can't obtain. I was wrong. DTPA chelates iron. True, it's harder to find, but I think it's obtainable with effort. For example, SpectrumChemical.com (in Los Angeles) sells 25 grams of pure DTPA (not a salt) for $17 here. I remember a posting where somebody mixed XTOL from the patent. He had a hard time finding DTPA, but finally located some in New York (I think). Also, it's worth calling Photo Formulary, as they stock chemicals not on their website.

I mixed a new batch of XTOL a couple of days ago, storing it in five 1-litre bottles. And I thought of a way of making it last longer: Store the extra bottles in the refrigerator. Sulfite will precipitate out of solution, but when it's time to start a new bottle, warming and shaking it should bring the precipitate back into solution. Kirk Keys reminded me that chemical reactions proceed half as fast with each 10C temperature-drop, so refrigeration should make XTOL last around 1.5-2 years instead of 6 months. Is there any reason why this would not work?

Mark Overton

 

[Photo Engineer]

Shaking entrains more oxygen.

PE

 

[albada]

Shaking entrains more oxygen. PE

Hmm. And if there's no air in the bottle, shaking is far less effective. Everything is more complicated than first appearances. And that's mostly what keeps us engineers employed.

Here's another question:
We saw earlier in this thread that a concentrate consisting of PG+SodiumMetaborate+AscorbicAcid+DimezoneS crystallizes. That developer behaved exactly like XTOL, so I want to use it. So my question is: Can another solvent be used instead of PG? How about glycerine? Or ethylene glycol? Anything else?

Mark Overton

 

[Rudeofus]

Mark, check this out for an overview over common chelating agents. The document includes EDTA, PDTA, DTPA and others. Note that it is not just important to chelate Fe-III and Fe-II, also the difference in complex stability between the two has an impact.

BTW: When you store your soup at cool temperatures, you also dissolve more air (and as a result oxygen) in the liquid in the long run. This could counteract the slowing down of the reactions you try to quench.

 

[Alan Johnson]

I mixed a new batch of XTOL a couple of days ago, storing it in five 1-litre bottles. And I thought of a way of making it last longer: Store the extra bottles in the refrigerator. Sulfite will precipitate out of solution, but when it's time to start a new bottle, warming and shaking it should bring the precipitate back into solution. Kirk Keys reminded me that chemical reactions proceed half as fast with each 10C temperature-drop, so refrigeration should make XTOL last around 1.5-2 years instead of 6 months. Is there any reason why this would not work?

Mark Overton

Mark,
Xtol in full sealed glass bottles (no oxidation from air) is reported to last 7 1/2 years:
(there was a url link here which no longer exists)
The 6 months figure you quote is for part full bottles with air oxidation of ascorbate.The chelating agent is to slow down this oxidation.
In full sealed bottles Xtol can only decompose by hydrolysis, not oxidation.Ascorbate hydrolysis is probably not significant ,Dimezone S is more resisistant to hydrolysis than phenidone.

 

[Photo Engineer]

Mark, I cannot suggest any other solvent. Sorry.

I can suggest that you fill bottles with nitrogen "caps" and then shaking will not be a problem.

PE

 

[Gerald C Koch]

It's erroneous to believe that using a chelating agent forever stops the Fenton reaction. Actually it only slows the reaction down. How well a particular agent works depends on its stability constant for iron (III). There will always be some uncomplexed iron (III).

 

[Photo Engineer]

A very good point Jerry. It is a matter of equillibria and nothing can stop this, it can only be slowed down.

PE

 

[albada]

A few weeks ago, I had a nice concentrate that matched XTOL in all respects -- density-curves, grain and even dev-times. But that work was halted when I discovered that the concentrate crystallizes and separates. This occurred with DimezoneS but not Phenidone, so I was back to using Phenidone. In that formula, using Phenidone was slightly grainier than DimezoneS.

After more experimenting, I've come up with the formula below. Its grain is slightly better than XTOL, sharpness is the same, and dev-times are longer. The 1-litre formula for mixing directly into water:

Sodium sulfite ................. 45 g
Sodium metaborate ........ 2 g
Ascorbic acid .................. 4.5 g
Phenidone ...................... 0.05 g
Target pH = 8.05 to 8.10.
13:40 minutes for TMY2.
​

For a concentrate, mix everything except sulfite into 16 ml of propylene glycol.
Here's the density-curve compared with XTOL, which is a perfect match:



Grain looks a tad better than XTOL in my 22x loupes. Here are full-resolution crops of neg-scans of XTOL (left) and concentrate (right):

-

An objective way to gauge grain is by the sizes of JPEG files, keeping dimensions and contrast identical. Finer grain (textures) produce smaller files. In the crops above, the concentrate's files are 2.25% smaller.

The sharpness of the high contrast horizontal line looks the same to me, but the low contrast vertical lines look a bit softer for the concentrate. Shutterspeed was 1/8 second (tripod), but there still might have been a bit of horizontal vibration. I'll have to watch future test-strips.

I discovered this formula back in February and posted it to apug on Feb 5, 2012 (there was a url link here which no longer exists). But I was investigating the effect of sulfite, and didn't pursue this discovery.
There's still much experimenting I want to do, varying ingredients by small amounts and gauging the effects. But I wanted to post this because it's a good milestone and will be my baseline for further work.

Mark Overton