PH increase when diluting developer, how can it happen?

Anon Ymous · Jan 26, 2019

Hi, lately I've been using a modification of the Xtol patent developer formula. The results so far are nice and I'm pleased with it. Yesterday I mixed a bit more and this involves using a pH meter. I calibrated it with pH 7 and 10 buffers, then proceeded to pH adjustment, to reach pH 8,2 that is the target pH. After completing pH adjustment I diluted the developer with an equal amount of tap water and I left the pH meter on, just to see what difference it would make in pH. I'd expect to see a slight drop in pH, but instead of dropping below 8,2, it raised to almost 8,3! Of course, I checked the tap water pH, which I found to be ~7,6. So, for the chemists out there, why did this happen?

Thanks in advance

Sirius Glass · Jan 26, 2019

It has to do with the further dilution allowing more H+ separating and ionizing thus lowering the pH. A good college level chemistry book will explain the mechanism more clearly.

Anon Ymous · Jan 26, 2019

Sirius Glass said:
It has to do with the further dilution allowing more H+ separating and ionizing thus lowering the pH. A good college level chemistry book will explain the mechanism more clearly.

There was a pH increase, not decrease.

Photo Engineer · Jan 26, 2019

As you diluted it, there was more ionization and the pH went up due to the increase in OH- ions.

If you dilute a concentrated acid, it becomes more acidic by the same method in reverse. There is an increase in H+ ions.

PE

EdoNork · Jan 27, 2019

Try to adjust initialñ pH with a buffer.

bernard_L · Jan 27, 2019

Photo Engineer said:
As you diluted it, there was more ionization and the pH went up due to the increase in OH- ions.

The number of OH- ions increases, but their concentration decreases
Example: ammonia, using the online calculator at https://sensorex.com/ph-calculator/

moles/litre	pH
0.1	11.12
0.01	10.61
0.001	10.09
0.0001	9.53

As one can see, when the ammonia is diluted , pH decreases, i.e; the concentration of OH- decreases. One can also see that for each successive dilution by a factor of 10 (by adding water, not discarding any liquid), the pH decreases by less than 1.0, meaning that the total number of OH- ions increases.

Anon Ymous · Jan 27, 2019

EdoNork said:
Try to adjust initialñ pH with a buffer.

The patent formula uses sodium metaborate, which I assume is used for buffering. Since I don't have any of it, I make it in the solution with the equimolar amount of borax and sodium hydroxide.

FWIW, other modifications I made was the substitution of Dimezone-S with Phenidone, sodium ascorbate with the equimolar amount of ascorbic acid + sodium hydroxide and DTPA-5Na (pretty much unobtainium) with sodium hexametaphosphate. This last modification is probably the only reason my pH is slightly less than the 8,2 target, close to 8,1. This requires the addition of ~2ml of 10% w/v NaOH per litre of developer, so 0,2g of NaOH.

Photo Engineer · Jan 27, 2019

There is also the ionic effect of the cations in solution on the electrode(s) and the common ion effect.

I gave just the simple solution to this problem. The actual solution is very complex, and for this we kept a data table next to our pH meter for just such problems.

PE

bernard_L · Jan 28, 2019

Photo Engineer said:
I gave just the simple solution to this problem.

So the simple solution is that when an alkaline solution is diluted, its pH goes up (more alkaline). Who'd have thunk? I need to digest this before I even dream of comprehending "The actual solution is very complex".

Anon Ymous · Jan 28, 2019

bernard_L said:
So the simple solution is that when an alkaline solution is diluted, its pH goes up (more alkaline). Who'd have thunk? I need to digest this before I even dream of comprehending "The actual solution is very complex".

This wouldn't be true for strong bases I suppose. Weak bases on the other hand, in a solution with all sorts of other chemicals? I guess that is what PE hinted at.

Alan Johnson · Jan 28, 2019

pH meter readings often fluctuate slightly with time , I am not convinced that the increase you report is real.

Raghu Kuvempunagar · Jan 28, 2019

Anon, on a different note, would you mind sharing your XTol modified formula?

Anon Ymous · Jan 28, 2019

Raghu Kuvempunagar said:
Anon, on a different note, would you mind sharing your XTol modified formula?

No, not at all, but I'm away from home and I won't be able to do it for some days.

Anon Ymous · Jan 28, 2019

Alan Johnson said:
pH meter readings often fluctuate slightly with time , I am not convinced that the increase you report is real.

If I had removed the probe, cleaned it, put it away with the rest of the meter and tried taking a measurement without recalibrating, then there would be reason to doubt. In my case, the calibration was performed minutes before, I waited the minutes needed for the reading to stabilise and the probe wasn't removed from the solution. After adding water, the reading went up and stabilised soon after the solution became homogenous at almost 8,3. I have very good reasons to believe it is real.

Photo Engineer · Jan 28, 2019

bernard_L said:
So the simple solution is that when an alkaline solution is diluted, its pH goes up (more alkaline). Who'd have thunk? I need to digest this before I even dream of comprehending "The actual solution is very complex".

This is a generalization. It is like saying that you have no temperature change when adding Sodium Carbonate to water. But there is a tiny one that becomes very great with stronger alkali and stronger acids such as NaOH and H2SO4. And Hypo causes a temperature drop as do other chemicals. So, there is no one answer. I'm saying that A pH change is possible, not that it goes up or down and not that there is none.

In Research at EK, we assumed no temperature change when mixing AgNO3 into Salt and this worked on a small scale, but at huge sizes in the plant, the temperature changes could be large and thus we had an "Enthalpy" calculation that told us what the temperature shift was likely to be when mixing huge volumes of chemicals that reacted with each other.

ENTHALPY: A thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.

PE

Rudeofus · Jan 28, 2019

If you add more and more NaOH to water, pH will go up. So no, there is definitely not always an increase in pH with higher dilution. The normal case is that an alkali increases pH with concentration, and that pH of an acid decreases with concentration.

There are special cases, in which ionic strength triggers secondary effects. This means, that because there is so much stuff in solution, some salts/ions do not behave as one would assume, or behave as if they were present in much lower concentration. Many photographic developers seem to be affected by these effects, because their high sulfite content alters the behavior of their other ingredients. If you dilute them, ionic strength effects from sulfite decrease, and alkaline compounds can suddenly raise pH of these solutions.

Anon Ymous · Feb 2, 2019

Raghu Kuvempunagar said:
Anon, on a different note, would you mind sharing your XTol modified formula?

Well, I'm back home, so I can post my modified formula, along with some general information. It is based on the formula found in patent US5853964A, example 1, page 5 of 7 and here are the modifications made:

DTPA-5Na (Diethylenetriaminepentaacetic acid, pentasodium salt) isn't easily obtainable and it was replaced by 2 g of Sodium Hexametaphosphate per litre. Since DTPA-5Na is quite alkaline and Sodium Hexametaphosphate is neutral, this will probably lower the pH of the solution, so pH adjustment will be required. This will also affect the keeping properties of the developer and the Fenton reaction that kills this developer is more likely to happen. I don't have any ambitions about making it a long shelf life developer and use it right after I make it. Keep in mind that this is optional and can be omitted if using distilled, or deionised water.
Sodium Metaborate isn't available to me, so I make it in solution using the right amounts of borax (Sodium Tetraborate decahydrate) and Sodium Hydroxide.
Dimezone-S (4-Hydroxymethyl-4-methyl-1-phenyl-3-pyrazolidone) is a derivative of Phenidone. I don't have Dimezone-S, so I replaced it with Phenidone. Molecular weights, as well as activity levels differ, so there's no easy way to tell how much will be needed. I just used the same amount per litre, 0,2g. I make a 1% w/v solution of Phenidone in Propylene Glycol, which has excellent keeping properties and use 20ml of it.
Sodium Isoascorbate isn't something I have. It can be replaced by the same amount of Sodium Ascorbate, which is an isomer. If you only have Ascorbic Acid, as I do, you can use an equimolar amount of it and neutralise it with sodium carbonate, or bicarbonate, or hydroxide. I opted for hydroxide, since I use it to make Sodium Metaborate, but also to adjust pH.

So, the resulting formula is this:

Sodium Hexametaphosphate	2g
Sodium Sulfite	85g
Borax	2,77g
Phenidone	0,2g
Sodium Metabisulfite	3,5g
Ascorbic Acid	10,67g
Sodium Hydroxide	3g
Water to make	1l
Target pH	8,2

In practice, I noticed that I need slightly more Sodium Hydroxide to reach the target pH of 8,2, 3,2 instead of 3g, but this also depends on the purity of the chemicals used.

You can simplify things quite a bit by making a long shelf life solution consisting of some of the ingredients listed above at double the concentration. So, you can mix Sodium Hexametaphosphate, Sodium Sulfite, Sodium Metabisulfite and Borax to a total volume of 500ml. When I want to make some developer, I measure half the volume I will need, then add the Phenidone, Ascorbic Acid and Sodium Hydroxide. I then top up to the volume of stock developer required. At this point, I adjust pH, after which I can further dilute the developer to 1+1.

Theoretically, I could make a solution consisting of everything except Ascorbic Acid, since this is what oxidises and kills makes the developer fail. If you do so, the solution will likely be copper coloured, because of the hydrolysis of Phenidone. The addition of Ascorbic Acid will make the solution water clear, something that you might experience when mixing part B of Xtol. In the end, I decided that it's better to add Phenidone (and hydroxide) just before use, because I don't know how well it would keep.

Raghu Kuvempunagar · Feb 2, 2019

Anon Ymous said:
Well, I'm back home, so I can post my modified formula, along with some general information.

This is very useful! Thanks a lot Anon.

Rudeofus · Feb 4, 2019

If you forego the iron/copper sequestering properties of DTPA anyway, is it even necessary to add the Hexametaphosphate ? Also, there a several resources about conversion between Dimezone-S and Phenidone, and the most common consensus I found was that two units of Dimezone-S can be substituted with one unit of Phenidone. Can you confirm, that your XTol version behaves the same as the original product, especially with respect to development times?

Anon Ymous · Feb 4, 2019

Rudeofus said:
If you forego the iron/copper sequestering properties of DTPA anyway, is it even necessary to add the Hexametaphosphate ?

Well, yes and no. If I use tap water with any solution containing sulfite, or carbonate, it will become cloudy within minutes. I could use deionised water, which is cheap, but IMHO hexametaphosphate is more convenient, that's all. It takes less space and 1kg is more or less a lifetime supply.

Rudeofus said:
Also, there a several resources about conversion between Dimezone-S and Phenidone, and the most common consensus I found was that two units of Dimezone-S can be substituted with one unit of Phenidone.

Thanks for the info, wasn't aware of that. Now, the question I have is what effect would different quantities of Phenidone have? Obviously, too little wouldn't be enough, but is there a threshold above which there would be no difference at all? Or would there be side effects?

Rudeofus said:
Can you confirm, that your XTol version behaves the same as the original product, especially with respect to development times?

This would be tough to say, since I don't have any real Xtol at hand and, so I can't make any side by side comparisons. That said, I've tried some films with this developer, following Kodak's times for Xtol and my methodology and characteristic curves can be found in this thread. The characteristic curve for TMax 400 nearly met the ISO criteria, it had slightly higher contrast. Slight deviations like that can IMHO happen in an amateur's lab, perhaps half a degree off. The one for (expired) APX100 met the ISO criteria for an ISO speed of 64. Obviously, I have to make more tests with other (fresh!) films, but so far it seems that it behaves reasonably close to Xtol. Sharpness and granularity are very nice too.

Alan Johnson · Feb 5, 2019

It is OK to use EDTA instead of DTPA just so long as the developer is kept in full sealed bottles or topped up with Protectan spray.
I know that EDTA promotes a Fenton-like reaction but so long as there is no oxygen there is no such reaction.
I made synthetic Xtol from the Film Developing Cookbook formula usimg EDTA and stored in full sealed glass bottles it lasted till it was used up, about 6 months IIRC.,

Rudeofus · Feb 5, 2019

Anon Ymous said:
Thanks for the info, wasn't aware of that. Now, the question I have is what effect would different quantities of Phenidone have? Obviously, too little wouldn't be enough, but is there a threshold above which there would be no difference at all? Or would there be side effects?

There is a lot of conflicting information about Phenidone vs. Dimezone-S out there, see e.g. this thread here. Recommended conversion factors are all over the place, anywhere from 1:1 all the way to 1:2. There may or may not be much of an effect from changes in Phenidone/Dimezone-S amount, and the effect may be pH dependent. So I guess a small set of test is all that's needed to find the optimum amount.

David Lyga · Feb 6, 2019

Sorry, wrong thread: meant "Rodinal exhaustion" thread. - David Lyga

PH increase when diluting developer, how can it happen?

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