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ph threshold of p-aminophenol

Jarin Blaschke

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Hi guys:

I'm still messing around with a replenishable motion picture developer, at least until I run out of time. Sadly, catechol was shot down by the lab. Being forced into a different look, I think I may have struck upon something nice, sharp and pleasantly grain-forward with p-aminophenol and carbonate. However, I may also want to test it as a buffered version with metaborate. Does this developing agent still work at the lower ph?

Thanks!

Jarin
 
Somewhere I have some additional information on the pH threshold of p-aminophenol. I will try to find it, but my vague recollection is that it is somewhere around pH 9.3 or so.

I think the threshold is not a hard on/off transition between inactive and active but more of a somewhat gradual transition.
 
Once I made a version od D76H with the metol replaced by p-aminophenol , pH ~ 8.6
https://unblinkingeye.com/Articles/Developers/Formulas/formulas.html -formula of D76H.
It did work but it took more than twice as long to develop the film.
Appears that you would need to slow down the rate of film feed into your continuous machine to about half to get the same development density?
If so , you might be better off with a metol carbonate sulfite developer suitably adjusted to give the same development time as the developer normally used in the continuous machine.
https://www.photrio.com/forum/threads/bjp-metol-carbonate-film-developer-any-experiences.106701/
 
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The chart posted by Alan suggests, that p-Aminophenol needs about 2 pH higher than Metol to have about the same activity. Since D-76 operates somewhere around pH 8.5, you'd have to set your p-Aminophenol version to pH 10.5, which is doable with Carbonate.
 
Thank you, great find!

What kind of ph drop can I expect in a switch from carbonate to metaborate?

Here’s what I tried last night in a hand tank:

P-aminophenol : 1g
Sodium Sulfite: 5g
Potassium Carbonate: 7.5g

The film was Double X. I’ve so far only observed it through a loupe, but grain seems prominent (good for us), good sharpness and and really good film speed. At only 5:45 at 68 degrees (constant agitation), the film is still somewhat overdeveloped, so I can afford a little drop in activity.

For consistency with replenishment, I’m considering the accelarator switch, as well as starting off the brew with a tiny bit of KBr.

J
 
Raghu Kuvempunagar said:
Using bicarbonate with carbonate should give you a buffer system whose pH can be set to any value between 9-11 by varying the ratio appropriately.
Click to expand...

Don't try to alter the pH of Rodinal working solutions. At a lower pH the activity will be but a fraction of the normal. The working solution should have a pH ~12 due to the presence of the phenolate.
 
Gerald C Koch said:
Don't try to alter the pH of Rodinal working solutions. At a lower pH the activity will be but a fraction of the normal. The working solution should have a pH ~12 due to the presence of the phenolate.
Click to expand...
I measured a pHof 9.7 for Rodinal 1+25 tap water.
 
I've been experimenting lately with Sodium Sulfite added to Rodinal. The Sulfite increases activity, or perhaps keeps the P-Aminophenol from oxidizing, either way developing times are significantly shorter with the Sulfite added. I don't see any reason why Rodinal with Sulfite can't be used as a replenished developer. With your application it might be just the thing you need. Tailor the dilution to the time/temp your lab uses and they won't need to change anything. Pretty cheap too...

And I almost forgot, the Sulfite increases film speed which is a nice little bonus. I've been using a 45% solution, so 45g Sulfite to one liter of water mixed with the Rodinal concentrate.

And I almost forgot again... I have been using it with 5222 which is what I shoot these days.

Hope that helps.
 
A bit of info that could be useful: both para aminophenol and bicarbonate have a pKa of 10.3, so as far as estimating pH is concerned you can lump the bicarbonate ion and neutral para aminophenol together and you can lump the negative ion of para aminophenol together with carbonate ion.

Also, a carbonate/bicarbonate buffer would put you right in the range where you can vary the negative para aminophenol ion concentration from being a fairly small percentage of the total para aminophenol to a fairly large percentage of the total para aminophenol. This should give you the opportunity to vary the activity of the developer over quite a large range.
 

Paraminophenol is a nonionic substance it does not producepH. Now the ion of the its phernolate is quite basic. The pH of the Rodinal concentrate is 12. Working solutions will be a bit less depending on dilution.

https://www.freestylephoto.biz/pdf/msds/agfa/Agfa_Rodinal.pdf

Measuring the pH of concentrated or high pH solutions is rather difficult and beyond the capibilitiy of cheap pH meters. You really need lab grade equipment.

To the OP. The lab will probably not be happy with a paramphenol based developer as the chemical is toxic to fish and does not meet EPA regulations
 
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At a lower pH as produced by carbonate such a developer woupld behave similarly to one made witrh metol. The two developing agents behave very similarly in this case.
 
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Gerald, let me provide an expanded discussion of the acid/base properties of Para aminophenol. It is actually an amphoteric compound, meaning that it can act as an acid under certain conditions and as a base under other conditions.

The amino group of the molecule is basic with a pKa of 5.48. The phenol end of the molecule is acidic with a pKa of 10.30 (listed as 10.46 in some references).

If the pH is much less than 5.48 it will exist primarily in a protonated (positive ion) form. at a pH around 5.48 both protonated and neutral forms are present in roughly comparable amounts. At a pH much higher than 5.48 but well below 10.3 it will exist primarily as the neutral molecule. At a pH around 10.3 it will exist in both neutral form and deprotonated (negative ion) form in roughly comparable amounts. At a pH much higher than 10.3 it will exist primarily in the deprotonated form.

In solid form it can exist as positive ion (e.g. in para aminophenol hydrochloride), neutral molecule (para aminophenol), or negative ion (e.g. ion sodium para aminophenolate).

The pH range of a pH meter is a more complex topic. Some cheap pH meters claim a range of pH 0-14 with fair accuracy (+/- 0.1 pH unit). I don't know how good those claims are. However, alkaline solutions do tend to etch glass, which can potentially damage meters that use glass electrode technology to measure pH.
 
Gerald C Koch said:
At a lower pH as produced by carbonate such a developer woupld behave similarly to one made witrh metol. The two developing agents behave very similarly in this case.
Click to expand...

That is also my understanding of the comparison between the two, with the caveat that at the same pH and same molar concentration para aminophenol is somewhat less active as a developer than metol.
 
alanrockwood said:
That is also my understanding of the comparison between the two, with the caveat that at the same pH and same molar concentration para aminophenol is somewhat less active as a developer than metol.
Click to expand...

True but the difference is not that great.
 

With Rodinal we are talking about the phenolate. An entirely difference type of animal.
 
Gerald C Koch said:
With Rodinal we are talking about the phenolate. An entirely difference type of animal.
Click to expand...
Hi Gerald,

The three forms of para aminophenol in solution are determined by simple acid base chemistry. Phenolate is simply the phenol-type molecule (para aminophenol specifically) which has lost an proton, leaving behind the phenolate anion. This is what phenols like to do in strongly basic solutions.

I don't really want to get into an argument, but just to explain the chemistry. I am a PhD chemist, and I know acid/base chemistry very well. In fact, I just finished teaching a chapter that included acid/base chemistry in the freshman chemistry class I am currently teaching at a local junior college.

I will try to dig up some structures to post to illustrate the points, but I have some things I need to take care of first, so I will have to do it later.
 
Gerald C Koch said:
True but the difference is not that great.
Click to expand...
Yes, I believe that it correct. The para aminophenol also produces somewhat less fog under similar conditions, probably as a result of it being a less active developer.
 

In the interim, before I can find good images of the chemical structures of the three forms, here are the chemical formulas for the three forms: C6H8NO+, C6H7NO, and C6H6NO-

The first formula represents the primary form in strongly acidic conditions, the last is the primary form under strongly basic conditions. the middle is the primary form under conditions that are neither strongly acidic nor strongly basic.

As an aside, as I noted above, this molecule is amphoteric, meaning it has both acidic and basic properties. The amino group in this compound is a weak base, with a pKa of 5.48 for the protonated form of the base, and the phenol group is a weak acid, with a pKa of 10.3. However, the phenol group is actually a very weak acid, so the compound tends to have stronger basic than acidic properties. Nevetheless, if the solution pH is high enough (much above 10.3) the phenol group can exist primarily in the de-protonated form, and the overall molecule (strictly speaking and ionic molecule under these conditions) has a negative charge.
 
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