p-Aminophenol Hydrochloride Solution

[Raghu Kuvempunagar]

Do aqueous solutions of p-Aminophenol Hydrochloride remain stable for long time? More generally, does a suitably acidified solution of p-Aminophenol base remain stable for long time?

 

[Donald Qualls]

In my experience, highly alkaline solutions of p-aminophenol remain stable for years, at least if they're labeled as "Rodinal" and have one of a couple different manufacturer names attached...

 

[relistan]

I don't know the answer, so just asking what comes to mind. Wouldn't oxidation likely be an issue like for other developing agents ? Or is the assumption that acidification would prevent oxidation?

 

[Rudeofus]

What's the dilution you are after? The closest thing to what you try to do here is E-6 CD-2 concentrate. It is, as the name implies, quite concentrated, and it contains a small amount of Sodium Metabisulfite to prevent trouble. It lasts for about a year in an unopened container.

 

[alanrockwood]

Do aqueous solutions of p-Aminophenol Hydrochloride remain stable for long time? More generally, does a suitably acidified solution of p-Aminophenol remain stable for long time?

Thanks for posting this question. I don't have the answer because I have the same question. However, my guess is that it would be quite stable. in acidic solution, provided that the pH is low enough, i.e. well under pH=5.48, which is the pKa of the amino group in para aminophenol.

 

[Raghu Kuvempunagar]

Thanks for posting this question. I don't have the answer because I have the same question. However, my guess is that it would be quite stable. in acidic solution, provided that the pH is low enough, i.e. well under pH=5.48, which is the pKa of the amino group in para aminophenol.

Alan, I'm using acetic acid and pH is definitely below 5.48. I hope the solution will remain stable for a few months at least; will do a test after 3 months. Interestingly, Catechol remains stable in acidified aqueous solutions when the concentration is not too low. Jay DeFehr's 25% solution of Catechol with some Metabisulphite added to it has apparently remained stable for over 9 years!!

 

[Raghu Kuvempunagar]

The closest thing to what you try to do here is E-6 CD-2 concentrate. It is, as the name implies, quite concentrated, and it contains a small amount of Sodium Metabisulfite to prevent trouble. It lasts for about a year in an unopened container.

Good to know this. Thanks for sharing this piece of information.

 

[Raghu Kuvempunagar]

I don't know the answer, so just asking what comes to mind. Wouldn't oxidation likely be an issue like for other developing agents ? Or is the assumption that acidification would prevent oxidation?

Pyrocat stock solutions are known to remain stable for reasonably long time due to the presence of Metabisulphite. Jay DeFehr's Obsidian Aqua is an even more concentrated solution of Catechol and Jay has a batch that has remained stable for over 9 years.

 

[relistan]

Pyrocat stock solutions are known to remain stable for reasonably long time due to the presence of Metabisulphite. Jay DeFehr's Obsidian Aqua is an even more concentrated solution of Catechol and Jay has a batch that has remained stable for over 9 years.

Ah I see, it's just an intent mismatch. I thought you were asking about a solution of only p-aminophenol hydrochloride and an acid, without metabisulfite (which is a weak acid, true).

 

[Raghu Kuvempunagar]

Ah I see, it's just an intent mismatch. I thought you were asking about a solution of only p-aminophenol hydrochloride and an acid, without metabisulfite (which is a weak acid, true).

Solution of p-aminophenol hydrochloride in water is itself an acidic solution. However, a solution of p-aminophenol base in water isn't and hence the acid. I've no idea whether Metabisulphite would extend the life of the solution of p-aminophenol base like it does for Catechol.

 

[Rudeofus]

Pyrocat stock solutions are known to remain stable for reasonably long time due to the presence of Metabisulphite. Jay DeFehr's Obsidian Aqua is an even more concentrated solution of Catechol and Jay has a batch that has remained stable for over 9 years.

The question appears to be: how much Metabisulfite may be in your intended solution? E-6 is a lot more limited than regular B&W developers.

 

[Raghu Kuvempunagar]

The question appears to be: how much Metabisulfite may be in your intended solution? E-6 is a lot more limited than regular B&W developers.

Good point.

 

[Alan Johnson]

https://www.flickr.com/groups/67377471@N00/discuss/72157632983815540/
Jay Defehr reports that bisulfite preserves metol so it will probably work with the chemically similar p-aminophenol.

 

[fgorga]

Here is a chemist's response...

Sorry for the scan of a hand written document... it is much faster than typing.

The p-aminophenol hydrochloride (A in the above) when dissolved in water partially dissociates to form "B". This adds protons to the solution thus lowering the pH a bit.

Conversely, when you dissolve p-aminophenol (B in the above) in water some if it "picks up" a proton to form "A". This removes protons from the solution thus increasing the pH.

The other equilibrium (between "B" and "C") does not enter into this situation significantly when adding these compounds to water in dilute solution since the pKa is quite far from the pH of water... in other words all of the phenolic groups are in the "OH" form.

With regard to pH and stability, the lower the pH the more stable the p-aminophenol solution should be. This is because the protonated amine (is in "A") and the protonated phenol (as in "A" and "B") are more stabile to oxidation than their unprotonated forms.

As for the bisulfate issue, I think that bisulfite would be effective with all phenols. Bisulfite is working indirectly to stabilize the phenolic solution by scavenging any oxygen present. Basically the bisulfite reacts with oxygen (to form sulfate) instead of the oxygen reacting with the phenol.

Hope this helps,

--- Frank

 

[Raghu Kuvempunagar]

Here is a chemist's response...


Hope this helps,

Frank, beautifully explained! Thanks so much!

 

[alanrockwood]

Here is a chemist's response...

Sorry for the scan of a hand written document... it is much faster than typing.
View attachment 275576

The p-aminophenol hydrochloride (A in the above) when dissolved in water partially dissociates to form "B". This adds protons to the solution thus lowering the pH a bit.

Conversely, when you dissolve p-aminophenol (B in the above) in water some if it "picks up" a proton to form "A". This removes protons from the solution thus increasing the pH.

The other equilibrium (between "B" and "C") does not enter into this situation significantly when adding these compounds to water in dilute solution since the pKa is quite far from the pH of water... in other words all of the phenolic groups are in the "OH" form.

With regard to pH and stability, the lower the pH the more stable the p-aminophenol solution should be. This is because the protonated amine (is in "A") and the protonated phenol (as in "A" and "B") are more stabile to oxidation than their unprotonated forms.

As for the bisulfate issue, I think that bisulfite would be effective with all phenols. Bisulfite is working indirectly to stabilize the phenolic solution by scavenging any oxygen present. Basically the bisulfite reacts with oxygen (to form sulfate) instead of the oxygen reacting with the phenol.

Hope this helps,

--- Frank

I agree, except that in the handwritten part I think it should say

RO- is more easily oxidized than ROH

where RO- indicates the negative ion cause by loss of a hydrogen ion from the phenol group. (I don't know how to write the - as a superscript in the post.)

 

[Raghu Kuvempunagar]

@alanrockwood, @Alan Johnson thanks for your inputs!

Though p-aminophenol base is only moderately soluble in water at neutral pH, it turns out that the solubility is much higher at acidic pH. I was able to dissolve 20g of p-aminophenol base using 33ml of glacial acetic acid and water to get 100ml concentrate.This gave me a 20% w/v solution of p-aminophenol base which is quite impressive IMO. Compare this with 8% w/v concentration achieved by Rodinal substitutes like Gainer's EZ Rodinal.

If this concentrate remains stable, then it can be very useful. The acid in the concentrate can be neutralized at the time of use by approximately 0.22g of Sodium Hydroxide per ml of the concentrate.

I added 2g Sodium Metabisulphite to the concentrate today and stored it full in a tightly sealed glass bottle. Will test the potency of the concentrate after three months.

 

[fgorga]

I agree, except that in the handwritten part I think it should say

RO- is more easily oxidized than ROH

where RO- indicates the negative ion cause by loss of a hydrogen ion from the phenol group. (I don't know how to write the - as a superscript in the post.)

Yes, of course! My proof reading skills have always been weak.

 

[fgorga]

@alanrockwood, @Alan Johnson thanks for your inputs!

Though p-aminophenol base is only moderately soluble in water at neutral pH, it turns out that the solubility is much higher at acidic pH. I was able to dissolve 20g of p-aminophenol base using 33ml of glacial acetic acid and water to get 100ml concentrate.This gave me a 20% w/v solution of p-aminophenol base which is quite impressive IMO. Compare this with 8% w/v concentration achieved by Rodinal substitutes like Gainer's EZ Rodinal.

If this concentrate remains stable, then it can be very useful. The acid in the concentrate can be neutralized at the time of use by approximately 0.22g of Sodium Hydroxide per ml of the concentrate.

I added 2g Sodium Metabisulphite to the concentrate today and stored it full in a tightly sealed glass bottle. Will test the potency of the concentrate after three months.

That p-aminophenol is more soluble in acidic solution than in neutral solution is expected, ionic species are generally more water soluble than neutral ones.

I would also expect the solubility in basic solution to also be higher than in neutral solution. I would also expect that if you dissolve p-aminophenol in a basic solution, the solution will be bright yellow. Phenolate anions tend to be highly colored.

 

[Raghu Kuvempunagar]

I would also expect the solubility in basic solution to also be higher than in neutral solution.

This is indeed the case in my own experience and literature. In fact, Haist writes thus on p-aminophenol: "Because of the poor stability in air, this compound is commercially available as the more stable hydrochloride salt. The salt is considerably more soluble in water than is the free base. The free base, however, is very soluble in strong alkali, such as sodium hydroxide, forming the sodium phenolate salt."

 

[Rudeofus]

To give you a point of reference: the CD-2 part in Kodak's E-6 5 liter kit contains 166.2 g/l CD-3 and 5.9 g/l Sodium Metabisulfite.

@fgorga : Sulfite does not only act as Oxygen scavenger. More importantly it acts as oxidized developer scavenger. The oxidation of (Amino)-Phenols tends to be autocatalytic, and Sulfite together with oxidized developer forms a Sulfonate and thereby removes it from the reaction.

 

[Raghu Kuvempunagar]

To give you a point of reference: the CD-2 part in Kodak's E-6 5 liter kit contains 166.2 g/l CD-3 and 5.9 g/l Sodium Metabisulfite.

Wow that's a lot! Is the solubility of CD-2 in water much lower if Metabisulphite is not added?

 

[Rudeofus]

Wow that's a lot! Is the solubility of CD-2 in water much lower if Metabisulphite is not added?

My writing was imprecise: the "CD-2" in my posting referred to "E-6 color developer concentrate part 2", which in turn comprises CD-3 "Kodak's color developing agent #3" and Sodium Metabisulfite.