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Hydrolyzing paracetamol, acetaminophen

alanrockwood

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Hi.

More questions about chemistry. In making home made Rodinal substitute one typically hydrolyzes paracetamol, also known as acetaminophen, in strong alkali such as NaOH or KOH. I am wondering if a weaker alkali would work for hydrolysis, i.e. sodium carbonate, possibly either at elevated temperature or longer reaction time. Has anyone tried using sodium carbonate in place of strong alkali? Actually, a 0.1 molar solution of sodium carbonate (10.6 g/liter) has a pH of about 11.3, so the pH of sodium carbonate is actually pretty high.

Reference for pH value: http://www.aqion.de/site/191
 
Dr Momme Andresen makes a point about Rodinal being free of Carbonate and also being the alkaline base of p-Aminophenol and containing no excess caustic alkali. Of course we know that modern Rodinal uses an excess of Hydroxide

I think it's Agfa R10 that uses p-Aminophenol Hydrochloride and Carbonate along with a second developing agent possibly Hydroquinone, my books in my darkroom.

Ian
 

Thanks for the comment. I wonder if there is some functional reason for Rodinal being free of carbonate or if it just happened to be the way they made it.

Actually though, the underlying reason for my question was to learn whether the pH of a carbonate solution would be high enough to catalyze the hydrolysis of paracetamol to para-aminophenol at a reasonable rate.

Can you tell us more about Agfa R10?

Considering the chemistry of Rodinal, from my reading about it I have come to the conclusion that the classic Rodinal was an equilibrium solution of neutral p-aminophenol and the anionic form of p-aminophenol, all in equilibrium with a small amount of undisolved neutral p-aminophenol. The chemistry would look something like this:

PAH(solid) <---> PAH(solution)
PAH(solution) <---> PAneg(solution) + Hydrogen ion

Where PAH is the neutral p-aminophenol, PAneg is the negative ion form of p-aminophenol formed by loss of a hydrogen ion from the phenol group, and <---> denotes an equilibrium reaction.

If you know the solubility of PAH, the pKa of PAH, and the total (i.e. combined) amount of all forms of p-aminophenol present, and make the assumption that the solid form is present in much smaller amount than the dissolved forms, then one can calculate the concentrations of everything.

I actually did this calculation once, and I concluded that the pH was alkaline, but not all that high. This is consistent with your comments. I also calculated that the concentrations of the two dissolved forms were of the same order of magnitude... not equal in concentration, but not too far different.

Also, with the two dissolved forms being present in amounts of the same order of magnitude, the para-aminophenol actually forms a buffered solution. This, of course, is very different from modern Rodinal, which is not buffered.

Also, I have a theory that one reason Rodinal can last so long (at least the classic Rodinal) is that if some of it gets oxidized then some of the solid p-aminophenol will dissolve to replace the oxidized fraction. Of course, this can only work as long as there is some undissolved p-aminophenol at the bottom of the bottle.

Anyway, that's my take on the chemistry of classic Rodinal, based on thermodynamic concepts. Does this chemistry sound right?
 
Agfa Rezepte 10

Part A
p-Aminophenol Hydrochloride . . . . . . . . 20g
water to . . . . . . . . 1 litre

Part B
Sodium Sulphite (anhyd) . . . . . . . . 125g
Potassium carbonate (anhyd) . . . . 120g
Water to . . . . . . 2 litres

To use 1 part A + 2 Part B Dev times approx 10-12 minutes.


This uses the cheaper p-Aminophenold Hydrochloride rather than the free base, Andresen is quite clear that Rodinal contains no Chloride which involves a step to form then filter the precipitated free base, R10 obviously doesn't keep well.

In the 1930's Agfa added a long chain wetting agent to Rodinal that has excellent anti-oxidant properties this helps with the legendary shelf life, it's covered by a German Patent that I referenced in a post about Modern Rodinal Substitutes.

The formula with Hydroquinone and Rodinal Andresen published has no number it does use Carbonate but again is 2 part, I just remembered that R10 was another p-Aminophenol developer. Mees did some research on p-Aminophenol and similardevelopers while working for Wratten and Wainwright about the same time Ilford released Certinal 1907 approx, Mees work was later used for Kodinol only made and sold by Kodak Ltd in Europe.

It looks like the issue with Carbonate, I have the Mees article somewhere but I'd don't think it'll be easy to find.

Ian
 
Theory of the Photographic Process, Mees & James, 3rd ed p292:
"Sulfite acts as a strong antioxidant on p-aminophenol and its n-methyl derivative {metol}"
I don't think it is known in detail why p-aminophenol apparently lasts so long, so that it is used in preference to Metol in high pH concentrates.
It might be related to differences in the response to sulfite or to the additive Ian mentioned (was that sodium benzene sulfonate?).
 
Developers like Rodinal depend on the fact that the hydrogen on the hydroxyl group is weakly acidic. In the presence of a strong base the paraminophenol forms what is known as a phenolate.

R-OH + KOH ---> ROK + HoH.

Alkalies such as carbonates are just no alkaline enough. Should you use a carbonate what you get is an ordinary MQ developer such as D-72 and not Rodinal. The phenolates are particularly resistant to aerial oxidation hence the long life of such developer concentrates are Rodinal and Kalogen.
 
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The pKa of 4-aminophenol (synonym for p-aminophenol and para-aminophenol) is 10.3. This implies that in a 0.1 molar carbonate solution, which has a pH of 11.3, 4-aminophenol is 90% ionized, so there is plenty of ionized form to act as developer.

At a pH of 10.3 it will be 50% ionized, which is still plenty of ionized form to act as a developer. It should be 50% active at this pH compared to maximum activity. (This statement may be an oversimplification if other factors besides the concentration of ionized form influence activity, but it should be at least qualitatively right.)

At a pH of 9.3 it will be 10% ionized, so there should be some activity, even at this pH.

As an interesting coincidence, the bicarbonate/carbonate system also has a pKa of 10.3, the same as 4-aminophenol, so bicarbonate/carbonate buffer systems should put the pH right in the range where 4-aminophenol is making the transition between little activity and nearly full activity, assuming of course that the concentration of the ionized form is what determines activity.

In any case, as far as pH calculations are concerned the fact that the two systems have the same pKa represents a simplification because carbonate concentration can be lumped together with the ionized form of 4-aminophenol and the bicarb concentration can be lumped together with the unionized form of 4-aminophenol for purposes of pH calculations.

Here's an interesting question: Is p-aminophenol superadditive with itself? This sound like an oxymoron, but hear me out on this. Let us assume that, as far as developer activity is concerned, unionized p-aminophenol would act like a superadditive molecule with respect to ionized p-aminophenol. In that case the residual developer activity of the system at low pH might be higher than expected if one considers only the concentration of the nominally active ionized form.

By the way, if there are any thermodynamic purists out there, another interesting complication comes from the concept of thermodynamic activity of components in non-ideal solutions. Solutions containing ions are highly non-ideal, even for solutions that contain only a small concentration of ions, and the non-ideality can be strongly dependent on the ionic strength. This means that equilibrium calculations must take into account these non-ideal effects, usually through something called an activity coefficient. I don't know if the quoted pKa of various species includes this correction or not. I suspect that it does not, in which case the various equilibrium calculations are going to be only qualitatively correct. This may account for the fact that, even though the bicarbonate/carbonate system has a pKa of 10.3, which should imply that an equimolar bicarbonate/carbonate buffer should have a pH of 10.3, the actual pH of equimolar bicarbonate/carbonate buffers is about 1 pH unit lower than this.
 
As far as this reaction is concerned, Metol would do the exact same, yet nobody rants and raves about long shelf life of D-23.

I think the secret behind Rodinal is the fact that it is made from components which can be dissolved in very high concentration, and that such highly concentrated solutions are generally less prone to aerial oxidation. Oxygen does not dissolve well in pure water, and much much worse in brine solution.

@alanrockwood : One of Rodinal's pedigrees is sharpness of results, and the poor buffering of hydroxides even at high pH may be one of the reasons. 0.1M Carbonate might match pH, but could still give you very different negatives.
 
There was another developer "Concentrated MQ" made from caustic soda , potassium metabisulfite, metol and hydroquinone described in the Dictionary of Photography ,ed FJ Mortimer 16th ed c1940 p210. But since neither this nor Kalogen ever caught on, the most likely explanation is that Rodinal is longer lasting. I don't think the reason for this has been fully explained and it remains a mystery.
 
I have read in some older article, that concentrated MQ developers are inherently unstable, since Metol and HQ tend to form some insoluble adduct. The Metolal developer concentrate described by Pat Gainer is not very concentrated - and you wrote yourself in that thread, that its shelf life was less than stellar.
 
Here is a calculation relevant to para-aminophenol.

Add 33 grams of para-aminophenol to 1 liter of water.

Titrate with NaOH or KOH until nearly all has dissolved. (Assume the undissolved fraction is about 3 grams).

This means that 30 grams are dissolved in some combination of neutral para-aminophenol and ionized para-aminophenol. That corresponds to 0.275 moles.

The solubility of neutral p-aminophenol is 15 grams/liter. Since there is undissolved p-aminophenol on the bottom of the bottle the solution is saturated in p-aminophenol, so the solution contains 15 grams of neutral para-aminophenol.

The amount of ionized para-aminophenol is 30-15 grams, so the concentration of ionized para-aminophenol is 15 grams/liter.

Converting to moles per liter, we have 0.138 moles/liter of the unionized para-aminophenol and the same concentration of ionized para-aminophenol.

Since para-aminophenol is a weak acid, and the solution contains the acid and the conjugate base then what we have is a buffer. The pKa of para-aminophenol is 10.3, so a pH calcuation on this buffer that is equimolar in the acid and conjugate base using the Henderson-Hasselbalch equation, the pH is the solution is predicted to be 10.3.

On the other hand, if we started with 23 grams per liter and left 3 grams undissolved we would end up with 15 grams/liter of neutral para-aminophenol and 5 grams/liter of ionized para-aminophenol. Applying the Henderson-Hasselbalch equation one predicts a pH of 9.8.

If we start with 63 grams and leave 3 grams undissolved we end up with 15 grams/liter of neutral para-aminophenol and 45 grams/liter of ionized para-aminophenol. We predict a pH of 10.8.

Obviously, this is not relevant to modern Rodinal, which contains an excess of strong alkali, but I gather that original Rodinal did not contain an excess of strong alkali, so calculations of this sort may be relevant to formulations similar to original Rodinal.
 


I have a PET bootle of Kalogen that is going on its twelfth year and shows no sign of age.
 
@Alan Johnson : the source was L. F. A. Mason, The Journal of Photographic Science, 1965, vol 13, p 205ff:

@alanrockwood : if you follow the Rodinal formula published by Anchell & Troop, you add 100 g p-Aminophenol Hydrochloride, 300 g Potassium Metabisulfite and about 150 g Sodium Hydroxide to about 1.3 l water. I dare say without doing any mole calculations, that this is far more concentrated than Gainer's Metolal soup.

This page claims, that Metol dissolves in water up to 50 g/l. Since it dissolves particularly slowly in in alkaline liquids, that appears to be the general upper practical limit for Metol. This article compares Oxygen solubility in water and brines at different temperatures and pressures. It is obvious that highly concentrated developers can expect much better shelf life than more dilute ones, regardless of which developing agent is used.
 
Mason, Photographic Processing Chemistry p76 notes to the effect that the first oxidation products of p-aminophenol (Rodinal) and metol (called semiquinoneimines) have very similar half-wave potentials at pH 10.
From this I infer that the first oxidation products are about equally stable so this is not the explanation for the longer life (if true) of Rodinal.
 

It would also explain why my control bottle of Kalogen is still good at 12 years and shows no sign of darkening from its original straw color.

I might also point out that other phenols like hydroquine also form phenolates.
 
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