P-Aminophenol HCL won't dissolve

The other day I attempted to make a liter of homemade Rodinal. I followed the formula by first adding 150g of sodium sulfite to 700ml water at 52C. I added 20g of p-aminophenol hcl from Photographers Formulary next, and the solution turned olive green and the crystals continued to stir without dissolving. I let it sit and stir for a few hours at the same temperature with seemingly no change. The only thing I can think of is the p-aminophenol is a few years old and maybe it went off.

I decided to give it another try by using p-aminophenol instead. I'd used up all my p-aminophenol hcl in the previous attempt. The solubility is listed as around 1.5g to 100ml, and 10g is listed as the equivalent to 17g of the hcl version. So I tried it with 800ml of water and 11.76g of p-aminophenol. The initial amount dissolved, but as I added more it refused to dissolve. I eventually upped the temperature to 65C and let it stir for several hours. The particle sized decreased, but as soon as I lowed the temperature, large fluffy crystals started falling out of solution. After it cooled to room temperature, it looked like almost nothing dissolved. This p-aminophenol is a bit older. It was slightly pink when I first got from Formulary. Now it's light tan/pink.

Any ideas?

p-Aminophenol is very poorly soluble in water. The same is probably true for this salt as well. With the addition of a hydroxide, it will dissolve completely, and as the pH increases you’ll obtain aminophenolate (your goal) and table salt…

Tumbles said:

I'd used up all my p-aminophenol hcl in the previous attempt.

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A suggestion from a fellow DIY-mixer of stuff: when trying a new formula, mix a small amount - e.g. 50ml, see how it goes and test the outcome. Then scale up if desired.

If you have not thrown the mixture away, you can add the alkali and complete the process (and use the developer).

lamerko said:

p-Aminophenol is very poorly soluble in water. The same is probably true for this salt as well. With the addition of a hydroxide, it will dissolve completely, and as the pH increases you’ll obtain aminophenolate (your goal) and table salt…

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In the Darkroom Cookbook, Steve Anchell says p-Aminophenol HCL dissolves readily in cold water - I have not tried it and don’t know if the age of the chemical may reduce that quality

The solubility of p-aminophenol is highly dependent on the pH and thus will depend on what other components are in the solution. In other words, the solubility in water value is of limited use in predicting how soluble the compound will be in any particular solution.

This compound can exist in three different ionic states as shown here:


The neutral form (middle) is least soluble. The ionic forms, the hydrochloride (left) and the phenolate (right) are much more soluble.

The pKa values indicate the pH at which there is a 50:50 mix of the two forms on either side of the equilibrium.

As a broad rule of thumb, I'd say this compound will show relatively low solubility between pH values between 6 and 8 and that the solubility will increase on either side of that range.

As a photographic developer, the anionic form (on the right above) is the active species. Thus, as others have pointed out, raising the pH by adding some sodium hydroxide will result in the compound going into solution.

farpointer said:

In the Darkroom Cookbook, Steve Anchell says p-Aminophenol HCL dissolves readily in cold water - I have not tried it and don’t know if the age of the chemical may reduce that quality

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But you’ve already added 150 grams of sodium sulfite to 0.7 liters of water, which means you’re almost at its solubility limit. Under those conditions, I’m not sure how much p-aminophenol HCl you can dissolve. However, the fact that precipitation starts immediately when the temperature drops is a clear sign that you’ve exceeded it. So just add the hydroxide, and everything should fall into place…