I think the precipitation occurs when the metol base is formed, when just enough hydroxide is added to form the nearly insoluble base, but not yet enough to form the soluble salt, just as happens when we make Rodinal. If we add the total calculated amount of hydroxide, the change from soluble to insoluble and back to soluble occurs continuously and so is not noticed.
If you see how much coast a bottle of Rodinal, I have some problems to find out my interest to do it by myself.
The development of my films takes me already enough time.
But it is nice to know that we can produce the Rodinal by ourself the day I won't find it any more on the market.
I just tested my Metolal stored in a 1/3 full sealed glass bottle for 28 months.
It seems not to work anymore,blank film.I tried adding more sodium hydroxide but this did not revive it, failure was not due to atmospheric CO2 reacting with hydroxide in the original mix.
I wonder if the sulfite content of the concentrate could be increased.
(1) Crawley (BJP Jan 6 1961) found sodium sulfite at 5g/L in the working solution is best with Metol as developing agent,it removes the oxidation products.
(2) More sulfite might be a better preservative if oxidation was the cause of the failure.
To get 5g/L at 1:50 dilution means 250g/L sulfite in the concentrate.
This is close to the solubility I looked up 270g/L.
I wonder if there is any opinion on increasing the sodium sulfite in the concentrate.
The opening words give the game away.
A 1/3 full bottle has enough oxygen present to allow the Metol to oxidise over that period of time.
Rodinal & R09 use slightly more Sulphite than Crawley suggest but as the more souble Potassium form
It seems quite likely the shelf life of this developer could be increased by keeping it in two parts,a lower pH one with metol and sulfite and one with sodium hydroxide.
It would end up something like FX-1 or Beutler with a hydroxide part B,something not tried very much.
Regarding oxidation: When I did my own processing... long ago... I used glass marbles to take up volume thereby displacing air space.
Who is afraid of the Rodinal Wolf...
To my failed Metolal I added enough metol to make the original concentration 40g/L.The metol appears slightly acidic and I had to add some sodium hydroxide to the 1:50 working solution to increase the pH from ~9 to ~11.
The metol addition restored the developing activity of the failed Metolal and I successfully developed a test film in the working solution at pH 11.
Thus,in the failed Metolal,it was the metol that failed not the sulfite or hydroxide.In Rodinal, p-aminophenol would not be expected to fail so rapidly IMO.
Only one experiment but it does suggest that metol undergoes some oxidation or hydrolysis to which p-aminophenol is not susceptible.
We are touching on this in the Modern alternatives to Rodinal thread.
There are good reasons why Afgfa, Ilford Kodak, Johnsons etc went down the p-Aminophenol route. Most of the early research is probably lost unfortunately.
Metol MW is 344.38, MW of NaOH is 40, so 40g of Metol = 0.114M and we need 4*0.116*40 = 18.584g of NaOH for create Metol.base.Na2 salt ((C7H10NO)2.Na2). now we can rewrite Metolal formula as:
Originally Posted by gainer
Sodium Sulfite 85g
Metol 40g + NaOH 18.584g = 34.18g of Metol.base.Na2 salt
i.e. we have 19.4g of free NaOH in concentrate!
I think that this formula is very different from EZRodinal where concentrate haven't any free of NaOH. When we dilute Metotal 1:50 we have:
Sodium Sulfite 1.7g
Metol.base.Na2 salt 0.68g
I know only one receipt like this. Here is the link (russian) and here is translation to english. receipt is simple:
20ml of D23 stock
12ml of 20% solution of NaOH
Water to 300ml
recalculate to 1l of all components:
Sodium Sulfite 6g
this is equivalent to
Sodium Sulfite 6g
Metol 0.45g + NaOH 0.2g = 0.385g of Metol.base.Na2 salt