Phenidone, glycol, alcohol, water and all that

tykos · Wednesday at 8:32 AM

I wrote here somewhere that i had some troubles with phenidone and pyrocat in glycol: i have quite a lot of things falling out of solution while everyone is ok, so maybe it's my fault.
I bought new phenidone from a more trusted source, i'll buy new glycol, i'll try again.

But, first of all, i'd like to have a few opinions about phenidone's keeping solutions, there seem to be quite a lot of theories out there, let's put all of it together.
Situation: i need to make a 1% phenidone solution.
- can i dilute it in propylene glycol, avoid it going bad on me for, let's say, 1-1.5 years?
- can i dilute it in isopropyl alcohol? i've seen someone say it will last less than with glycol, is it true?
- can i dilute it with water? looking at some old sandy king's posts i think it'll need a few drops of alcohol to dissolve, and i think it will last for a short amount of time.

last:
- if i dilute it in alcohol, can i use it to mix a glycol developer (e.g.: pyrocat)? will it make a mess or will it be fine?
- the other way around doesn't make a lot of sense, so we won't do that
- alcohol or glycol will be both fine to mix a water developer (like any paper developer)? (i suppose so, because even pyrocat is to be diluted with water, but you never know)

pentaxuser · Wednesday at 11:37 AM

It will be interesting to see if anyone has conducted an experiment to see which will last the longest, an alcohol mix or a glycol mix or can point to a known and reliable source for the conduct of such an experiment

pentaxuser

koraks · Wednesday at 11:46 AM

tykos said:
- can i dilute it in propylene glycol, avoid it going bad on me for, let's say, 1-1.5 years?

Yes, 1% phenidone in glycol will be stable over at least 1.5 years, usually more.

tykos said:
- can i dilute it in isopropyl alcohol? i've seen someone say it will last less than with glycol, is it true?

Yes, this is true; a stock solution of phenidone in IPA or plain ethanol tends to be less stable. This likely has something to do with the water that's inevitably present in the mix.

tykos said:
- can i dilute it with water?

Yes, but it will have a short lifetime.

tykos said:
- if i dilute it in alcohol, can i use it to mix a glycol developer (e.g.: pyrocat)? will it make a mess or will it be fine?

You could, but I don't see any sense in doing this as pyrocat is a fairly concentrated developer to begin with, so just dissolve the phenidone in the glycol directly when making the pyrocat concentrate. Moreover, if you have glycol for a glycol-based phenidone, why on earth wouldn't you just use the same glycol for your phenidone stock solution (which you could use for other developers)?

tykos said:
- alcohol or glycol will be both fine to mix a water developer (like any paper developer)?

Yes.

pentaxuser said:
It will be interesting to see if anyone has conducted an experiment to see which will last the longest, an alcohol mix or a glycol mix

'Mix' is a confusing term here. Let's just call it a solution of phenidone in a solvent, with the solvent being water, glycol or an alcohol (ethanol, isopropyl alcohol etc.). Many of us have made such concentrates and observed the keeping differences. The general tendency in the fairly common threads discussing this is that the glycol solution is stable, the alcohol solution a little less so and the water solution is not very stable in the long term.

tykos · Wednesday at 12:33 PM

Ok, general guidelines are more or less as I thought, good.

koraks said:
You could, but I don't see any sense in doing this as pyrocat is a fairly concentrated developer to begin with, so just dissolve the phenidone in the glycol directly when making the pyrocat concentrate.

I don’t use a ton of developer, so I’m going to mix 100ml of pyrocat to begin with.
That calls for 0,2g of phenidone: my scale is not an high end lab scale, so it’s more convenient to have that 1% solution.
Alcohol is often recommended so a “what if” question came to mind, especially given my past experiences. I’m aware it’s almost a nonsense.
I’ll try glycol again and see what happens.

koraks · Wednesday at 4:51 PM

Sounds good, and yes, your original understanding is really fine.
Note that these days, we can get very cheap digital scales that are sufficiently accurate even for as small an amount as 200mg; you could consider getting one of them. Like you, I generally mix small batches of developer concentrate. The amount you mention is quite typical in my practice as well.

Alan9940 · Wednesday at 5:30 PM

I've mixed phenidone in both isopropyl alcohol and glycol, and it's fine in either but I prefer the glycol because it lasts longer.

As for measuring small amounts of phenidone, I've found my cheap electronic scale to be acceptably accurate, if I apply a little weight on the pan along with the chemical being measured.

tykos · Thursday at 4:20 AM

koraks said:
Sounds good, and yes, your original understanding is really fine.
Note that these days, we can get very cheap digital scales that are sufficiently accurate even for as small an amount as 200mg; you could consider getting one of them. Like you, I generally mix small batches of developer concentrate. The amount you mention is quite typical in my practice as well.

i have a 10€ digital scale, 50gr x 0.01gr precision: Theoretically ok, in reality sometimes it behaves a little erratically so i'd prefer being on a safer side.
I'll try some different measures to check the consistency.

relistan · 2025-09-12T13:05:03-0400

Oxygen is almost insoluble in propylene glycol, whereas it's quite soluble in the other solvents you mentioned. Propylene glycol also tends not to contain other metallic agents that can lead to the breakdown of developing agents when put into aqueous solution. There is no comparison in terms of which solvent will give better longevity.

Developing agents at the air/glycol interface _will_ oxidize, however, so you want to be sure to top off your bottle of phenidone in popylene glycol with butane (e.g. from a lighter) or other gas that will be effectively more inert.

If you are having trouble getting phenidone to dissolve, you probably haven't heated the solution enough. That said, don't overheat it. 60-80C tends to be the right range.

Hope it helps

tykos · 2025-09-12T13:16:22-0400

I didn’t have any troubles dissolving it, but after a few days (at room temperature) I get some deposit that dissolves when reheated.

Patrick Robert James · 2025-09-12T13:50:22-0400

I use 91% isopropyl alcohol, put it in glass and top off with gas. I don't recall ever having a problem.

pentaxuser · 2025-09-12T15:50:29-0400

tykos said:
I didn’t have any troubles dissolving it, but after a few days (at room temperature) I get some deposit that dissolves when reheated.

I had always thought that once dissolved in glycol there isn't normally any deposit at room temperature? If there is this kind of problem then that suggests that storage at more than room temperature is advisable

When I have seen videos on mixing phenidone with glycol no-one I can recall warns that deposits can occur at room temperature

What was the room temperature?

pentaxuser

john_s · 2025-09-12T18:35:27-0400

relistan said:
........

If you are having trouble getting phenidone to dissolve, you probably haven't heated the solution enough. That said, don't overheat it. 60-80C tends to be the right range.

Hope it helps

I have found that a lower temperature works but it takes longer (40degC).

If it comes out of solution afterwards would that not mean that there was too much phenidone? I think I read that at room temperature the solubility limit is 3g/L, but depending on storage temperature that might be at its limit.

pentaxuser · 2025-09-12T18:43:03-0400

john_s said:
I have found that a lower temperature works but it takes longer (40degC).

If it comes out of solution afterwards would that not mean that there was too much phenidone? I think I read that at room temperature the solubility limit is 3g/L, but depending on storage temperature that might be at its limit.

However it doesn't seem from what tykos said that he needs anything like 3g/L so I am still puzzled

pentaxuser

relistan · 2025-09-13T02:05:54-0400

The only thing that I can see happening is that it didn’t dissolve. It went into suspension. The liquid should be clear if it is dissolved. The only other idea I have is If you were near solubility limit and you stored it somewhere cold then it could have dissolved and come out of solution. But otherwise, it never dissolved I’d be pretty sure. If it’s propylene glycol.

Vetus · 2025-09-13T02:12:42-0400

Is there any way of testing if phenidone has lost any of its potency and if it has can using an increased amount compensate?

koraks · 2025-09-13T04:17:12-0400

Vetus said:
Is there any way of testing if phenidone has lost any of its potency and if it has can using an increased amount compensate?

You could rig up a test where you use the phenidone in a developer and use that to develop a control strip. Compare the density of the developed control strip to measurements made with known-good phenidone solution. This is probably the most practical for a home darkroom user - and as you can gather, it's already a bit of a chore.

So instead I personally just rely on visual assessment of the color (the solution shouldn't be dark) and the knowledge that at least in glycol (which is cheap and easy to obtain) the solution will be stable for a long time. Combine that with a habit of not making more than I can use up in 1-2 years and you've got a pretty dependable process that doesn't involve any testing.

Phenidone, glycol, alcohol, water and all that

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Phenidone, glycol, alcohol, water and all that

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