PMK - how does it work?

[Kimberly Anderson]

I've been using PMK for about six years now and love using it with just about every film I've shot in that time. As I was mixing up a fresh batch tonight before processing my 12x20's from today, I was wondering how it works. I mean, I know it works WELL, I'm just wondering how exactly does it *work*. I am using a double-strength mixture of PMK for my negs that I am making salted paper prints with right now. The film looks AMAZING and I'm thinking of standardizing on that dilution instead of the 1:2:100. Apparently Rollo Pyro is essentially double-strength PMK?

I am not a chemist, but I'm trying to understand more about the chemistry of photography. I had to hunt for some Sodium Bisulfate tonight even though have about 50 lbs of Sodium Sulfite. I did a little digging and understand somewhat the difference between those two and did not substitute the SB with SS.

Anyway, as I was mixing I was just wondering how it all works and when you mix the A+B, what is it about the Kodalk that kicks the A mixture into action.

So, just curious if anyone can give a PMK chemistry 101 primer.

[Andrew O'Neill]

Do you have Gordon Hutchings' book, The Book of Pyro? He created PMK. You can get them cheap on Amazon.

[Kimberly Anderson]

I have that book as well as The Darkroom Cookbook.

[Doremus Scudder]

Almost all developers have:

Developing agent(s) (PMK has Metol and Pyrogallol)
Preservative (usually sodium sulfite; to scavenge oxygen and prevent oxidation)
Restrainer (potassium bromide, to prevent development of unexposed parts of the neg and thus prevent fogging)
Activator (an alkaline, since most developing agents only work in an alkaline environment. This is the Kodalk in your PMK).

That's basically it. A developer works by chemically changing silver halides in your film that have been exposed to light into metallic silver.

Read the PMK book and the opening chapters of the Darkroom Cookbook.

Doremus

[Kimberly Anderson]

Reading today.... Thanks Doremus.

[bob carnie]

I too use double strength PMK for certain applications

[Michael R]

Kimberly - make sure you don't confuse sulfite with sulfate.

Here is a quick "101" summary of the basic operation of PMK:

How it works
It contains two developing agents - (1) pyrogallol and (2) metol. On a weight/weight basis there is 1/10th the amount of metol compared with pyrogallol. Metol is a more "active" developing agent than pyrogallol, and the two agents form what is called a superadditive combination. In this particular case, metol likely does most of the direct chemical development (reduction of silver halide to metallic silver). As development proceeds, metol is oxidized as it develops silver. The role of pyrogallol is primarily to regenerate the metol so that it can keep doing its job.

Pyrogallol is oxidized by regenerating the metol. The oxidation products of pyrogallol are special in that they can both tan (harden) the gelatin in the film emulsion, and dye the emulsion where development is taking place. This dye adds extra optical density to the negative, proportional to exposure and development. In other words, the total optical density in any area of the negative is that of the metallic silver plus dye. The dye is commonly referred to as "stain" - hence the term "staining developer". A prerequisite for this type of imagewise staining is a very low level of preservative (sulfite) in the developer. If too much preservative is present, it will prevent the oxidation products of pyrogallol from tanning/staining. However a small amount of sulfite can improve certain working characteristics and stability.

Nearly all developing agents require an alkaline pH (pH is a very important concept in photochemistry) to be active. In PMK, "Kodalk" (sodium metaborate) provides the alkalinity. However the alkaline environment also accelerates the oxidation of developing agents (in particular pyrogallol, which is unstable at an alkaline pH), which is why there are two separate stock/concentrate solutions for developers such as PMK.

PMK solution A
Contains the metol and pyrogallol and sodium bisulfite. All three of these are acidic compounds in water. This means the pH of solution A is mildly acidic, which helps keep the developing agents (particularly pyrogallol) from oxidizing too quickly with storage.

PMK solution B
The sodium metaborate alkali. When mixed with solution A the total solution pH becomes alkaline enough for the developing agents to work. At the same time, once the solution becomes alkaline the sodium bisulfite from solution A provides the sulfite ion in the combined working solution.

[Kimberly Anderson]

Michael R....now *that* is what I was looking for. Thank you.

BTW, here's what I used. I may have typed it wrong, but I believe the contents of the metal box are correct.

[Taija71A]

... Apparently Rollo Pyro is essentially double-strength PMK?

__

Quick Answer:

No.

The following thread... May perhaps be of further interest to you:
(I know... More reading...)

http://www.largeformatphotography.in...perience/page2

Regards,

-Tim.
_________

[Kimberly Anderson]

Sweet. Pyro-nerds abound. Thanks Tim.