Developer concentration and PH

MMfoto · Feb 2, 2006

If you take a two part stock developer and equally increase or decrease both parts in relation to working volume aren't you both changing developing agent concentration and developer activity?

For example, take a two part stock developer like Pyrocat HD that you might normally use 1:1:100. Now let's say you decide to reduce developer concentration by one half. Instead of diluting further to 1:1:200, why not use it 1:2:200? Doesn't the PH of the working solution, and developer activity, depend more so on the ratio of accelerant to overall volume, than on the ratio of accelerant to developer? If you have a target PH for any given developing agent, wouldn't it would make more sense to change developer concentration by changing the ratio of A:B+water, instead of A+B:water?

sanking · Feb 2, 2006

It is called buffering capacity. The alkalii (potassium carbonate) of Part B of the Pyrocat-HD Sock serves to keep the pH at about the same value over a fairly wide range of dilutions between 1:1:50 and 1:1:200.

Develpers that are not well-buffered show a significant change in pH with dilution.

Sandy

MMfoto said:
If you take a two part stock developer and equally increase or decrease both parts in relation to working volume aren't you both changing developing agent concentration and developer activity?

For example, take a two part stock developer like Pyrocat HD that you might normally use 1:1:100. Now let's say you decide to reduce developer concentration by one half. Instead of diluting further to 1:1:200, why not use it 1:2:200? Doesn't the PH of the working solution, and developer activity, depend more so on the ratio of accelerant to overall volume, than on the ratio of accelerant to developer? If you have a target PH for any given developing agent, wouldn't it would make more sense to change developer concentration by changing the ratio of A:B+water, instead of A+B:water?

MMfoto · Feb 2, 2006

OK, I understand. Thanks Sandy.

I looked up "buffering" in the Darkroom Cookbook: "A buffer acts to keep the PH of the solution fairly constant. A buffered developer will maintain a constant PH whether it is diluted 1:1 or 1:3." Anchell continues: "Carbonate and Balanced Alkali are the most stable alkalies. Borax does not buffer well against a rise in PH, and hydroxide loses it's alkalinity rapidly as the solution is diluted."

"...hydroxide loses it's alkalinity rapidly as the solution is diluted" Is it safe to assume this explains the loss of film speed with high dilutions of Rodinal?

Developer concentration and PH

MMfoto

sanking

MMfoto

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