Bleach equations

[sfaber17]

I was trying to look up those ionic equations posted one time on the bleaching process but couldn't find them. Basically I'm trying to figure out the pH changes with bleaching and regeneration via air bubbling. I believe bleaching would be Ag(0) + NH4(+) [Fe(+3)PDTA(-4)]-1 + NH4(+) + Br(-) --> AgBr + (NH4(+))2 [Fe(+2)PDTA(-4)]-2 so no pH change, but uses up the Bromide. Then O2 regeneration:

2((NH4(+)2 [Fe(+2)PDTA]-2) + 1/2 O2 + 2 H+ + 2 Ac- ----->
2(NH4(+) [Fe(+3)PDTA(-4)]-1) + 2 NH4+ 2Ac- + H2O

is something like the above and would use up the acetic acid leaving ammonium acetate and making the solution more basic - higher pH. So even if you have no developer carryover and use a stop bath and wash, the bleach pH would increase with use. Does it look right?

 

[Rudeofus]

AFAIK the relevant reactions are something like this:

[FeEDTA]- + Ag + Br- <===> [FeEDTA]-- + AgBr​

The NH4+ counter cation to the Ferric/Ferrous EDTA complex maintains high solubility of the salt, and with its mild solvent action somewhat accelerates bleaching. At the pH most Ferric EDTA bleaches operate, we can assume that the NH4+ counter cation is mostly dissociated.

T.H. James states the restoration equation roughly as this (Theory of the Photographic Process, 4th edition, page 451):

2 [FeEDTA]-- + 1/2 O2 + H2O <===> 2 [FeEDTA]- + 2 OH-
​

which is essentially what you suggested.

If you consider, that typical Ferric EDTA bleaches use lots and lots of Ferric EDTA compared to the amount of silver which is bleached, and given that EDTA is a fairly good buffer at typical bleach pH, I would not expect a large change in pH unless you use this bleach until the bitter end.

 

[sfaber17]

Thanks Rudi. I did get somewhat lax on not separating out the ammonium ion. I used the PDTA version. It takes 120ml of a bleach starter that is a sodium acetate solution, so it is heavily buffered with acetate as well. The raw bleach is pH 3.6 or so and the tank version with the starter is pH 4.57. After I ran 6 rolls or so it was 4.67. I could not change the pH by bubbling air, so I'm thinking the oxygenation is fairly quick and may occur in the tank as I think was mentioned before.

 

[Rudeofus]

Bleach strength decreases slowly with rising pH. This is more of an issue with the already very weak Ferric EDTA bleach. Ferric PDTA is much stronger by itself, therefore a shift from 4.57 to 4.67 shouldn't be an issue. If pH goes too low, PDTA/EDTA will precipitate.