What's the sodium carbonate doing in the bleach?

[walbergb]

I'm into making my own bleach (potassium ferricyanide + potassium bromide) for indirect toning. I have seen recipes that include sodium carbonate (e.g., Photographers' Formulary recipe). What function does the sodium carbonate serve?

 

[RattyMouse]

I'm into making my own bleach (potassium ferricyanide + potassium bromide) for indirect toning. I have seen recipes that include sodium carbonate (e.g., Photographers' Formulary recipe). What function does the sodium carbonate serve?

Just a guess here...pH buffer?

 

[ajs77306]

Perhaps it has something to do with contrast?

http://www.largeformatphotography.info/twobath/

 

[jm94]

That's what I thought too, a pH buffer.