Barry Thornton--Perceptol-- weak acid

[lensmagic]

Barry Thornton, in Edge of Darkness, page 94, speaks of Perceptol containing a "mild acid" to inhibit development, giving the sodium sulfite in Perceptol more time to dissolve grain. What is the weak acid to which he refers? Sodium chloride? I thought sodium chloride was pH neutral.

 

[Ian Grant]

He's referring to Sodium Metabisuphite or Sodium Citrate both of which have been used is in the Kodak equivalent Microdol-X and also D25 (metabisulphite). In addition Boric acid can be used to lower the pH.

The Perceptol formulae hasn't been published but is somewhere between D23/D25 with NaCl.

Ian

 

[lensmagic]

It sounds like Mr. Thornton confused Perceptol and D-25. I have never heard of Perceptol containing anything other than sodium sulfite, metol, salt (NaCl) and some oddball anti-sequestering agent.

 

[Alan Johnson]

It sounds like Mr. Thornton confused Perceptol and D-25. I have never heard of Perceptol containing anything other than sodium sulfite, metol, salt (NaCl) and some oddball anti-sequestering agent.

10% sodium sulfite has a pH of 9.7.
I measured the pH of some old Perceptol, it is just under 8.
Mr Thornton appears to be right to suggest that a mild acid salt is present.

 

[john_s]

10% sodium sulfite has a pH of 9.7.
I measured the pH of some old Perceptol, it is just under 8.
Mr Thornton appears to be right to suggest that a mild acid salt is present.

Would the acid in Metol be enough to drop the pH?

 

[Ian Grant]

Would the acid in Metol be enough to drop the pH?

No, but the levels of something like Metabisulphite or Boric acid needed could be below the 1% threshold for inclusion in an MSDS and Ilford's current MSDS sheets are less inclusive than Kodak's

Ian

 

[lensmagic]

10% sodium sulfite has a pH of 9.7.
I measured the pH of some old Perceptol, it is just under 8.
Mr Thornton appears to be right to suggest that a mild acid salt is present.

9.7 seems high. Stroebel, et al., Photographic Materials and Processes, lists borax at 9.6.

 

[Ian Grant]

It is to high, but some sources give typical pH's of developers containing 100gm/litre Sulphite, a saturated solution of Sodium Sulphite has a pH of around 9.

Ian

 

[Kirk Keyes]

Remember that sodium sulfite has little buffering power, and other ingredients will most likely change the pH rather quickly, even with 100g/L of sulfite.