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Barry Thornton--Perceptol-- weak acid

lensmagic

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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.
 
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
 
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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.
 
lensmagic said:
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.
Click to expand...
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.
 
Alan Johnson said:
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.
Click to expand...

Would the acid in Metol be enough to drop the pH?
 
Alan Johnson said:
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.
Click to expand...

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