Funny you should mention weighing. I find myself weighing all liquids once I know their sg, as in the formula below. For me, it's more accurate.
Originally Posted by Photo Engineer
Here's an interesting discovery:
The pH of a TEA-based developer changes much with temperature.
I know that pH changes with temperature, but I'm used to XTOL and my similar developers that use borate buffers. For them, the change in pH from 20C to 30C is .02 to .04, which is not significant. But when I replaced the borate with TEA, the pH-shift from 20C to 30C was .17, which is substantial. Here's the TEA-based developer I was testing:
80 g Sodium sulfite
8.3 g Ascorbic acid
13.6 g TEA-99%
0.2 g Dimezone-S
20 g Propylene glycol (to simulate a concentrate)
Water to 1 L
(I don't suggest you mix this. pH is too high at 20C and dev-times too short)
As temperature rises, the large pH-drop will partially offset the effect of the rise in temperature. I wonder if such a developer would allow users to be sloppy with temperature-control. Anyway, this aspect of TEA is nice to know about. As always, comments are welcome.
Has anyone seen this happen?
This is the same beaker of developer, developing test-strips for 9.5 and then 10.25 minutes:
What's odd is the left half of the curves are identical, despite the time-difference. The right half of the chart behaves as we'd expect. Any idea why?
I would need more information the film and agitation scheme, but offhand depending on the processing temperature, the difference between 9.5 and 10.25 minutes is not much when it comes to materially changing the lower densities (depending on the inherent shape of the toe for a particular film). So while one might expect a more gradual increase in contrast with a small increase in time, rather than the slightly "kinked" curve you posted, it doesn't look out of line to me. Of course there can also be small measurement errors, plus the fact your test conditions might have significant flare which would tend to flatten the toe contrast.
You have probably caught the film at maximum (or optimum) development in the larger faster grains, just before the fog starts. The upper scale is just a bit underdeveloped. Thus the upper scale contrast increases.
Originally Posted by albada
This is just a guesstimate, but I have seen something like it before.
Oh, great... yet another variable in the system decreasing the likelihood of us ever getting what we think we're getting...
Originally Posted by Photo Engineer
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Well, this might take place in any developer. You just have to test and see if you are concerned.
Oops - didn't mean to imply it was specific to Mark's developer.
NP, just clarifying my comment combined with yours.
A question for chemists and chemist-wannabees
The question is:
What is the pH of equal weights of borax and boric acid dissolved in distilled water? I used 15 g/L of both.
To help with pH measurements, I decided to create a reference-buffer near the pH of the developer I'm working on, and noticed that equal weights of borax and boric acid were close to what I wanted according to the Henderson-Hasselbalch equation. I calculated a pH of 8.28 (at 30C), but measured 8.05, so I was far off. I used a pKa of 9.139 for boric acid at 30C. Furthermore, I know that borax splits into boric acid and its conjugate base upon dissolution, so I added the concentration of borax to that of boric acid. Here's my math:
g = grams of powder per litre
b = 381.37 = mw of Borax
a = 61.83 = mw of boric acid
B = g / b
A = g * (1/b + 1/a)
pH = pKa + log(B/A) = pKa + log(1/(b*(1/b + 1/a)) = pKa + log(1/(1+b/a)) = pKa - log(1 + b/a)
Plugging in the numbers: 9.139 - log(1 + 381.37/61.83) = 8.28
But boric acid is polyprotic, and according to wikipedia, has pKa2=12.74 and pKa3=13.8. I don't know how to account for those. Would they cause my calculation to be in error by 0.23?
Also, before adding the boric acid, I measured the pH of the borax solution as 9.164 (at 30C). It should be 9.14, so my (home-made) pH meter was off by .024, which is accurate enough. So the measurement of 8.05 should be close, so I really am .23 off. Any clue why?
Home made pH meter? Sounds pretty good for home made.