My question then is: Does pH-metering then make sense at all?
Hi, I think that if someone is mixing from some formula with possibly marginal chemicals, maybe different purities than what the formulator used, then result will likely be a little different. So the problem is, how does the user deal with this?
Now the original formulator probably had to experiment by processing film, etc., while tweaking the formula, and ideally they can report an aim pH. Now if they had an inadequate meter reading then it sorta leaves another user with not many options - they'll have to tweak the formula by trial and error. Probably the most useful modification would be pH adjustments. So once they find a good working formula, they could use their own meter to find an aim pH value for their own use. (It might not be a "correct" pH, but it should be repeatable for them.)
With respect to a "proper" C-41 formula, one that closely approximates the "real" formula, this is more than likely gonna need to operate at the pH spec given by Kodak. (I've looked at more than a couple of these over the years; they're close to the Kodak pH spec.) Ok, now the user has a problem; they should ideally adjust pH to spec, but they're not sure their meter is accurate. This gets back to what I suggested previously - if they buy an actual Kodak mix (or other mainstream mix) they can use their own meter to measure the Kodak mix. This then becomes the aim pH value for their self-mix.
Some people may think the meter will be accurate, but this is not necessarily so. Kodak has a specific pH electrode that they recommend, and they caution that any other electrode should be verified by the user.
Now I have personally observed a pH difference in other meters/electrodes, albeit back in 1990s or so. Previously user Anon Ymous advised me to calibrate the meter with pH buffers (as though I didn't know this). So let me clarify the methods used. First, the pH reading will vary by temperature. So you must always take a pH value at a specific temperature. For Kodak this is usually 25 deg C. Now to calibrate a meter you must do two things - zero the meter and then set a slope (this is probably semi-automatic with modern meters). Here's how it works, sort of: a pH value of 7 is a "neutral" pH value, and this is where you zero the meter. The electrode pair is gonna have a certain theoretical response characteristic to changes in the pH , but they vary slightly. So to calibrate your meter/electrode combination, here's what you do. First obtain a reference pH buffer solution for pH 7 ( the exact values will be listed on the buffer package). Now, to set a slope value you ideally use another reference pH buffer with a value similar to your sample. Since the color developer is near pH 10, you would normally use a pH 10 buffer. So obtain a pH 10 (approximately) buffer. Ok, for the calibration: note the specified temperature for the developer pH; also note the exact pH values of your buffers at that temperature. You ideally want your sample AND the buffers at that same temperature. Ok, use the pH 7 buffer to zero your meter for the exact value for the temperature. Next, use the pH 10 buffer to set the meter slope for the exact value for the temperature. (Note: follow standard procedures while doing this, rinsing electrodes with distilled water when changing solutions, blot off excess water, never ever wipe, etc. pH 10 buffers are subject to changing when exposed to air, so it should be fresh from the container for critical work.)
Ok, finally, read the developer sample. As a note, you will typically find that the buffers give a stable pH value very quickly, meaning in perhaps 5 or 10 seconds (? I don't recall clearly). But the developer samples will probably take quite a bit longer.
When we read seasoned developer samples it would typically take between 1 and 2 minutes. This is with a 3-decimal research-grade pH meter, a high-flow reference electrode, and the Kodak-standard pH electrode. We watched the third decimal digit to judge the rate of drift. If only two decimal places are shown one might judge the drift to have stopped earlier. I believe that more modern meters are calculating the rate of drift internally, and possibly predicting how far it will go, so they might obtain readings much quicker - I dunno.
With respect to different pH electrodes, we would get different values for the developer with some electrodes. So we take the Kodak-standard electrode as "correct," and reject other electrodes as not useful. The values we got would (typically) concur with the published Kodak spec.
With all this out of the way, I'll repeat: I think that the user of a non-Kodak-standard meter can probably get an aim pH reference value, FOR THEIR METER, by mixing a packaged Kodak developer, then measuring it. (Congratulations if you get the "correct" value.)
Ps, I DO have a question for the other proponents of the low-priced meters: how did YOU know if your meter was accurate or not? I mean, if you think your meter was useful in measuring pH, you must have had some idea how close it was, right? (Presumably you are then adjusting the developer to some pH value? If not, what was the use of the meter?)
With that I'm gonna try to step out of this thread; I don't really have much else to say)