Help with specific gravity calculation

Terrence Brennan · Mar 16, 2022

I could use some help with a specific gravity calculation. I don't need help with the actual meaurement of the specific gravity; rather, I need to get some input on making an offset for a seasoned tank solution, as opposed to a fresh tank solution.

I use an AGFA Rapiline 66 processor, with Ilford 2000RT developer and fixer, and have the Ilford specifications at hand for the pH and specific gravity aims. I have contacted Ilford, and they confirm that the published information refers soley to fresh tank solutions. I know from 19+ years in the photo lab industry, plus another 20+ years as a photographer, that the two sets of aims are not the same.

My concern at this time is for the fixer. I had a look at an old data sheet, from the Kodak manual on the E-6 process, and there is a section on the pH and specific gravity aims. While I am reasonably sure that the Kodak Fixer used in that process is not the same fixer that Ilford manufactures as 2000RT Fixer, I used that as a model to calculate an offset.

The method I used is listed below; if I am way out in the weeds, or operating under a false assumption, I would love to hear from anybody who has a chemistry/engineering/industrial background. My chemistry studies only go as far as High School, which was 50+ years ago, plus what I learned in my career.

Ilford recommends a specific gravity of 1.050 +/- 0.005 for the developer, and 1.075 +/- 0.005 for the fixer, when freshly mixed. I looked at the specifications for E-6 Fixer, and they are 1.041 +/- 0.003 for a freshly mixed solution, and 1.065 +/- 0.025 for a seasoned solution. Using that as a model, I made the following calculations:

For Kodak E-6 Fixer, I divided the aim for seasoned tank solution by the aim for a fresh tank solution, 1.065/1.041 = 1.02305; I then multiplied that number by the recommended aim for 2000RT Fixer for a fresh solution, 1.075 x 1.02305 = 1.09978, which I rounded up to 1.100. Is that valid? It should be noted that when evaluating the specific gravity for Ilford chemicals, 20°C/68°F is specified, and for the Kodak chemicals 25°C/77°F is specified.

For the tolerance, I used the same calculation, the tolerance for the seasoned tank solution of the Kodak Fixer by the tolerance for the fresh tank solution, 0.025/0.003 = 8.33, and then multiplied that by the tolerance for the fresh tank solution of the 2000RT Fixer, 0.005 x 8.33 = 0.042.

Okay, is there anybody who can enlighten me on this topic? And how's my math?

Scott J. · Apr 13, 2022

TL;DR -- Based on the info you have available to you, I think what you did was perfectly reasonable.

In essence, you're anticipating that the specific gravity of your fixer (Ilford 2000RT) will slightly increase over time as it becomes seasoned due to the accumulation of denser components that slough off the film as it goes through the fixing bath (and also perhaps because of chemical carryover). Because Ilford doesn't publish a value for the specific gravity of seasoned 2000RT fixer, you've used the percent increase in specific gravity expected between fresh and seasoned Kodak E-6 fixer, then applied that percent increase to the specific gravity for your fresh Ilford fixer. That's pretty sound logic, with the obvious caveat being that the components in black and white film and chemistry are slightly different than the components in their E-6 counterparts, so we might not expect a perfect, one-to-one correlation. Still, you gotta start somewhere, and this approach seems perfectly sound.

As for the temperature issue, I wouldn't worry about that. The percent change in the specific gravity will be pretty linear with respect to any change in the temperature at which the measurements are made (i.e., a +2.305% change observed at 25 degrees C will likely still be a +2.305% change observed at 20 degrees C). Furthermore, the real-world density difference between a measurement made at 20 degrees C and one made at 25 degrees C is going to be negligible for most water-based solutions. Any difference you actually see is likely going to be the result of the inherent limitations in the precision and accuracy of the measuring device (I'm assuming you're using a hydrometer).

As for the tolerances, that starts to get a little tricky. The general observation here is that the tolerance is smaller (i.e., more stringent) for fresh solutions and wider (i.e., more relaxed) for seasoned solutions, likely owing to the fact that a parameter like specific gravity is going to change by a different amount depending on the volume and type of film that have been run through the fixer in the process of seasoning it. Since a manufacturer has no way of predicting what emulsions each user is going to process with their own chemistry, the manufacturers allow a wider tolerance on what an "acceptably good" seasoned fixer should look like. Again, what you did seems perfectly reasonable to me.

This sort of opens up an academic question for me personally -- i.e., how useful is specific gravity for determining the quality of photo chemistry, really? My gut instinct is that it's probably not as good as doing something like a clip test (to check B&W fixer) or running control strips (for C-41 and E-6), but manufacturers have suggested it because it's a quality-control step that users can perform with relative ease and little expense. Again, the precision and accuracy of the measuring device are potential weak points.

ludwighagelstein · Apr 18, 2022

Scott J. said:
TL;DR -- Based on the info you have available to you, I think what you did was perfectly reasonable.

In essence, you're anticipating that the specific gravity of your fixer (Ilford 2000RT) will slightly increase over time as it becomes seasoned due to the accumulation of denser components that slough off the film as it goes through the fixing bath (and also perhaps because of chemical carryover). Because Ilford doesn't publish a value for the specific gravity of seasoned 2000RT fixer, you've used the percent increase in specific gravity expected between fresh and seasoned Kodak E-6 fixer, then applied that percent increase to the specific gravity for your fresh Ilford fixer. That's pretty sound logic, with the obvious caveat being that the components in black and white film and chemistry are slightly different than the components in their E-6 counterparts, so we might not expect a perfect, one-to-one correlation. Still, you gotta start somewhere, and this approach seems perfectly sound.

As for the temperature issue, I wouldn't worry about that. The percent change in the specific gravity will be pretty linear with respect to any change in the temperature at which the measurements are made (i.e., a +2.305% change observed at 25 degrees C will likely still be a +2.305% change observed at 20 degrees C). Furthermore, the real-world density difference between a measurement made at 20 degrees C and one made at 25 degrees C is going to be negligible for most water-based solutions. Any difference you actually see is likely going to be the result of the inherent limitations in the precision and accuracy of the measuring device (I'm assuming you're using a hydrometer).

As for the tolerances, that starts to get a little tricky. The general observation here is that the tolerance is smaller (i.e., more stringent) for fresh solutions and wider (i.e., more relaxed) for seasoned solutions, likely owing to the fact that a parameter like specific gravity is going to change by a different amount depending on the volume and type of film that have been run through the fixer in the process of seasoning it. Since a manufacturer has no way of predicting what emulsions each user is going to process with their own chemistry, the manufacturers allow a wider tolerance on what an "acceptably good" seasoned fixer should look like. Again, what you did seems perfectly reasonable to me.

This sort of opens up an academic question for me personally -- i.e., how useful is specific gravity for determining the quality of photo chemistry, really? My gut instinct is that it's probably not as good as doing something like a clip test (to check B&W fixer) or running control strips (for C-41 and E-6), but manufacturers have suggested it because it's a quality-control step that users can perform with relative ease and little expense. Again, the precision and accuracy of the measuring device are potential weak points.

Specific Gravity measurements are useful for determining Solution Crossover Rates and replenishing. Its better in my opinion to use the ilford aim and from there make a Series of measurements taken after a known amount of processed Material at a given Replenishmrnt rate. correlate That to a known sensitometric aim and from there on determine your own sg aim. With That you can Then Monitor Crossover and replenishment accuracy.

Help with specific gravity calculation

Terrence Brennan

Scott J.

ludwighagelstein

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