another sodium carbonate question

The 2% could also be water of crystallization. I don't know that either water or NaCl should be required to be specified on the MSDS, but neither is. I suppose one could get an idea of how much NaCl would be needed to affect printing paper by adding some to a test solution of Dektol.

PS:
It can't be very much, or one could not make home-brewed laundry detergent. Try washing clothes in sea water.

2% NaCl will do more to photographic papers than it will to a laundry detergent.

Dirty clothes are a lot less sensitive to minor variations like 2%.

PE

Yes, but we are not talking about 2% of the developer solution, but about something like 2% of 5 or 10 % in the working solution. Besides, we don't know if the 2% is salt. It could be silicon dioxide or water. It could also be that the weight of the contents of the package is not guaranteed to be more than 98% accurate.

Patrick, you are correct, and the uncertainty is the problem.

If it is 2% of the purity though then if you weigh out 100 g of sodium carbonate, there are 2 grams of 'something else' in that developer. If inert, then it will affect the pH and buffer capacity by 2%. So whatever it is, if it is an impurity it will cause a 2% variation of some sort in your formulation and it may be in any direction possible.

PE

I wonder, though, if after you do your lab work with, perhaps, reagent grade chemicals, and come up with the cat's meow of developers, what happens when it is marketed? Does the lab also do analyses of sensitivity to common impurities in tech or commercial stuff? Or is the formula marketed with highest grade components?

Furthermore, it is customary here in the country to assume that anyone who gets a chance will be pulling your leg, as the saying goes. Needless to say, I do that upon occasion, and you all should probably take anything I say with a grain, or maybe 2%, of salt.

Patrick;

The answer to that question is to build a developer formulation that is bullteproof and can tolerate errors of 2% or perhaps 10% with no shortcomings. Unfortunately, few developers today have that capability.

PE

I think D-76 is quite tolerant of measurement errors. That is, an error in measurement in home-brewed D-76 has to be fairly large before the average user will see it. Precise sensitometric work is a different world. When I measure the brightness range of a scene, my results might be more different from another's than any likely errors in measuring the developer constituents. We know many fine photographs were produced long before we had the ability to do the precise measurements we can do now. It is probably a good thing, considering the open loop character of negative production, that it B&W is a two stage process. We don't always know why the negative's CI wasn't exactly as predicted, but a fairly large variance is adjustable in the printing.

Sodium chloride in the developer must be in quite large concentraion to affect greatly the negative characteristics of modern film emulsions. Some water softeners produce water with enough salt content to be of concern to those who must avoid salt. My well water is hard enough that sometimes I think I could walk on it. I use dehumidifier condensate in developers with carbonate. The hard water is good for the heart, they say. It may have some bad effects on memory, though. I have heard that there are some benefits from being 80 years old, but I can't remember what they are.