Chemical Equivalence

[sergio_caetano1]

Does somebody know the equivalence between sodium carbonate and borax ? In other words, trying to use borax instead of sodium carbonate in the developer .

Thanks a lot

Sergio

[glen_h]

I would suggest adjusting for pH.

 

They are both weak bases, which suggests that they are pH buffers.

There are ways to calculate the pH of concentrations for buffers, but I haven't thought

about them for many years.

[Sandy Vongries]

Variable Litmus strips?

[glen_h]

Variable Litmus strips?

 

When I used to work in chemistry labs (not related to photography), I used to have

a roll of pH test paper, which I think is what you mean. It will turn different colors based

on pH, and there is something to match the color against.

 

Mostly I used it so that I would know what not to stick my hand into.

 

Not so long ago, I was thinking that it might be good enough to test the pH

of rapid fixer, which seems to change fast.

 

I suspect that in the case of developers, it might not be close enough without

testing of the result. Developer action is somewhat pH sensitive, so it might

need different times if you are a little off.

 

In the case of both carbonate and borate, you need to check the hydration

of the form that you have. Carbonate comes anhydrous, monohydrate,

or decahydrate:

 

Sodium carbonate - Wikipedia

 

Sodium tetraborate also comes in different hydrations:

 

Borax - Wikipedia

[rodeo_joe1]

Borax is generally a weaker base (alkali) than carbonate. It's also more stable and self-buffering than carbonate. Borates tend to resist pH change, while carbonates are less stable once in solution.

 

By buffering sodium carbonate with sodium metabisulphite for example, you can achieve a more stable and lower pH, but on its own a carbonate salt tends to vary in alkalinity quite readily; starting out with quite a high pH (>10) that drops noticeably with use.

 

To put it in practical terms. Developers that use sodium or potassium carbonate tend to have high activity (contrasty and grainy) and a relatively short shelf life. While Borax is more suited to lower activity and fine-grained formulae that have good keeping properties.

 

The two aren't really interchangeable; they give a developer different characteristics. Carbonate tends to be used in print or 'universal' developers that are used once, and Borax in developers suited only for film that can be re-used.

 

Why not just look at a few published developer formulae to get an idea of which alkali suits which application, and the comparable amounts of chemical used?

Edited October 11, 2019 by rodeo_joe|1

[glen_h]

I suspect that the OP is in a country where one is less available.

 

In the US, both are readily available in grocery stores, as they are used to help laundry detergents.

(Though, as above, maybe unknown hydration.)

 

They should also be readily available, and for a low price, from photography supply

stores, as well as art stores, as they are used in other arts.

 

But in some countries, they are not so easy to find.

 

But yes, the results will be different. Maybe close enough, maybe not.

[ben_hutcherson]

There are ways to calculate the pH of concentrations for buffers, but I haven't thought

about them for many years.

 

The Henderson-Hasselbach equation can give you a ballpark, but in the real world when I make buffers it only ever gets me close and I still have to adjust to the desired pH.

 

We call pH paper "Hydrion" paper after the brand name we buy(I think there are others out there, but that's the most common). It's generally a yellow-orange color, comes on a roll, and should have a card in the roll holder that tells you the pH of whatever you've put on it/put it in. It's hideously expensive(at least compared to litmus), usually only accurate to 1-2 pH units, and is basically useless at the extreme ends. It's good for quick and dirty gross characterization, but I use a pH meter when I need to make buffers. I've actually been campaigning to phase it out as much as possible due to the cost and the fact that most of what we do with it now can be done with litmus.

[glen_h]

Yes, looking at a picture, that is the one like I used to have. (Maybe still have, somewhere.)

 

I used to work in a semiconductor electrochemistry lab, so both semiconductor physics

and electrochemistry in the same lab. It seems to be about $6/roll, which at the rate I

used it, was pretty cheap. They claim 15 feet, and 100 tests, per roll, so $0.06/test.

 

But yes, 1 or 2 pH units accurate. As I wrote above, I mostly used it with unknown solutions.

I believe I had one roll, each, at the two labs I worked in.

 

In recent years, I have had my diluted Ilford Rapid Fixer silver plate the insides of bottles, and

way within the rated capacity. I suspect that it is due to pH changes, and Ilford does give

a range, and also specific gravity, for diluted fixer. One could test. I don't know if 1 unit

is close enough, though. Otherwise, it would be nice to have a working pH meter.

[glen_h]

The Henderson-Hasselbach equation can give you a ballpark, but in the real world when I make buffers it only ever gets me close and I still have to adjust to the desired pH.

 

(snip)

 

I have seen many formulae for developers, but never one that required pH adjustment after mixing.

 

That seems to suggest that they are close enough, if one is careful with weighing.

[Alan Marcus]

Development goes forward best in an alkaline solution. Most every developer formula contains an alkaline activator. The most common are --- Sodium hydroxide pH 12 --- Sodium carbonate pH 10.2 – Kodak Balanced Alkali (Sodium boric acid) pH 9.8--- Borax pH 9.7 (pH is approximate).

 

Film contains both exposed and unexposed light-sensitive crystals. The developer chemically differentiates between the two and reduces those silver salts that have been exposed. If development proceeds aggressively, both exposed and unexposed crystals will be reduced. In other words, the pH of the developer controls the rate of this reaction.

 

Additionally ionized materials in solution will act to buffer the solution. In other words, a buffer retard changes in the value of the pH of the solution. Because a buffered solution can tolerate considerable quantities of alkali or acid and still maintain the specified pH. Because it of the importance to keep the developer’s pH steady, Kodak formulated what it called “Kodak Buffered Alkali” and included repeatedly in its kit mixes. Borax is a close second in this regard.

[glen_h]

As well as I know it, the Polaroid pack films, and maybe others, use sodium hydroxide.

 

For black and white, the developer develops the exposed grains (fast!), and also

dissolves the unexposed grains. Dissolved silver ions then diffuse through the

solution to the positive sheet, and are converted to silver metal, forming the

positive image. All that in 15 seconds!