FX2 - crystalline or anhydrous potassium carbonate?

[Kevin Caulfield]

For almost a year now I have been mixing my own home made developers. I'm very happy with FX37 and FX15, which are my main developers, and also traditional Rodinal. Seeing Photographers Formulary seems to have pinacryptol yellow back in stock, I am going to mix up some FX2, and try stand development. The Film Developing Cookbook specifically states that crystalline grade potassium carbonate should be used, not the anhydrous grade, as the crystalline grade contains some bicarbonate buffer. The Photographers Formulary FX2 kit contains the anhydrous grade. Any opinions on whether the potassium carbonate needs to be the crystalline grade?

 

[Kevin Caulfield]

I did notice an earlier thread on this -

(there was a url link here which no longer exists)

But still interested in recent experiences and opinions, thanks.

 

[Gerald C Koch]

Crawley specified the crystalline form BECAUSE of the small amount of bicarbonate that it contains. As mentioned in another thread the bicarbonate ion is a weak restrainer. Crawley was particularly concerned that the crystalline be used and not the anhydrous.

I personally would not recommend that FX-2 be used as a stand developer due to the danger of over-emphasized edge effects which can be very unpleasant in appearance. There is also a problem that this developer whose formulation is over 50 years old may not work well with modern films. So test before you use it on any important negatives.

As far as edge effects are concerned less is more. In other words the effect should usually not be evident to the viewer.

 

[Rudeofus]

Crawley specified the crystalline form BECAUSE of the small amount of bicarbonate that it contains. As mentioned in another thread the bicarbonate ion is a weak restrainer.

I keep reading that Bicarbonate is a restrainer, yet from my little knowledge of chemistry I wouldn't call it that. Unlike organic restrainers (BTAZ, PMT, ...) Bicarbonate doesn't cover Silver Halide crystals, nor does it directly shift the balance of the development reaction like most inorganic restrainers (Bromide, Iodide, to some extent Chloride). All it does is lower the pH of developers which would normally operate at medium to high pH, while an acidic Amidol developer would go medieval on your film/print if you added enough Bicarbonate ...

 

[Murray Kelly]

From what I've read there isn't a lot of bicarbonate in the crystalline form but it would be nice to find out. Anyone know how much?

Last time I used FX-2 I added a generous pinch of Sodium bicarb the the 250ml tank full. That's a bit less than 100mg (I measured 15 pinches to the gram ) but can't say it looked any different to usual. The bicarb/carbonate buffer pair would be pretty weak at such low levels but I doubt Crawley was pulling our collective leg.

 

[Alan Johnson]

Here's what Crawley wrote, BJP Jan 6 1961 p12:

"This use of potassium carbonate (crystalline) as in FX2 deserves comment.
Potassium carbonate (cryst) B.P. K2CO3 1 1/2 H2O gives an individual type of alkalinity not matchable with any direct equivalent of potassium carbonate (dried)-as was observed by Dalzell in the twenties;the B.P. crystals must therefore be used in these formulae when specified.
The type of alkalinity provided is useful, as it is in practice less energetic than the other carbonates (due to the formation perhaps of some restraining bicarbonate?) and therefore allows a fairly large concentration to be present, which stabilises the activity of the solution.It seems to work very well with glycin, and was mentioned in this context 60 or 70 years ago."

I think it would be a pity if mysticism was entirely removed from developer formulating so I'll shut up now.

 

[Gerald C Koch]

Hi Michael,

Two points.

The amount of bicarbonate in potassium carbonate sesquihydrate is about 1.5%. Really too small to have a practical function as a buffer.

There is a very brief mention of the restraining action in one of my photochemistry books. I have tried to find the passage but without success. Mention was made that early photographers mixed their developers from sodium carbonate and bisulfite lye solution (sodium bisulfite solution). In doing so a small amount of bicarbonate is created. When sodium sulfite became readily available it was observed that the resulting developers did not work as well. This was attributed to the restraining action of the bicarbonate ion. Certainly simple electrostatics would predict that the HCO3- ion can attach to the Ag+ ion in the crystal lattice. This is exactly what happens with bromide ions.

Whether or not this produces a restraining effect I don't now. There was just this one passage. But certainly the author thought enough of it to include it in the book.Just reporting what I have read.

Jerry

 

[Rudeofus]

Certainly simple electrostatics would predict that the HCO3- ion can attach to the Ag+ ion in the crystal lattice. This is exactly what happens with bromide ions.

The interaction between Bromide and Silver is not determined by their electrostatic charge alone, otherwise NO3- would behave just the same as Br-, and we know it doesn't. Since any developer containing lots of Carbonate automatically contains plenty of HCO3-, it's restraining effect would hardly go unnoticed if it existed. The funny thing, though, is that Carbonate is called accelerator.

 

[Gerald C Koch]

The interaction between Bromide and Silver is not determined by their electrostatic charge alone, otherwise NO3- would behave just the same as Br-, and we know it doesn't. Since any developer containing lots of Carbonate automatically contains plenty of HCO3-, it's restraining effect would hardly go unnoticed if it existed. The funny thing, though, is that Carbonate is called accelerator.

The charge distribution on a multl-atom ion can be very complex. So one cannot simply say whether one ion is similar to the other.

Perhaps my use of the word restrainer is confusing to some. It is intended to apply to those chemicals that act as antifoggants. If bicarbonate ion has a antifoggant property it is a weak one which would be overwhelmed by any bromide present in the developer. The claim is based on empirical data. I am unaware of any scientific study. The web yields nothing for a short search.

The concentration of bicarbonate ion present in a carbonate solution is dependent on the concentration of the carbonate ion and the pH of the solution.

 

[Rudeofus]

The charge distribution on a multl-atom ion can be very complex. So one cannot simply say whether one ion is similar to the other.

Fluoride would be a single atom ion just like Bromide, yet the attraction of Silver to Bromide is orders of magnitude above the attraction of Silver to Fluoride. And sure enough, one is an effective restrainer while the other one is not. Before you bring up that HF is a weak acid compared to HBr: H2S is even weaker and wow, does H2S stick to Silver!

Perhaps my use of the word restrainer is confusing to some. It is intended to apply to those chemicals that act as antifoggants.

Any mineral acid added to a developer will reduce fog until it completely inactivates the developer, but AFAIK nobody would call them restrainers.

The concentration of bicarbonate ion present in a carbonate solution is dependent on the concentration of the carbonate ion and the pH of the solution.

The concentration of HCO3- in any reasonable developer should be pretty close to the original concentration of Carbonate, assuming Carbonate was the predominant alkali to begin with. A pKa2 of 10.32 suggests that little CO32- is present below pH of 10.

 

[Gerald C Koch]

Fluoride would be a single atom ion just like Bromide, yet the attraction of Silver to Bromide is orders of magnitude above the attraction of Silver to Fluoride. And sure enough, one is an effective restrainer while the other one is not. Before you bring up that HF is a weak acid compared to HBr: H2S is even weaker and wow, does H2S stick to Silver!

The effectiveness of halides as restrainers is inversely proportional to the solubility products. So I > Br > Cl. Nothing can be determined for silver fluoride since it reacts violently with water.

Any mineral acid added to a developer will reduce fog until it completely inactivates the developer, but AFAIK nobody would call them restrainers.

Any restrainers depress the development of fog centers to a greater extent than for other activation sites. If they didn't then they would have no purpose. So I don't really see the point in this statement. The addition of an acid would tend to depress development equally for all parts of the crystal.

The concentration of HCO3- in any reasonable developer should be pretty close to the original concentration of Carbonate, assuming Carbonate was the predominant alkali to begin with. A pKa2 of 10.32 suggests that little CO32- is present below pH of 10.

Any activity of bicarbonate ion would only be seen in low alkalinity developers that do not contain any restrainer.

 

[Kevin Caulfield]

Thanks, everybody, for the comments so far. Gerald, thanks for the warning about stand development - I will test carefully.