Kodalk: NaBO2 anhydrous or tetrahydrate?

[WAn]

As far as I know Kodalk is Sodium Metaborate. Is the anhydrous (just

NaBO2) or tetrahydrate (NaBO2*4H2O) salt meant in developer formulas

(e.g. in PMK)?

 

Thank you.

[scott walton]

I agree! With Tetra you need to do some conversions and add more due to the added water molecules. Essentially, tetra is a watered down version giving you a lesser of what you need.

[WAn]

Gentlemen, I realize that both will work ;)

 

My point was: does an amount mentioned in developer formulas relate to anhydrous or tetrahydrate version? -- there is a difference in weight, as Scott pointed out.

You both seem to agree that 'Kodalk' is the anhydrous version.

What still confuses me is the information I found today at http://www.jackspcs.com/chemdesc.htm#SOD_METABO :

 

'SODIUM METABORATE

Other names: Sodium metaborate tetrahydrate; trade name: Kodalk balanced alkali.'

 

Well, are you sure?

Thanks again.

[pete_andrews]

According to <a href="http://www.binbooks.com/books/photo/i/l/5D1C6AF668&nf=1">this bulletin board</a>, (the guy seems to know exactly what he's talking about), Kodalk is Sodium Metaborate <b>Octahydrate</b>.<p>The article goes into detail about how to constitute Sodium Metaborate in solution from Borax and Sodium Hydroxide, but <b>read the correction that follows his first post</b>.<p>Andrey: There may not even be a truly anhydrous form of Sodium Metaborate, since some compounds like this completely break down in the absense of any water molecules.<p>It's interesting to follow what happens to chemicals like Borax, Boric acid and Boric Anhydride once they're in solution, since you always seem to end up with Boric Acid and Sodium Hydroxide as the two self-buffering components.<br>I find it amusing that some people would turn their nose up at a developer which used Sodium Hydroxide as its alkali content, but don't mind a bit using a developer with Kodalk or borax in it!

[ryuji_suzuki]

> It's interesting to follow what happens to chemicals like Borax, Boric acid and Boric Anhydride once they're in solution, since you always seem to end up with Boric Acid and Sodium Hydroxide as the two self-buffering components.

 

Some of the standard methods to make pH buffer is to use boric acid + strong base, borax + strong acid, and borax + strong base. Also, you don't get sodium hydroxide in aqueous solution - they nearly completely dissociate, and sodium ion is pretty inert.

 

> I find it amusing that some people would turn their nose up at a developer which used Sodium Hydroxide as its alkali content, but don't mind a bit using a developer with Kodalk or borax in it!

 

If you use hydroxide without any acid you get almost no buffering effect and I would rather avoid such a developer unless there is another reason to do so.

 

Even pure water contains free hydroxides!

[WAn]

Hmmm.. octahydrate...

Purchase of pH checker seems to be inevitably...

Right now I can only hope that dissociation constant of sodium metaborate is low enough and the error in weighting is not too critical (in other words, certain degree of dilution doesn't change pH drastically. -- Ryuji, do you agree?)

 

Pete: special thank for the link.

[ryuji_suzuki]

The solution's pH is largely determined by the ratio of boric acid to its conjugate base, so it's relatively independent of absolute concentration. However, when other stuff (ascorbic acid, sodium sulfite, etc.) come into picture, the pH difference might be observed depending on the amount of metaborate you put in.

 

If you don't know what grade of metaborate you have, you should run a titration with standardized HCl and appropriate pH indicator dye. Pipette is rarely found in standard darkrooms but be creative. I don't know if a cheap pH meter is a good purchase because their proble is incompatible with silver-reactive material like photographic developer. I recommend you check in with manufacturer before purchasing. Some of more expensive models overcome this problem but maintaining the probe is a pain in the azz and the lifespan of the expensive probe is relatively short.

 

The way I do: I don't stock sodium metaborate. I make it from borax and hydroxide or trisodium phosphate.

 

If you made up your mind that you won't use alum hardening fixer (or stop bath, for that matter) then you should also take advantage of phosphate buffers :-) With disodium and trisodium phosphates, you can cover developer's pH range almost entirely. Plus, phosphates also have some sequestering effects.

[WAn]

I know what grade of metaborate I have: my is the tetrahydrade.

I don't know what is meant when a receipt tells: "Kodalk --- XXX g"

 

"I don't know if a cheap pH meter is a good purchase because their proble is incompatible with silver-reactive material like photographic developer" --- Ryuji, could you please be a bit more specific? Indeed I saw a relatively cheap pH meter ($60); it resembles a digital thermometer with probing rod. Why can it be incompatible with developers?

 

Re phosphate buffers: thank you for advice, I'm aware of other buffering receipts, but in my particular case I'm playing with PMK; G.Hutchings emphasizes in his book that most buffering solutions are not as good as kodalk. I'm very inclined to take in on trust ;)

[ryuji_suzuki]

<a href=http://www.omega.com/toc_asp/frameset.html?book=Green&file=ELECTRODE_SELECTION_REF>

Check this link</a>

for pH probe selection. You should select double junction type, which I never saw with inexpensive pH meters. $60 for a pH meter is really cheap.

<p>

I'm curious why metaborate is considered the best buffering agent.

[WAn]

Ryuji, thanks for the link.

 

Why -- I don't know. Here is the quote from G.Hutchings book. He speaks about accelerators, but in PMK formula the sodium metaborate is obviously not only an accelerator, but also a biffering agent.

 

"A variety of alkaline chemicals including trisodium phosphate, ammonia, trichloroethylene, sodium hydroxide and more exotic compounds like barium hydroxide were tested as accelerators. Most caused serious problems and none worked as well as sodium metaborate. Sodium carbonate, the most commonly used alkali for all pyro developers, worked fairly well but it increased the alkalinity to about pH 10.5-11.0. This increased the aerial oxidation of the pyro which shortened solution life and caused increased general fog. The buffered sodium metaborate maintains sufficient alkali at a lower pH level. Since there is so little aerial oxidation, long "plus" development times are possible. PMK solutions can be used in an open tray in excess of one hour without development problems."

[pete_andrews]

Andrey: A number of authorities on development chemistry seem to agree that it theoretically doesn't matter HOW the pH of the developer solution is achieved; it's ONLY the pH that's important. This is obviously aside from other considerations such as the suitability of the chemicals used, and their compatibility with one another.<br>In other words, a given developer accelerated with a small amount of NaOH should give the same practical results as one accelerated with a larger amount Carbonate, or an even larger amount of Borax.<p>Among the practical considerations are that a small amount of NaOH will exhaust quickly (except that one of the byproducts of developer oxidation in the presence of Sodium Sulphite is NaOH); Carbonate can generate CO2; etc.

[WAn]

Pete: no objections.

My understanding is that G.Hutchings' point was that these "other considerations" you mentioned do matter (buffering capacity, oxidation resistance, less fog, desired edge effect...).

 

Pete and Ryuji, please dont get me wrong: I don't blame other buffering agents, developers etc. I just try to honestly follow the G.Hutchings' receipt. May be I could find something that works equally well (or even better) but generally I don't like experimenting in this area: I prefer to spend my time for photography and by all the means I try to avoid any chemistry experiments (if possible).

 

P.S. I made some PMK treating the Kodalk as the tetrahydrate and printed my first negaties. May be I'm not at optimum, but the specific of PMK negatives is visible!

[ryuji_suzuki]

> it's ONLY the pH that's important

 

There's one more thing that matters. It's the concentration of titratable base. Sure you can adjust pH to be 8.5 to 9 (about comparable to D-76) with sodium sulfite and a tiny amount of NaOH, but such an approach finds a problem in very low titratable base concentration. First of all, you can't add much NaOH because it will bump up the pH way too high. The pH is also very sensitive to the amount of NaOH added. Although enough OH- ions are present in the fresh developer, they quickly exhaust during development, both in bulk concentration and at the development site. Pete's talking about reaction products of hydroquinone at moderate-high pH, but if the pH drops way low, everything can slow down to the extent that they become negligible for practical purposes.

If you remove borax from D-76 and add just enough amount of NaOH to adjust the initial pH to be 9, and if you process HP5+ in it, I bet you'll need MUCH longer development time or maybe you won't even get normal contrast because the pH will drop quite sharply during the development. The pH that matters at the development site is even lower (the base must be transported from the bulk solution through diffusion through the gelatin layer). Even with genuine 2g/litre borax formulation, the pH drop is a few tenths of a pH scale.

 

 

> a given developer accelerated with a small amount of NaOH should give the same practical results as one accelerated with a larger amount Carbonate, or an even larger amount of Borax.

 

With these three agents, most developers behave differently because of the reason I said above. NaOH only developer will be least stable while borax and carbonate will have much larger buffering capacity. This is because NaOH will dissociate almost completely in water to release same number of OH- as the number of NaOH you add. This is very different from borax and carbonate -- only a small portion of them dissociate to provide OH-, but as OH- ions are consumed through the reaction previously undissociated molecules now dissociate to provide more OH-, practically stabilizing the pH. That is why they are called "buffering" agents. The difference is quite pronounced in terms of image quality.

 

Also, even if you make saturated borax solution, its pH won't be as high as a few percent carbonate solution.

 

If you want to give enough OH- only with NaOH, the initial pH will be pretty high. One objectionable quality of such a formulation is that the film gelatin will swell a lot during development, and grain clumping is also promoted through such a low-salt, high-pH solution. As is necessary, if the developer is formulated to prevent fog at the initial high pH, the developer activity sharply drops as the solution becomes exhausted, and therefore one doesn't quite get benefit of stable high-pH formulation or that of stable low-pH formulation. This unstable pH may contribute to accutance, but accutance is better obtained through halide ion gradient and developer exhaustion instead of pH gradient because so many other things depend on the operating pH.

 

In fairness, Pete mentioned these issues in a less formal way:

 

> Among the practical considerations are that a small amount of NaOH will exhaust quickly

 

This factor is sufficiently described by "titratable base." In photographic chemistry literature, the developer composition is usually described in terms of how many molarity of sodium carbonate, for example. One reason NaOH is out of favor as a developer accelarator is its difficulty of providing high titratable base at desired low pH without combining with weak acids such as boric acid or phosphoric acid. If you combine with these, one would rather begin with borax or sodium phosphate (dibasic for most purposes). Another reason has to do with hazard rating of NaOH compared to these other agents.

 

> I just try to honestly follow the G.Hutchings' receipt.

 

I understand, although I disagree with the discussion quoted from Hutchings's book. What I meant is that if you are experimenting with developers, it is much easier to deal with systematic buffer formulation to the designed pH and titratable base concentration using one of standard acid-base pairs such as carbonate-bicarbonate or phosphates instead of such an ad-hoc self buffering agents like borax and metaborate. If you are not experimenting, it is not worth considering, but then you won't even need a pH meter then :-)

 

Anyway, pH buffer is an important part in typical freshman chemistry classes, and this topic is well covered in most textbooks, for those who are interested. (That is, get a copy of an obsolete edition of a decent book for $5 instead of wasting time searching for unreliable information on the web.)

[ryuji_suzuki]

Just because Andrey took some effort to type it in...

 

"A variety of alkaline chemicals including trisodium phosphate, ammonia, trichloroethylene, sodium hydroxide and more exotic compounds like barium hydroxide were tested as accelerators."

 

Trisodium phosphate will provide much higher pH, and therefore, for the pH range he is talking about, disodium phosphate should be considered instead. Ammonia reacts with silver halides and should be considered incompatible unless he is looking for some special secondary reactions. NaOH is problematic if used by itself, as discussed in my previous posting.

 

 

"Most caused serious problems and none worked as well as sodium metaborate."

 

I might say that is because he didn't try better competitors :-)

 

"Sodium carbonate, the most commonly used alkali for all pyro developers, worked fairly well but it increased the alkalinity to about pH 10.5-11.0."

 

The pH of carbonate solution is easy to control by changing the ratio of carbonate and bicarbonate, especially in the range of 9 to 11. It's a very standard technique.

 

He was lucky because he was dealing with pyro. If he were dealing with catechol, he would have problem with metaborate because of incompatibility (pyrocatechin and borates form inactive complex that throw these agents out of the development reaction equation) and he would have to seek something else...