Sodium metaborate solubility problem

Yesterday I started making a batch of PMK. All went well with part A, but I struck problems with part B - a 300g/L solution of sodium metaborate (Kodalk). At this concentration the solution is virtually saturated, as the solubility of the chemical is 28.2 g/100 mL at 25 deg C. However I found that a large amount of the chemical wouldn't dissolve and was still there after 24 hours. I checked Gordon Hutching's book; he points out the difficulty dissolving Kodalk and recommends making the volume up to 2 L if having difficulty with solubility, and using the developer 1+4+100 instead of 1+2+100. I did this this morning and apparently still have the same amount of undissolved substance. In my reasoning two possibilities might account for this: (i) the metaborate is heavily impure, or (ii) the water I used is contaminated. I used commercial deionised water, which I've recently used from the same bottle for mixing other chemistry, without ill effect. I have also used a small portion of the metaborate from the same bottle for another batch of developer, again without ill effect. This has me stumped. The only thing I can think of trying is to heat it somewhat, but this shouldn't be necessary. I've been mixing chemistry for years made have PMK before without problems. I'd be glad of any advice. Thanks.

Sometimes theoretical solubility limits don't really hold up in practice due to various reasons (temperature mostly). Increasing the volume should have helped, so yeah, somewhat perplexing. I'd suggest doing what you intended yourself as well - heat it up. I wouldn't hesitate to heat the solution to 60C or so; in fact, anything up to boiling point will be OK as the metaborate will be stable at these temperatures still. See if it all dissolves and hope/pray that it doesn't recrystallize upon cooling.

You really need distilled water. De-ionized might still have some impurities, IDK. At any rate, try heating your solution and see if you can get things to dissolve completely. If not, you've possibly created some insoluble compounds somehow. In that case, start over. If you've bought a kit and don't have anymore metaborate on hand, then you might want to invest in a jar of metaborate (or an old jar of Kodalk if you can find it on eBay or wherever) for such problems in the future.

Best,

Doremus

This won't help your current problem, but for future reference you might want to make sodium metaborate in the solution: see the first few posts of this thread.

I greatly appreciate all your answers. I tried heating it and achieved some increase in solubility, as would be expected, but still couldn't get it all into solution. And on cooling there is some re-crystallisation. So back to square One. Why this has happened I cannot say... a real problem. I've been aware for some time that sodium metaborate can be made from NaOH and borax, but have not tried it - yet. I think this is my next step, but I'm a bit hesitant as I've read that some peoples' experiences have produced much the same result as I've just had. The thread you mentioned Jonathan is very interesting, and as you have done this I feel encouraged to give it a try. Which method do you recommend, mixing the chemicals dry first then adding to water, or dissolving the NaOH first then adding the borax? Thank you all again.

How critical is Metaborate to PMK? Would a substitute such as Potassium carbonate - bicarbonate buffer give vastly different results?

@OP: I'm sure you're aware that there is no strict necessity to prepare a concentrated solution of Metaborate. Though it's a little bit of more work and slightly less convenient, you can dissolve just enough Metaborate each time (equivalent of whatever volume of Part B you prefer to use) you prepare the working solution.

Raghu according to The Book of Pyro, many substances were tested as accelerators but sodium metaborate was found to be the best. Sodium carbonate was apparently the best of the others, but was found to reduce tray life, increase aerial oxidation of the pyro and increase general fog. I would much prefer to stick with metaborate as the original recipe has stood the test of time. Also I've used it for over 20 years without issues - until now. It's a good point you make about making Part B as required. As an alternative a 10% solution as suggested by Steve Anchell in The Darkroom Cookbook, made with NaOH and Borax might be the way to go. Such a low concentration solution should be less problem to make than the 30% solution. Thanks for your thoughts.

Barry Kirsten said:

I've been aware for some time that sodium metaborate can be made from NaOH and borax, but have not tried it - yet. I think this is my next step, but I'm a bit hesitant as I've read that some peoples' experiences have produced much the same result as I've just had. The thread you mentioned Jonathan is very interesting, and as you have done this I feel encouraged to give it a try. Which method do you recommend, mixing the chemicals dry first then adding to water, or dissolving the NaOH first then adding the borax?

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Barry, to the best of my knowledge the order in which you mix the two components doesn't matter. I weigh and dissolve them in the order stated using water at room temperature. The NaOH raises the temperature as it dissolves but the concentration I use is only 12g/litre so it's a trivial effect. Over several years of experience with the same developer formula, both components have always dissolved easily. I've not noticed any batch inconsistencies when measuring small amounts of components - I use an old-fashioned beam balance whose sensitivity exceeds the tolerance on the weights!

Barry,

Bite the bullet and order some sodium metaborate from the Formulary or some other reputable supply house. Mix a new PMK B solution and be done with it.

Doremus

Barry I just checked Vanbar and they have it for $9.90 for 220 gm including GST.

https://www.vanbar.com.au/catalogue/product.php?id=79009#overview

If you order it, then once it is in their store from their warehouse, you can take a train to the city, then tram to Vanbar and you'll have some fresh chemistry which should work.

Just a thought, Mick.

Thanks Mick, but I got my metaborate there. And I think it's what's caused my problems. But short of analysis I can't be certain. The water I used was de-mineralised from refresh in Sydney, and they advertise 2-8 ppm tds, which is better than rainwater, I reckon. Anyhow I've used the same water for other brews without ill effect.

Doremus, I went ahead and made a 10% solution of metaborate using NaOH and Borax, using Patrick Gainer's recipe. There was a faint excess of borax undissolved after mixing, and I was able to correct this by adding a small amount of NaOH. I figured this came about because of absorbed water in the NaOH at the time of weighing, reducing the effective amount of chemical.

I agree, getting the right chemical for the job is preferable, but when it doesn't work out there has to be a Plan B. Unfortunately in Australia there is not a lot of choice for photographic supplies. At least I have a workable solution at this time, I just have to use 3 times the amount of B each batch of Pyro I make. Thanks everyone for your input.