[Alt-photo] potassium oxalate behaving strangely

Keith Schreiber keith at jkschreiber.com
Wed Jan 25 21:42:59 UTC 2017

Dear group,

Some of you may have seen my inquiry yesterday on the Facebook Pt/Pd group. I'd like to compare notes with anyone who is using potassium oxalate from Bostick & Sullivan, particularly if you purchased it recently. 

Back in October, I bought 2500gm of potassium oxalate. When I went to mix some up 2 or 3 weeks later I noticed that rather than the coarse crystals that I had been getting from B&S for the last 25 years, this was almost a powder. When I called to ask if maybe there had been a mistake, Melody told me that their supplier recently changed from the small pea sized crystal form that they have been selling forever to a finely crushed powder. So I proceeded to mix gallon of it which I use to replenish my working bottle. 

As I've gone through the first gallon of fresh new PO, in the last week my working solution has changed from a deep but clear amber color (which has been normal and expected over the 25 years I've been doing this), to a nearly opaque black. It has also always been normal for greenish crystals to form in the storage bottle (probably a sign of saturation), but yesterday I found instead a layer of fine black sludge and no crystals. The sludge stuck to the bottom of the container. It is so fine that what is suspended passes through my filter paper. I'm no chemist (1 intro class in college) but it seems to me that this is indicative of something different in the chemistry.

I am going to take the advice I received from several printers who I respect and mix my own KO from oxalic acid and potassium carbonate which I have already ordered. But I am still curious to figure out what has happened. I received a message this morning from a printer in Italy who said he just experienced the same thing, but he gets his KO locally, not from B&S. He thinks he may have been given sodium oxalate by mistake, rather than potassium oxalate.

I have used potassium oxalate for 25 years, and rarely ammonium citrate, but nothing else. Anyone else noticing anything like this? Do other developers ever do this? Are there any chemists in this group who might be able to explain why this would happen? 

Some of you are probably wondering at this point, but does it affect the print? The short answer is I'm not sure. If it does it is not dramatic. Prints are still decent looking, though slightly less warm, and I think slightly depressed from the lower mid-tones into the highlights. I will have to run some test to be sure of that.


Keith Schreiber

PS - David Michael Kennedy, if you are still a member of this group, Melody suggested I contact you since she thinks you have been using this new PO in fairly large quantities.

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