Maleic acid buffer: Help me make it

Scott C. Johnson scj at calshp.cals.wisc.edu
Thu Feb 15 10:09:12 EST 1996


I have long used the Genius detection system, however, B-M has changed
things on me a bit. Namely, one of the new buffers calls for 100mM Maleic
acid, 150mM NaCl, pH 7.5. Seems easy right. Well, the 1x buffer is, only
problem is that you need alot of it, and a 10x stock would be much more
convenient. B-M sells a 10x stock, but there's not much sense in buying
the buffer, when it's easy to make, or so I thought. Before I describe the
problem, I'll tell you that I've been on the line with B-M tech services
several times, and they cannot solve it. Although they can make the
buffer. 

So, basically the 10x sotck requires 1M Maleic acid, right now let's
forget about the NaCl, actually we'll never even get to it. We'll make one
liter, and since the FW of Maleic acid is 116, we'll use 116 grams. Stir
it up, and in it goes. So now we have about 800 mls of water with 116
grams of Maleic acid completely disolved. Next the recipe calls for pHing
with NaOH pellets or concentrated solution. I've tried both, and get the
same result. The pH of the acid solution is about 1.3, and we've got to
get it to 7.5. I start adding NaOH, and before the pH hits 2.0, all the
Maleic acid starts falling out, crystalizing like crazy. I've tried heat,
and tons of stirring, but that stuff is not going back in.

Well, that's about all you need to know. If you know how to suceffully
make a 1M stock of Maleic acid pH 7.5 I would appreciate you sharing. B-M
says they use the same stuff I did, the free acid FW 116. I even ordered
new Maleic acid, as ours was old. As would be expected. Both the old Sigma
stuff and the new VWR stuff did the same thing. So the tech services guys
said to try a 5x solution. Not really a reasonable explanation, but I
tried, and even 500mM Maleic acid precipitates at pH 2.0. 

Am I an idiot? What am I missing? What are they doing differently?

Thanks,
Scott

-- 
Scott C. Johnson
AHABS
Univ. of Wisc. - Madison
scj at calshp.cals.wisc.edu



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