can't stop the precipitate! - adding cacl2 to pk buffer (Jul/12/2006 )

hi,
i'm having trouble keeping my solution from precipitating out. i am making a .2M potassium phosphate buffer and adding 1% sucrose and 1mM cacl2. i know it's the calcium playing with the phosphate. how can i stop this? i usually bring my solution up to about 900mL and then add the right amount of .1M cacl2, and then top off. i thought it was because of the cold, but leaving it out on a bench does the same thing versus refrigeration. any suggestions?
thank you
cheers
dez

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Sounds like a pH issue. Does your recipe indicate a certain pH?

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yeah, this buffer is pH 7.2

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try adding the ca solution very slowly, dropwise, with vigorous stirring.

if this doesn't work then try using a lower concentration ca solution added the same way.

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would i be better off making the 1mM cacl2 solution and then adding the buffer i need? when i add the cacl2 to the pk buffer, it clouds up, but then dissolves. it usually takes 30-45 minutes before stuff precipitates out

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same problem if you add high concentration phosphate buffer to the ca solution.

try adding 100 ml of a 10 mM cacl2 solution to 900 ml phosphate solution, slowly, with vigorous stirring.

the problem is that the cloudiness that you see is insoluble ca-phosphate. you disperse it, that's why it seems to disappear, but you aren't dissolving it. so it settles. you need to prevent it from coming out of solution in the first place. do this by working closer to the final concentration. warming the solution during preparation may also help.

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great! thank you, i will try it tomorrow

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