I need to make a series of 6M and 8M guanidinium HCl buffers for a protein purification, but I can't seem to get the GdnHCl into solution. I'm able to make a clear 8M solution, but when I make the 6M solution, theres some sort of cloudy precipitate that eventually settles.

 

I've tried using the 8M solution as a stock and diluting it down to 6M, but I've noticed that it starts to crash out at around 7.3M. Heating it to 35C doesn't make a difference, nor does stirring it for several days.

 

Is there some trick to it or has my GdnHCL probably gone bad?

This is weird. Normally you have to slightly heat the solution to around 37 C to prepare the 8M solution. The 6M should work fine on RT. The reverse does not make much sense to me. Is the water you are using OK?

The water should be fine. It's from a milliQ system with a measured resistance of around 18MOhm

Then I am out of ideas. Maybe you should check the pH of the solutions. But quite frankly I would not know why a higher concentration should have less impact than a low one.