Why is my solution precipitating after neutralization with NaOH and HCl?

[Nix13]

We're attempting to measure the immunogenic affects of a particular molecule after subsequent acid and base treatments. As a first version, we're keeping it simple. After all treatment and neutralization, we wind up with 400 uL 0.2 N HCl + 400 uL 0.2 N NaOH + 200 uL H2O. We are getting massive precipitation upon neutralization! To test whether this was due to our compound, I did a water control and the same thing happened. Considering the solubility of NaCl is in excess of 6 M, what the hell could be going on? Any ideas would be greatly appreciated. This is basic gen chem stuff, so I am baffled.

 

[Quantum Defect]

We're attempting to measure the immunogenic affects of a particular molecule after subsequent acid and base treatments. As a first version, we're keeping it simple. After all treatment and neutralization, we wind up with 400 uL 0.2 N HCl + 400 uL 0.2 N NaOH + 200 uL H2O. We are getting massive precipitation upon neutralization! To test whether this was due to our compound, I did a water control and the same thing happened. Considering the solubility of NaCl is in excess of 6 M, what the hell could be going on? Any ideas would be greatly appreciated. This is basic gen chem stuff, so I am baffled.

You have contamination in your solutions if you are seeing precipitation with "pure water". The end-point of a titration of HCl with an equal amount of sodium hydroxide is a sodium chloride solution, which is a homogeneous liquid.

Assuming that your precipitation is with hydroxide, you likely have some kind of metal cation impurity that is precipitating out.

http://cpe.njit.edu/dlnotes/che685/cls06-2.pdf

 

[Nix13]

You have contamination in your solutions if you are seeing precipitation with "pure water". The end-point of a titration of HCl with an equal amount of sodium hydroxide is a sodium chloride solution, which is a homogeneous liquid.

Assuming that your precipitation is with hydroxide, you likely have some kind of metal cation impurity that is precipitating out.

http://cpe.njit.edu/dlnotes/che685/cls06-2.pdf

With some experimentation it appears our NaOH solution was contaminated, though how and by what I am puzzled, since the precipitation filled as much as half the volume.

 

[Bystander]

NaOH solution was contaminated ... as much as half the volume.

Stored in "soft glass" for any length of time? Sounds like a rediscovery of silica solubility in base.