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

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SUMMARY

The discussion centers on the unexpected precipitation occurring after neutralizing 0.2 N HCl with 0.2 N NaOH, resulting in a mixture of 400 uL HCl, 400 uL NaOH, and 200 uL H2O. Participants concluded that contamination in the NaOH solution is likely responsible for the precipitation, possibly due to metal cation impurities or silica solubility in base. The end-point of the titration should yield a homogeneous sodium chloride solution, indicating that the observed precipitation is not typical for pure solutions.

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  • Understanding of acid-base neutralization reactions
  • Familiarity with titration techniques and endpoints
  • Knowledge of solubility principles, particularly regarding NaCl
  • Basic chemistry concepts related to contamination and impurities
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Chemists, laboratory technicians, and researchers involved in immunogenic studies or acid-base chemistry who need to understand precipitation phenomena and solution purity.

Nix13
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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.
 
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Nix13 said:
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
 
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Quantum Defect said:
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.
 
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Nix13 said:
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.
 
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