Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Water molecules effect on Hydrogen bonds in precipitation

  1. Oct 11, 2017 #1
    Cheers everyone. Can the amount of water molecules added to a solution impact the formation of a salt with its ions mainly bonded through hydrogen bonds with electrostatic like behaviour?

    Let me give Some background to the question first:
    I am studying the precipitation of Phosphorous(P) as Struvite (MgNH4PO4.6H2O). One of the common steps is increasing pH of solution to a level between 7-11. I thought the solution I prepared would have a higher pH and prepared a 0,5M NaOH solution to correct the pH, but during the experiment(with some p extractions made with this concentration) I decided to use 1M to accelerate the process cause the pH of solution was sligthly lower than what I supposed) repeating all essays with 1M even those I had already did with 0,5M.

    I thought using 1M instead of 0,5M would have no significant impact on the outcome of P recovered as the number of moles needed to achieve a certain pH is more or less the same, right? What happened was the amount of P precipitated in struvite conditions with 1M was lower than with 0,5M tests (with the same remaining conditions)

    Studying the structure of struvite, it is mostly electrostatic like interactions, where H2O in Mg.6H2O octahedra is hydrogen bonded to a O in the PO4 tetrahedra, and where NH4 is also connected to the tetrahedra by Hydrogen bonds, though hydrogen bond donor nature of NH4.

    When we add 0,5M instead of 1M of NaOH, despite the moles of NaOH being approximately the same, we add more moles of the solvent molecules(in this case water) to the solution, so my main question is: can the amount of water molecules we add to solution have a significant impact in precipitation of salts which are formed though hydrogen bonding? Can the reason for this behaviour be connected to the Solvation shells because water helps in formatting those hydrogen bonds? I am still trying to understand if the addition of more water molecules may have an impact or if I am totally mistaken and water here is not so important and I should search other reason.

    Also thought of the common ion effect(because of Na+ ion) for why 0,5M seems to be better than 1M but it should be out of question has the amount of Na+ added is also ~ the same in both scenarios through the same reason as before.

    The solution I used is a complex system with Na+(0,07M), K+(0,055M), Cl-(0,150M), SO42-(0,009M), Ca2+(0,002M) along with Mg2+(0,002M), NH4+(0,070M) and PO43-(0,019M, value sin brackets are approximate just if the composition of solution is useful)

    I tried to be as clear as possible hope to not have made it all too confuse.
    Thanks in advance for any insight on this topic
    Last edited: Oct 11, 2017
  2. jcsd
  3. Oct 12, 2017 #2


    User Avatar
    Science Advisor

    How can you correct too high a pH adding base?
  4. Oct 12, 2017 #3
    sorry bad English of my part. Yeah what I write sounds like the pH was too high and I on top of that added a base, my bad.
    What I was trying to say is, when I was studying the type of solution to prepare for the tests, I saw the pH level tends to be around 6 so I was expecting to have that pH in my solution, but in the end the solution I used had an initial pH of 5, so the pH was lower than what I expected it to be, so I needed a stronger concentration of Base(1M instead of 0,5M).
    Well that was only to explain Why I tried two different NaOH concentrations in the first place.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted