Agra Yuba Bachtiar There is actually very little difference between pH 7 and 9 in terms of ions that would cause this change. You can calculate the concentration of OH- ions needed to effect this change. You'll find it very small. Is pH 9 the normal external pH for Fe3O4? What does the Pourbaix diagram tell you? That is, is the material actually basic in suspension from slight dissolution? Is it a basic solid? So, what options do you have:

• Do nothing. Why is there a problem with pH 9? Why does it have to be pH7? Many people are obsessed with washing precipitates - one guaranteed outcome is loss of (expensive?) material...
• Continue washing with DI water. This could take a long time.
• Wash with very dilute HCl. Start at 10-6 molar to begin with and maybe increase to 10-3 M

I also did a similar synthesis, which can be referred to in my paper. Washing can be done only with DI water, which is also OK. You will get a pH of around 7. But need to wash 5-6 times at least. It's a really easy process so don't get confused with the procedure. If it's alkaline pH is also no issue because you can adjust the pH during your experiment and it won't affect the result.

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