The solution contains: ZnSO4 + Water+ trisodiumcetrate,? NH4OH was added drop wise to this solution to raise the pH to 6.5. Could anyone help me to find out the problem?
While you are adding the NH4OH, you are probably getting a little bit of Zn(OH)2 precipitate, which makes your solution cloudy. You may want to try adjusting the pH using a more dilute source of NH4OH and add it more slowly.
The solubility of zinc(II) sulfate in water is much higher than that of zinc(II) citrate; which seemingly could possibly have precipitated from enough concentrated zinc sulfate aq. solution, by means of adding the tri-sodium citrate solution. Less likely seems, in principle, the precipitation of zinc(II) ammonium sulfate for the pH considered (6.5); and also of ammonium citrate dibasic, ammonium sulfate, sodium citrate, sodium sulfate, zinc(II) ammonium citrate, or of zinc(II) citrate. This is no more than a preliminary generic opinion, that might have to be considerably revised after considering the actual concentrations, volumes, and temperature.
You have to prevent that carbon dioxide from air to enter all the solutions you use. Zinc carbonate is not very soluble. Therefore deaerate all the water by bubbling pure nitrogen or argon in the water you use to prepare the solution. There might be a lot of carbonate already in your ammonia solution as well as this solution is certainly already contaminated by carbonate. If the ammonia is not clean and its is already a long time on the shelf you have to get a new clean solution and protect it when you use it. Working possibly under an argon sheet is advisable or use citron in a protected work area.
In the last sentence of my answer replace the meaningless 'citron' by 'nitrogen'. So sorry.
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