# Why does dissolved oxygen concentration decrease with increasing atomic number of metals?

I am looking at how metal ions ($$\ce{Fe^2+}$$, $$\ce{Ni^2+}$$, $$\ce{Cu^2+}$$, $$\ce{Zn^2+}$$, and $$\ce{Ag+}$$) affect the biochemical oxygen demand in water and used the Winkler's method to find out the oxygen concentration of each solution. I graphed this with oxygen concentration ($$y$$ axis) and increasing atomic number ($$x$$ axis) and found that the order: $$\ce{Fe^2+}$$, $$\ce{Ni^2+}$$, $$\ce{Cu^2+}$$, $$\ce{Zn^2+}$$, and $$\ce{Ag+}$$ had decreasing value of oxygen concentration (almost a decreasing exponential graph like an "l" shape rather than a straight negative linear line) but I do not really know the specific chemistry behind it.

I thought with an increase in atomic number, there will be an decrease in dissolved oxygen concentration because as the atomic number increases, the proton number increases. The greater nuclear charge attracts the water molecules more and a ligand can be formed. Thus, as the atomic number increases, the metal ion will form a ligand with water molecules more easily, and the dissolved oxygen concentration will decrease as more water molecules will form ligand with the metal ion.

But I am confused because with increasing atomic number, the atomic size gets smaller and wont that make it harder to form a ligand bond with water?

• What is your sample matrix i.e., what else do you have in the sample besides metal ions? – M. Farooq Feb 20 at 16:59
• It is Winkler's method not Wrinkler of oxygen determination. – M. Farooq Feb 20 at 17:01
• @M. Farooq : I think OP should elaborate his method. This seems a modification to the Winkler's method. – Mathew Mahindaratne Feb 20 at 19:45
• @M.Farooq Thank you so much for your reply i only have distilled water in the sample besides metal ions as a controlled variable - first data point in the graph (atomic number = 8). For the method, the metal ions were in solution as a metal nitrate and distilled water was added. the wrinkler's method was then used and sample was titrated w sodium thiosulfate. The ratio of the steps in wrinkler's method was used to find the oxygen concentration. I am a bit confused here. How is this a modification of the wrinkler's method? and how would that affect my data? – user105060 Feb 21 at 3:53
• Does your comparison make sense ? You present a curve going from $x = 13$ to $x = 47$. But this curve is based on the only points where $x = 26, 28, 29, 30$. The point at x = 47 (Silver $\ce{Ag+}$) may not be part of the same line as the corresponding ion is singly charged, and the four previous points are doubly charged ions. Si to be sure that the dependance does exist as you imagine, I would recommend to repeat this measurements with maybe Magnesium ($x = 12$), Calcium ($x = 40$), Cadmium ($x = 48$), Baryum ($x = 56$), and forget about silver. – Maurice Feb 21 at 10:35