Should cadmium sulfate affect the pH of my buffer?

[philip041]

I have a 0.2M solution of Sodium Acetate, (which surprisingly seems to be at pH7.8, (we calculated it should be around pH9)). We then add Cadmium Sulfate, and adjust the pH using acetic acid to pH5. However, when I add the Cadmium Sulfate the pH drops.

Surely this can't happen?

 

[Borek]

Are you adding solid sulfate, or solution? If solution - what is its pH?

Cations of many metals are able to lower pH of the solution reacting with water.

 

[philip041]

Solid Cadmium Sulfate

 

[philip041]

How come a metal can affect pH if pH is a measure ph hydrogen ions?

 

[Borek]

See my previous answer. Hydrolyzis.

 

[lightarrow]

I have a 0.2M solution of Sodium Acetate, (which surprisingly seems to be at pH7.8, (we calculated it should be around pH9)). We then add Cadmium Sulfate, and adjust the pH using acetic acid to pH5. However, when I add the Cadmium Sulfate the pH drops.

Surely this can't happen?

$$Cd^{++}\ +\ H_2O\ \rightarrow\ CdOH^+\ +\ H^+\ \ \ pKa\ =\ 7.6$$

So with Cadmium the reduction of pH is slight. Try to do the same with Hg++ or Fe+++.

 

[philip041]

$$Cd^{++}\ +\ H_2O\ \rightarrow\ CdOH^+\ +\ H^+\ \ \ pKa\ =\ 7.6$$

So with Cadmium the reduction of pH is slight. Try to do the same with Hg++ or Fe+++.

I love you

 

[Borek]

You would love yourself, you just ignored what I have posted earlier. This is metal cation reacting with water, lowering pH. This reaction is called hydrolysis. It is all in this thread.

 

[philip041]

Yes, I read your post. I also read about hydrolysis. I also understand it. I also have a sense of humour. Get a grip.

 

[philip041]

HA! I just saw you have an award for humour!

 

[symbolipoint]

The target is becoming lost here. Cadmium would be reacting with something, not necessarily water, affecting the pH.

 

[Borek]

Cadminum will also get complexed by acetate, up to four ligands per cation. Looks like in solution that is 0.1M in both Cd and acetate about 40% of cadmium will be complexed. However, concentration of undissociated acetic acid goes up (even if concentration of acetate anion goes down; one may expect that will shift dissociation right), so it must be cadmium hydrolysis that is responsible for the pH change.

0.1M CdCl₂ and 0.1M NaAcetate.