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Beer's Law (Calibration Curve?)

I am making a series of complexes, and I need to determine the concentration of nickel in each one.

These are my reactants.

Metal solution = NiCl2 (aq)

Ligands: EDTA, Oxalic Acid, conc. Ammonia, conc. HCl

I'm going to plot a Beer's Law calibration curve by taking 0.05M, 0.1M and 1M Nickel chloride solution and measuring the absorbance, before plotting my results on a graph.

I reacted the metal ion solution (using unknown concentration of NiCl2) and the ligand together in different ratios, and I'm going to plot an average of the maximum absorbance (for each complex) on the graph.

My question is, what will this actually tell me the concentration of in the complex? Is it the Ni2+ ions, or NiCl2, because NiCl2 doesn't actually exist in any of the complexes?

Thank you for your help.
Reply 1
Avatar for yodude888
yodude888
16
Hmmm Beeeer.....

*hic*
You are measuring the concentration of the complexes themselves - which will be equal to the concentration of the Ni2+ ions from NiCl2.
Reply 3
Avatar for Rikriel
Rikriel
OP
5
Ah, so the concentration of the complex and the concentration of the Ni2+ ions will be the same? Sorry to sound dumb, but how is this so?
Indeed - as when you dissolve NiCl2 it dissociates into it's respective ions - Ni2+ and Cl-, it is the Ni2+ you then react the ligands with (via ligand substitution) to give you a complex with n ligands NiLn(depending on what type of ligand it is):

Ni2+ + nL ----> NiLn so as you can see the ratio of complex to Ni2+ is 1:1 :smile:
In dilute solution, the measured absorbance is a measure of the Ni-complex. But how the Ni is partitioned between free and complexed forms doesnt actually matter. This will vary depending on the equilbrium constant for the complexation reaction and the concentrations used. But you dont need to know these details and can use the callibration curve totally operationally. So having got a curve relating the measured absorbance to the total nickel in standards you can work out the total nickel in an unknown sample by reference to the callibration curve. However, if the curve is linear with a flat top you can assume that the reaction is essentialy irreversible so that limiting amounts of Ni are completely complexed (but equally it wont matter if that is not the case)
Reply 6
Avatar for Rikriel
Rikriel
OP
5
I have an update. It doesn't look good.

Here is the email I sent to my chemistry teacher. I'm not expecting a response.

"It looks like my earlier work has been setting me up rather badly for this.

I tested the absorbance of NiCl2 and EDTA using a manual / not a digital / analogue colorimeter. I know I used 0.05M, and my absorbance is averaged at 3.5!

Doing the calibration curve today, with the digital colorimeter, the absorbance of 0.05M NiCl2 was 0.06.

If I plotted the Ni/EDTA on the graph, it would result in a concentration far too high!

Also, I looked at my results today and couldn't believe that I had used CuSO4, as this meant that I should have done a separate calibration curve just for that.

And the results of most of the other complexes seem random, at:

NiCl2 / Oxalic Acid = 0.21M

NiCl2 / conc. Ammonia = 0.14M (and this doesn't take into account whether these concentrations are actually showing the nickel concentration.)

The NiCl2 / conc. Hydrochloric acid was actually perfect, because the maximum absorbance contained no hydrochloric acid at all...

Yeah, I really need to think about what I'm going to do, as at the moment I'm really not sure.

So much for the "simple experiment" that the Salters version promised. "

There's no way to salvage this, is there?

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