Question about absorbance (beer's law)

[Rujano]

Homework Statement

Hi. I know this will sound silly, but I really don't get it.Let's say that I want to determine phosphate in a given sample by using molybdate.
.

After plotting the calibration curve using the absorbance and concentration of the phophate standard, I calculate the concentration of phosphate in the sample like this :

Csample = Asample - intercept/slope

But... why does the concentration of the sample is equal to the concentration of phosphate?

I know that when molybdate is added to phosphate a complex is formed... so, why does this happen?

In fact I found this: Phosphate will readily react with ammonium molybdate in the presence of suitable reducing agents to form a blue coloured complex, the intensity of which is directly proportional to the concentration of phosphate in the solution... Why is it proportional to the concentration of phosphate?

 

[Borek]

Think about the reaction stoichiometry. Conditions are selected in such a way phosphate is a limiting reagent and equilibrium is shifted far to the right.

 

[Rujano]

Hi. Thank you for your reply. What would be the stoichometric reaction, though? (I mean, why is to proportional to the concentration oF Phosphate instead of the whole complex?)

 

[Borek]

Google for the phosphate molybdate determination. I am not sure about the exact product - some kind of phosphomolybdate, I don't have time to check the details at the moment.

 

[Rujano]

Thanks again. When I google that I get a bunch of labs reports that show experimental data about it. However, my question is why do I only have to take into account the phosphate instead of the whole complex that is formed? Take a look at this please:

The monitoring of phosphate involves the addition of a chemical
reagent solution to the sample under constant temperature
conditions. The result is a chemical complex, in solution, which
has a predetermined color. The absorbance of this colored
complex is proportional to the concentration of the phosphate in
the original sample (WHY?), thus making it possible for the measurement
to be made optically.

 

[Borek]

I already told you it is a matter of stoichiometry. Amount of phosphomolybdate created is directly proportional to the amount of phosphate present, just like amount of water produced is directly proportional to the amount of hydrogen reacting with oxygen.

 

[Rujano]

I already told you it is a matter of stoichiometry. Amount of phosphomolybdate created is directly proportional to the amount of phosphate present, just like amount of water produced is directly proportional to the amount of hydrogen reacting with oxygen.

Thanks! I got it now