Photometric titration

If we have $\ce{A + T -> P}$ 
where $\ce{A}$ is the analyte, $\ce{T}$ is the titrant, and $\ce{P}$ is the product,
photometric titration happens only in one condition, that one of the following substances will absorb the electromagnetic radiation. 


When stable complexes formed such as $\ce{Bi}$-EDTA, they will not absorb the radiation (or in small amounts), but when unstable complexes formed such as $\ce{Cu}$-EDTA, they will absorb highly. The same goes also for the acids. Strong acids will absorb much *less* the electromagnetic radiation than the weak acids. 
The reason for this is that *strong acids are completely dissociated*.


Why does that indicate the photometric titration ? 





> 
> photometric titration happens only in one condition, that one of the following substances will absorb the electromagnetic radiation.
> 
> 
> 


Almost ;-)


Regardless whether you monitor a complexometric titration by eye or with the help of a spectrometer, it is a good idea to use an indicator, such as **Eriochrome Black T**. Then, it doesn't matter, if or how strong the EDTA complex of the metal absorbs!


Instead, you monitor the absorption of the indicator, that is liberated from the metal-indicator complex upon addition of EDTA.


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