[unknown_analysis]

We are doing a spectrophotometry, and we need to do a blank reading first. The blank solution is made by mixing FeCl₃ and HCl. I am wondering, does a blank solution need to be colorless, meaning it has no absorbance?

I guess the answer is due to acidity of the solution, or because FeCl₃ is very diluted that its yellow color vanishes, or beacuse FeCl₃ + HCl --> FeCl₄^-, a colorless complex? I don't know any more.

Thanks for the replies.

[Borek]

I am wondering, does a blank solution need to be colorless, meaning it has no absorbance?

No, you make a blank solution to know what corrections are needed in case it HAS an absorbance.

[unknown_analysis]

I see. So it is just 'coincidental' that the blank solution in our experiment is colorless.
Another question (though not related to blank solutions anymore): how come the yellow solution, FeCl3, becomes colorless when mixed with HCl? What is FeCl3 + HCl --> ? I'm just confused.