Question

Why is a blank solution used in absorption spectroscopy? I know it is to nullify the reflection, refraction, and diffraction that occurs, but if the blank solution was a dilute solution rather deionized water, how would the concentration of Cr(VI)in a sample be affected?

Answer 1

When a blank is used the spectrophotometer is made to read 100% with the blank inserted (you may make it read A = 0 which is the same as 100%T). So if you set the unit to read 100%T (0 Absorbance) with the blank, what will it read if you next insert a dilute Cr solution. We don't know exactly what it will read but we know it will read less %T (more absorbance) because the dilute Cr solution absorbs some of the light. Now suppose you make that dilute Cr solution read zero with the adjustment knob. Then you measure an unknown with that setting. Suppose the unknown SHOULD HAVE READ 50
%T but it won't read that now. It will read something like 60%T (less absorbance). Less absorbance means less Cr so the results will be low. Check my thinking.