Question

Why is isopropyl alcohol used to zero the spectrophotometer?

Answer 1

I'm not sure where you got this question, but maybe something like here (pg. 9):
http://www.chem.purdue.edu/teacher/table_of_contents/uvvus/uvvis.plant%20pigments_ch.pdf

To zero the spectrophotometer is to set a standard for the blank spectrum.

The blank spectrum is like a "reference point" for actual sample spectra that you take. Some spectrophotometers let you subtract the blank spectrum from the sample spectrum so that you can remove stray light and other background interferences with the spectrum's accuracy to the sample itself.

This can be hard to understand without having done it yourself. I'm going to try to illustrate it with two `"NMR"` spectrums. It's not the same kind of spectrum, but it demonstrates the same idea.

This is the `""^1 "H"` `"NMR"` for `"CDCl"_3`.

Notice how there are three peaks between `8` and `7` `"ppm"`. The tallest peak is at about `"7.26 ppm"`.

This is the `""^1 "H"` `"NMR"` for acetonitrile in `"CDCl"_3`, so normally we would expect that peak at `"7.26 ppm"` to be there:

Notice how even though acetonitrile is in `"CDCl"_3`, that very tall peak at about `"7.26 ppm"` is, however, not there. It was subtracted from the spectrum (or maybe it was edited out using a graphics editor, but you get the idea).