As part of my work, I am testing the amount of impurity of methanol in an ethanol sample,

the standard method is as follow:

1. Making a 5% alcohol from a methanol free alcohol= solution A

2. making different concentration of solution A

3. picking 2ml of these concentrations (standard solutions for calibration curve)

4. adding 1 ml of (potassium permangenate + acid) and subsequent 30 min wait in ice for all concentrations

5. adding enough amount of sodium meta bisulfite to the concentration to reaching to a colorless solution.

6. adding 10 ml of chromotropic acid solution (diluted with sulfuric acid) and subsequent holding of these concentration in water bath (70 .C)

7. UV-Vis assay at 575-570 nm.

the problem is the UV-Vis instrument does not show the absorbance amount of the standard samples and therefore the would not be a calibration curve for the determination of samples. I see some stars like (*****) instead of number when I test these standard concentrations!!!

I am using a shimadzu UV1800 instrument.

what composition of compounds should I use for reference for Auto-zero?