The purpose of using distilled water in the cuvette of the spectrophotometer is to calibrate the instrument. It is generally considered that distilled water shows zero absorbance and 100 % transmittance. So, to zero out the absorbance of compounds other than the analyte being determined, distilled water is used as a blank.
If the unknown being determined is prepared using ethanol as the base solution, the blank used must be ethanol. This is because absorbance if any from the solvent, ethanol must be zeroed out as when the measurement of the actual unknown is being made, the absorbance of the solvent does not interfere.
A blank in a spectrophotometry is the sample consisting of everything except analyte. The blank is a sample of solvent. The distilled water is used in the cuvette to calibrate. The distilled water shows zero percent absorbance or emittance.
If the unknown has to be determined using ethanol as base solution, then ethanol should be considered as blank. The measurement of any unknown can be calculated, as ethanol will zero out the absorbance from solvent.
Therefore, the amount of unknown can be measured as the absorbance of solvent would not interfere.
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