The best choice for a blank when calibrating the spectrophotometer is distilled water.
Distilled water works best as a "blank" for calibrating the spectrophotometer. A blank is used to zero the spectrophotometer before measuring the absorbance of an unknown analyte/sample.
Before measuring the absorbance of an analyte, the spectrophotometer must be calibrated. This is because substances other than the specimen absorb light. If the instrument is not calibrated properly, then the absorbance obtained will be the sum of the absorbance of the analyte and these other substances. Distilled water does not interfere with the absorbance measurement of the analyte and is ideal for zeroing the absorbance of other substances.
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