I don't believe you can do that. Pure or not. I don't remember the exact explanation behind it but to properly calibrate your refractometer, you need the calibration solution of 1.026 SG. I believe the reason being is that the refractometer is not as accurate the farther out of its normal range would be, ie in the 1.001 - 1.030 range. Calibrating to zero I believe throws the accuracy off of the range you really want the most accurate, 1.023 - 1.026.
Someone correct me if I am wrong though.
