The ASTM D852 standard can be used to determine the purity of benzene. The closer the freezing point is to pure benzene, the purer the sample.
Meaning and use

5.1 This test method can be used as a standard to determine the purity of benzene. The closer the freezing point is to pure benzene, the purer the sample.

ASTM D852-2016 Standard Test Method for Freezing Point of Benzene

Step 1: Scope

1.1 The test method includes determination of the freezing point of benzene.

1.2 When using this method to determine the consistency of test results with applicable specifications, the results should be rounded according to the rounding method of Practice E29.

1.3 Values expressed in SI units shall be regarded as standard values. No other units of measurement are included in this standard.

1.4 This standard is not intended to address all safety concerns, if any, associated with its use. It is the responsibility of users of this standard to establish appropriate safety and health practices and to determine the applicability of regulatory restrictions prior to use. See Section 7 for specific hazard descriptions.