This part of ISO 6271 specifies a spectrophotometric method for estimating the colour, in Pt-Co units, of clear liquids. It is applicable to clear liquids having a colour characteristic similar to those of the reference platinum-cobalt scale specified in ISO 6271-1. The method described provides a more precise way of measuring Pt-Co colour than that described in ISO 6271-1. NOTE The term “platinum-cobalt colour (Pt-Co colour)” used here is preferred over the terms “Hazen colour” and “APHA colour”.