The APHA/Pt-Co/Hazen and Gardner visual color scales were both originally based on liquid chloroplatinate color standards but have different history and intended use.
EP – European Pharmacopoeia Color is a visual liquid color scale used primarily in the pharmaceutical industry. It consists of 3 primary color standard solutions (yellow, red and blue) mixed with hydrochloric acid to make 5 standard color solutions – B (brown), BY (brownish-yellow), Y (yellow), GY (greenish-yellow) and R (red).
FAQ: ” We are currently using a 10 mm path length and read in the 0 – 50 region for APHA/Pt-Co/Hazen Color, usually in the area of 10. Would you recommend a 10 mm path length with our equipment for this application?”