The reference document that defines the visual EP Opalescence scale is:
EP 2.2 Physical and Physico-Chemical Methods for color and opalescence
EP – European Pharmacopoeia, Section 2.2 Physical and Physico-Chemical Methods, Unit European Pharmacopeia, Strasbourg, France (1997: 15-16) www.pheur.org
This method describes the visual evaluation of scattering or opalescence in near clear liquids, typically pharmaceutical, relative to distilled water being a perfect clear.
There are two types of physical liquid standards for visual turbidity or opalescence – Formazin solution (with or without stabilizer) and polymer beads (polystyrene micro spheres). The Formazin solution is the historical liquid scattering standard but the polymer beads is considered more stable and homogenous.
Section 2.2.1 Clarity and Degree of Opalescence of Liquids in the EP 4th edition defines a Formazin Primary Opalescent Liquid Suspension (rated at 4000 NTU per EP 5th edition) as a solution of hydrazine sulphate solution and hesamethylenetetramine solution which is stable for 2 months stored in glass.
The EP 4th edition further defines a Formazin Standard of Opalescence (rated at 60 NTU per EP 5th edition) as a dilution of 15.0-ml of the Formazin Primary Opalescent Liquid Suspension (4000 NTU) to 1000.0–ml of water. This suspension must be freshly prepared and stored for no more than 24 hours.
To make the EP Reference suspensions or OP – Opalescence standards, the Formazin Standard of Opalescence (60 NTU) is mixed with distilled water in the following proportions to define 4 levels of liquid EPOP Opalescence Standards. Distilled water is nominally a fifth EPOP standard defining no opalescence or scattering.
Table 2.2.1-1 EPOP StandardsIIIIIIIV Formazin Standard of Opalescence (60 NTU)0.0 ml5.0 ml10.0 ml30.0 ml50.0 mlDistilled Water (fill to 100.0 ml mark)100.0 ml95.0 ml90.0 ml70.0 ml50.0 mlNTU Rating361830
