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) http://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 Standards | 0 | I | II | III | IV |
| Formazin Standard of Opalescence (60 NTU) | 0.0 ml | 5.0 ml | 10.0 ml | 30.0 ml | 50.0 ml |
| Distilled Water (fill to 100.0 ml mark) | 100.0 ml | 95.0 ml | 90.0 ml | 70.0 ml | 50.0 ml |
| NTU Rating | 0 | 3 | 6 | 18 | 30 |
Sources for EPOP Liquid Opalescence standards are:
Hach Company
Loveland, CO 80538 USA
+970-669-3050
www.hach.com
Hach offers the STABLCAL Reference Suspension Set that consists of a range for EP Opalescence Standards per EP is 0 (distilled water >0.1), 3, 6, 18, 30 NTUs. Given the 2-year stability, stabilizers will have been added to these liquid EP standards.
Hach STABLCALC set of EP Opalescence Standards
Another source for the Formazin Primary Opalescent Liquid Suspension (rated at 4000 NTU) and EPOP Liquid Opalescence standards (rated at 0 – 30 NTU) is:
RICCA Chemical Company
Arlington, TX 76094 USA
+817-461-5601
www.riccachemical.com
Reporting of EP Opalescence using a HunterLab Sphere Instrument and EasyMatch QC Software
As of EasyMatch QC version 4.82 and higher HunterLab has implemented a correlation method to the EP Opalescence scale based on these standards and is able to report EPOP-10mm (D65/10) for liquid samples measured in a 10 mm path length transmission cell. EPOP values are reported to tenths of a unit, along with NTU values as well.
Measure True Color 