There are no physical liquid standards specifically designed to verify the Yellowness Index of liquid samples over time. Yellowness Index is defined as a mathematical function such that a perfect clear of 100% transmission, typically represented by the transmission cell filled with DI water, will have a YI value of 0.
However, a good workaround solution is to use a very stable APHA/Pt-Co liquid standard in the yellowness range of your product as an application diagnostic to monitor your liquid measurement over time.
- Purchase an APHA/Pt-Co 500 color standard that conforms to ASTM D1209. Prepare an intermediate APHA/Pt-Co liquid standard close in yellowness to your sample by diluting an APHA/Pt-Co 500 color standard with DI water following ASTM D1209.
- Standardize the HunterLab sphere instrument in TTRAN LAV mode using your cell filled with DI water.
- As a PQ (Performance Qualification) step after standardization, read the cell filled with DI water. You should obtain CIE L* = 100, a* = 0, b* = 0, YI = 0.0 if the instrument is set up and standardized correctly.
- Read the cell with the intermediate APHA/Pt-Co liquid color standard and baseline the Yellowness Index value of this stable yellow liquid on the first day. From that point on, read this APH-Pt-Co liquid color standard after standardization as a further Application Diagnostic. The YI value should match the baseline value closely over time.
- You can create a statistical process control (SPC) chart with the YI values of both the DI water and the dilute APHA/Pt-Co liquid color standard over time as a record of instrument performance in the same region of color space as your sample.