What Path Length Cell should I use for APHA/Pt-Co/Hazen Color Measurement?

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?”

Nessler Tube Rack with  APHA Pt-Co Hazen 0-30-100-200-300-400-500 Color Standards

APHA Pt-Co Hazen    0-30-100-200-300-400-500 Liquid Color Standards in Nessler Tubes representing the range of this yellowness scale.

“APHA Color” is also known by two other names – “Pt-Co Color” and “Hazen units (HU)”. All three names represent the same color scale. As the APHA/Pt-Co/Hazen Color is dependent on the path length or thickness of the sample, it is important to:

  • Use the same cell path length for all measurements.
  • Report the cell path length as part of the measurement method if inter-instrument agreement of APHA/PtCo/Hazen Color values is important at multiple sites.

HunterLab’s instrument correlation to the visual APHA/Pt-Co/Hazen Color scale per ASTM D5386 allows the user to select a 10-mm, 20-mm or 50-mm path length cell by configuring the Color Data View in EasyMatch QC software to display “APHA-10mm”, “APHA-20mm” or “APHA-50mm”.

EZMQC Configuration of Color Data View for APHA Color for different cell path lengths

While any of these three cell path lengths can be used to effectively measure APHA/Pt-Co/Hazen Color, the optimal choice for cell path length choice is based on the following criteria:

  • A 10-mm path length cell is typically used for highly saturated or chromatic liquids, typically with APHA/Pt-Co/Hazen Color values > 300.
  • The easy-to-fill 20-mm path length cell is used for moderately chromatic liquids, and is the default cell path length when a wide APHA/Pt-Co/Hazen Color range of liquid colors is being measured from 0 (distilled water) to 500.
  • The longer 50-mm path length cell is preferred for clear or near clear liquids where APHA/Pt-Co/Hazen Color values < 30, and especially when APHA < 10.

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