This application note explains how to measure APHA color (also called Hazen) using a photometer, either online or in a laboratory setting.
The APHA color scale, also known as the Platinum-Cobalt (Pt/Co) or Hazen scale, serves as a “yellowness index.” It compares the intensity of yellow-tinted samples to assess the quality of clear to yellowish liquids. Originally developed to assess municipal water purity, APHA color monitoring is now widely used across industries such as water treatment, chemical, oil, plastics, and pharmaceuticals. APHA color measures trace amounts of yellowness, which can indicate product degradation from light or heat, impurities, or processing effects. As a result, many industries rely on APHA color as a key product release specification.
APHA is a single-number yellowness index, based on dilutions of a 500 ppm PtCo solution. Distilled water has an APHA value of zero, while the stock solution measures 500. The ASTM D1209 standard defines the methodology for measuring the color of clear liquids.
ASTM D1209 APHA COLOR MONITORING Using Process Spectroscopy
Our GUIDED WAVE™ ClearView® db Dual Beam Photometer provides accurate APHA color measurements, both online and in the lab. This photometer’s dual-beam design ensures precise and repeatable measurements with excellent linearity. It can be configured to measure two separate sample locations, reducing the cost per sample point, and is designed for flexibility in various applications, offering stable, long-term performance.
To calibrate the ClearView db photometer for APHA color monitoring, a series of standards ranging from 100 to 500 were used. The ClearView db photometer allows for real-time, online APHA color monitoring at two sample locations. It is configured with one analytical wavelength and one reference wavelength, connected via fiber optics to a sample cell (flow cell or inline probe) with a 30 mm pathlength. This setup provides highly accurate APHA color measurement in real-time.
Furthermore, a simple linear regression was performed between absorbance at 456nm and APHA color values, as shown in Figure 1, with an R2 value of 0.999. An R2 value of 1 indicates a perfect correlation between the model and the reference method. The ClearView db photometer offers a long-term photometric drift of less than 500 μAU rms, ensuring excellent stability for long-term APHA color measurements. For cases requiring lower APHA value ranges, a longer pathlength cell can be used to achieve the necessary sensitivity.
In summary, the ClearView db photometer provides a robust and cost-effective solution for APHA color monitoring across a range of industries. With its real-time capabilities and reliable accuracy, it ensures that APHA color values meet stringent quality standards, maintaining product consistency and compliance.
