Clear liquid purity is measured using the APHA color scale and is essential for ensuring product safety and consistency and many various industries. Image Source: Flickr user Nathan Forget

Purity is an important factor imperative in the foods we eat, the medicines we take, and the water we drink. However, visual analysis of clear liquid color measurement is highly subjective, leaving a high margin of error in purity analysis. Color measurement using the APHA color index allows these various industries to maintain safety and consistency in their products.

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What Is the APHA Color System?

The American Public Health Association (APHA) color scale — or the Hazen scale — is a color scale that was developed using the Hazen Color Index. The APHA scale is a way of interpreting water — white or near-colorless liquids — and measuring wastewater using gradient values of yellowness. It measures the yellowness in liquid based on dilutions of platinum-cobalt (PtCo) solution.

The APHA color scale ranges from 0 to 500, with 0 as distilled white water and 500 as distinctly yellow water. Higher-purity liquids have less yellow and lower PtCo concentrations. Samples can be visually compared to a yellow reference solution, but more accurate results are obtained from instrumentation technology.

The APHA scale assesses the quality of yellow-to-clear liquids to:

  • Evaluate the purity of wastewater.
  • Detect any traces of organic substances or other unwanted impurities.
  • Assess pollution levels in wastewater.
  • Determine product quality.

Though it was created in 1892 by the chemist A. Hazen for public water reserves, the APHA color scale was quickly adopted by other industries. Many applications involve using Hazen’s PtCo base measurement system to evaluate the purity, consistency, and safety of products. The APHA scale is also used to compare the intensity of yellow-tinted samples.

APHA Color Standards

The APHA color measurement method measures the yellow hue in liquids in comparison to a platinum cobalt reference solution. Often referred to as the platinum/cobalt color, APHA color, or Hazen color scale, this system is one of the most common standards for the color measurement of liquids. These standards have been refined through the International Organization for Standardization (ISO) to create precise guidelines for industrial applications. These new standards rely on spectral absorption measurement for calibration to the APHA color standards, which rely on advance spectrophotometric instrumentation. American Society of Testing and Materials (ASTM) also relies on APHA/Pt-Co/Hazen color scale for standardization and is referenced in many industrial applications as well.

Original liquid color standards used visual comparison to reference samples to the APHA/Pt-Co/Hazen color scale. These systems were primarily used as a “performance qualification check of the instrument prior to measuring liquid product samples, or to validate the instrumental-to-visual correlation.” New technology relies on advanced spectrophotometers to create an internal diagnostic standard for instrument performance verification. The ability to verify these standards with advanced spectrophotometry offers a diagnostic tool for measuring samples in relation to the APHA color scale, keeping these results precise and consistent every time.


Measurement of the yellow hue in liquids in comparison to a platinum cobalt reference solution using spectrophotometers for calibration is required to meet the APHA color scale standards. Image Source: Flickr user Martin Kozák

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APHA Color System Industry Applications

Being able to assess the purity of clear liquids has applications in numerous industries, such as:

Besides water purity, the APHA color system also has applications for solutions such as:

  • Polymer additives
  • Hexane
  • Mineral oil
  • Glycerol
  • Resin solutions
  • Methanol

The color property for the products and solutions in these industries is unique. Color in these applications varies in importance based on its intended use and the amount of color that can be tolerated. Since color can be a sign of contamination, the presence or absence of a color can indicate how well the liquid has been refined or kept hygienic throughout production.


Sphere instrumentation technology has the ability to precisely measure clear liquid purity and maintain consistency with industry standards.

APHA Color Measurement Instrumentation From HunterLab

HunterLab leads the industry in sphere instrumentation technology, which has the ability to precisely measure visual ASTM D1209 APHA/Pt-Co Color scale in accordance with the instrumental ASTM D5386 method. Applications and standardizations vary from industry to industry, but as a leader in color technology, HunterLab offers a variety of instrumentation specifically designed for individual purposes and needs.

Contact HunterLab to see how color measurement instrumentation can help you meet purity and quality standards today.