APHA Color System: A Measurement of Liquid Purity

Purity is an important word that we often take for granted in our daily lives, but it’s imperative in the foods we eat, the medicines we take, and the water we drink. The purity of clear liquids has applications in numerous industries, such as petroleum, chemicals, plastics, textiles, and pharmaceuticals. Color measurement using the APHA color index allows these various industries to maintain safety and consistency in their products.

The APHA (American Public Health Association) color scale was developed using the Hazen Color Index. Created  in the late 1800’s by the chemist A. Hazen, the APHA color scale was used as a way of interpreting “white water” and measuring waste water using gradient values of yellowness. This system was used to evaluate the purity of waste water and detect any traces of organic substances or other unwanted impurities. The APHA color scale was quickly adopted by other industries that were able to use Hazen’s platinum-cobalt (PtCo) base measurement system to evaluate the purity of their products.

clear liquid medicine APHA color
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

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 ISO (International Organization for Standardization) 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. ASTM (American Society of Testing and Materials) also relies on APHA/Pt-Co/Hazen color scale for standardization and is referenced in many industrial applications as well.

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semi-clear color liquid APHA in a bottle
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

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.

APHA color measurement instrumentation

Visual analysis of clear liquid color measurement is highly subjective, leaving a high margin of error in purity analysis. “The instrumental method provides an objective quantification of the APHA Color Scale with much better precision than the visual method, particularly for near colorless samples.” 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.

vista benchtop sphere APHA color measurement device
Sphere instrumentation technology has the ability to precisely measure clear liquid purity and maintain consistency with industry standards.

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. Our friendly staff is knowledgeable and we value our customers, working hard to develop relationships that meet the ever-changing needs and regulations of the color measurement industry. Contact HunterLab to see how color measurement instrumentation can help you meet purity and quality standards today.

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  • Tara Shanker Kumar

    I want to use the APHA test method to compare the color the various grades of “Cresylic Creosote”. Is it possible to compare and grade them.

    Please advise

  • hunteradmin

    What color is your “Cresylic Creosote”? APHA is used to measure the yellow samples. If your sample is in this color range, then you can use APHA to grade your sample. At the same time, you can always use the color scale (e.g. L* a* b*) to differentiate their samples. We recommend you to email us at support@hunterlab.com for further questions. Thank you.

  • Rajdipsinh vansadiya

    Can we measure our liquid Color APHA value with UV-Vis spectrophotometer ? Which wavelength to be used.

    Please suggest, Is that any standard solution which have 10 max value.

  • rashi shetty

    Thanks for sharing this article. I think UV water analyzer imported, high reliability, Long lifetime, suitable for long time on-time monitoring. The photoelectric measurement method makes accurate sampling with low injection error.

  • swathisingh

    thankyou for sharing your blog

  • Dr. Sunil Bhongale

    Is measurement of APHA value related to liquid substance only?
    Can we measure it for solid substances as well?
    If yes, which solvent the solid sample should be dissolved in ? and what should be the concentration level (w/V) of such solution?

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