APHA/Hazen/Pt-Co vs. Gardner Color Scale
Chemicals are used in a variety of industries including automotive, food and beverage, personal care and electronics. These chemicals need to undergo stringent quality control (QC) testing to meet its intended quality specifications and color measurement is an important part of this test procedure. Beside color space like CIE L*a*b* and L*C*h, industrial standard color indices like APHA Color and Gardner Color are commonly used to measure the color of liquid chemicals.
APHA Color, also called Hazen or Platinum Cobalt (Pt-Co) scale, has a range of 0 (achromatic clear) to 500 (light yellow) and was originally developed to evaluate water quality. Since then, its usage has extended to liquid chemicals such as petroleum, distillates, solvents and alcohols which are either colorless or slight yellowish. Gardner Color, with a range of 1 (light yellow) to 18 (red), is commonly used to evaluate color of drying oils, varnishes, lecithins, fatty acids, resin solutions and liquid chemical products which are yellowish or reddish.
Although APHA Color and Gardner Color have different intended use, some applications may require the combination of both indices. For instance, a slight yellowish chemical may start off using Gardner Color, but after refinement, it turned colorless which then required the use of APHA color. Generally, an APHA value of 500 is equivalent to the Gardner value of 2-3.
The earliest form of both APHA Color and Gardner Color were liquid standards and rely comparing sample against the liquid standards visually. This method is subjective and difficult to maintain consistent color judgement. Today, with advanced color measurement technologies, color measurement of chemicals can be done objectively and consistently using color measuring instruments like Spectrophotometer CM-5.
The CM-5, capable of both reflectance and transmittance mode, can measure the color of samples in various forms like solid, pastes, granular and liquid. It offers all major colorimetric systems, including industrial standard color indices like APHA, Gardner Color Scale and Iodine Color Number. When used together with SpectraMagic NX software, indices such as Correlated Haze is also avaie.
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