OptLab-SPX   Pt-Co /APHA /Hazen Color Values

The colors of technical liquids are frequently classified according to the APHA-/Hazen-/Pt-Co color scale. These three designations are commonly used in different application areas, but they are based on identical procedures. The APHA-/Hazen-/Pt-Co color values complement the Iodine color scale for weak yellow/brown hues.

Technical liquids often have a slight yellow color due to contamination or decomposition products. Traditionally, color classification for this type of sample is performed by visual comparison of the sample with yellow reference solutions in defined vessels. According to a proposal by A. Hazen in 1892, the Pt-Co/APHA-/Hazen color scale uses an acidic solution of potassium hexachloro- platinate(IV) and cobalt(II) chloride. The reference solutions are designated according to their platinum content in mg/L in the range 0–500. The solutions can be obtained from commercial suppliers.

       Spectrum of a Pt-Co reference solution (APHA color value 10) in a 1 cm cell

There are several national and international standards which describe the procedure in more detail:
ASTM D 1209            Standard Method for Color of Clear Liquids (Platinum-Cobalt
                   Scale)
BS 5339:76 (1993)    Measurement of Color on Hazen Units /Platinum-Cobalt
                   Scale
DIN 53409                  Bestimmung der Hazen-Farbzahl (APHA-Verfahren)
DIN ISO 6271              Einstufung der Farbe nach der Platin Cobalt Skala

Typical application areas of the Pt-Co/APHA-/Hazen color scale include polymer additives, water and waste water, resin solutions, and solvents such as hexane, glycerol, methanol, mineral oils , etc. The recommendation of this procedure by the American Public Health Agency (APHA) has established the respective naming.

As in other areas of color classification, there is a trend nowadays to replace the complex and subjective visual comparison with reference solutions by objective procedures. For APHA-/Hazen reference solutions a degradation must also be taken into account since the solutions are not stable with regular use. An objective procedure can be based on the sample spectrum in the visible spectral range, i. e. 380–780 nm.

The OptLab-SPX application software includes an appropriate algorithm, which is based on a unique calibration of the Pt/Co reference solutions. The calibration data are contained with the software. Using these data, the sample’s Pt-Co/APHA-/Hazen value is calculated from the color values obtained for the sample spectrum. Calibration data are included for 1 cm, 5 cm, and 10 cm cells, so that also very light coloration can be determined. The detection limit can be estimated to 0.5 units. This gradation is much finer than recommended by DIN 53409 for example, which describes the rounding of the result to the nearest ten . OptLab also allows new calibration data to be added, the designations for the report output to be modified, or the numerical results to be reformatted.

Each one-dimensional color scale has its limitations, if the color of the sample differs markedly from the color of the reference solutions. With the integral algorithm included, the OptLab procedure is less sensitive to color variations compared to a procedure based on a single measurement wavelength, for example. Nevertheless color variations must be taken into account. For each result, OptLab-SPX calculates the hue angle, which allows an estimation of the color difference to be made. Sample solutions should be within a typical range.

Summary: With pre stored calibration values for the Pt-Co reference solutions, OptLab allows the Pt-Co/APHA-/Hazen color values to be automatically evaluated from a visible-range (380–780 nm) sample spectrum. This procedure avoids the tedious handling of expensive and strongly acidic reference solutions, and also makes the procedure substantially more objective and faster. Above the procedure is open for automation. The problem of color deviations to the color scale is solved by a corresponding sample parameter.