As the chemical, petroleum, plastic, and pharmaceutical industries have grown, they developed criteria to judge the color of their products. Descriptions of slightly colored clear liquids were based on visual comparisons to dilutions of platinum-cobalt (Pt-Co) stock solutions. The whiter the solution, the more desirable the purity of the product and the more dilute the stock solution. Related to this, A Hazen documented in 1892, the American Public Health Association (APHA) color index. APHA ranges from near zero for clean water and up to 500 for waste water discolored by undesirable impurities and organic materials. This application note considers the use of the APHA index to measure the color of water white liquids.
What is APHA?
APHA is a single number yellowness index where each APHA unit is based on a dilution of the 500 ppm stock solution of PtCo. A detailed description of solution preparation and measurement procedures may be found in ASTM D1209 and ASTM D5386.
While instrumental determination of APHA is allowed by D1209 and D5386, no formula is prescribed by either method. For APHA, HunterLab uses a custom correlation based on known APHA values compared to instrumental measurements. Please note that these correlations apply only to HunterLab instruments able to read in Transmittance (2° Standard Observer/ Illuminant C).
Preparation of APHA Standards
The APHA zero (0) standard is physically represented by distilled water (for water-based products, toluene or benzene for resins, and mineral oil for oils), can be used is a blank during the standardization process. Pre-mixed and certified intermediate standards can be purchased from RICCA Chemical (www. riccachemical.com). The APHA platinum-cobalt 500 ppm stock solution is available from Fisher Scientific. Intermediate standards can also be prepared from the stock as described in ASTM D1209.
All APHA samples are to be prepared the same way and measured at the same temperature, after the same amount of mixing, etc.
In order for the APHA index to be meaningful, samples must be clear or slightly colored liquids that are similar in hue to the PtCo standards. In addition, steps should be taken to ensure that samples are non-scattering, or the APHA evaluation will be biased. Based on HunterLab‘s years of experience, a measured haze value above 5 % is visually hazy. The sample should be labeled as such or filtered before measurement.
If an APHA value is negative or much lower than expected, the sample may be off-hue. If an APHA value is very high, the sample may be of the correct yellow hue, but more saturated than the upper limit of the stock solution. In this case, the Gardner Index, which is designed for liquids that are darker yellow or brown may be used. Alternately a tristimulus color scale like CIE L*a*b* or Hunter L, a, b could be used.
APHA Sample Measurement
The instrument must be standardized in Total Transmittance (TTRAN) mode using a transmittance cell of the same path length as will be used in the measurement. Samples may be measured using a 10, 20, or 50 mm cell, and the calculations performed are specific to the cell size. Consistency with both the cell size used and the cell size indicated in the software is important or the measurements will not be comparable. For example, if a transmittance cell with a path length of 20 mm is used this should also be selected in the software as APHA-20 mm.
Precision of the visual method is non-linear to the increase in APHA value. For example, repeatability for a single operator at an APHA value of 25, is determined to be 3 units. At an APHA 475, repeatability was 16 units. Experimental reproducibility among 10 analysts is 10 units at APHA 25 and 49 units at APHA 475. A similar phenomenon is observed instrumentally. Single operator repeatability when determining APHA instrumentally is within 0.9 unit under APHA 30 and multiple operator reproducibility is within 2 units, to a 95 % confidence level.
The APHA/Pt-Co/Hazen Color Index is useful for measuring the trace yellowness associated with the degree of contamination or processing quality of transparent liquid samples that are near-colorless.
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