This article provides a history and overview of the APHA scale to measure clear liquids.
Prior to the 1890s, descriptions of slightly colored clear liquids were based on the concept of "water white." There were many interpretations of "water white" and it became necessary to have a more specific method for describing the color of wastewater. In 1892, chemist A. Hazen described the American Public Health Association (APHA) color index. The index was initially used to evaluate the color of wastewater by comparison with dilutions of a platinum-cobalt (PtCo) stock solution. The index was used as an indication of purity, as the color of wastewater is produced by undesirable impurities and organic materials.
As the chemical, petroleum, plastic, and pharmaceutical industries grew, they also developed the need to measure the color of their products, some of which were similar in hue to the PtCo solutions in A. Hazen's method. In many cases, the color of their almost "water white" products was an indication of purity, the level of refinement, or the cleanliness of the product container. In 1952, ASTM Test Method D1209 was adopted using a PtCo scale similar to that described by A. Hazen.
APHA is a single number yellowness index where each APHA unit is based on a dilution of the 500ppm stock solution of PtCo. Distilled water has an APHA value of zero. The stock solution has an APHA value of 500. The PtCo scale and Hazen scale are also based on this sample reagent dilution and have units equivalent to APHA units. APHA is the name used in HunterLab systems, although it could also be called the PtCo scale. A detailed description of solution preparation and measurement procedures may be found in ASTM Designation D1209, 'Standard Test Method for Color of Clear Liquids (Platinum-Cobalt Scale).'
ASTM Designation D5386, "Standard Test Method for Color of Liquids Using Tristimulus Colorimetry," describes how color measurement instruments correlate to the physical APHA/PtCo color standards described in ASTM D1209.
In order for the values of this index to be meaningful, the samples to be measured must be clear, slightly colored liquids that are similar in hue to the PtCo standards. Using Hunter Lab's EasyMatch QC or Universal Software with a Vista Sphere or UltraScan, samples may be measured using a 10, 20, or 50-mm cell, and the calculations performed are specific to the cell size. The instrument must be standardized in Total Transmittance (TTRAN) mode using a transmission cell of the same path length as will be used in the measurement. The solvents suggested for use in standardization are distilled water for water-based products, toluene or benzene for resins, and mineral oil for oils.
Sources of the PtCo 500 Color Standard
The APHA/PtCo 500 color standard can be made using the chemical formula described in ASTM D1209 or it can be purchased from the following vendors:
Fisher Scientific Company, Telephone: (800) 766-7000 FAX: (800) 926-1166, WEB: http://www.fishersci.com
RICCA Chemical Company, Telephone: (888) GO-RICCA, www.riccachemical.com
Spectrum Chemical Manufacturing Company, Telephone: (800) 525-2299, Telephone: (800) 813-1514, www.spectrumchemical.com/retail/
Conditions for Measurement
Instrumental: Vista, UltraScan PRO, or UltraScan VIS
Illuminant: C
Standard Observer Function: 2-degree
Transmittance and/or Reflectance: Transmittance only.
Formulas
APHA values are calculated from Yellowness Index E313 in accordance with ASTM D5386 using a proprietary formula that correlates well with the APHA standard solutions as defined in ASTM D1209.
Typical Applications
This index is often used by the water processing, chemical, petroleum, plastic, and pharmaceutical industries for the measurement of color in products that are nearly clear or "water white."
What is the Formula for APHA?
There is no published formula for APHA. The ASTM method on APHA (D1209) describes only visual evaluation of APHA, where samples are visually compared to known standards. While instrumental determination of APHA is allowed by D1209 and D5386, no formula is prescribed by either method. In order to allow you to determine APHA with your instrument, HunterLab uses a correlation it developed by comparing solutions of known APHA values to instrumental measurements. Since APHA is a yellowness scale, HunterLab based its correlation on yellowness index (YI E313) values. This correlation was optimized for each of its instruments that is capable of measuring APHA. These correlations are proprietary and apply only to HunterLab instruments.
Why Don't My APHA Value Make Sense?
If your APHA value is negative or much lower than you expected, your sample may be off-hue.
The APHA index was originally designed to evaluate the color of near-colorless wastewater by visual comparison of the water to dilutions of a platinum-cobalt (PtCo) stock solution. PtCo solutions are yellow, as were the wastewater samples of interest. While other types of samples, such as liquid chemicals, petrochemicals, plastics, and pharmaceuticals, are now evaluated using the APHA index as well, the expectation that the liquids measured would be of a yellow hue similar to PtCo solutions has not changed. APHA is a yellowness index, and cannot be used in evaluating liquids of other hues, such as blue or red. Even those liquids that appear colorless, yet when measured by a spectrophotometer that is more sensitive than the human eye are actually determined to be of a hue other than yellow, should not be evaluated using the APHA index. Off-hue samples can yield negative or meaningless APHA values.
ASTM D1209 states that the method is 'applicable only to materials in which the color-producing bodies present have light absorption characteristics nearly identical with those of the platinum-cobalt color standards used.' It later directs, 'If, owing to differences in hue between the specimen and the standards, a definite match cannot be obtained, report the range over which an apparent match is obtained, and report the material as 'off-hue'.'
If your APHA value is very high, your sample may be of the correct yellow hue, but more saturated than even the APHA 500 PtCo stock solution that marks the upper limit of the scale. In this case, you might be better off evaluating your sample using the Gardner Index, which is designed for liquids that are darker yellow or browner than PtCo solutions, or you could use a tristimulus color scale like CIE L*a*b* or Hunter L, a, b.
Examine the values in the table below. The PtCo 500 ppm stock solution is still very light (L = 97.35 on a scale of 0 to 100) and not that saturated in b (yellowness; 14.39). It is also slightly green (small negative a). Many, many yellow solutions will be more saturated than the stock solution and will not be well-described by the APHA index.
For readings that don't match expectations, it is also possible that your sample is hazy, and the light scattering caused by suspended solids is affecting your measurements. ASTM D1209 section 7.1 states that you should 'pass the specimen though a filter if it has any visible turbidity.' In other words, steps should be taken to ensure that the sample is clear and non-scattering, or the APHA evaluation will be biased. Based on Hunter Lab's years of experience, a measured haze value above 5% is visually hazy. The sample should be labeled as such or filtered before measurement to make it clear.
Sample Preparation
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 Hunter Lab'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 20mm is used this should also be selected in the software as APHA-20 mm.
Method Precision
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.
Why Do My APHA Values Seem to Vary So Much?
Unless there is a problem with your instrument or measurement method, the variation is probably not as significant as you think.
Table 4 in ASTM D1209 reports precision obtained in a study of APHA values assigned visually. At APHA 25, the repeatability for a single operator was determined to be 3 units. At APHA 475, repeatability was 16 units. Reproducibility among 10 analysts was 10 units at APHA 25 and 49 units at APHA 475. The difference is because it is easier to visually determine differences between similar solutions in the lower APHA ranges than in the higher ranges. A similar phenomenon is observed instrumentally.
According to ASTM D5386, single operator repeatability when determining APHA instrumentally is within 0.9 unit under APHA 30 (corresponding with ASTM D5386's direction to report APHA to the nearest whole unit under APHA 30), and multiple operator reproducibility is within 2 units, to a 95% confidence level. This means that if your instrument is operating properly and your methodology is sound, one operator should be able to make multiple measurements of the same solution that match within 0.9 APHA unit and multiple operators should be able to make measurements of the same solution that match to within 2 APHA units when reading solutions with APHA values (ppm concentrations) below 30. If you are unable to measure to within this repeatability, you should consider that either your instrument or your method may be faulty.
Why is a one-to-two-unit variation in APHA allowable when much smaller variations in tristimulus color values are achievable? It's due to the math.
Small variations in tristimulus values (such XYZ or L, a, b) normally occur in instrumental measurements. Remember, though, that APHA is determined by a correlation to YI E313, which is a calculation using X, Y, and Z.
Small variations in X, Y, and Z yield larger variations in YI (and therefore APHA) when plugged into this formula, due to the multiplication and division. Therefore, an instrument range (such as 2 units) that seems very large when applied to a tristimulus value (such as X or L), is not as extreme when applied to APHA. To check the repeatability of your instrument on a known, stable standard, run the repeatability test described in your user's manual.
As far as methodology, ensure that you are always using the same size of sample cell when making APHA measurements, and that the APHA index selected in your software matches the cell size in use. For example, if you measure your solutions in a transmission cell with a path length of 20 mm, be sure that your software lists 'APHA-20 mm' as the selected index. You should be consistent with both the cell size used and the cell size indicated in the software or else your measurements will not be comparable. Make sure all samples are prepared the same way and are measured at the same temperature, after the same amount of mixing, etc.
How Close Should My Instrument Read to the Known Values of the Stock Standards?
Distilled water should yield an APHA value of zero to within a unit or two. A difference less than 2 units is not visually significant at this level, according to ASTM D1209.
The 500 ppm PtCo stock solution as purchased should yield an APHA value of 500 to within twenty units or so.
Why aren't they closer? Additive error.
Dilutions of the stock solution theoretically have APHA values equivalent to their ppm concentrations. However, the dilution itself introduces some error into the equation due to calibration error in the glassware (both the pipette and the volumetric flask), as well as operator imperfections. Add to that the instrumental variations described in the last question, and you'll see why the raw numbers come out a little further from the 'known' than you might expect.
When in doubt as to whether your numbers are acceptable, run the diagnostic tests described for your instrument (such as the green tile check and the didymium filter test) in its user's manual to ensure that your instrument is in good working order.
Where Can I Get APHA Standards?
The APHA zero (0) standard is distilled water, which should also be used as a blank during the standardization process.
The APHA platinum-cobalt 500 ppm stock solution is available from Fisher Scientific at www.fishersci.com. Search on 'platinum cobalt.' Intermediate standards can then be prepared from the stock as described in ASTM D1209.
Pre-mixed and certified intermediate standards can be purchased from RICCA Chemical, www.riccachemical.com. Search on 'APHA.
What is an APHA/Pt-Co/Hazen 500 Color Standard?
Today, APHA is used as a metric for purity in the chemical, oil, plastics, and pharmaceutical industries. This scale serves to quantify the appearance of yellowness, a visual indicator of product degradation due to exposure to light and/or heat, the presence of impurities, and negative effects of processing.
This visual yellowness color scale is based on a chemical reagent solution, best defined in ASTM D1209 Standard Test Method for Color of Clear Liquids (Platinum Cobalt Scale).
The instrumental APHA/Pt-Co/Hazen method designed to correlate to the ASTM D1209 visual APHA/Pt-Co/Haze color scale is best defined in ASTM D5386 Standard Test Method for Color of Liquids Using Tristimulus Colorimetry.
APHA/Pt-Co/Hazen is a single number, low chroma yellowness metric based on the dilution of a reagent Pt-Co (potassium chloroplatinate) stock solution. Each APHA unit is based on a dilution by volume of 500 parts of Pt-Co stock solution per million parts of distilled water. Distilled water ('water-white') is rated as APHA/Pt-Co 0. The reagent APHA/Pt-Co/Hazen stock solution carries a rating of 500 APHA/Pt-Co/Hazen.
The definition of what is a 500 APHA/Pt-Co/Hazen reagent color standard is found in Section 6 of ASTM D1209.
Section 6 has two parts. A 500 APHA/Pt-Co/Hazen reagent color standard must have a particular chemical composition, and an absorbance criterion. After blanking spectrophotometer on the 10 mm cell and DI water, the measured absorbance of the 500 APHA/Pt-Co/Hazen reagent color standard must be within a defined acceptance window at 4 spectral points in the blue region of the visible spectrum. These absorbance points happen to correspond slopes for the 500-color standard and are quite sensitive to concentration.
If an Pt-Co solution meets both chemical and absorbance criteria, it is an APHA/Pt-Co/Hazen 500 reagent color standard, suitable for visual APHA/Pt-Co/Hazen evaluation of sample solutions per ASTM D1209.
If three (3) sites have their own APHA/Pt-Co/Hazen 500 reagent color standards, would them all measure them as 500 on their instruments?
Given that there is a range of absorbance acceptance criteria per D1209, there will be some variation in the measured values of multiple APHA/Pt-Co/Hazen 500 reagent color standards when measured on the same instrument, or on multiple instruments.
The saving grace is that while there may be some measurable difference among multiple 500 color standards, this largely disappears at the low end of the APHA/Pt-Co/Hazen color scale where most clients are measuring samples.
What are Some Good References on APHA?
'APHA,' HunterLab Applications Note, November 16-30, 1996.
Designation D1209-00, 'Standard Test Method for Color of Clear Liquids (Platinum-Cobalt Scale),' ASTM International, West Conshohocken, Pennsylvania, 2000.
Designation D5386-04, 'Standard Test Method for Color of Liquids Using Tristimulus Colorimetry,' ASTM International, West Conshohocken, Pennsylvania, 2004.
Hazen, A., 'A New Color Standard For Natural Waters,' American Chemist Journal (14:300), 1892.
Hazen, A., 'The Measurement of the Colors of Natural Waters,' American Chemist Journal (18:264), 1896.
ISO 2211, 'Liquid Chemical Products -- Measurement of Colour in Hazen Units (Platinum-Cobalt Scale),' International Organization for Standardization, Geneva, Switzerland, 1973.
ISO 6271, 'Clear Liquids -- Estimation of Colour by the Platinum-Cobalt Scale,' International Organization for Standardization, Geneva, Switzerland, 1997.
Method 110.2, 'Color - Colorimetric - Platinum-Cobalt,' Methods for the Chemical Analysis of Water and Wastes, U.S. Environmental Protection Agency, Washington, D.C., 1983.
Method 2120, 'Color by Visual Comparison,' Standard Methods for the Examination of Water and Wastewater, American Public Health Association, Washington, D.C., 1998.
Conclusion
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.
See attached pdf files for the complete article with illustrations.
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