APHA10 Testing with Datacolor Tools QC
Background
APHA is a visual scale for the determination of color of near-white yellow liquids and other materials. APHA is an acronym which stands for the American Public Health Association. The APHA scale was developed in the 1880’s to evaluate the quality of America’s water supply. It is a visual scale comparing the color of a liquid (water) to a standard reference material. Later the use of the scale expanded to assess the color of treated waste water. The modern day APHA10 Color Scale is based on work by Richard Hunter in the 1980’s.
The procedure involves comparing the color of water visually to standardized solutions (APHA) containing increasing concentrations of platinum cobalt. Platinum cobalt is slightly yellow as is wastewater.
APHA Scale
The APHA is a scale derived on the measurement of Platinum Cobalt, PtCo, at various concentrations in water. ASTM D1209-05 states in the Scope that: “It is applicable to materials in which the color-producing bodies present have light absorption characteristics nearly identical to these of platinum cobalt color standards.” A word of caution here-This means that the APHA scale cannot be used to evaluate materials that are not the same hue.
The APHA scale is defined in terms of Illuminant C using the 2o Observer function. This is the only illuminant/observer condition supported.
The APHA scale is designed to correlate well (0.9999) with the ASTM E313 Yellowness Index, YI function.
Instrumentally
The specimens to be examined are to be measured in transmission using a 10mm path length absorption cell.
Scale Determination
The determinations of the APHA10 Color Scale values are based upon correlating to ASTM E313 YI values.
Absorption Cells
ASTM D1209 specifies a 10mm light path spectrophotometric cell in the apparatus section. Only the 10mm path length absorption cell can be used to measure an APHA scale value. The path length of the absorption cell is indicated by the subscript immediately following the scale; for instance APHA10. Currently only the 10mm path length is supported. Absorption cells of other path lengths will be supported in the future.
Scale Limitations
This scale is valid only for the measurement of near white solutions that are slightly yellow. Other specimens that have a slightly different hue will report values that may (will) not correlate with the anticipated result.
The current scale is defined over the limits of 0 to a maximum of 500 units. Measured values above this range should be evaluated using another tristimulus metric; such as, CIELAB.
Specimens must be clear and exhibit no-scattering as defined in ASTM D1003. ASTM D1209 states that: “. . . pass the specimen through a filter if it has visible turbidity.” Any haze or cloudiness caused by suspended particles will adversely affect the measurement results, the data, and its analysis should be questioned.
How to Obtain APHA10 Readings in Datacolor Tools QC
You must use the spectrophotometer’s transmission configuration to make these readings.
You must use a transmission cell/cuvette with a light path no greater than 10 mm.
You will need reagent grade water.
These calculations require you use illuminant/observer combination C/2.
You will need to open and use the Tools QC screen form APHA 10 Scale.
APHA10 Scale Transmission Measurement Procedure
1. Insert the Cuvette Holder on the Transmission Sample Holder Base, Position 3. This is the position for the measurement of regular transmittance.
- Fill the 10 mm transmission cell, Datacolor PN 2950-0004, with Type 1 Reagent Grade Water.
- Place the 10 mm transmission cell (Transmission Cell) in the Cuvette Holder.
- Press the “Clear Liquid” button on the form (“APHA10 Scale” )
- Remove the transmission cell and dispose of the liquid (water), appropriately.
Re-measure the cell every time the new cell is utilized the system.
Fill the transmission cell with the liquid specimen to be measured.
- Press the “APHA Std :Instr” or “APHA Bat: Instr” buttons.
The APHA10 measurement process is complete. The APHA value for the standard sample will be displayed there. If you also measured a batch sample, its APHA value will be displayed also, as well as the difference between the standard’s and the batch’s APHA values.