Wastewater Treatment Terminology: Colority (Color)
Wastewater Treatment Terminology: Colority (Color)
Human visual perception of color is highly complex. In order to describe color quantitatively, the International Commission on Illumination (CIE), based on experiments, records the sensations triggered in the human visual system by the radiant energy of different wavelengths within the visible spectrum using the color-matching functions of the three primary colors red, green and blue. These color-matching functions are then used to describe and apply color.
This standard specifies two methods for the determination of color. It determines the color of samples after 15 minutes of clarification. The pH value has a large influence on color, so pH shall be measured at the same time as the color determination.
1.1 Platinum-cobalt colorimetric method. This references the international standard ISO 7887—1985 "Examination and determination of the color of water". The platinum-cobalt method is applicable to clean water, lightly polluted water with a slight yellow tint, and relatively clean surface water, groundwater and drinking water.
1.2 Dilution multiple method. This is applicable to heavily polluted surface water and industrial wastewater.
The two methods shall be used independently and are generally not comparable.
This standard is not applicable when the hue of the sample and the standard solution are inconsistent.
The definitions in this standard are taken from CIE Publication No. 17 and are as follows:
Color: an optical property that changes the spectral composition of transmitted visible light.
True color: the color produced by dissolved substances and undissolved suspended matter, determined using the original, unfiltered or uncentrifuged sample.
Apparent color: the color produced only by dissolved substances, determined using a sample filtered through a 0.45 µm membrane filter.
2.4 Standard unit of colority, degree: the color produced when each litre of solution contains 2 mg of cobalt(II) chloride hexahydrate and 1 mg of platinum (in the form of hexachloroplatinic(IV) acid) is defined as 1 degree.
A color standard solution is prepared from potassium chloroplatinate and cobalt chloride and visually compared with the sample to determine the color intensity, i.e. the colority.
The colority of a sample is expressed as the degree value of the color standard solution to which it is equivalent.
Note: the standard degree derived from this unit is sometimes called the "Hazen unit" or "Pt–Co scale" [GB 3143 "Method for Determination of Color of Liquid Chemical Products (Hazen Unit — Platinum-Cobalt Color Number)"], or milligrams of platinum per litre.
Unless otherwise stated, only optically pure water and analytical reagent grade chemicals are used in the determinations.
3.2.1 Optically pure water: soak a 0.2 µm membrane filter (as used in bacteriological studies) in 100 mL of distilled or deionized water for 1 hour, filter 250 mL of distilled or deionized water through it, discard the first 250 mL, and use this water to prepare all standard solutions and as dilution water.
3.2.2 Colority stock solution, equivalent to 500 degrees: dissolve 1.245 ± 0.001 g of potassium hexachloroplatinate(IV) (K2PtCl6) and 1.000 ± 0.001 g of cobalt(II) chloride hexahydrate (CoCl2·6H2O) in about 500 mL of water, add 100 ± 1 mL of hydrochloric acid (ρ = 1.18 g/mL) and dilute to the mark in a 1000 mL volumetric flask.
Store the solution in a sealed glass bottle in the dark at a temperature not exceeding 30 °C. This solution is stable for at least 6 months.
3.2.3 Colority standard solutions: into a series of 250 mL volumetric flasks, pipette 2.50, 5.00, 7.50, 10.00, 12.50, 15.00, 17.50, 20.00, 30.00 and 35.00 mL of the stock solution respectively and dilute to the mark with water. The resulting colorities are 5, 10, 15, 20, 25, 30, 35, 40, 50, 60 and 70 degrees.
Store the solutions in tightly closed glass bottles in the dark at a temperature not exceeding 30 °C. These solutions are stable for at least 1 month.
3.3.1 Common laboratory apparatus and the following instruments.
3.3.2 Stoppered color-comparison tubes, 50 mL, of consistent specification and optically clear glass with no shadow at the bottom.
3.3.3 pH meter, accuracy ±0.1 pH unit.
3.3.4 Volumetric flasks, 250 mL.
All glassware that comes into contact with the sample shall be cleaned with hydrochloric acid or a surfactant solution and finally rinsed with distilled or deionized water and drained.
Collect the sample in a glass bottle of at least 1 L capacity and carry out the determination as early as possible after sampling. If storage is necessary, store the sample in the dark. In some cases also avoid contact of the sample with air, and avoid temperature changes.
Pour the sample into a 250 mL (or larger) measuring cylinder, allow to stand for 15 minutes, and decant the upper layer as the test portion for determination.
Fill a set of stoppered color-comparison tubes with the colority standard solutions to the mark. Fill another set of stoppered tubes with the test portion to the mark.
Place the stoppered tubes on a white surface at a suitable angle so that light is reflected upward through the liquid column from the bottom of the tubes.
Observe the liquid columns vertically from above and find the standard solution closest to the test portion in colority.
If the colority is ≥70 degrees, dilute the test portion appropriately with optically pure water so that the colority falls within the range of the standard solutions before determining.
Determine the pH of the sample separately.
Report the value of the standard solution closest to the test portion in standard units. In the range 0–40 degrees (excluding 40), report to the nearest 5 degrees; in the range 40–70 degrees, report to the nearest 10 degrees.
Report the pH value together with the colority of the sample.
The colority A0 of a diluted sample, in degrees, is calculated as follows:
A0 = A1 × V1 / V0
where V1 is the volume of the diluted sample in mL; V0 is the volume of the sample before dilution in mL; and A1 is the observed colority of the diluted sample in degrees.
Dilute the sample with optically pure water until, by visual comparison, it is just indistinguishable in color from optically pure water; the dilution multiple at this point expresses the color intensity, in multiples.
At the same time visually observe the sample and note the nature of the color: depth (colorless, light or dark), hue (red, orange, yellow, green, blue, purple, etc.), and, where possible, transparency (transparent, turbid or opaque). Describe in words.
The result is expressed as a combination of the dilution multiple value and a verbal description.
4.2.1 Optically pure water.
4.3.1 Common laboratory apparatus and stoppered color-comparison tubes, pH meter.
Take the test portion and optically pure water respectively into stoppered color-comparison tubes to the mark, place the tubes on a white surface at a suitable angle so that light is reflected upward through the liquid column, observe vertically from above, compare the sample with the optically pure water, and describe the colority and hue of the sample, including transparency where possible.
Dilution method: when the colority of the test portion exceeds 50 multiples, pipette an aliquot of the test portion into a volumetric flask and dilute to the mark with optically pure water, using a large dilution ratio each time so that the diluted colority is within 50 multiples.
Dilute the test portion stepwise with optically pure water to different multiples, placing each into a stoppered tube to the mark. Place the tubes on a white surface and compare with optically pure water using the same method as above. Dilute the test portion until it is just indistinguishable from optically pure water, and record the dilution multiple at this point.
When the colority of the test portion is below 50 multiples, take 25 mL of the test portion in a stoppered tube and dilute to the mark with optically pure water, using a dilution multiple of 2 each time.
When the test portion or its dilution reaches a very low colority, pour an appropriate amount of the test portion from the stoppered tube into a measuring cylinder, measure the volume, then dilute to the mark with optically pure water, using a dilution multiple of less than 2 each time. Record each dilution multiple.
Determine the pH of the sample separately.
Multiply the successive dilution multiples together; the integer value of the product expresses the colority of the sample.
At the same time describe the depth and hue of the sample's color in words, including transparency where possible.
Report the pH value together with the colority of the sample.
This standard was proposed by the Standards Office of the State Environmental Protection Administration. It was drafted by China Textile University. The chief drafters were Xi Danli and Chen Jihua. The interpretation of this standard is entrusted to the China National Environmental Monitoring Center.
Colority is a sensory indicator. Generally, pure natural water is clear and transparent, i.e. colorless. But wastewater carrying colored pollutants such as metal compounds or organic compounds appears in various colors. Dilute the colored wastewater with distilled water and compare it with a reference water sample until the two show the same color difference; the dilution multiple of the wastewater at this point is its colority.
