Natural color, like turbidity, is due to a wide variety of substances, and it has been necessary to adopt an arbitrary standard for its measurement. This standard is employed directly and indirectly in the measurement of color. Many samples require pretreatment to remove suspended matter before true color can be determined. The method of pretreatment must be carefully selected to avoid introduction of errors.
Waters containing natural color are yellow-brownish in appearance. Through experience, it has been found that solutions of potassium chloroplatinate (K2PtCl6) tinted with small amounts 'of cobalt chloride yield colors that are very much like the natural colors. The shading of the color can be varied to match natural hues very closely by increasing or decreasing the amount of cobalt chloride.
The color produced by 1 mg/L of platinum (in the form of K2PtCl6) is taken as the standard unit of color. The usual procedure is to prepare a stock solution of K2PtCl6 that contains 500 mg/L of platinum. Cobalt chloride is added to provide the proper tint. The stock solution has a color of 500 units, and a series of working standards may be prepared from it by dilution. A matched set of color-comparison tubes, commonly called Nessler tubes, as shown in Fig. 14.1, are usually used to
jCulur-companson tubes, commonly called Nessler tubes.
¡fentain the standards. A series ranging from 0 to 70 color units is employed and Mil serve for several months, provided that it is protected from dust and evapora-|,on. The color-comparison tubes should be a matched set conforming to American pblic Health Association standards as described in the introductory chapter of ¡¡Standard Methods."
P Samples subjected to analysis may contain suspended matter that will interfere pith the measurement of true color. Apparent color is determined on the sample "as ¡1" Suspended matter must be removed to enable determination of true color. This |an usually be accomplished by centrifuging the sample to separate the suspended l^ids. Analysis is performed on the clarified liquor. Filtration is not recommended ¡fcause of possible adsorption of color on the filtering medium. | Satnp)es with color less than 70 units are tested by direct comparison with the pre-■ standards. For samples with a color greater than 70 units, a dilution is made with JjTMneraljzed water to bring the resulting color within the range of the standards, and I llation of color is made, using a correction factor for the dilution employed.
A number of instruments have been developed for the measurement of color to " j eliminate the need for renewing standard color solutions from time to time. Most of ;; these instruments employ colored glass disks that simulate the various color stan- " dards when used in the particular instrument. '
The proprietary devices find their greatest use in water works laboratories where trained chemists are not employed, or for field measurements where the use of standard color solutions is not practical. They are not accepted as a standard procedure for measuring color because of variations in the color of the glass disks and their tendency to change characteristics, owing to fingerprints, dust, and so on. They should always be standardized against standard solutions of K2PtCl6 for highly important work.
Spectrophotometric Methods Applicable to Domestic and Industrial Wastewaters
Many industrial wastes are highly colored, and some contain colored substances that are quite resistant to biological destruction. Regulations concerning the color of effluents that may be discharged to streams are common. Evaluation of the color of ! yellow-brownish-hue wastes can be made by the standard procedures previously described. Other systems of measurement have to be used to measure and describe ; colors that do not fall into this classification. "Standard Methods" lists three possible spectrophotometric methods for color determination. One involves use of a normal spectrophotometer with an operating range from 400 to 700 nm and the collection of percent transmissive values on a sample at several different wavelengths. A calculation procedure together with a color table is then used to express sample color in terms of dominant wavelength, luminance, and purity. The other two procedures make use of filter photometers and three different color filters to achieve similar characterization as provided by the spectrophotometer approach. These procedures obviously provide much more detail about color than given by the simple platinum standard approach. They may be appropriate for use when looking for the source of color in a water supply or in evaluating the effectiveness of color removal procedures. The results, however, do not translate directly into the U.S. EPA secondary standard or WHO guideline of 15 color units for drinking water. Thus they may not be appropriate for routine monitoring to ensure compliance with drinking water requirements.
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