Read Chapter 10 in Simplified Procedures for Water Examination.
Introduction to Color
What Causes Color?Color in water is the result of dissolved extracts from metals in rocks and soil, from organic matter in soil and plants, and occasionally from industrial by-products. When color is caused by metals, it is usually due to iron, copper, or manganese ions in the water. Leaves and peat may add tannin, glucosides, and their derivatives to the water, resulting in a yellow or brown hue. Industries can add a variety of chemicals with various colors.
In water and wastewater treatment, we make a distinction between true color and apparent color. True color is the result of dissolved organics, minerals, or chemicals in water, as noted above. When testing the color of a water sample, the goal is to measure the true color of the water. However, suspended materials in the water (turbidity) can change the apparent color of the water (the color of the water before filtration.) As a result, the first step of measuring true color is to remove the water's turbidity by filtration or centrifugation.
You should also be aware that changes in pH can change the true color of water. As a result, pH is always measured along with color during color testing.
Two types of procedures can be used to measure the color in water - the visual method or the instrumental method. The visual method is the simplest since it consists of a water sample being compared to a series of colored slides or tubes. This method can be used in most cases, but it is not appropriate for use on water which has been contaminated by industrial wastes. Various instrumental methods can be used to create a more accurate portrayal of the water's color if the color cannot be matched using the visual method.
The visual method
When using the visual method, color is usually measured in color units. Water with color as high as 20 units will not ordinarily be noticed by the consumer, but the water plant should be operated so that the color will be less than 20 units. The EPA Drinking Water Standards limit for color, as related to filtered water, is 15 units.
Color in water is nearly always harmless. However, for aesthetic reasons, color in water should be maintained at an unnoticeable level. A plant will have little difficulty in producing an effluent with color of 10 units or less since coagulation followed by chlorination or ozonation tends to remove the majority of the color from water.
For any color testing method, the first step will be to remove turbidity, so we will describe this procedure first. Then we will briefly introduce two different visual methods and one instrumental method used for measuring color. All of these methods are treated in much greater depth in Standard Methods.
Before testing for color, you must remove the turbidity from the water. Removal of suspended matter by centrifuging has been found to give the best results. Although filtration will remove suspended matter, it also tends to change the true color of the water.
You can read more about removing turbidity by centrifuge in section 2120 C. 3. a. on page 2-4 in your text. The virtual lab demonstrates the removal of turbidity by filtration.
The Platinum-Cobalt Method
The standard method used for measuring color is a visual method known as the Platinum-Cobalt Method which is described in section 2120 B. of Standard Methods. In the Platinum-Cobalt method, color is measured by comparison of the sample to standards prepared with various concentrations of potassium chloroplatinate and cobaltous chloride. Both the sample tube and the standards are viewed against a white background and the standard tube closest in color to the sample is chosen.
Comparison with Glass Discs
The Platinum-Cobalt method is not well adapted for field work, so water is sometimes tested by comparing the sample to glass discs. These discs are produced by various laboratory supply companies in a series of hues, calibrated to correspond to colors on the platinum scale. Experience has shown that the glass discs used by the U.S. Geological Survey give result in substantial agreement with those obtained by platinum determinations, and their use is recognized as a standard procedure.
When using this method, the glass discs are held at the end of metallic tubes. The operator views the discs through the tubes while looking toward a white surface. The disc which most closely matches the color of the sample gives the number of color units found in the sample water.
Standard Methods explains the procedure used for a variety of instrumental methods of color determination.
You should view the virtual lab to familiarize yourself with this procedure. Notice that the operator treats the water by filtration to remove turbidity, then uses an instrument to read the amount of true color remaining in the water. There is an assignment that needs to be completed concerning this lesson and the virtual lab, so please print the assignment first and then answer the questions as you perform the lab. Once you have the assignment completed, log in and complete the assignment online to be entered directly into the database.
American Public Health Association, American Water Works Association, and Water Environment Federation. 1998. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, D.C.
Kerri, K.D. 2002. Water Treatment Plant Operation. California State University: Sacramento.