Q15. When taking samples from a standing water supply, you take the samples from how far below the surface and at what angle?
LEARNING OBJECTIVE: Understand procedures and analysis for different types of water tests.
Various analyses of water must be performed by trained chemists or skilled laboratory technicians. As a Utilitiesman, however, you must be able to perform various types of treatment control tests. These tests are used during treatment to ensure proper operation and the output of safe water of acceptable quality. We will describe the procedures to follow in carrying out a number of treatment processes, such as chlorination, corrosion control, and clarification.
Before proceeding, note that certain tests which we will cover are based on the simple principle of adding a chemical to the sample that forms a color with the substance to be measured and matching the treated sample with color standards containing known amounts of the substance. There are several colorimeter sets available commercially which vary slightly in use and operation. For that reason, make a careful study of the manufacturer's instructions before using such equipment. Other tests are performed by titration or by special instruments. "Titration" means finding out how much of a substance is in a given solution by measuring how much of another substance or reagent has to be added to the given solution to produce a given reaction.
Various reagents required for the tests discussed below are available from a number of manufacturers and laboratory supply houses. Some of these reagents require special preparation and handling before test use. This is customarily the responsibility of the laboratory technician, since, in some cases, the preparation of reagents requires a thorough knowledge of the chemical procedures. For complete information on the preparation of reagents, refer to the manufacturer's instructions or consult your supervising petty officer.
Two tests are frequently used in testing water for chlorine resid uals. They are the orthotolidine test and the orthotolidine-arsenite (OTA) test. Each of these tests is discussed separately below.
Chlorine residuals can be measured easily by using a commercial comparator and orthotolidine reagents.
EQUIPMENT. - Either a disk or slide comparator may be used in performing the orthotolidine test. A disk comparator is shown in figure 7-13. This comparator consists of a standard color disk and two sample tubes. Water to be tested is placed in both tubes. Reagent is added to one and the resulting color matched with the disk. The other tube is placed behind the disk to eliminate any color error that might be caused by turbidity in the test sample.
A slide comparator, also referred to as a block comparator, is shown in figure 7-14. This comparator consists of standard color ampoules for more accurate color matching. The other two sample tubes are used as compensators and are placed behind the color ampoules.
- only reagent used is a standard orthotolidine solution.
Figure 7-13. - Disk comparator with the front removed show construction.
Figure 7-14. - Slide comparator for chlorine residual test.Continue Reading