TM 10-6630-245-13&P
(c)
Gently squeeze Valve S(3) to allow liquid to slowly fill the pipet(1) to the level mark on the pipet.
Keep the tip of the pipet(1) below the surface of the sample. Otherwise, you will lose the suction and
possibly introduce surface particles.
(3)
Expel Liquid:
(a)
Hold the tip of the pipet(l) at a slight angle against the container wall.
(b)
Press Valve E(5) to allow the liquid to drain from the pipet(l).
NOTE
Do not attempt to discharge that portion of the sample of reagent that remains in the tip of the
pipet after draining. Pipets are manufactured so that a reproducible amount of sample always
remains in the tip of the pipet.
2.6.1.8. Titrating.
a.
Purpose. In the WQAS-PM, titrating is a technique used to find out how much of a specific chemical is in water.
In some cases, the water may first be treated with one or more reagents. Then a special reagent called the
titrating reagent, or titrant, is added to the solution in small measured amounts. The amount of titrant added
indicates the amount of the test chemical present in the water.
b.
Method. The titrant is added a little bit at a time until a visible chemical change occurs in the solution. Two
methods of adding titrant are used in the WQAS-PM: (1) The drop count method in which a dropping pipet is
used to add titrant drop by drop, and the drops are counted to determine the amount of titrant used, and (2) with
the mini burette where the volume added is read from the burette as described in the test procedure in para
2.6.8.8.
c.
End Point. The titration end point, or end of the titration, happens when just enough titrant has been added to
turn the water test sample to the color indicated in the procedure. Sometimes a little bit of color will form and
then disappear. This means the end point is near but has not yet been reached. The color change must occur
and remain after mixing for the true end point to have been reached.
2.6.1.9. Sources of Error.
Knowing things that might cause errors in tests can help the operator to develop good test habits. This knowledge can
also be useful in deciding whether to repeat tests and how much to rely upon the results. Several events and conditions
that might cause errors in the results are explained below.
a.
Temperature. Very hot or very cold temperatures can lower the accuracy of results, can lead to false results, or
can prevent the test from working at all. The best temperature (unless otherwise specified in the test procedure)
for tests is between 68F (20C) and 77F (25C). The further from this temperature the less reliable the test is.
b.
Chemical Interferences. When testing for one chemical, the presence of another chemical can interfere with the
test. It can block the test from working, or it can falsely indicate the presence of something that is not there.
Individual test procedures describe common chemicals that interfere with the test. If there is a method of by-
passing the interference using the WQAS-PM, it will be described in the test procedure.
c.
Too Much Elapsed Time. If too long is taken in performing the test, results can be compromised. Except where
specifically noted in the test procedure it is not necessary to rush the tests. However, once the test is begun, it
should be completed without stopping for other tasks. With the exception of the Dissolved Oxygen Test (para
2.6.8.8) water samples can be held for several hours after they are taken before beginning a test.
d.
Contamination. A common source of error is contamination from dirty labware or foreign matter getting into a
reagent. This can be avoided by using only clean dry labware, cleaning all items before storing, and keeping lids
securely on all reagents.
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