TM 5-6630-215-12Figure 2-3.Rubber Bulb Pipet Filter2-5.DROP COUNT METHOD.When titrating by drop count, the mostaccurate results will be obtained with the sample and titratingsolution at room temperature and with the solution dispensed from avertical dropper at the rate of 1 drop per second.2-6.REAGENT POWDER PILLOWS.Dry powdered reagents are used whereverpossible to minimize the problems of leakage and deterioration.Forgreatest convenience, the powders are packaged in individual, pre-measured,polyethylene or saran “Powder Pillows.”Each pillowcontains sufficient reagent for one test and is easily opened withnail clippers or scissors.2-7.REAGENT BLANK-COLORIMETER TESTS.The term “reagent blank”refers to that effect wherein the reagent itself adds some color orturbidity to the sample being tested, thus giving erroneously highreadings.In several of the tests, the blank is of such magnitudethat compensation must be made each time the test is performed.Thisis done by zeroing the instrument on a blank sample, as in the case ofthe ammonia nitrogen test, paragraph 2-18d(1)(d). In most cases,however, the reagent blank is so very small that the instrument iszeroed on either an untreated portion of the original water sample ordemineralized water.This is done routinely without any significantloss of accuracy, except where extremely small amounts of a consti-tuent are sought.In such a case,it is best for the analyst todetermine the reagent blank by performing the test on a sample ofhigh-quality, turbidity-free demineralized water.The result isexpressed as the milligrams per liter reagent blank and is subtractedfrom the results of subsequent tests using that particular bottle ofreagent.It is necessary to determine the reagent blank only when thematerial is first used, and at intervals of several months, unlesssubsequent contamination is suspected.2-6
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