INTERFERENCES

TM-5-6630-215-12 Alarm-Monitor Group (AMG) OA-9431/F88-9(V) CAGEC 97403 Manual
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TM 5-6630-215-12 2-8. REAGENT BLANK-TITRIMETRIC TESTS. Although a reagent blank is of lesser influence in titrimetric tests other than in calorimetric tests, occasionally it must be considered. In the usual procedure, the required amount of buffer and/or indicator are added to a small amount of demineralized water in the titrator flask to be used. This mixture is very carefully adjusted with titrant to obtain the desired end point. The amount of titrant used represents the reagent blank and should be deducted from the amount used in the test on the unknown sample. 2-9. INTERFERENCES. Analytical determinations are subject to inter- ference from other substances present in the samples. a. Interference may also be caused by a high concentration of the constituent under analysis. For example, the presence of a large excess of iron (say 30 mg/l) will cause the test to read less than full scale. Dilution of the sample to 5 mg/l will result in a reading higher than full scale. This indicates the need for even further dilution until meter indication is “on scale.” b. When a result is suspect (unusual answer is obtained, a color other than that expected is formed, or an unusual odor or turbidity is noticed), the test should be repeated on a sample diluted with deminer- alized water (para 2-10a.) and the result compared with the result of the original test. If these two results are not identical the orig- inal result is probably in error and a further dilution should be made to check the second test (first dilution). 2-10. SAMPLE DILUTION TECHNIQUES. a. Colorimetric Tests (Spectrophotometers). Twenty-five milliliters (ml) is the specified size for the calorimetric tests. However, in some tests, the color that develops in the sample may be too intense to be measured, in other tests, colors other than those expected may occur. In both cases, it is necessary to dilute the original sample or investigate for possible interfering substances. (1) For example, when performing a test, the spectrophotometer (operation described in para 2-17) may read above 0.5 mg/l on the pertinent meter scale. Since this is beyond the last division of the meter scale, a sample dilution is necessary. The test must be repeated, but with a 25-ml graduated cylinder filled to the 12.5-ml mark with the sample and then to the 25-ml mark with demineralized water. Since the sample was diluted to twice its original volume (12.5 to 25-ml), the meter reading should be multiplied by 2 to give the correct concentration. (2) To accomplished the sample dilution conveniently, pipet the chosen sample portion into a clean graduated cylinder (or clean volu- metric flask for more accurate work), and fill the cylinder (or flask) to the desired volume with demineralized water. Mix well, then use this diluted sample when running the test. 2-7

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