Table 2-7. SPADNS Method Errors Due to Interference

TM-5-6630-215-12 Alarm-Monitor Group (AMG) OA-9431/F88-9(V) CAGEC 97403 Manual
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WARNING TM 5-6630-215-12 NOTE (Continued) A simple method of checking for interference by aluminum is to read the concentration one minute after mixing with the SPADNS reagent, wait 15 min- utes, and read it again. An appreciable increase in concentration indicates the presence of alum- inum as an interference. At this point, allowing the reacted sample to stand undisturbed for two hours before making the final reading will elim- inate the effect of up to 3.0 mg/l aluminum. Table 2-7. SPADNS Method Errors Due to Interference Interference Substance Alkalinity (asCaCO₃) Aluminum Chloride Iron, ferric Sodium Hexametaphosphate Phosphate, ortho Sulfate Concentration 5000 mg/l 0.1 mg/l 7000 mg/l 10 mg/l 1.0 mg/l 16 mg/l 200 mg/l Error -0.1 mg/l F -0.1 mg/l F +0.1 mg/l F -0.1 mg/l F +0.1 mg/l F +0.1 mg/l F +0.1 mg/l F (a) Take a water sample by accurately filling a clean gradu- ated cylinder to the 25.0-ml mark. Pour the sample into a clean, dry sample cell. (b) Accurately measure 25 ml of fluoride standard solution, (1.00 mg/l of fluoride (F)), by filling another graduated cylinder to the 25.O-ml mark. Pour the standard into the other sample cell. The SPADNS reagent is a very strong acid. (c) Using a pipet filler, pipet 5.00 ml of SPADNS reagent into each sample cell and swirl to mix immediately. Allow one minute for the color to fully develop and proceed with step (d). (d) Place the sample cell containing the Fluoride Standard Solution into the cell holder. Insert the Fluoride (SPADNS method) Meter Scale into the meter and adjust the wavelength dial to 580 nm. Adjust the LIGHT CONTROL for a meter reading of 1.0 mg/l Fluoride. (e) Replace the standard blank with the prepared sample in the cell holder and read the mg/l fluoride. 2-42

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