Fluoride Test

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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TM 5-6630-215-12 a. Fluoride Test - SPADNS Method, 0-2.0 mg/l range (para 2-18 and figs. 2-11 and 2-12). (1) Fluoride occurs naturally in some ground waters and a 1 mg/l level is normally maintained in public drinking water supplies for the prevention of dental cavities. Excessive amounts of fluoride cause an objectional discoloration of tooth enamel called fluorosis, though levels up to 8 mg/l have not been found to be physiologically harmful. (a) The SPADNS methods for fluoride analysis involves the re- action of fluoride with a dark red zirconium-dye lake. The fluoride combines with part of the zirconium to form a colorless zirconium-flu- oride complex with the net effect of bleaching the color in an amount proportional to the fluoride concentration. The SPADNS method is the preferable calorimetric method due to its instantaneous reaction with fluoride and the stability of the SPADNS reagent. (2) Fluoride Test Procedure. NOTE The fluoride test is sensitive to small amounts of interference (table 2-7). Glassware must be abso- lutely clean and it is advisable to mark the sample cells and graduated cylinders so that they do not become mixed. Repeating the test using the same cylinders and sample cells is recommended to insure they are free of interferences and the results are accurate. Polyethylene bottles are preferred for collecting and storing water samples for fluoride analyses. Rinse the bottle several times with portions of the sample before collecting the test sample. It is important that the water sample and the flu- oride standard solution be at the same temperature (+ or - 1°C) for test. The temperature of the standard solution and the water sample should be 20°C + or- 1° (68°F) for best results. Temperature adjustments should be made after the addition of the SPADNS reagent in step (c), below. Volume measurements of the sample and the SPADNS reagent are extremely critical and should be made very carefully. For greater accuracy, the sample and the fluoride standard solution should be measured with 25-ml volumetric flask. 2-41

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