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
