Determination of Fluoride in Water

Aim
To determine the fluorides present in water.

Principle
Fluorides in excessive quantities and absence of fluorides in water, both create problems. A disfigurement in teeth of humans known as mottled enamel or dental fluorosis is occured in those, who consume waters with fluoride content in excess of 1.0 mg/L. It has been scientifically established that 0.8-1.0 mg/L of fluorides is essential in potable water. Thus, absence or low fluoride content may cause dental caries in the consumers.

Fluorides are measured by colorimetric methods. Fluorides are separated out by distillation, if interfering substances are present. Fluorides are analysed by a method that involves the bleaching of a performed colour by the fluoride ion. The performed colour is the result of the action between zirconium ion and alizarin dye. The colour produced is referred to a lake and the intensity of colour produced is reduced if the amount of zirconium present is decreased. Fluoride ion combines with zirconium ion to form a stable complex ion ZrF₆^-- , and the intensity of the colour lake decreases accordingly. The reaction is as follows:

Zr_alizarin lake + 6F (reddish colour)

alizarin + ZrF6-- (yellow

The bleaching action is the function of the fluoride ion concentration and is directly proportional to it. Thus, Beer's law is satisfied in and inverse manner.

Apparatus

1. Spectrophotometer or colour comparator

Reagents (click to check the preparation of reagents)

1. Standard fluorides solution 1mL = 10µgF.
2. Zirconyl-alizarin reagent.
3. Mixed acid solution.
4. Acid-zirconyl-alizarin reagent.
5. Sodium arsenite solution.

Procedure

1. If residual chlorine is present, remove the same by adding one drop of arsenite per 0.1 mg Cl and mix.
2. Prepare a series of standard by diluting various volume of standard fluoride solution (1 ml =10 µgf) to 100 mL in tubes. The range should be such that it is between 0 and 1.4 mg/L.
3. To 50 mL of each standard add 10 mL mixed acid-zirconyl-alizarin reagent.
4. Set the spectrophotometer to a wavelength of 570 nm.
5. Adjust the spectrophotometer to zero absorbance with the reference solution i.e., distilled water with reagent.
6. Plot the concentration along x-axis and absorbance along y-axis and obtain a calibration curve.
7. Take 50 mL of the sample and add 10 mL of mixed acid-zirconyl-alizarin reagent and mix well.
8. Place the solution in the spectrophotometer and read the absorbance.
9. By referring the calibration curve, the concentration for the observed absorbance is read out.
10. Repeat the procedure with dilute samples.

Observation
The observation is presented in Tables A and B respectively.

Table A: Observation for calibration

Table B:

Calculation

F in mg/L =	A x B
	V x C

where,
A = μgF determined
B = sample dilute to this volume
C = portion taken for colour development
V = mL of sample.

Results