DETERMINATION OF FLUORIDE IN VEGETATION

Fluoride is extracted from the dried vegetation by stirring with 0.1M perchloric acid and then determined by the standard addition technique.

Equipment Required;

1. Corning pH/Ion meter OR pH meter with millivolt scale
2. Double junction reference electrode
3. Reference electrode lead
4. Plastic bottles
5. Accurate balance weighing to ±0.0005g

Reagents:

Perchloric acid (62% reagent grade) Sodium fluoride (analar)

Standard Preparation:

Dissolve 4.421g of analytical reagent grade sodium fluoride (dried at 110°C for 2 hours) in distilled water and dilute to 1 litre. This is the 2000mg/L fluoride standard.

0.1M Perchloric Acid

To a 1 litre volumetric flask half filled with distilled water add 10.3mls of 62% reagent grade perchloric acid (wt per ml 1.52g). Dilute to the mark with distilled water.

Method:

Dry the sample of vegetation for 24 - 48 hours in an oven set at 80°C. Then grind it to pass through a number 60 sieve and store in clean, dry, lightly closed plastic bottles.

Before removal of the sample, rotate the bottle to ensure thorough mixing.

Using an accurate balance weigh accurately about 1.0g of powdered sample into a wide mouthed plastic bottle. Add 25ml of the 0.1M perchloric acid and stir on a magnetic stirrer for 20 minutes.

Add a further 25mls of the perchloric acid and immerse the fluoride ISE and reference electrode in the solution while continuing to stir.

Record the stable reading (E₁). Then add 200ml (0.2ml) of the fluoride standard from a micro-burette or hand held pump pipette and record the new stable meter reading (E₂).

Calculation;

Using a Corning pH/Ion analyser with a known addition mode, the reading can be obtained directly in concentration units from the display.

If a pH meter in the millivolt mode is used, then E is firstly calculated by:

E = e₂ – e₁

The known parameters are then substituted into the following equation.

Where:     Cu= concentration of the unknown

Cs = concentration of the standard

Vs = volume of the standard

Vu = volume of the unknown

E = change in electrode potential in mV

S = slope of the electrode in mV