Detection of Fluorides

Qualitative Test for Fluorides (Gettler Test)
Aim
To detect the presence of fluorides in the given sample.
Materials and chemicals
Sodium hydroxide solution, conc. sulphuric acid, glass beaker, heater, 5% sodium chloride, glass plate, microscope.
Procedure
1. To a small quantity (1-3 ml) of urine or stomach contents, add 3 ml of sodium hydroxide solution in a glass or porcelain dish.
2. Add little quantity of powdered glass and mix thoroughly with the residue.
3. Apply moderate heat till dry.
4. Add 10 ml of conc. sulphuric acid and cover the dish with a small glass plate from the under surface of which is suspended a drop of 5 per cent sodium chloride solution. Place a small piece of ice on top of the plate to prevent evaporation of the drop.
5. Heat gently for 3-5 min carefully remove the drop and examine under low power of microscope.
Inference
If the given specimen contains fluorides, silicon fluoride is formed which appears as small, light pink, hexagonal crystals along the rim of the drop, whereas sodium chloride crystals are large and square shaped.