Tests for fluoride ion, Fluorine, HF
There are tests to identify Fluoride (F-) anion in qualitative analysis. Fluoride ion
forms aqueous solutions with some metal cations and sometimes forms precipitates with some cations.
Fluorine and fluoride differs from other halogens and halides respectively. Hydrogen fluoride (HF) is a weak acid. Other hydrogen halides
such as HCl, HBr, and HI are strong acids. Fluorine's oxidation power is higher than other halogens.
In this tutorial, we will learn followings.
- Hydrofluoric acid, HF
- Qualitative tests for fluoride ion
- Calcium fluoride and concentrated sulfuric acid
- Formation of SiF4
- Solubility of fluoride ion
- Concentrated sulfuric acid and solid fluoride compound reaction
- Differences of fluorine and fluoride comparing with other halogens and halide ions
- Differences of fluorine and other halogens
- Differences of fluoride and other halides (chlorides, bromides and iodide)
Hydrofluoric acid, HF
- Volatile liquid
- Boiling temperature is 19.50C
- HF vapour has a pungent smell and is highly poisonous
- A weak acid
- F- ions cannot be oxidized by chemical reagents to free F2 because F2 is the most powerful oxidizing agent
Qualitative tests for fluoride ion
Fluoride ion has some difference behavior respect to other halide ions. Therefore we cannot do same qualitative tests for fluoride ion
like other halide ions.
Qualitative analysis of halide
ions, chloride, bromide, iodide
Experiments to identify fluoride (F-) ion
There are several experiments to test fluoride anion we will discuss them in detail here.
Calcium fluoride and concentrated sulfuric acid | CaF2 + H2SO4
It will give Hydrogen fluoride (HF(g)) vapor which is highly corrosive.
Formation of SiF4
A characteristic peculiarity of HF is its reaction with SiO2 and silicates to form gaseous silicon fluoride, SiF4.
Solubility of fluoride ion
- Silver fluoride (AgF) is readily soluble in water. But other Silver halides
of Silver(I) chloride, Silver bromide(I) and Silver iodide(I) are insoluble in water.
- Fluorides of alkali metals,
aluminium, tin and mercury are soluble in water.
- Fluorides of alkaline earth metals are
insoluble in water.
- Potassium fluoride (KF) is soluble in HF.
Concentrated sulfuric acid and solid fluoride compound reaction
Then, Hydrogen fluoride (HF) vapor is released. Hydrogen fluoride is a highly corrosive vapor. That vapor can turn glass bristle
into gelatinize state.
Differences of fluorine and fluoride comparing with other halogens and halide ions
There are very different characteristics of fluorine (F2) and other halogens(Cl2, Br2, I2).
Also F- ion also shows different characteristics than other
halides (Cl-, Br-,
Differences of fluorine and other halogens
- Fluorine only shows -1 and 0 oxidation states. But other halogens show positive oxidation states too.
- Oxidation power of F2 is much higher than other halogens.
- F2 forms compounds with inert gases such as Kr, Xe. But other halogens doesn't.
- Fluorine's ionization enthalpy is greater than other halogens.
- Electronegativity of Fluorine is much higher than other halogens.
- Fluorine makes strong bonds with non metals.
- F2 can oxidize water in two ways and this will be explained in detail in a next chapter of this tutorial.
Water and fluorine reaction | H2O + F2
Reaction of water and
Fluorine gas are occurred in two ways because product vary according to the amount and rate of fluorine is fed in to the water.
Reaction of water and fluorine gas when fluorine gas is fed slowly through water
If fluorine gas is sent slowly to the water, Hydrogen fluoride and Oxygen
gas are given as products.
Oxidation numbers of oxygen atoms and fluorine atoms are changed during the reaction.
Send excess fluorine quickly through water
When fluorine gas is sent quickly and in excess amount to water, Hydrogen fluoride and
Ozone (O3) are given as products.
Hydrolysis of fluorine
F is a non metal. Therefore F2 hydrolyses. Due to F's higher electronegativity F2O is formed.
When producing F2O by bubbling F2 in lower temperatures, solubility is less. Therefore F2 is bubbled
through 2% dilute NaOH to prepare F2O efficiently.
Differences of fluoride and other halides
- Hydrogen fluoride is a weak acid
in water because it partially dissociates to Hydronium ion (H3O+) ion
Fluoride ion (F-) ions. Other hydrogen halide acids are strong acids.
- Silver fluoride (AgF) and Lead fluoride (PbF2) are soluble in water. But other halides of Ag and Pb are insoluble in water.
- Lithium fluoride (LiF) is insoluble in water due to its high lattice energy because of small cation and small anion size.
- Fluorides of alkali earth metals are sparingly soluble in water.
More to learn about fluoride
- Hydrogen fluoride formed acts on the silicates of the wall of the test tube. It forms gaseous silicon fluoride, SiF4.
- Barium chloride (BaCl2) forms a bulky white precipitate of Barium fluoride (BaF2) which is soluble in
excess mineral acid and in ammonium salts, with the
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