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

1. Volatile liquid
2. Boiling temperature is 19.5⁰C
3. HF vapour has a pungent smell and is highly poisonous
4. A weak acid
5. F^- ions cannot be oxidized by chemical reagents to free F₂ because F₂ is the most powerful oxidizing agent known.

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, ammonium, 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^-, I^-).

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 F^- ion

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