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What is lattice energy and how does it influence flame photometry?

Lattice energy is the energy needed to separate a mole of an ionic solid into gaseous ions.

Its internationally recognised definition is “a measure of the energy contained in the crystal lattice of a compound, equal to the energy that would be released if the compound were brought together from infinity”.

So, the assortment of positive and negative ions released when combined together in sum total is the lattice energy.

How is lattice energy formed?

Lattice energy is held by a crystal structure. However, it may not be indicative that energy is actually released when these ions are combined.

This is due to entropy - a quantitative measure of what the second law of thermodynamics describes: the spreading of energy until it is evenly spread.

A crystal structure is a very rigid and ordered structure so, to form this structure, energy must be taken from elsewhere; i.e. the magnetic pull of a positive and negative ion.

As the kinetic energy of the pulling motion is changed into a “holding” energy between ions, this forms the crystal lattice.

What is an example of lattice energy?

One of the most common examples of lattice energy is found in sodium chloride (NaCl).

The lattice energy for this is the energy released when gaseous sodium (Na+) and chlorine (Cl–) ions come together to form a lattice of alternating ions in the NaCl crystal.

Here, the negative sign of the energy is indicative of an exothermic reaction.

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What influence does lattice energy have on flame photometry?

Some types of interference commonplace in flame photometry are due to the formation of compounds which the temperature of the flame is unable to break down.

From our knowledge, these ionic compounds are ligand-based. This means they have strongly-bound ionic bonding from multiple negatively charged ‘ligands’ onto the central metal ion.

This in turn pulls on the electronic shells of the metal, thus affecting the light emissions and the spectra produced from the ion when heated in a flame.

The strong binding force of some of these ligands is known as the lattice energy of a crystalline structure. It could be considered the equivalent of the pull between two magnets.

The value of these lattice energy is usually a negative figure, meaning that energy needs to be put into the system to break this bonding energy.

All ionic species are crystal structures, and a valid example of this type of interference in photometry is the presence of sulphates when analysing calcium samples.

Where the signal produced is significantly depressed by the presence of sulphates, due to their strong lattice energies.