There are two aspects in the analysis of chemical compounds.

• What is the compound? This is known as the qualitative analysis.
• What is the quantity (how much) is the compound present? This is known as the quantitative analysis.

There are two distinct categories in chemical analysis.

• Analysis of ionic compounds.
• Analysis of covalent compounds.

For ionic compounds it is difficult to determine what the compound is if you have a mixture. For example a mixture of sodium chloride and barium nitrate in a solution. What you get in solution is a mixture of:

All you can determine is the presence of the ions in the solution, not the particular compound.

For covalent compounds we can determine exactly which compound is present in a mixture or solution as the molecule exist as a unit.

You may wonder why we have to assume that the mixture of ionic compounds must be analysed in solution form. There are two reasons.

• Most analytical technique required the use of a solution.
• The more important reason is that in its usage the formulation is mostly applied in a solution form. Reaction in solid form is very, very, very limited.

QUALITATIVE ANALYSIS of IONIC COMPOUNDS

Currently most analyses are conducted with high-tech instruments. They are fast, more reproducible (less human skill required) and do not require an in-depth understanding of basic chemistry. However old fashion chemistry allows for greater intellectual exercise and challenge. So for the understanding of chemistry we will discuss the classical methods here and the high-tech approach much, much later.

To make the discussion simple we will just focus on a single ionic compound. For a mixture you will then have to be creative and work out the answers (by deduction) yourself.

THERMAL STABILITY

CARBONATES

Carbonates and bicarbonates (of all elements) are not stable when heated. They will decompose to their oxides and carbon dioxide gas is released.

If a carbonate or bicarbonate is heated in a closed hard glass tube with the glass delivery tube outlet immersed into a solution of calcium hydroxide, any carbon dioxide released will react with the hydroxide to form calcium carbonate. Let us illustrate using potassium carbonate.

Since the calcium carbonate is only slightly soluble in water, after a while white particles of calcium carbonate can be seen precipitating out of the solution.

On further absorption of carbon dioxide, the calcium carbonate reacts to give the calcium bicarbonate, which is soluble in water.

The white particles then disappear. This is strong evidence that the compound heated is either a carbonate or a bicarbonate.

Note:

• Make sure you move the outlet out of the calcium hydroxide solution before you stop the heating. Otherwise the calcium hydroxide will be sucked backwards into the hard glass tube. This can be dangerous.
• You can actually tried this series of reactions out by bubbling your breathe into a solution of calcium hydroxide. Make sure you do not suck and drink any of the calcium hydroxide as it is corrosive.

SOLUBILITY

To analyse for the anion we first put of little (a few grains) of the ionic compound in about 15ml of water in a test tube. Stir it gently with a glass rod for about 10 minutes. If you can see some solid, place the test tube for 15 minutes in a warm water bath (ca 60°C). If you can see some solid than consider the compound insoluble in water.

The insolubility in water should narrow down for you the type of anion present.

If it is soluble use a dropper and add two drops of barium nitrate solution into it. If solid precipitated out of the solution, that should be barium sulphate. You can then conclude that the anion present is a sulphate.

Alternatively you can add two drops of silver nitrate solution into it. If solid precipitated out of the solution can then conclude that the anion present is a halide. Also the chloride is normally white in yellow, bromide reddish yellow, and iodide blackish.

We will stop here for now and continue when we come to other properties we can use. The point to make is, we use the chemistry of similarity and difference in the analysis of chemicals.