Aromatic Waters and Dilutions: The Practical

What are aromatic waters?

An aromatic water is water holding a small amount of a volatile (easily evaporating) oil or another aromatic substance. It has the taste and smell of the oil but is clear or slightly cloudy.

Aromatic waters are used as the vehicle in mixtures, so that the medicine tastes and smells pleasant.

How they are supplied

Making an aromatic water from the oil is slow, and the oil dissolves poorly in water. In most pharmacies the water is made by diluting a concentrated aromatic water with purified water. The label of the concentrate gives the dilution, and common ones are around 1 in 40. Always use the dilution stated on the bottle you are using.

Making an aromatic water from a concentrate

1. Read the label for the dilution.
2. Calculate the volume of concentrate for the volume wanted.
3. Measure the concentrate in a small measuring cylinder, because the volume is small.
4. Transfer it to a larger measuring cylinder.
5. Add purified water and make up to the final volume. Stir.
6. Transfer to a clean bottle and label it.

Worked example

Dilution 1 in 40. Make 200 mL.

• Concentrate = 200 ÷ 40 = 5 mL
• Add purified water to 200 mL, which is 195 mL of water.

Notice that the water is added to make up to 200 mL. It is not 200 mL added to the concentrate.

Dilutions in general

Often a strong solution has to be made weaker. The amount of drug stays the same, but the volume increases.

strength wanted × volume wanted = strength of stock × volume of stock

Worked examples

Example 1. Make 100 mL of a 2% solution from a 10% stock.

1. 10 × V = 2 × 100
2. V = 200 ÷ 10 = 20 mL of the 10% solution
3. Add water to 100 mL. Water used = 100 − 20 = 80 mL.

Example 2. Make 250 mL of a 1% solution from a 5% stock.

1. 5 × V = 1 × 250
2. V = 250 ÷ 5 = 50 mL
3. Add water to 250 mL. Water = 200 mL.

Example 3. How many times has Example 1 been diluted?

• 10% to 2% is a dilution of 10 ÷ 2 = 5 times, which is a 1 in 5 dilution.

Check. The amount of drug in Example 1 is 10 g per 100 mL × 20 mL = 2 g. In the final solution, 2 g per 100 mL = 2%. It agrees.

Storage and labelling

• Use clean, dry equipment, and purified water.
• Water supports the growth of germs and moulds, so aromatic waters have a short shelf life. Make only the amount needed.
• Store in a well-closed container, in a cool place and out of strong sunlight.
• Label with the name, the strength or dilution, the date and the discard date.
• A bottle that becomes cloudy, grows a skin or smells off must be thrown away.

Safety and records

• Check the label of the concentrate: name, strength and expiry date.
• Record the volumes used and the discard date.
• Chloroform water and similar waters are used only in the amounts the formula allows. Ask the pharmacist if you are unsure.