Simple Solution Mixtures: The Practical

What is a simple solution mixture?

A simple solution mixture is an oral liquid in which every ingredient is dissolved in the vehicle, usually water or an aromatic water. The liquid is clear, so the patient does not need to shake the bottle, and every dose has the same amount of drug.

Solubility

Solubility tells you how much liquid is needed to dissolve a solid. It is often given as the number of millilitres of water needed to dissolve 1 g.

If a drug is insoluble it cannot go in a solution mixture. It needs a suspension instead. See the lessons on diffusible and indiffusible mixtures.

Worked example

1 g of a drug dissolves in 3 mL of water. How much water is needed for 5 g?

• 5 × 3 = 15 mL

If the volume of the dose is too small for this much water, the drug will not all dissolve. Ask the pharmacist.

Equipment

Balance and weights, mortar and pestle, beakers, a glass rod, measuring cylinders, a funnel with filter paper or muslin, a bottle and a label. Everything must be clean. Wash with purified water at the end.

Method

1. Calculate the quantity of every ingredient, and have it checked.
2. Weigh the solids. Crush any large crystals in a mortar, because small particles dissolve faster.
3. Dissolve the solids in part of the vehicle in a beaker, starting with the slowest to dissolve. Stir with a glass rod. Do not heat unless the formula allows it, because heat can break down some drugs.
4. Strain or filter if the liquid is not clear.
5. Add the other liquids, such as syrups, tinctures and flavourings, after the solids have dissolved.
6. Transfer to a measuring cylinder.
7. Rinse the beaker and mortar with some vehicle and add the rinsings.
8. Make up to the final volume with the vehicle, reading at eye level.
9. Transfer to the bottle, close it and label it.
10. Clean up and record the preparation.

Why add syrup late?

Syrup is thick and slows the dissolving of solids. It is also measured by volume, and a thick liquid clings to the cylinder, so measure it carefully and rinse the cylinder with some of the vehicle.

Worked example: scaling the formula

Illustrative practice formula, not a pharmacopoeial one.

Sodium citrate mixture, 100 mL: sodium citrate 3 g, syrup 10 mL, water to 100 mL. Make 150 mL.

1. Scale factor = 150 ÷ 100 = 1.5
2. Sodium citrate = 3 × 1.5 = 4.5 g
3. Syrup = 10 × 1.5 = 15 mL
4. Water to 150 mL

Check. Strength of sodium citrate = 3 ÷ 100 × 100 = 3% w/v. In the scaled mixture, 4.5 ÷ 150 × 100 = 3% w/v.

For a 10 mL dose there are 150 ÷ 10 = 15 doses, so each dose holds 4.5 ÷ 15 = 0.3 g of sodium citrate.

Labelling

A solution mixture label carries:

• the patient's name, the date, and the name of the mixture;
• the dose in millilitres or 5 mL spoonfuls, and how often to take it;
• the discard date and storage advice;
• any warning that the pharmacist adds.

Because the liquid is a clear solution, "Shake the bottle" is not needed. Give the patient a 5 mL spoon or an oral syringe.

Faults and what to do

Safety and records

• Check each ingredient against the formula twice.
• Never taste an ingredient or the mixture.
• Record the weights, the volumes and the discard date.
• The pharmacist checks the preparation and the label before it is handed out.