Diffusible Mixtures: The Practical
9 minute read
What you will learn
- Recall what makes a solid diffusible
- Set out the equipment and ingredients for a diffusible mixture
- Make a diffusible mixture following the correct steps in order
- Scale a formula up or down for the quantity prescribed
- Label the finished mixture correctly
- Recognise common faults and how to avoid them
Key points
- A diffusible solid is a light powder that spreads easily in the liquid. It needs no suspending agent.
- Powder the solid finely, then add the vehicle a little at a time to make a smooth cream before adding the rest.
- Dissolve the soluble ingredients in part of the vehicle first, and strain if needed.
- Transfer to a measuring cylinder, rinse the mortar, and make up to the final volume.
- Always scale every ingredient by the same factor.
- Label: name, directions, date, discard date and 'Shake the bottle well before use'.
- Clean equipment, accurate weighing and a second check protect the patient.
Before you start
A diffusible solid is a light powder that spreads easily through the liquid and stays spread long enough for a dose to be measured, so it needs no suspending agent. Common examples are light kaolin, light magnesium carbonate and magnesium trisilicate. See the lesson on mixtures for the theory.
Good practice at the bench:
- Wash your hands and wear a clean coat. Tie back long hair.
- Clean the bench, the mortar and pestle and the measuring cylinder.
- Read the prescription and the formula twice. Ask the pharmacist if anything is unclear.
- Calculate the quantities before you weigh anything, and have the pharmacist check them.
- Work calmly. A mistake in weighing cannot be undone after the powder is in the mortar.
Equipment
| Item | Use |
|---|---|
| Balance and weights | Weighing the solids |
| Mortar and pestle | Powdering and mixing |
| Measuring cylinder | Measuring liquids and the final volume |
| Glass rod | Stirring and transferring |
| Funnel, strainer or muslin | Transferring and straining |
| Spatula and weighing boats or papers | Handling powders |
| Bottle and closure | Final container (clean, and amber for light-sensitive drugs) |
| Labels | Labelling the bottle |
Method
- Calculate the quantity of each ingredient for the volume to be made.
- Weigh each solid. Use a clean boat or paper for each, and note down the weights.
- Dissolve the soluble solids, such as sodium bicarbonate, in part of the vehicle in a beaker. Strain if the liquid is not clear.
- Powder the diffusible solid in the mortar until it is fine.
- Add a little of the vehicle to the powder and rub to a smooth cream, with no lumps.
- Add more vehicle gradually, stirring all the time.
- Add the solution from step 3, and any liquid ingredients such as flavouring.
- Transfer to a measuring cylinder using the glass rod.
- Rinse the mortar with a little of the vehicle. Add the rinsings to the cylinder.
- Make up to the final volume with the vehicle and stir.
- Transfer to the bottle, close it and label it.
- Clean up, return the ingredients to the shelf, and record the preparation.
Check the measuring cylinder at eye level, reading the bottom of the curve of the liquid (the meniscus).
Worked example: scaling a formula
This is an illustrative practice formula, not a pharmacopoeial one. Use the formula your school or pharmacist gives you.
Antacid mixture, 100 mL: magnesium trisilicate 5 g, sodium bicarbonate 5 g, peppermint water to 100 mL. The prescription is 10 mL three times a day for 5 days.
- Volume needed = 10 × 3 × 5 = 150 mL
- Scale factor = 150 ÷ 100 = 1.5
- Magnesium trisilicate = 5 × 1.5 = 7.5 g
- Sodium bicarbonate = 5 × 1.5 = 7.5 g
- Peppermint water to 150 mL
Check. Each 10 mL dose should contain 7.5 ÷ 15 = 0.5 g of each solid, because there are 150 ÷ 10 = 15 doses. This agrees with the original formula, which has 5 g in 100 mL, or 0.5 g in 10 mL.
Always multiply every ingredient by the same factor, and check one dose at the end.
Labelling
The label of a diffusible mixture shows:
- the patient's name, the date, and the name of the mixture;
- the directions, for example "Take TWO 5 mL spoonfuls THREE times a day";
- "Shake the bottle well before use";
- the discard date, and storage advice;
- the name and address of the pharmacy.
A pharmacist checks the finished mixture, the label and the prescription before it is handed out.
Faults and what to do
| Fault | Likely cause | What to do |
|---|---|---|
| Lumps in the mixture | Liquid added too quickly | Work the powder into a cream with a little vehicle first, then add the rest gradually |
| Powder left in the mortar | Mortar not rinsed | Rinse with the vehicle and add the rinsings |
| Volume too low or too high | Wrong reading of the cylinder | Read at eye level, and make up to the mark |
| Powder cakes at the bottom | The powder was not diffusible, or the wrong ingredient was used | Report to the pharmacist before dispensing |
| Fizzing or gas when ingredients meet | A reaction between ingredients | Stop and ask the pharmacist |
Safety and records
- Clean every item between ingredients to prevent cross-contamination.
- Weigh carefully, and write the weights down.
- Keep a record of what was prepared, the batch, and the discard date, as the pharmacy requires.
- Never taste an ingredient or the mixture.
Exam focus
- Describe the method for making a diffusible mixture, in the correct order.
- Scale a formula to a different volume. Every ingredient is multiplied by the same factor.
- State the label instructions for a suspension.
- Common mistake: adding all the liquid at once to the powder, which leaves lumps.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.Which is a diffusible solid?
2.When making a diffusible mixture, the powder is first mixed with:
3.A formula for 100 mL needs 5 g of a powder. How much is needed for 150 mL?
4.Why is the mortar rinsed with some of the vehicle before the final volume is made up?
5.Which instruction must appear on the label of every suspension?