Mixtures
9 minute read
What you will learn
- Define a mixture and state what it contains
- Distinguish solution mixtures from suspension mixtures
- Explain the difference between diffusible and indiffusible solids
- State why a suspending agent is used and name some
- Outline how solution and suspension mixtures are prepared
- Calculate the volume of mixture to supply and the amount of drug it needs
Key points
- A mixture is an oral liquid in which one or more drugs are dissolved or suspended in a vehicle, usually water.
- In a solution mixture every ingredient dissolves. In a suspension mixture an insoluble powder is spread through the liquid.
- Diffusible solids are light powders that spread easily and stay spread long enough for a dose to be measured. They need no suspending agent.
- Indiffusible solids are heavy powders that sink fast. They need a suspending agent.
- A suspending agent, such as compound tragacanth powder, thickens the liquid and slows settling.
- All suspensions must be labelled 'Shake the bottle well before use'.
- Water-based mixtures spoil quickly, so they have a short shelf life and a discard date.
What is a mixture?
A mixture is a liquid for taking by mouth, in which one or more drugs are dissolved or suspended in a vehicle. The vehicle is usually water. A mixture may also contain a sweetener such as syrup, a flavour, a colour and a preservative.
Mixtures are measured with a 5 mL medicine spoon, an oral syringe or a measuring cup. Common doses are 5 mL and 10 mL. A mixture is supplied in a bottle, usually glass or plastic, with a tight closure.
Types of mixture
| Type | What it contains | Example |
|---|---|---|
| Solution mixture | Every ingredient dissolves. The liquid is clear | A simple antacid or expectorant mixture in which everything dissolves |
| Suspension mixture | An insoluble powder is spread through the liquid. The liquid is cloudy | A chalk or kaolin mixture |
Suspension mixtures are divided again according to how the powder behaves in the liquid.
Diffusible and indiffusible solids
| Diffusible solids | Indiffusible solids | |
|---|---|---|
| Nature of the powder | Light and fine | Heavy, or very fine and hard to wet |
| In the liquid | Spreads easily and settles slowly | Sinks quickly and may form a hard cake |
| After shaking | Spreads evenly again, and stays spread long enough to measure a dose | Does not stay spread unless something holds it |
| Suspending agent | Not needed | Needed |
| Examples | Light kaolin, light magnesium carbonate, magnesium trisilicate | Chalk, calamine, zinc oxide |
Whether a powder counts as diffusible can depend on its particle size and weight, so use the classification in the textbook your school follows.
Suspending agents
A suspending agent makes the liquid thicker and coats the particles, so they settle slowly and are easy to shake back into the liquid. Common suspending agents are:
- compound tragacanth powder
- tragacanth and acacia
- sodium carboxymethylcellulose
- bentonite
The amount of suspending agent is taken from the formula in the pharmacopoeia or reference, and it should never be guessed.
How mixtures are prepared
The practical steps are covered in the dispensing practicals. In outline:
Solution mixture
- Weigh or measure each ingredient.
- Dissolve the solids in part of the vehicle, using the ingredient that dissolves most slowly first.
- Strain or filter if the liquid is not clear.
- Add the liquid ingredients and flavour.
- Make up to the full volume with the vehicle and transfer to the bottle.
Suspension mixture
- Powder the insoluble solid finely in a mortar.
- For an indiffusible solid, mix the powder with the suspending agent first.
- Add the vehicle a little at a time, stirring to a smooth cream, then add the rest.
- Add the other ingredients, then transfer to a measuring cylinder.
- Rinse the mortar with some of the vehicle, add the rinsings and make up to volume.
- Transfer to the bottle and label it "Shake the bottle well before use".
Problems to watch for
| Problem | Cause | What to do |
|---|---|---|
| Precipitate forms | Two ingredients react or one is not soluble enough | Tell the pharmacist. Do not dispense it |
| Powder settles and cakes | Indiffusible solid without enough suspending agent | Use the right suspending agent at the stated amount |
| Germs grow | Water-based mixture with no or weak preservative | Use clean equipment, a preservative if suitable, and a short discard date |
| Colour or smell changes | The drug is breaking down | Do not dispense it |
Shelf life, labelling and storage
Water supports the growth of germs and the breakdown of drugs, so water-based mixtures have a short shelf life. The pharmacist sets the discard date, and the label states it. See the lesson on extemporaneous terms.
A mixture label carries:
- the name and strength of the mixture, and the patient's name;
- the directions for use, in plain words, and the dose volume;
- the date of preparation and the discard date;
- "Shake the bottle well before use" for every suspension;
- the storage advice, such as "Keep in a cool place" or "Store in a refrigerator".
Give the patient a 5 mL measuring spoon or oral syringe, because kitchen spoons vary in size.
Calculations
Example 1: volume to supply. Mixture, 10 mL three times a day for 5 days.
- 10 × 3 × 5 = 150 mL
Example 2: drug needed. Each 10 mL dose contains 500 mg of drug. Make 150 mL.
- Number of doses = 150 ÷ 10 = 15
- Drug needed = 15 × 500 mg = 7 500 mg = 7.5 g
Example 3: number of spoonfuls. How many 5 mL spoonfuls are in 150 mL?
- 150 ÷ 5 = 30 spoonfuls
Example 4: strength of the mixture. 7.5 g of drug in 150 mL.
- 7.5 ÷ 150 × 100 = 5% w/v
Exam focus
- Classify a named powder as diffusible or indiffusible. Learn the examples in the table below.
- Know why indiffusible solids need a suspending agent, and name two suspending agents.
- Quantity questions: dose volume × doses per day × days.
- Common mistake: forgetting that suspensions must be shaken, and that shaking does not replace a suspending agent for heavy powders.
Quick check
Test yourself before you move on. Pick an answer to see the explanation.
1.A mixture is best described as:
2.Which of these is a diffusible solid?
3.Why is a suspending agent added to a mixture containing an indiffusible solid?
4.A mixture is prescribed as 10 mL three times a day for 5 days. What volume is supplied?
5.Each 10 mL dose must contain 500 mg of a drug. How much drug is needed to make 150 mL?