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ALIQUOT CALCULATION INVOLVING VERY SMALL QUANTITIES (ALTERNATIVE METHOD)

Solid liquid aliquot

Example 1:

Drug A 20 mg

Distilled water to 50 ml

Steps;

1. MWQ?

200mg

2. Solubility of drug A?

Say it`s 1:4 vs. 1:100

3. Aliquot?

MWQ/Drug = 200mg/20mg = 10

4. Dissolve Drug A in convenient volume of water

If the solubility of drug A is 1 in 4 (1:4), then

0.2 g drug can dissolve in 0.8 ml water (minimum volume needed),

And 0.08 ml of the solution will contain 20mg Drug A.

However it is more convenient to dissolve 0.2 g in a volume bigger than

0.8 ml. We can use 10ml, 20ml etc

0.2 g in 10 ml 1 ml of solution contain 20mg drug A

0.2 g in 20 ml 2 ml of solution also contain 20 mg Drug A

If the solubility is 1 in 400 (1:400) then

0.2g in 80ml (min. vol.) 8 ml

0.2g in 100ml 10 ml

0.2g in 200ml 20 ml

𝑀𝑖𝑛 𝑉 =𝑀𝑊𝑄

𝑆

𝑉𝑟 =𝑄𝑟 × 𝑉𝑢

𝑄𝑢

𝑀𝑖𝑛 𝑉 =𝑀𝑊𝑄

𝑆

𝑀𝑖𝑛 𝑉 = 0.2𝑔

1𝑔4𝑚𝑙

= 0.8 𝑚𝑙

𝑉𝑟 =𝑄𝑟 × 𝑉𝑢

𝑄𝑢

𝑉𝑟 =20𝑚𝑔 × 10𝑚𝑙

200𝑚𝑔 = 1 𝑚𝑙

alternative method*

Steps;

1. Calculate the least volume of

solvent required;

S = solubility in BPC under

solubility of the drug

2. Decide on the convenient

volume of water to be used.

3. Calculate the volume of the

solution to be taken for

compounding;

Vr = volume to be taken from

the solid liquid aliquot solution.

Qr = quantity of drug required

in the prescription.

Vu = volume of solvent used in

the solid liquid aliquot.

Qr = quantity of drug used in

the solid liquid aliquot.

Example 1;

Step 1

Step 2

Use 10 ml

Step 3