pcp4- dilutions and pharmaceutical concentrations

calculation s- solid, solution preperationcallculate the ratios of two products that are mixed to give a concentration

Why do we dilute a solution ?

to give a desired strength needed for a patient use 

What is the key dilution equation?

c1x v1 = c2 x v2

the number of moles stays the same

1- original

2-new

general method

step1 - use equation , change any units

step2- find out what is missing

step3- solve

example calculation - how many millilitres of a 10% w/v solution of drug A is needed to make 500ml of 4% w/v solution ?

1. c1= 10 , v1=? , c2= 4, v2=500

2. 10v1=2000

3. v1= 200

example calculation using - in ratios

How many ml of a 1 in 2000 solution of drug c can be made from 500ml of a 0.25% w/v solution of a compound

1.change units  - 1g  in 2000ml  ( what is in 1ml )

1/2000 × 100 = 0.05%

2.use equation - 0.25× 500 = 0.05 v2 

3. solve- v2= 2500ml

Example - how many of water must be added to 300ml of 20% w/v stock solution of drug d to prepare a 0.8% w/v solution

note- added ( must takeaway 300 from answer)

1. 300 × 20 = 0.8v2

2. v2 = 75000

3. 7500-3000= 7200ml

what to we need to do when mixing two solutions together our product ?

if we know the concentration or amount of the being mixed , we can use the following approach to calculate the concentration of the final product:

1. calculate the weight of active ingredient in each of the solutions being mixed

2. calculate the total volume of mixture produced 

Mixing solutions - example calculation

what is the conc of active ingredient in a solution prepared by mixing 100 ml of 20% w/v , 50 ml of 10% w/v and 200ml of 5% w/v ?

1.find the total weight of active ingredients

100ml of 20% w/v : 20/100 × 100 = 20g

50ml of 10%:w/v 10/100 × 50= 5g

200ml of 5% w/v : = 5/100 × 200= 10g

total weight = 20+10+5= 35g

2.find the total volume

total = 50+100+200= 350 ml

3.combine them and find out what is in 100ml

35g in 350 ml

in 100 35/350× 100= 10g

4.answer

10% w/v

How do we obtain a desired concentration?

• mix two preparations of differing concentrations to obtain - DESIRED CONCENTRATION

• usually when there is no ready access to the stock solution

• but there is a supply of differing concentrations of the products

• =ALLIGATION

What are the key principles when doing allegations?

• one is greater than the desired concentration and one is less than

• use alligation grids to help work out proportions then amounts 

Alligation grid 

calculation using alligation grid- general. method

1- work out proportions of higher

2- work out proportions of lower

3- apply how much does each part weigh

4- work out each weight using their proportion/parts

Example calculation - alligation grid

You re asked to produce 150g of a 2% cream using stock creams of 0.5% and 4% .How much of each stock cream will you require?

1. 2-0.5= 1.5 parts

2. 4-2 = 2 parts

3. ratio of higher to lower 1.5 to 2

4. 150 for 3.5 parts , 1 part = 42.857 g

5. 42.8× 1.5= 64.3 g , 42.8 × 2= 85.7 g