The concentration of the components in a pharmaceutical preparation is based on its quantity relative to the total quantity of the preparation .

The aim : to learn pharmacy students how to preparepharmaceutical preparation of wanted concentration from anotherpharmaceutical preparationof different concentration.

Contents :

  1. Dilution & concentration of liquids:
  2. stock solution
  3. Dilution of alcohol
  4. Dilution of acids
  5. Dilution of solids & semisolids
  6. Trituration
  7. Alligation

Dilution:

By addition of diluents .

By admixture with solutions or mixture of lower strength .

Concentration :

By addition of active ingredient

By admixture with solutions or mixture of greater strength .

Evaporation of the diluents.

Calculating the percentage or ratio strength of the solution made by diluting or concentration a solution of given quantity & strength by equation:

  1. Inverse proportion .
  2. The equation: ( Q1 C1 = Q2 C2)

(1st quantity 1stconcentration) = (2nd quantity 2nd concentration).

  1. By determining the quantity of active constituents ( solute ) needed & then calculating the quantity of the available solution (usually concentrated or stock solution ) that will provide the needed amount of constituents.

Dilution & concentration of liquids:

Example :If 500 ml of 15% (v/v) solution of methyl salicylate in alcohol are diluted to 1500 ml . What will be the percentage strength ?

x= 5%

Or :

Q1 C1 = Q2 C2

500 ml 15%=1500 ml x %

x= 5%

Or :

500 ml of 15% (v/v) solution contains 75 ml of solute

Ex.: If 50ml of 1:20 w/v solution are diluted to 1000 ml , what is the ratio strength (w/v)?

1:20 = 5%

Or:

Or:

Q1 C1 = Q2 C2

50ml 5% = 1000mlx%

x =0.25% = 1:400

Or:

50ml of a 1:20 solution contains 2.5g of solute

Ex: How many grams of 10 (w/w)ammonia solution can be made from 1800g of 28% w/w strong ammonia solution?

Or :Q1 C1 = Q2 C2

180028% = x g10%

x = 5040g

Ex: How many milliliters of a 1:5000 w/v solution of drug can be made from 125ml of a 0.2% solution ?

1:5000 = 0.02%

Or :

0.2% = 1:500

Or:

Stock solution :

Stock solution are concentrated solution of active ( e.g. drug ) or inactive ( e.g. colorant ) substances are used by pharmacists as a convenience to prepare solution of lesser concentration (strong solutions from which weaker once may be made conveniently ).

Examples :

How many milliliters of a 1:400 w/v stock solution should be used to make 4L of a 1:2000 w/v solution ?

4L=4000ml

1:400=0.25%

1:2000=0.05%

Or

Or 4000 ml of 1:2000 w/v solution requires 2g of active constituent ;

Or:Q1 C1 = Q2 C2

4000ml x 0.25%= x x 0.05%

x =800ml

Ex:How milliliters of a 1:50 stock solution of phenylephrine HCl should be used in compounding the following prescription ?

Rx: Phenylephrine HCl0.25%

Rose water ad 30 ml

Sig. for the nose.

1:50 = 2 %

Or :30g x 0.0025=0.075 g ofPhenylephrine HCl needed

1:50 means 1g in 50ml of stock solution

Calculation of active constituent:

Ex: 500 ml of 15% v/v solution in alcohol are diluted to 1500ml . What will be the % v/v?

Q1 x %1 = Q2 x % 2

500 x 15 % = 1500 x % after dilution

or 1500 ml 15%

500 ml x %

x =5 % v/v

Ex: How many grams of 10% w/w Ammonium can be made from 1800 g , 28% w/wAmmonium ?

1800 g x 28 % = x g x 10 %

X = 5040 g

Ex: How many mls of water should be added to 375 ml of solution containing 0.5g of benzylkonium Cl to make 1:5000 solution ?

5000 x 0.5= 1 x x

x =2500ml 2500 -375 = 2125 ml of water added

Ex: How many mls of water should be added to 450 ml of solution 1:900 w/v of methyl salicylate to make solution of 1:3000 w/v ?

1:900 = 0.11% , 1:3000 =0.033%

450ml x 0.11 = V2 x 0.033

V2 1500 ml

1500-450= 1050 ml of water added

Dilution of alcohol:

When water & alcohol are mixed , there is a physical contraction such that the resultant volume is less than the total of the individual volumes of the two liquids .

Ex: how much water should be mixed with 5000 ml of 85% v/v alcohol to make 50% v/v alcohol ?

Therefore, use 5000ml of 85% v/v alcohol & enough water to make 8500 ml .( 8500-5000=3500 ml water added).

Ex: How many mls of 95% v/v alcohol & how much water should be used in compounding the following prescription :

Boric acid 1g

Alcohol 70% 30ml

Sig. Ear drops. ?

70% x 30ml = 95% x Xml

X= 22.1 ml of alcohol % dissolve 1g of boric acid then complete the volume to 30 ml by water .(30ml-22.1ml=7.9 ml of water )

Dilution of acids :

The strength of an official undiluted (concentrated) acid is expressed as percentage w/w .

Ex: How many mls of 85% w/w phosphoric acid having a specific gravity of 1.71 should be used in preparing 1 gallon of ¼% w/v phosphoric acid solution to be used for bladder irrigation ?

1 gallon=3785 ml

3785 x 0.0025= 9.46 g of phosphoric acid (100%) in 1 gallon

11.13g of water measures 11.13 ml

11.13 ml /1.71=6.5 ml

Dilution & concentration of solids & semisolids:

Dilution of solids in pharmacy occurs when there is need to achieve a lower concentration of an active component in a more concentrated preparation .

Ex: how many grams of opium containing 15% w/w of morphine & how many grams of lactose should be used to prepare 150g of opium containing 10% w/w of morphine?

150g -100 g= 50g of lactose.

Ex: if 7.2%w/w active ingredient contained in crude plant & 21.6 % water , what is the percent of active ingredient after the drying ?

100 g – 21.6 = 78.4 g after drying

Ex: how many grams of zinc oxide should be added to 3200g of 5%zinc oxide ointment to prepare an ointment containing 20% of zinc oxide ?

3200g x 0.05 =160g of zinc oxide of 5%

3200g – 160g =3040g of base in 3200g of 5% ointment

Because the 5% oint. Already contains 160g of zinc oxide

760 – 160 =600g

1