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 :
- Dilution & concentration of liquids:
- stock solution
- Dilution of alcohol
- Dilution of acids
- Dilution of solids & semisolids
- Trituration
- 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:
- Inverse proportion .
- The equation: ( Q1 C1 = Q2 C2)
(1st quantity 1stconcentration) = (2nd quantity 2nd concentration).
- 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
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