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Hello every one ...... The very first blog I would like to place in this April is on calculations.
Generally most of the students are confused while dealing with calculations. During my course of time till now, here are few calculations which I have come across. Before proceeding into the calculation the pre requisite step is to read the data thoroughly, understand the given information and select the required procedure for calculation.
I would like to touch up few important concepts in these calculations.
A) DOSAGE CALCULATIONS
B) ALCOHOL DILUTIONS
C) ISOTONICTY CALCULATIONS
D) PROOF SPIRIT CALCULATIONS
E) DOSAGE REPLACEMENT FACTOR
F) TEMPERATURE CONVERSION
A) DOSAGE CALCULATIONS1
Based on Age
Young Rule (Children under 12 yrs of age)
Child's dose = Age (yrs)/ Age (yrs) +12 x Adult dose
Example: 1. Let us consider a drug whose dose is 350 mg, then the dose for a three year old child will be:
Sol: Child dose = 3/3+12x 325 = 65mg
Dose for the child = Age in years/20 x Adult dose
Dose for the child = Age in months/150 x AD
Consider that the dose of a drug for adult is 100 mg then the dose for a 9 month old infant and a boy of 16 yrs age can be calculated as below:
Ans i) Fried's Rule 9/150x100 Dose = 6 mg
ii) Dillings formula: Dose = 16/20 x 100 = 80 mg
Based on Body Weight:
Dose for the child = Weight of the child/ 150 x Adult dose
Example 3: If the adult dose of a drug is 10mg the dose for a 40 lb child will be:
Child's Dose = 40/150 x 100 = 2.67mg
Based On the Body Surface
Child's Dose = Body surface area of a child/Body surface area of Adult x Adult dose
Average body surface area for adult = 1.73 m2
Example 4: Let us see how to calculate the dose for child who has a body surface equal to 0.57 sq.m. when an average adult dose of a drug is 50 mg
Child dose (approx) = 0.57/1.73 x 50 = 16.5 mg.
B) ALCOHOL DILUTIONS 2
This method is used to find out the intermediate strength by mixing two preparations which are similar and having different strengths.
Example 5 Calculate the volume of 95% alcohol required to prepare 600 ml of 70% alcohol?
Volume required = 600ml
Percentage of alcohol required = 70
Percentage of alcohol used = 95
By allegation method Alcohol Water
70-0 95-70 =25
To produce required % of alcohol, 70 parts of 95% alcohol and 25% of water mixed.
Quantity of 95% alcohol required = 600x70/95 = 442.10ml
Quantity of water required = 600 x 25/95= 157.90ml.
C) ISOTONICTY CALCULATIONS3
Freezing point method or Cryoscopic method:
% w/v of adjusting substance needed = [0.52-a]/b
a= Freezing point of the unadjusted solution
b=Freezing point of a 1% W/V solution of the adjusting sub.
Using croscopic method how will you prepare 1% solution of boric acid, iso-osmotic with blood plasma. (Hints: freezing point of 1% w/v solution of boric acid is -0.288o C). The freezing point of 1% w/v solution of NaCl is - 0.576 o C)
Solution: % w/v of NaCl required = [0.52 -0.288]/0.576 = 0.402% w/v.
So, weigh 1 gm of Boric acid and transfer to a volumetric flask then weigh 0.4 g of NaCl and add it to the boric acid in flask. Then add small quantity of water, dissolve and make up the volume up to 100 ml. Now we will have an iso-osmotic solution.
NaCl Equivalent method:
Quantity of NaCl required to adjust tonicity = 0.9 - (n x E) n = number of grams of unadjusted substance E = NaCl equivalent (quantity of NaCl i.e. equivalent to 1 gm of drug)
White Vincet method:
V = w x E x 111.1
Where V = volume of isotonic solution in ml that can be prepared by dissolving drug in water w = wt. of drug
E = NaCl equivalent
% w/v of adjusting substance required = 0.03M/N M= Gram molecular weight
N=Number of ions
How to prepare an iso-osmotic solution using dextrose.
Molecular wt of dextrose=180
Dextrose is non ionizing substance so no ions.
W= 0.03 x M = 0.03 x 180 = 5.4 g/100ml
D) PROOF SPIRIT CALCULATIONS2
Any alcoholic solution which has 57.1% v/v alcohol is proof spirit.
Steps which are involved in calculating.
Firstly let us known how 1.753 is obtained. 57.1 Volume of ethyl alcohol = 100 Volume of P.S Volume of ethyl alcohol= 100/57.1= 1.753 volume of PS
Step1: The % strength of alcohol is to be multiplied with 1.753 and substract100 from the value obtained after multiplying.
Step2. If value obtained is + then it is over proof
Step3. If value obtained is - then it is under proof.
What is the proof strength for 95% v/v alcohol?
% strength of alcohol x1.753 -100 = 95 x 1.753-100 = 166.53-100=
+66.53 o So 66.53 o OP
E) DOSAGE REPLACEMENT FACTOR5
Number of parts by weight of drug that replaces one part by weight of base.
Displacement factor= 100 (E-G)/Gx + 1
E = Wt of pure suppositories
G = Wt of suppositories containing x % of drug
For preparation of suppositories in uniform and accurate weight this factor is used
9C = 5F - 160C
C = Degree centigrade
F = Degree Fahrenheit
1. Remington the science and practice of pharmacy, 21st edition, volume1 pg no 116 [accessed date: 12th march, 2010]
2. Remington the science and practice of pharmacy, 21st edition, volume1 pg no 122-123 [accessed date: 12th march, 2010]
3. Physical pharmacy and pharmaceutical sciences by Martin, Fourth edition, Pages-182-185 [accessed date: 13th march, 2010]
4. Dispensing Pharmacy by RM Mehta, 3rd edition, Page. No. 94 [accessed date: 13th march, 2010]
5. Theory and Practice of Pharmacy, 20th edition, Page No. 585 [accessed date: 14th march, 2010]
6. http://en.wikipedia.org/wiki/Temperature[accessed date: 14th march, 2010]
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