T he various scales and balances used in pharmaceutical weighing have ascribed to them di erent degrees o precision. As described previously in this chapter, knowledge o the sen- sitivity requirement o the balance being used is critical in weighing to a specif ed degree o accuracy. T he sensitivity requirement o a balance may be used to determine the percentage o error in a given weighing.
(1) W hen the maximum potential error is ±4 mg in a total o 100 mg, what is the percentage o error?
4 100 100
mg
mg
× %
= 4%
(2) A prescription calls or 800 mg o a substance. A ter weighing this amount on a balance, the pharmacist decides to check by weighing it again on a more sensitive balance, which registers only 750 mg. Because the f rst weighing was 50 mg short o the desired amount, what was the percentage o error?
50 100 800
mg
mg
× % = .
6 25%
3 • Pharmaceutical Measurement 45
Examples of Measurement Applications in Pharmaceutical Compounding
T he following are examples of the calculations applied in weighing and measuring in the compounding of pharmaceutical formulas or medication orders. Many additional problems are found in Chapter 17, Selected Calculations in Contemporary Compounding.
(1)2 Misoprostol 400 µg
Polyethylene oxide 200 mg
H ydroxypropyl methylcellulose 15 g Compounding Instructions:
(1) Accurately weigh each of the ingredients.
(2) Place the misoprostol in a mixing vessel, and add the polyethylene oxide in equal portions until thoroughly blended.
(3) Add the hydroxypropyl methylcellulose in portions until all ingredients are thoroughly blended.
(4) Label and dispense.
(a) Would a torsion prescription balance allow the accurate direct weighing of each ingredient? Explain.
(b) Explain how the misoprostol might be accurately obtained using a torsion prescrip- tion balance and the aliquot method of weighing.
(c) How many misoprostol tablets, each containing 0.2 mg, could be used in com- pounding this order? How would they be combined?
Answers:
(a) N ot for the misoprostol. T he least weighable quantity using a torsion bal- ance is 120 mg (with an SR of 6 mg and an acceptable error of ±5% ), and the misoprostol required is 400 µg or 0.4 mg. An analytical balance could be used.
(b) T he pharmacist could weigh 300 times the required amount of misoprostol, 120 mg (300 × 0.4 mg = 120 mg); then, mix that with 35,880 mg of polyethyl- ene oxide to make 36,000 mg of mixture from which a 120-mg aliquot portion could be taken to provide the 0.4 mg of misoprostol (and 119.6 mg of polyeth- ylene oxide). H owever, this would be very wasteful of the ingredients, so the better option is provided by (c).
(c) Two misoprostol tablets each containing 0.2 mg (200 µg) would provide the 400 µg required. T he tablets would be pulverized using a mortar and pestle and the other ingredients combined in portions as described in the compound- ing instructions as stated above.
(2)2 Fentanyl citrate 2.5 mg
Methylparaben 10 mg
Propylparaben 10 mg
Propylene glycol 0.2 mL
N ormal saline solution 10 mL Compounding Instructions:
(1) Accurately weigh and measure each of the ingredients.
(2) D issolve the methylparaben and the propylparaben in the polyethylene glycol.
46 Pharma eu i al c al ula ion
(3) D issolve the entanyl citrate in the normal saline solution.
(4) Slowly add the solution o the parabens to the entanyl citrate solution and mix well.
(5) Sterilize by f ltering through a sterile 0.2-µm f lter into a sterile metered spray bottle.
(6) Label and dispense.
(a) W hat type of balance should be used to weigh the fentanyl citrate and the parabens?
(b) W hat are the best options in measuring the propylene glycol?
Answers:
(a) An analytical balance should be used.
(b) A graduated pipette or a graduated syringe.
c ASE In Po In T 3 .1 A pharma i i a ked o ompound he following formula for he prepara ion of 1 00 ap ule 3:
Estriol 200 mg
Estrone 25 mg
Estradiol 25 mg
Methocel E4M 10 g
Lactose 23.75 g
U ing a balan e ha ha an s R of 6 mg, he aliquo me hod of weighing, la o e a he diluen , and an error in weighing of 4 % how, by al ula ion , how he orre quan i y of e rone an be ob ained o a ura ely ompound he formula.
c ASE In Po In T 3 .2 A phy i ian pre ribed 25 4-mg ap ule of a drug for a pe- ial need pa ien , knowing ha he do e pre ribed wa on idered “ ub herapeu- i .” t he lowe reng h ommer ially available able on ain 25 mg.
t he pharma i de ided o ele he minimum required number of 25-mg ab- le (4 able ), redu e hem o a powder wi h a mor ar and pe le, weigh he powder (280 mg), and on inue he pro e u ing he aliquo me hod. s he alled upon her pharma y uden in ern o al ula e (a) he minimum quan i y of la o e (diluen ) o u e in preparing he ru hed able –diluen mix ure and (b) he quan i y of he mix ure o u e o fill ea h ap ule.
t he pre rip ion balan e had an s R of 6 mg, and a weighing error of 5% wa a ep able.
s how your al ula ion for (a) and (b), and ( ) prove ha your an wer o (b) i orre by demon ra ing ha ea h ap ule would indeed on ain 4 mg of drug.
3 • Pharmaceutical Measurement 47
PRAc TIc E PRo Bl EMS Calculations of Aliquot Parts by Weighing
1. A prescription calls for 50 mg of chlorpheniramine maleate. U sing a prescrip- tion balance with a sensitivity requirement of 6 mg, explain how you would obtain the required amount of chlorpheniramine maleate with an error not greater than 5% .
2. A prescription balance has a sensitivity requirement of 0.006 g. Explain how you would weigh 0.012 g of a drug with an error not greater than 5% , using lactose as the diluent.
3. A torsion prescription balance has a sensitivity requirement of 4 mg. Explain how you would weigh 5 mg of hydromorphone hydrochloride with an error not greater than 5% . Use lactose as the diluent.
4. A torsion prescription balance has a sensitivity requirement of 0.004 g.
Explain how you would weigh 0.008 g of a substance with an error not greater than 5% .
5. A prescription balance has a sensitivity requirement of 6.5 mg. Explain how you would weigh 20 mg of a substance with an error not greater than 2%.