Unit #3
Q: Solid in Solid Aliqout We need 30 mg Klonopin (clonazepam) to prepare a capsule prescription. Assume that you have a torsion balance with a sensitivity of 5 mg and that you do not want to exceed the 5 % error. How would you prepare this prescription?
#1 MWQ 5 mg / 5 % = 100 mg #2 Multiple factor that = or is larger than the MWQ 30 needs to multiple to 100 Factor could be 4 30 x 4 = 120 mg #3 Select aliquot weight lets select 200 mg which is more than the MWQ and now times that by 4 #4 Multiple by first factor 200 x 4 = 800 mg #5 Calculate the amount of diluent to add 800 - 120 mg = 680 mg #6 Divide diluent by factor 800 / 4 = 200 mg and in that is the 30 mg we need
Percent Error Formulas
% Error = Sensitivity of the Balance x 100% / Quantity Desired % Error = Actual Weight - Theoretical Weight / Actual weight x 100% If a (-) number is calculated KEEP IT
2 types of balances used in the preparation of pharmaceutical dosage forms
- torsion (actual balance, levels out, older) - digital (new)
Q: Working Backwards Through an Aliquot A pharmacist is preparing a batch of suppositories. The pharmacist needs to weigh a total of 25 mg of promethazine. He is using a balance with a MWQ of 125 mg. Because the 25 mg is below the MWQ for his balance, he instead weighs 150 mg of the drug. He takes the 150 mg of promethazine and mixes it with 600 mg of silica gel. What amount of this mixture must he weigh out in order to deliver the 25 mg of drug originally needed?
150 / 25 = 6 so thats the factor Total / factor = amount you weigh out 750 / 6 = 125 mg will contain the 25
Liquid in Liquid Aliquot Steps
#1 Determine the MWQ #2 Find Factor that is equal or greater than MWQ #3 Select an Aliquot Volume #4 Multiply by Factor #5 Minus the diluent #6 Take the Total divided by the factor
Steps for a Solid in Solid Aliquot Problem
#1 Determine the MWQ #2 Find multiple that equals or is larger than the MWQ #3 Select aliquot weight (random dilution #) #4 Multiply by the first factor you found #5 Calculate amount of diluent to add (total diluent - active ingredient) #6 Divide total amount of dilution by your factor
Q: Liquid in Liquid Aliquot, A vet has calculated that the appropriate dose of a certain concentrated wormer for a kitten is 0.03 ml. The drug is given twice by mouth, first on the day of infestation is diagnosed and then a second dose is sent home with the owner for administration 14 days later. If the owner will use a 1 ml syringe to administer the drug, how should the wormer be diluted with tuna water to prepare the dose?
#1 MMV 1 ml x 20 % = .2 ml #2 Factor equal or greater than MMV .2 ml / .03 = 6.666 Lets use 10 #3 Select a Volume 0.5 ml as volume #4 Multiply by factor 0.5 x 10 = 5 ml #5 Minus the diluent 5 ml - .2 ml = 4. 7 ml #6 Total divided by factor 5 ml / 10 = 0.5 ml in which you will fidn the .03 ml in
Q: Solid in Solid Aliquot, You need to weigh out 4 mg of a drug on a compounding balance with a linearity of 3 mg (use 5% error). How would you prepare this prescription?
#1 MWQ 3 / 5% = 60 mg #2 Find a factor above the MWQ 4 x 15 = 60 so factor of 15 #3 Select an aliquot weight 100 mg #4 Aliquot weight x the factor 100 x 15 = 1500 mg #5 Amount of dilutent to add 1500 - 60 = 1440 mg #6 Divide diluent by factor 1500 / 15 mg = 100 mg which has the 4 mg in it
Q: Liquid in Liquid Aliquot A pharmacist is preparing a compounded prescription that calls for 2 ml of a concentrated drug solution. Because this volume is less than the MMV of her smallest graduate (25 ml) she instead measures 6 ml of concentrated drug solution and brings it to a volume of 15 ml with propylene glycol. What amount of this mixture in ml must she measured out in order to deliver the 2 ml of concentrated drug solution that was originally needed?
25 x 20% = 5 ml is the MMV 6ml / 2ml = 3 which is the factor total / factor = 15 / 3 = 5 ml has the 2 ml in it 6 + X = 15 ml
% Error in prescription compounding is usually
5% - you can use lower cut off if weighing is more critical for a drug
Q: Percent Error, A prescription requires the pharmacist to weigh 500 mg of a drug. After weighing the 500 mg, the pharmacist checks the weight on another balance. It registers at 475 mg. Whats the Percent error
Actual - Theoretical / theoretical x 100% (475 - 500) / (500) x 100% = -5% error
Maximum weight that can be placed on the balance is called the
Capacity
Digital balances, the smallest weight increment that can be read is the
Readability
Torsion balances, the smallest weight that gives one subdivision deflection on the pointer is the
Sensitivity
Q: Torsion balance, Sensitivity is 5-6 mg and the capacity is 120 g Whats the MWQ?
Sensitivity / % Error 6 g / 5% = 120 mg which is equal to the weight capacity so we can weigh it
Q: Percent Error, if we weigh out 4 mg on the torsion balance with a sensitivity of 6 mg? what is the % error??
Sensitivity of balance x 100% / Quantity desired 6 mg x 100% / 4 mg = 150% error
Aliquot Methods
a way to measure quantities of ingredients that are less than your MWQ or MMV 1. we dilute drug to a quantity that can be measured 2. we then take an "aliquot" of the dilution to get weight or volume that contains the desired amount of the drug
in preparation of dosage froms we are required to measure
drugs or solvents
Digital Balance, the minimum reproducible readable value is the
linear or linear accuracy
MMV
minimum measurable volume Total Capacity x 20% For: graduate cylinders, syringes
MWQ
minimum weighable quantity is the smallest weight that can be accurately measured on a balance
Aliquots
small amount
2 types of aliquots
solid in solid liquid in liquid
when making measurements the pharmacist must recognize what
the limitations of the instruments used
Q: Compounding Balance, Linearity is 10 mg and the Capacity is 200g. What is the MWQ?
Linearity / % Error 10 g / 5% = 200 mg Which is equal to the capacity, so we can weigh it
What 2 things help calculate a minimum weigh able quantity
Linearity and Sensitivity
Digital Balance Equation:
MWQ = Linearity / % Error
Torsion Balance Equation:
MWQ = sensitivity / % Error
