Does anyone know if there were any aliquot questions in their NAPLEX this year?? i HATE aliquot, and just don't get it and it's making me super depressed... If someone could help me out with the following i would really appreciate it.
A prescription calls for 50mg chlorpheniramine maleate. Using a prescription balance with a SR of 6mg, explain how you would obtain the required amount of chlorpheniramine maleate with an error no greater than 5%?
I get that:
-> If MWQ = 6/.05 = 120mg
-> And the multiple is 3 (50*3=150)
After this, the numbers and the equations i set up lead to the wrong answers...
50mg (needed amount) = 150mg (actual quantity weighed out > 120mg) (Atropine + Lactose)
120mg (MWQ) BULK (atropine + lactose)
bulk = 360; which means lactose = bulk (360) - quantity weighed(150) = 210mg lactose
So 360 x (1/3) = 120 --> 50mg Atropine + 70mg Lactose
But apparently this is WRONG... I have NO idea how the answer became:
weight of Chlorpheniramine = 150mg
Dilute with 450mg (I'm assuming lactose)
to make 600mg (I'm assuming bulk)
weigh 200mg (I'm assuming aliquot amount)
Could someone please explain this to me, like step-by-step and majorly dumbed down? Apparently I don't remember anything about aliquots...
To Anomaly285,
You asked:
Could someone please explain this to me, like step-by-step and majorly dumbed down? Apparently I don't remember anything about aliquots...
My answer: I think your answer is okay.
Let's review the answer:
weight of Chlorpheniramine = 150mg
Dilute with 450mg (I'm assuming lactose)
to make 600mg (I'm assuming bulk)
weigh 200mg (I'm assuming aliquot amount)
Let's mirror that with your answer:
weight of Chlorpheniramine = 150mg
Dilute with 210mg (I'm assuming lactose)
to make 360mg (I'm assuming bulk)
weigh 120mg (I'm assuming aliquot amount)
Let's check your answer:
weight of Chlorpheniramine = 150mg (We can weigh this accurately because this is over 120 mg)
Dilute with 210mg (We can weigh this accurately because this is over 120 mg)
to make 360mg (We can weigh this accurately because this is over 120 mg)
weigh 120mg (We can weigh this accurately because this is 120 mg)
A very good explanation in plain human friendly language is attached, please see page 9, find section with title:
Aliquot Measurement Calculations.
(Credit to:
http://courses.washington.edu/pharm309/calculations/Lesson3.pdf
Emphasized and clarified excerpt from that math guide:
Note, the guide did not explain well about a term called:
“multiplication factor”
This is also called MAGIC NUMBER by RXPREP.
For active drug, to find Weigh-able Number, you use this MAGIC NUMBER.
For diluent, to find Weigh-able Number, you use this MAGIC NUMBER.
After mixing,
For each capsule, to find Weigh-able Number, you use this MAGIC NUMBER.
Okay, MAGIC NUMBER is important. How do I find this MAGIC NUMBER?
You guess.
Or you can do detailed steps.
Let’s see the question again:
A prescription calls for 50mg chlorpheniramine maleate.
Using a prescription balance with a SR of 6mg, explain how you would obtain the required amount of chlorpheniramine maleate with an error no greater than 5%?
The balance has Error of 6 mg.
The lab wants me to have Error as 5% or less.
Setup math:
6 mg counts as 5%.
X mg counts as 100%.
Answer: x = [(6 TIMES 100 %) / 5%] = 120 mg.
We call this 120 mg as minimum weight we must measure.
If we weigh 120 mg, we may have error as 5%, acceptable.
If we weigh 100 mg, we may have error as 6%, not acceptable.
If we weigh 90 mg, we may have error as 6.7%, not acceptable.
If we weigh 80 mg, we may have error as 7%, not acceptable.
If we weigh 70 mg, we may have error as 8%, not acceptable.
If we weigh 60 mg, we may have error as 10%, not acceptable.
If we weigh 130 mg, we may have error as 4.60%, acceptable.
If we weigh 140 mg, we may have error as 4.28%, acceptable.
If we weigh 150 mg, we may have error as 4.00%, acceptable.
If we weigh 200 mg, we may have error as 3.00%, acceptable.
Start: I know after I mix a bunch of powder, I will have to weigh the powder for each capsule. What is the weight of powder for each capsule?
That means: find what’s inside the capsule.
What do we have to have?
Active Drug.
Diluent.
Active drug is 50 mg.
Diluent…I don’t know yet.
But, I know, each time I weigh something, the weigh must be big enough. In this case of this balance, weigh must be as big as 120 mg or more.
So, powder of 1 capsule must be 120 mg or more.
Active drug of 1 capsule must be 50 mg.
Diluent is what?
120 mg – 50 mg = 70 mg.
Diluent must be 70 mg (or more).
How many capsules do I make?
3 capsules. Why 3 capsules?
I want to make 1 capsule, but, how do I weigh 50 mg of Active drug? I can’t.
Why? (This is NOT weigh-able because this balance has Minimum Weighable Quantity as 120 mg.)
How about 2 capsules?
I want to make 2 capsules, but, how do I weigh 100 mg of Active drug? I can’t.
Why? (This is NOT weigh-able because this balance has Minimum Weighable Quantity as 120 mg.)
How about 3 capsules?
I want to make 3 capsules, but, how do I weigh 150 mg of Active drug? I CAN.
Why? (This is weigh-able because this balance has Minimum Weighable Quantity as 120 mg.)
So, by guessing, the decision is: 3 capsules, at least. I can do 4, but, who will buy the left over 1 pill? Drug will expire before someone buys. So, I make minimum allowed by law and force buyer to buy minimum of 3.
Therefore, MAGIC NUMBER is 3.
(Side note: If you really really have to make only 1 pill because Active Drug is super expensive, I guess you can QS active drug. I guess: start with 120 mg of lactose, the balance shows 120, doctor wants 50 mg of Active drug?....slowly....snail...slowly... add Active Drug until you see exactly 170 mg. You just added 50 mg of Active drug into the powder. Put ALL into 1 capsule shell, you have exactly 50 mg of Active drug. Any comment, please teach me. Thank you.)
Let's go back to main concept: make 3 capsules, each capsule has 50 mg of Active Drug.
What is total Active drug?
[3 capsules TIMES (50 mg / 1 capsule)] = (150 mg / 3 capsules) (of Active Drug)
What is total weight of powder of 3 capsules? I choose to make a capsule to be 120 mg (This is weigh-able because this balance has Minimum Weighable Quantity as 120 mg.)
[3 capsules TIME (120 mg / 1 capsule)] = (360 mg / capsules) (of Total powder.)
Then, what is total weight of Diluent? I do math:
(360 mg / 3 capsules) (of Total powder.)
Minus
(150 mg / 3 capsules) (of Active Drug)
Equals
210 mg / 3 capsules (of Diluent.)
Okay,
Step 1: walk to balance.
Step 2: slow put Active drug on balance, weigh 150 mg. This is weigh-able because this balance has Minimum Weighable Quantity as 120 mg.
Step 2: slow put Diluent on balance, weigh 210 mg. This is weigh-able because this balance has Minimum Weighable Quantity as 120 mg.
Step 3: Mix Active Drug and Diluent, I get total powder as 360 mg.
Step 4: I will make 3 capsules, so, I will do math:
360 mg DIVIDE BY 3 capsules = (120 mg / 1 capsule)
Step 5: To make 1 capsule, weigh 120 mg (This is weigh-able because this balance has Minimum Weighable Quantity as 120 mg), put in capsule shell, close capsule shell, and get 1 capsule.
Step 6: Still got powder, repeat 2 more times, get 2 more capsules. Done. End of movie.
(Imagine you have to repeat that step to make 4000 capsules a day. In real lab, I get 3 capsule shells that are empty, stand them up, spread evenly total powder of 360 mg into 3 shells evenly, close shell. Done.)
++++++++++++++++++++++++++++++++++++++
Emphasized and clarified excerpt from attached math guide:
Here’s how you do it.
Let’s say that you are using the old-fashioned mechanical torsion balances found in the laboratories of most cash-strapped pharmacy schools (the new electronic balances weigh over $1000 apiece at this time – ouch!) These torsion balances have a sensitivity requirement of 6mg. The Minimum Weighable Quantity is 120 mg (this obtained by dividing 6 mg by the acceptable error rate of 0.05).
You get a prescription for a compounded drug containing 20 mg of drug in each solid dosage form. You decide to use lactose as a diluent after checking that the patient is not lactose-intolerant. This patient will take one capsule daily for 5 days, so you know you need to make 5 capsules.
You cannot weigh 20mg on your balance with an error rate of 5% or less, nor can you weigh 100mg (the total amount of drug in all 5 capsules). What you will need to do is to take your 20mg and multiply it by some number that will allow you to weigh 120mg or more of drug.
It’s probably easiest to choose the multiple of six, since that means you will weigh 120mg of drug and thus will have the least amount of wastage, but will have enough for one extra capsule, just in case of spillage. You weigh 120mg on your balance.
You now need to choose an amount of diluent that you will use. You can choose any amount as long as that amount meets each of two criteria.
First, the total amount of drug and diluent in each dosage unit must weigh more than your minimum weighable quantity. In our example, then, we will need to measure enough diluent so that there is at least 100mg of diluent in each dosage form
(100mg diluent + 20mg drug = 120 mg of drug in each dose (120 mg is the Minimum Weighable Quantity = MWQ).
Second, you cannot use more diluent in each dosage form than will fit into the dosage form container, in this case, an empty gelatin capsule. The easiest thing to do is to:
for each dose, determine the how much diluent needs to be added to the drug in order to meet your minimum weighable quantity for each dose,
then multiply that number by the same multiple you used for the drug, which in this case is six.
This means you will weigh 600mg (100mg x 6) of lactose on your balance.
1. Place the measured drug and lactose in a mortar and triturate well with the pestle.
2. Weigh out 120mg of the resulting mixture and place in an appropriately-sized capsule shell. Do this 5 times and you’re set.
To summarize the steps in aliquot calculations:
STEP 1. Identify the sensitivity requirement of your balance. Divide by 0.05 to determine the Minimum Weighable Quantity (MWQ).
(SR) / (0.05) = (MWQ)
STEP 2. Multiply the strength of each drug dose by a number that will produce a weighable quantity at or above the minimum weighable quantity.
(drug dose)(multiplication factor) = QUANTITY (this QUANTITY must equal MWQ or over MWQ).
STEP 3. Choose an amount of diluent to weigh. The most logical way to choose the amount is to subtract the drug dose from the minimum weighable quantity and use this number for your diluent amount per dose, then multiply by the same factor as the one used in step #2.
(MWQ – Drug Dose Weight)(multiplication factor) = total diluent amount to weigh.
4. Mix the drug and diluent well. Divide the total weight by the multiplication factor (to see how much of the drug-diluent mixture to use for each dosage form).
++++++++++++++++++++++++++++++++++++++
Good luck and please help others....Thank you very much in advance.
