NTA Level 4 • Semester 1 • PST04105

Calculations of Weight by Aliquot Method

Pharmaceutical Calculations • Source Session/Topic 19

Full source-text version: all educational wording from the extracted learning source is retained; only presenter/tutor metadata and web-layout noise are removed, while formatting is improved for readability.

Session 19: Calculations of Weight by Aliquot Method

Total Session Time: 120 minutes

Prerequisites

None

Learning Tasks

By the end of this session students are expected to be able to:

• Define Aliquot Method

• Calculate weight by using Aliquot Method

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board and chalk/whiteboard markers

SESSION OVERVIEW

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

ASK students if they have any questions before continuing.

STEP 2: Definition of Aliquot Method (20 Minutes)

|Activity: Brainstorming (10 minutes) |

| |

|Ask students to brainstorm on the following question: |

| |

|What is Aliquot Method? |

| |

|ALLOW few students to respond |

| |

|WRITE their responses on the flip chart/ board |

| |

|CLARIFY and SUMMARISE by using the content below |

• An aliquot is a fraction, portion, or part that is contained an exact

number of times in another.

• Aliquot method is useful when a degree of precision in measurement is

beyond the capacity of the instrument at hand, hence the

pharmaceutical personnel may achieve the desired precision by

calculating and measuring in terms of aliquot parts.

STEP 3: Weighing by Aliquot Method (70 Minutes)

• The aliquot method of weighing is a method by which small quantities

of a substance may be obtained within the desired degree of accuracy

by weighing a larger-than-needed portion of the substance, diluting it

with an inert material, and then weighing a portion (aliquot) of the

mixture calculated to contain the desired amount of the needed

substance.

• A stepwise description of the procedure is depicted in Figure 1 below

and is described as follows:

[pic]

Figure 1: A stepwise description of weighting by Aliquot method

Preliminary Step. Calculate the smallest quantity of a substance that can

be weighed on

the balance with the desired precision.

• The equation used:

100% x Sensitivity Requirement (mg) = Smallest Quantity (mg)

Acceptable Error (%)

• Example:

On a balance with an SR of 6 mg, and with an acceptable error of no greater

than 5%, a quantity of not less than 120 mg must be weighed.

100% x 6 mg = 120 mg

Step 1. Select a multiple of the desired quantity that can be weighed with

the required

precision.

• If the quantity of a required substance is less than the minimum

weighable amount, select a‘‘multiple’’ of the required quantity that will

yield an amount equal to or greater than the

• Minimum weighable amount (A larger-than-necessary multiple may be used to

minimum accuracy desired.)

For example:

• If the balance in the example in the preliminary step is used, and if 5

mg of a drug substance is

• required on a prescription, then a quantity at least 25 times (the

multiple) the desired amount, or

• 125 mg (5 mg x 25) must be weighed for the desired accuracy.

Step 2. Dilute the multiple quantity with an inert substance.

• The amount of inert diluent to use is determined by the fact that the

aliquot portion of the drug-diluent mixture weighed in Step 3 must be

equal to or greater than the minimum weighable quantity previously

determined.

• By multiplying the amount of the aliquot portion to weigh in Step 3 by

the multiple selected in Step 1, the total quantity of the mixture to

prepare is determined.

Example:

• According to the preliminary step, 120 milligrams or more must be

weighed for the desired accuracy.

• If we decide on 120 mg for the aliquot portion in Step 3, and multiply

it by the multiple selected in Step 1 (i.e., 25), we arrive at 3000 mg

for the total quantity of the drug-diluent mixture to prepare

• Subtracting the 125 mg of drug weighed in Step 1, we must add 2875 mg

of diluent to

prepare the 3000 mg of drug-diluent mixture

Step 3. Weigh the aliquot portion of the dilution that contains the desired

quantity.

Since 25 times the needed amount of drug substance was weighed (Step 1), an

aliquot part

equal to 1⁄25 of the 3000-mg drug-diluent mixture, or 120 mg, will contain

the required

quantity of drug substance.

Proof: 1⁄25 x 125 mg (drug substance weighed in Step 1) = 5 mg

1⁄25 x 2875 mg (diluent weighed in Step 2) = 115 mg

120 mg aliquot part

|Activity: Small Group Discussion ( 30 minutes) |

| |

|DIVIDE students into small manageable groups |

| |

|ASK students to discuss on the following question |

|A torsion prescription balance has a sensitivity requirement of 6 |

|milligrams. Explain how you would weigh 4 milligrams of atropine |

|sulfate with an accuracy of 5%, using lactose as the diluent. |

| |

|ALLOW students to discuss for 15 minutes |

| |

|ALLOW few groups to present and the rest to add points not mentioned |

| |

|CLARIFY and SUMMARIZE by using the contents below |

• The following are the solution of the above exercise:

o Because 6 milligrams is the potential balance error, 120 milligrams is

the smallest amount that should be weighed to achieve the required

precision.

o If 120 milligrams, or 30 times the desired amount of atropine sulfate,

is chosen as the multiple quantity to be weighed in Step 1, and if 150

milligrams is set as the aliquot to be weighed in Step 3, then:

1. Weigh 30 x 4 mg, or 120 mg of atropine sulfate

2. Dilute with 4380 mg of lactose to make 4500 mg of dilution

3. Weigh 1⁄30 of dilution, or 150 mg of dilution, which will contain 4 mg

of atropine sulphate Answer:

Proof: 4500 mg (dilution) = 120 mg (atropine sulfate)

150 mg (dilution) x mg (atropine sulfate)

= 4 mg, answer.

STEP 4: Key Points (10 minutes)

• The dilution of solid dosage forms such as powders or semisolids dosage

forms such as ointments and creams may be performed to alter the dose or

strength of the product

• The concentration of solid and semisolids dosage forms also known as

fortification, by addition of calculated quantity of additional

therapeutic agent, remains a viable practice in pharmaceutical

compounding

STEP 7: Evaluation (15 minutes)

• What is aliquot?

• What are the steps of weighing by aliquot method?

References

Ansel, H. C & Stocklosa, M. J. (2001). Pharmaceutical Calculations (11th

ed.). Philadelphia, United States: LIPPINCOTT WILLIAMS & WILKINS

Ansel, H. C (2010) Pharmaceutical Calculations (13rd ed.). Philadelphia,

United States: LIPPINCOTT WILLIAMS & WILKINS

Senya, S. S, Mwasha, C.Y, Muyinga, A. M, Amiri,R. I. and Mauga E.A.S.K.

(2011) Tanzania Pharmaceutical Handbook (2nd ed.). Dar eS Salaam,

Tanzania: School of Pharmaceutical Sciences.

Zatz, J.L and Teixeira, M.G. (2005). Pharmaceutical Calculation (4th ed.).

New Jersey: John Wiley & Sons

← Previous Topic Next Topic →View all Pharmaceutical Calculations topics Open Complete Full Notes

PDF / OFFLINE NOTES

Unataka kutumiwa notes hizi kupitia WhatsApp?Kwa notes zilizopangiliwa vizuri kwa kusoma offline au PDF, bonyeza kitufe hapa chini. Ujumbe wenye Level, Semester, Module na Topic utaandaliwa moja kwa moja.TUMIWA NOTES WHATSAPP

WhatsApp: 255620339260