Calculations of Weight by Aliquot Method – PST04105 Pharmaceutical Calculations
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 |Step |Time |Activity/ |Content | | | |Method | | |1 |05 minutes |Presentation |Introduction, Learning Tasks | |2 |20 minutes |Presentation |Definition of Aliquot Method | | | |Brainstorming | | |3 |70 minutes |Presentation |Calculations of Weight by Aliquot | | | |Group |Method | | | |discussion | | |4 |10 minutes |Presentation |Key Points | | 5 |15 minutes |Presentation |Evaluation | 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 5% 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