NTA Level 4 • Semester 1 • PST04105

Calculations of Quantities of Water to Use for Reconstituting

Pharmaceutical Calculations • Source Session/Topic 10

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Session 10: Calculations of Quantities of Water to Use for Reconstituting

Powders for Oral and Parental Preparations

Total Session Time: 120 minutes

Prerequisites

• None

Learning Tasks

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

• Calculate the Volume of Dry Powder and Quantity of Drug Present

• Calculating the amount of water to be used for reconstitution of oral and

parenteral preparations

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: Calculate the Volume of Dry Powder, Total Drug Present (40 minutes)

• Some drugs lose their potency in a relatively short period of time when

prepared in a liquid dosage form,

o Thus to enhance its shelf life, manufacturers provide products to

the pharmacy in dry powder form for constitution with purified

water or special diluents at the time a prescription or medication

order is received

• After constitution, the resultant solution or suspension is stable in the

quantities usually dispensed, for up to 10 days at room temperature or 14

days if maintained under refrigeration

• For children and adult, constituted products for oral solution or

suspension generally are formulated such that the usual dose of the drug

is contained in teaspoonful amounts of product

• Calculating the amount of water to be used for reconstitution of oral and

parenteral preparations

o This calculation involved in calculating the amount of diluents

required to reconstitute the powder for oral solution or injectable in

order to get the desired concentration

o Dry powder for constitution are packed in self-contained bottles of

sufficient size to accommodate the additional of the required volume

of diluents

o In additional to the quantitative amount of therapeutic agent, the

powder contains such pharmaceutical ingredients as solubilizing

agents, stabilizers, colorants, sweeteners, and flavorants

|Activity: Small Group Discussion ( 20 minutes) |

| |

|DIVIDE students into small manageable groups |

| |

|ASK students to discuss on the following question |

|The label instructions for an ampicillin product call for addition of |

|78 mL of water to make 100 mL of constituted suspension such that each |

|5mL contains 125 mg of Ampicillin. What is the volume occupied by dry |

|powder and the total drug present? |

| |

|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 is the answer for the above:

Volume of dry powder

Because the additions of 78 mL of water result in the preparation of 100 mL

of product, the volume occupied by dry powder is:

100 mL-78 mL= 22 mL

22 mL is the volume occupied by dry powder.

Total drug (ampicillin) present

If in the constituted product, each 5 mL contains 125 mg of ampicillin, the

total amount of ampicillin in the 100 mL product is:

If 5 mL contains 125 mg of ampicillin

100 mL will contain how many mg of ampicillin?

(100 mL × 125 mg)/ 5 mL

=2500 mg of ampicillin is in the drug product.

STEP 3: To Calculate the Amount of Water to be Used for Reconstitution of

Oral and Parenteral Preparations (40 minutes)

|Activity: Small Group Discussion ( 20 minutes) |

| |

|DIVIDE students into small manageable groups |

| |

|ASK students to discuss on the following question |

|The label instructions for an ampicillin product call for addition of |

|78 mL of water to make 100 mL of constituted suspension such that each |

|5mL contains 125 mg of Ampicillin. If the physician desires an |

|ampicillin concentration of 100 mg/5 mL (rather than 125 mg/5 mL), how |

|many milliliter of water should be added to the dry powder? |

| |

|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 answers of the above exercise:

Because it was determined that 2500 mg (from step number 2 above) of

ampicillin are in the dry product, the volume of product that can be made

with a concentration of 100 mg/5 mL may be calculated by,

If in every 5 mL of suspension contains 100 mg of ampicillin,

Then X mL of suspension contains 2500 mg of ampicillin.

X mL= (5 mL ×2500 mg)/ 100 mg

X mL= 125 mL of the drug product (ampicillin suspension)

Then, because it had been determined that the dry powder occupies 22 mL of

volume, it is possible to determine the amount of water to add:

125 mL-22 mL=103 mL Answer

STEP 4: Key Points (10 minutes)

• A number of drugs are unstable in an aqueous environment, even when

exposed for a short duration, thus requiring packaging, storage, and

shipping in a powder or lyophilized state to keep the product stable

during its shelf life

• Drugs used for parenteral administration, are commonly known as powder

for injection (PI), powder for reconstitution, dry powder injection, or

powder for constitution

• Typically, PI drugs are supplied in glass vials with rubber plugs and are

mixed or reconstituted with a diluent (usually 5% dextrose solution,

normal saline, bacteriostatic water, or sterile water for injection)

before administration

STEP 5: Evaluation (25 minutes)

• What are the common diluents used for diluting powders for injection?

• The label amoxicillin oral suspension calls for the additional of water

into a bottle (100 mL) with a 30 mL of a dry powder such that when mixed

with water will produce a 250 mg / 5mL amoxicillin oral suspension.

How many milliliters of water were added?

How many milligram of amoxicillin are in the suspension?

The following is the answer for the above question:

Volume of the dry amoxicillin powder is 30 mL

Then volume of water needed will be determined as follows

Total volume (100 mL) – volume of dry powder (30mL) = 70 mL Answer

Amount ofamoxicillin in the suspension:

Each 5 mL contains 250 mg of amoxicillin

100 mL will contain X mg of amoxicillin.

X mg = (100 mL×250 mg )/5 mL

X mg = 5000 mg of amoxicillin powder

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, Inc

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