Calculations Involving Constituted Solutions – PST05208 Pharmaceutics Theory and Compounding

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NTA Level 5 • Semester 2 • PST05208

Calculations Involving Constituted Solutions

Pharmaceutics Theory and Compounding • Source Session/Topic 19
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Session 19: Calculations Involving Constituted Solutions

Total Session Time: 120minutes + 6 hours of Practices

Prerequisites

• None

Learning Tasks

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

• Give overview of calculations involving constituted solutions
• Perform calculations involving constituted solutions

Resources Needed:

• Flip charts, marker pens, and masking tape
• Black/white board, chalk and whiteboard markers
• LCD projector and computer

SESSION OVERVIEW

Activity/

Step Time Content

Method

1 05 minutes Presentation Introduction, Learning Tasks

45minutes Introduction to Calculations Involving Constituted

2 Presentation Solutions

60 minutes Presentation Performing Calculations Involving Constituted

3 Solutions

Demonstration

4 05 minutes Presentation Key Points

5 05 minutes Presentation Evaluation

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SESSION CONTENTS

STEP1: 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: Introduction to Calculations Involving Constituted Solutions (45

Minutes)

• Process of mixing and diluting solutions
• Parts of Solutions

Terms

• Solute

o Substance to be dissolved or diluted

o Can be either solid or liquid

• Solvent

o Substance (liquid) that dissolves another substance to prepare solution

o Often referred to as diluent

• Solution

o Resulting mixture of solute plus solvent

Caution

o Before reconstituting injectable drugs, read and follow label or package insert directions

carefully

o Check drug

o Check diluent dates

• When reconstituting injectable medications, must determine both type and amount of diluent to be

used

o Sterile water and 0.9 percent NaCl commonly used

o Some drugs supplied with special diluent

o Determine volume in mL of diluent to be used

132

o Check that route noted on drug label matches route ordered

o Reconstitute drug and note resulting supply dosage on vial

o Note if single-dose or multiple-dose vial

Example Drug Label

133

STEP 3; Performing Calculations Involving Constituted Solutions (60 Minutes)

Activity: Small Group Discussion ( 30 minutes)

DIVIDE students in small manageable groups

ASK students to discuss in groups on the following questions

• Reconstituting Parenteral Solutions: Single Strength
o Order: Zithromax 400 mg IV daily for 2 days
o Available: Zithromax 500 mg vial for IV infusion
o Drug is in powder form with directions on label that state, “Constitute to 100 mg/mL
with 4.8 mL of Sterile Water for Injection

REFER Students to Pharmaceutical Calculation. 13th Edition by HOWARD C.

ANSEL: Chapter 11, for reference

ALLOW students to discuss for 20 minutes

ALLOW each groups to present for 5 minutes

CLARIFY and SUMMARIZE by using the contents below

Procedure:

1. How much and what type of diluent is needed?

• 4.8 mL of sterile water

2. What is the dosage concentration after reconstitution?

• 100 mg per mL

3. What is the total volume after reconstitution?

• 5 mL

4. Given the ordered dose, how many doses are available in the vial?

• One

To reconstitute Zithromax:

• Choose 5 mL syringe
• Withdraw 4.8 mL of sterile water
• Add 4.8 mL of sterile water to Zithromax powder and shake well

Determine amount needed for dose ordered

1. Convert

• No conversion needed

2. Think

• Need 400 mg

134

Have 100 mg per mL
• Need four times that amount

3. Calculate

Dosage on hand = Amount on hand

Dosage desired X Amount desired

100 mg = 400 mg
1 mL X
100 X = 400
100 X= 400 X = 4 mL

100 100

Withdraw 4 mL of reconstituted Zithromax using 5 mL syringe
• Further dilute and give IV
• Since single-dose vial, discard any remaining drug

Reconstituting Parenteral Solutions: Multiple Strength

• Some parenteral powdered medications have directions for preparing several different solution

strengths

• Penicillin G potassium 1,000,000 units vial

Reconstitution instructions note four different solution concentrations as determined by amount of

diluent added

Units per mL

mL Diluent

20 mL 50,000
10 mL 100,000
4 mL 250,000
1.8 mL 500,000
• Order: Penicillin G potassium 300,000 units IM every 6 h for adult patient

o Available: Penicillin G potassium 1,000,000 unit vial

o Given the reconstitution concentrations on the previous slide, which should you use when

preparing to administer the ordered dose?

• Consider:

o Dose ordered

o Patient receiving dose

o Consider:

o Volume and concentration that results with each noted diluent volume

o Smaller the amount of diluent added, stronger the resulting solution concentration

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o Consider maximum recommended volumes for injection by patient and parenteral route

• Considering previously mentioned factors, adding 4 mL of diluent results in reasonable volume

and medication concentration

o Results in concentration of 250,000 units per mL
o How many mL are needed to deliver ordered dose of 300,000 units?

1. Convert

o No conversion necessary

2. Think

o 250,000 units in 1 mL need 300,000 units
o Slightly more than what is available in 1 mL
250,000 units = 300,000unit
1mL x mL
250, 000x = 300,000
250,000 = 300,000 x = 1.2 mL

250,000 250,000

o Rule

o When reconstituting multiple-dose injectable medications:

o Label reconstituted drug noting resulting supply dosage

o In previous example, 250,000 units per mL

o Verify length of drug potency and storage directions

o Label on penicillin G potassium notes that solution “may be kept in refrigerator for one (1)

week”

o Complete label of reconstituted multiple-dose vial, noting:

 Date and time of preparation

 Supply dosage

 Length of potency

o Expiration date

o Complete label of reconstituted multiple-dose vial, noting:

 Storage directions

 Own initials

o Different IM and IV Reconstitution Instructions

 Reconstitution instructions can differ in amount and/or type of diluent based on

administration route

 Must carefully check route ordered and related reconstitution directions

o Complete label of reconstituted multiple-dose vial, noting:

 Storage directions

 Own initials

o Different IM and IV Reconstitution Instructions

136

o Reconstitution instructions can differ in amount and/or type of diluent based on

administration route

o Must carefully check route ordered and related reconstitution directions

• Reconstitution of Non-Injectable Solutions
• Examples:

o Nutritional formulas

o Irrigating solutions

o Usually need to dilute liquid concentrate to weaker solution

• Solution Concentration

o Amount of solvent used determines final solution concentration or strength

 Fraction expresses strength of solution made from liquid concentrate

o Numerator

o Number of parts of solute

• Fraction expresses strength of solution made from liquid concentrate

o Denominator

• Number of parts of solution

o Difference between denominator (final solution) and numerator (parts of solute) is number of

parts of solvent

• Solution Concentration Example:

1 Strength nutritional formula

3

1 part concentrate

3 parts total solution

3 – 1 = 2 parts of solvent
• Water
• Calculating Solutions
• To prepare solutions:

1. Apply ratio-proportion to find amount of solute (X)

Solution strength = X amount of solute

Quantity of desired solution

2. Quantity of desired solution – Amount of solute = Amount of solvent

137

• Solution Calculation Example

Physician orders patient‟s wound irrigated with 2

3

strength hydrogen peroxide and normal saline solution every four hours while patient is awake

• 60 mL per irrigation for three irrigations during 12-hour shift
o Prepare 60mL x 3 irrigations = 180mL total solution
• How much stock hydrogen peroxide and normal saline is needed?

1. Convert

o No conversion necessary

2. Think

o Need2 strength

3

2 parts solute (concentrated hydrogen peroxide) to 3 total parts solution

o Amount of solvent is 3 – 2 = 1 part saline
o For 180 mL of solution, need2 as solute (120 mL) and1 as solvent (60 mL)

3. Calculate

2 = x mL

3 180

3x = 360
3 x = 360 x = 120mL

3

o 120 mL of full strength hydrogen peroxide and 60 mL of normal saline are needed to make

desired solution

STEP 4: Key Point (Minutes 5)

• Solute is a substance to be dissolved or diluted and can be either solid or liquid, solvent is a

substance (liquid) that dissolves another substance to prepare solution and often referred to as

diluent and solution is a resulting mixture of solute plus solvent

• Caution; Before reconstituting injectable drugs, read and follow label or package insert directions

carefully, check drug and check diluent dates

STEP 5: Evaluations (Minutes 5)

• What are the solvents?
• What are the Cautions before reconstituting injectable drugs?

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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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