ISOTONICITY TUTORIAL
CALCULATING E VALUE
Number of
dissociated
ions
Dissociation
factor (i)
1
1
2
1.8
3
2.6
4
3.4
5
4.2
(58.5) 𝑖
E value = (𝑀𝑊 𝑜𝑓 𝑑𝑟𝑢𝑔) (1.8)
E value tells you how many grams of NaCl equivalents there are in 1 g of drug
Ex: Boric acid has an E value of 0.52
This means there are 0.52 g of NaCl equivalents per 1 g of boric acid. This also means there are 0.52
mg of NaCl equivalents per 1 mg of boric acid.
0.52 𝑔 𝑁𝑎𝐶𝑙
1 𝑔 𝑏𝑜𝑟𝑖𝑐 𝑎𝑐𝑖𝑑
E value is a conversion factor used to convert g NaCl ↔ g drug
■
■
You need an E value for every ingredient in your order that is not water or NaCl.
You also need an E value if your isotonicity agent (i.e. what you are adding to make
the solution isotonic) is not sodium chloride
1. Calculate the E value for ephedrine sulfate (MW = 429), which dissociates into 3 ions.
(Round the final answer to the nearest TENTH)
STEPS TO COMPLETING ISOTONICITY PROBLEMS
1. Calculate the necessary E values. If you already have them, skip this step. (E)
2. Calculate the total amount of NaCl needed to make the product isotonic. (Goal: Vol, NS)
a. Pretend your volume of product = volume of NS.
b. How many g of NaCl are in that volume of NS?
c. Find out by multiplying your desired volume by 0.9 g NaCl/100 mL. This is how many
total grams of NaCl are needed to make your product isotonic.
3. Calculate the total amount of NaCl equivalent contributed by each ingredient. (Have:
gdrug x E)
a. Multiply the total amount of each drug/ingredient (in grams) by the E value of the
drug. This is how many g NaCl are being contributed by the ingredient
b. If there are multiple ingredients, repeat for each ingredient and then add up the
total amount of NaCl equivalents.
4. Find the amount of NaCl that needs to be added to make the product isotonic. (g NaCl
Needed = G - H)
a. Subtract Goal - Have to find how many grams of NaCl should be added to make the
product isotonic. (Need = Goal - Have)
5. Any other conversions you may need to do. (Ask)
a. Ex:
i.
If they are asking for mg of NaCl, convert g NaCl from step 4 to mg
ii.
If you are asked for mL of 3% NaCl, convert g NaCl from step 4 to mL 3% NaCl
iii.
If you are asked for g of an isotonicity agent that is NOT NaCl (common ones
include boric acid and zinc chloride), use the E value of the isotonicity agent to
convert g NaCl from step 4 → g of the agent (i.e. divide by the E value) For
example, if you were to use zinc chloride (E value = 0.62) to adjust tonicity, you
would divide by 0.62.
SAMPLE PROBLEM:
Part 1
A pharmacist receives the following order:
Ephedrine sulfate 0.3 g
Sodium chloride
qs
Purified water ad 30 mL
Make isotonic solution
Sig. Use as directed
If the E value of ephedrine sulfate is 0.2, how many milligrams of sodium chloride should be used
in compounding the prescription? (Round the final answer to the nearest WHOLE number)
1. Calculate the necessary E values (E value)
2. Calculate the total amount of NaCl needed to make the product isotonic. (Goal: Vol, NS)
3. Calculate the total amount of NaCl equivalent contributed by each ingredient.
(Have: ging x E)
4. Find the amount of NaCl that needs to be added to make the product isotonic. (Need
to add = Goal - Have in g NaCl)
Part 2
Suppose the pharmacist will use 3% sodium chloride to adjust the tonicity in the above scenario.
How many mL of 3% sodium chloride should be added to the product? (Round the final answer to
the nearest WHOLE number)
Part 3
Sodium chloride 3% is on backorder. How many milliliters of a 5% solution of boric acid (E value =
0.52) should be used in compounding the prescription? (Round the final answer to the nearest
WHOLE number)
Part 4
If the pharmacist used 5% boric acid to adjust the tonicity of the solution, how many mL of
purified water should be used to prepare the order?
PRACTICE
1. The pharmacist receives the following order:
R
Cocaine HCl
Sterile water for injection qs
4%
25 mL
The sodium chloride equivalent for cocaine hydrochloride is 0.16. How many milligrams
of sodium chloride would be used to make this solution isotonic? (Round to the
nearest WHOLE number)
9 0.22s
Is
xigxo.io onegnace
0.065race 1655rad
How many milliliters of 3% NaCl should be used to prepare this order? (Round to the
nearest TENTH)
0 59 300
2.167mi
12.2Mt
2. The pharmacy receives the following order:
R
Tetracaine HCl
Zinc sulfate
Boric acid
Sterile water ad
Make isotonic solution
Sig. Instill 1 drop in each eye
0.2 g
0.1 g
qs
60 mL
How many milliliters of 10% boric acid solution should be used to prepare this order?
The E values of tetracaine, zinc sulfate and boric acid are 0.18, 0.16, and 0.52
respectively. (Round to the nearest TENTH)
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98 8
gg
I.am
3. Lidocaine hydrochloride (MW = 289) is a 2-ion electrolyte. Calculate its sodium chloride
equivalent. (Round to the nearest TENTH)
ce
E crosscuts
C5B.s
Csis.SC
casaX
10.202
1
4. A pharmacist receives an order for 100 mL of procaine hydrochloride 1% solution for
injection. Procaine HCl dissociates into 2 ions and has a molecular weight of 273. How
many milligrams of NaCl are needed to make this solution isotonic? (Round to the
nearest WHOLE number)
to
EE
E o.ua
sxig
o.amasgnac oagnace
o.aiuaisgnace o.ess7gnace
Tale
How many milliliters of 3% sodium chloride need to be added to make this solution
isotonic? (Round to the nearest WHOLE number)
0.615579
58
HDMI
How many mL of 5% boric acid (E value = 0.52) needs to be added to make this solution
isotonic? (Round to the nearest TENTH)
0.61557g
Nall
0.52
BA
1.318g
go
IET
5. How many milligrams of sodium chloride may be used in the preparation of 15 mL of an
eye drop containing 1% tropicamide (E value - 0.09) and 0.5% chlorobutanol (E value 0.24) to render the solution isotonic with tears? Round to the nearest TENTH.
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ISOTONICITY key
CALCULATING E VALUE
Number of
dissociated ions
Dissociation
factor (i)
1
1
2
1.8
3
2.6
4
3.4
5
4.2
(58.5) 𝑖
E value = (𝑀𝑊 𝑜𝑓 𝑑𝑟𝑢𝑔) (1.8)
E value tells you how many grams of NaCl equivalents there are in 1 g of drug
Ex: Boric acid has an E value of 0.52
This means there are 0.52 g of NaCl equivalents per 1 g of boric acid. This also means there are 0.52
mg of NaCl equivalents per 1 mg of boric acid.
0.52 𝑔 𝑁𝑎𝐶𝑙
1 𝑔 𝑏𝑜𝑟𝑖𝑐 𝑎𝑐𝑖𝑑
E value is a conversion factor used to convert g NaCl ↔ g drug
■
■
You need an E value for every ingredient in your order that is not water or NaCl.
You also need an E value if your isotonicity agent (i.e. what you are adding to make
the solution isotonic) is not sodium chloride
1. Calculate the E value for ephedrine sulfate (MW = 429), which dissociates into 3 ions.
(Round the final answer to the nearest TENTH)
0.2
STEPS TO COMPLETING ISOTONICITY PROBLEMS
1. Calculate the necessary E values. If you already have them, skip this step. (E)
2. Calculate the total amount of NaCl needed to make the product isotonic. (Goal: Vol, NS)
d. Pretend your volume of product = volume of NS.
e. How many g of NaCl are in that volume of NS?
f. Find out by multiplying your desired volume by 0.9 g NaCl/100 mL. This is how many
total grams of NaCl are needed to make your product isotonic.
3. Calculate the total amount of NaCl equivalent contributed by each ingredient. (Have:
gdrug x E)
c. Multiply the total amount of each drug/ingredient (in grams) by the E value of the
drug. This is how many g NaCl are being contributed by the ingredient
d. If there are multiple ingredients, repeat for each ingredient and then add up the
total amount of NaCl equivalents.
4. Find the amount of NaCl that needs to be added to make the product isotonic. (g NaCl
Needed = G - H)
a. Subtract Goal - Have to find how many grams of NaCl should be added to make the
product isotonic. (Need = Goal - Have)
5. Any other conversions you may need to do. (Ask)
a. Ex:
i.
If they are asking for mg of NaCl, convert g NaCl from step 4 to mg
ii.
If you are asked for mL of 3% NaCl, convert g NaCl from step 4 to mL 3% NaCl
iii.
If you are asked for g of an isotonicity agent that is NOT NaCl (common ones
include boric acid and zinc chloride), use the E value of the isotonicity agent to
convert g NaCl from step 4 → g of the agent (i.e. divide by the E value) For
example, if you were to use zinc chloride (E value = 0.62) to adjust tonicity, you
would divide by 0.62.
SAMPLE PROBLEM:
Part 1
A pharmacist receives the following order:
Ephedrine sulfate 0.3 g
Sodium chloride
qs
Purified water ad 30 mL
Make isotonic solution
Sig. Use as directed
If the E value of ephedrine sulfate is 0.2, how many milligrams of sodium chloride should be used
in compounding the prescription? (Round the final answer to the nearest WHOLE number)
1. Calculate the necessary E values
Given, E = 0.2
2. Calculate the total amount of NaCl needed to make the product isotonic.
0.27 g NaCl
3. Calculate the total amount of NaCl equivalent contributed by each ingredient.
0.06 g NaCl
4. Find the amount of NaCl that needs to be added to make the product isotonic.
0.21 g = 210 mg
Part 2
Suppose the pharmacist will use 3% sodium chloride to adjust the tonicity in the above scenario.
How many mL of 3% sodium chloride should be added to the product? (Round the final answer to
the nearest WHOLE number)
7 mL
Part 3
Sodium chloride 3% is on backorder. How many milliliters of a 5% solution of boric acid (E value =
0.52) should be used in compounding the prescription? (Round the final answer to the nearest
WHOLE number)
8 mL
Part 4
If the pharmacist used 5% boric acid to adjust the tonicity of the solution, how many mL of
purified water should be used to prepare the order?
22 mL
PRACTICE
1. The pharmacist receives the following order:
R
Cocaine HCl
Sterile water for injection qs
4%
25 mL
The sodium chloride equivalent for cocaine hydrochloride is 0.16. How many milligrams
of sodium chloride would be used to make this solution isotonic? (Round to the
nearest WHOLE number)
65 mg
Note:
We got 70 mg because we rounded the total grams of NaCl needed to 0.23 g. If you keep the 0.225 g, and round
at the end, you’ll get 65 mg. If you round to 0.23 g, you’ll get 70 mg
How many milliliters of 3% NaCl should be used to prepare this order? (Round to the
nearest TENTH)
2.2 mL
Note:
This is for 65 mg of NaCl.
2. The pharmacy receives the following order:
R
Tetracaine HCl
Zinc sulfate
Boric acid
Sterile water ad
Make isotonic solution
Sig. Instill 1 drop in each eye
0.2 g
0.1 g
qs
60 mL
How many milliliters of 10% boric acid solution should be used to prepare this order?
The E values of tetracaine, zinc sulfate and boric acid are 0.18, 0.16, and 0.52
respectively. (Round to the nearest TENTH)
9.4 mL
3. Lidocaine hydrochloride (MW = 289) is a 2-ion electrolyte. Calculate its sodium chloride
equivalent. (Round to the nearest TENTH)
0.2
4. A pharmacist receives an order for 100 mL of procaine hydrochloride 1% solution for
injection. Procaine HCl dissociates into 2 ions and has a molecular weight of 273. How
many milligrams of NaCl are needed to make this solution isotonic? (Round to the
nearest WHOLE number)
686 mg
Hint: E value = 0.214
How many milliliters of 3% sodium chloride need to be added to make this solution
isotonic? (Round to the nearest WHOLE number)
23 mL
How many mL of 5% boric acid (E value = 0.52) needs to be added to make this solution
isotonic? (Round to the nearest TENTH)
26.4 mL
5. How many milligrams of sodium chloride may be used in the preparation of 15 mL of an
eye drop containing 1% tropicamide (E value - 0.09) and 0.5% chlorobutanol (E value 0.24) to render the solution isotonic with tears? Round to the nearest TENTH.
103.5 mg