ELECTROLYTE SOLUTIONS

INTRODUCTION

Electrolytes in human body fluids are prominent in

maintaining acid-base balance in body.

Electrolytes help regulate metabolism in the body and

control volume of water in the body.

Compound

Dissociate

Donot

Dissociate

Non

ElectrolytesUrea, Dextrose

Electrolytes NaCl Na+ Cl-

Cation Anion

Molecules

BODY FLUIDS - 60% BODY WEIGHT

WATER IS LARGEST

SINGLE COMPONENT

Dec. to 45-50 % body

weight in elderly

Variations occur based

on age, gender & amt.

of body fat

80% neonate is water*

MAJOR COMPARTMENTS FOR FLUIDS

INTRACELLULAR

FLUID (ICF)

Inside cell

Most of body fluid

here - 40% weight

Decreased in elderly

EXTRACELLULAR

FLUID (ECF)

Outside cell

Intravascular fluid -

within blood vessels (5%)

Interstitial fluid -

between cells & blood

vessels (15%)

Transcellular fluid -

cerebrospinal, pericardial ,

synovial

ELECTROLYTES

Substance when dissolved in solution separates

into ions & is able to carry an electrical current

CATION - positively charged electrolyte

ANION - negatively charged electrolyte

# Cations must = # Anions for homeostatsis to

exist in each fluid compartment

Commonly measured in milliequivalents / liter

(mEq/L)

PHARMACEUTICAL APPLICATION

Various electrolyte ions in the human blood are

Electrolyte Preparations employed to treat fluid and

electrolyte imbalance in the body.

Available as oral solutions, syrups, dry granules to be

dissolved in water/juice, capsules, tablets and also

intravenous infusions.

Cations Na+, K+, Ca++, Mg++

Anions Cl-, HCO3-, HPO4

- -, SO4- -

MILLIEQUIVALENTS - SIGNIFICANCE

Most exclusively employed unit by pharmacists, physicians,

manufacturers and clinicians across USA to express

electrolyte concentration in solution is “Milliequivalents

(mEq)”

Internationally (European and many other nations), molar

concentration (mmol/L or µmol/L) are employed.

A mEq represents amount in milligrams taking into account

valence of the ions.

A mEq expresses the chemical activity / combining power

of a substance relative to the activity of 1 mg of hydrogen.

Equivalent

Weight =

Atomic / Formula /

Molecular weight--------------------------------

Valence

Converting milligrams to milliequivalents

The following formula can be employed:

Atomic/formula/molecular weight

Valence

Problem:

Given the molecular weight of calcium is 40, Represent 10

mg of calcium as mEq? Also, how many mg will be there in

1 mEq of calcium?

= Equivalent Weight

mEq =

mg ×Valence

Atomic/formula/molecular weight

Solution

10 mg of calcium = 0.5 mEq

1 mEq will contain --- 10 / 0.5 = 20 mg of calcium

mEq =

mg ×Valence

Atomic/formula/molecular weight

mEq =

10 ×2

40

•To convert milliequivalents to milligrams

Convert 8 mEq of potassium to mg?

Majority of electrolyte substitutes are available in liquid

form. The concentration of electrolytes in i.v. infusion

fluids is represented as mEq/L

mg =mEq × Atomic/formula/molecular weight

Valence

mg =8 × 39

1

= 312

To convert mEq/mL to mg/mL

Calculate the concentration in mg/mL, of a solution

containing 3 mEq of Kcl per mL?

mg/mL =mEq/mL × Atomic/formula/molecular weight

Valence

mg/mL =3 × 74.5

1

= mg/mL223.5

What is the concentration in g/mL of a solution containing 4

mEq of Calcium chloride / mL?

Calcium chloride = CaCl2.2H2O

Molecular weight = 147

Eq. Wt = 147 / 2 = 73.5

mg/mL = 4 × 73.5 = 294 mg/mL = 0.294 g/mL

Estimation of % w/v from mEq

What is the % w/v conc. of a 100 mEq/L solution of

NH4Cl?

M.W of NH4Cl = 53.5

Eq. Wt of NH4Cl = 53.5

1 mEq of NH4Cl = 53.5g/1000 = 0.0535 mg

100 mEq of NH4Cl = 5.35 g/L or 0.535 g/100 mL

= 0.535%

Problem

A solution has 10 mg / 100 mL of K+ ions. Express the

concentration in mEq/Lit?

mg/mL =mEq/mL × Atomic/formula/molecular weight

Valence

100 =mEq/mL × 39

1

mEq/mL = 2.56 mEq/L

A solution contains 10 mg/100 mL of Ca++ ions. Express

the concentration in mEq/Lit

10 mg/100 mL == 100 mg / Lit

A mg++ ion level in plasma is found to be 2.5 mEq/L.

Express the conc. in mg?

Model Problem – Solve – Imp*

How many mEq of Kcl is present in a 15 mL dose of a

10% w/v Kcl elixir?

Solution

M.W of Kcl = Eq.Wt of Kcl = 74.5

1 mEq of Kcl = 74.5/1000 = 0.0745 g = 74.5 mg

15 ml dose of 10% w/v elixir formulation = 1.5 g of Kcl

74.5 mg ---- 1 mEq

1500 mg ---- ?

Ans: 20.1 mEq

Model Problem – Solve – Imp*

Calculate the mEq of Na+ present in 30 ml of following

solution?

Disodium Hydrogen Phosphate 18g

(Na2HPO4.7H2O - 268)

Sodium Biphosphate 48 g

(NaH2PO4.H2O - 138)

Purified Water 100 mL

Solution

Eq.Wt - (Na2HPO4.7H2O - 268) – 268/2 – 134

18 g / x g = 100 mL / 30 mL

x = 5.4 g / 30 mL

1 mEq – 134 mg

mEq of Na2HPO4.7H2O – 40.3

Eq.Wt - (NaH2PO4.H2O - 138) – 138

mEq of NaH2PO4.H2O – 104.3

Total mEq of Na+ -- 144.6

Clinical Application – Model Problem

A person will receive 2 mEq of NaCl / kg of body weight.

He weighs 132 lb. Calculate the ml of 0.9% sterile NaCl

that should be administered?

Solution

M.Wt / Eq. Wt of NaCl = 58.5

1 mEq of NaCl = 0.0585 g

2 mEq - 0.117 g

Wt in Kg – 132 / 2.2 – 60 kg

Drug is administered at 2 mEq (0.117 g) / kg

Total amt of drug in body – 0.117 * 60 - 7.02 g of NaCl

0.9% sterile NaCl solution has – 9 g of NaCl / Lit

9 g --------- 1000 ml

7.02g ---- ? = 780 mL

Millimoles and Micromoles

A mole is the molecular weight of substance in grams.

A millimole – one thousandth of a mole

A micromole – One millionth of a mole

SI expresses, electrolyte conc. in mmol/L

How many millimoles of monobasic sodium phosphate

is present in 100 gms of substance?

Solution

Mol.wt - (NaH2PO4.H2O - 138)

1 mole – 138 g

100 g represent ---- 100/138 = 0.725 moles

= 725 mmoles

Model Problems

Calculate the weight in mg of 1 mmol of HPO4-

1 mole weighs --- Atomic weight -- 95.98 g

1 mmol of HPO4- weighs - 0.09598 g = 95.98 mg

Convert plasma levels of 0.5 µg/mL of tobramycin (mw –

467.52) to µmol/L?

1 mol - 467.52 gms

1 µmol - 467.52 µg

0.5 µg/1 ml * 1 µmol/467.52 µg * 1000 ml/1 L

= 1.07 µmol/L

OSMOSIS

Movement of the solvent or water across a

membrane

Involves solution or water

Equalizes the concentration of ions on each side

of membrane

Movement of solvent molecules across a

membrane to an area where there is a higher

concentration of solute that cannot pass through

the membrane

Osmolarity

Osmotic pressure is an important biologic parameter

which involves diffusion of solutes or the transfer of fluids

through semi permeable membranes.

Per US Pharmacopeia, knowledge of osmolar

concentrations of parenteral fluids is important.

Labels of pharmacopeia solutions providing intravenous

replenishment of fluids, nutrients, electrolytes and

osmotic diuretic mannitol are required to state osmolar

concentration.

Information provides the doctor to decide, if the solution

is, hypoosmotic, iso-osmotic or hyperosmotic with regard

to biological fluids and membranes

Osmotic pressure is α (total number of particles in

solution)

Unit of measurement is milliosmoles

For non-electrolytes like dextrose, 1 mmol represents 1

mosmol

However, with electrolytes, as total number of particles in

solution depends on degree of dissociation of a

substance.

For e.g. Assuming complete dissociation, 1 mmol NaCl

represents 2 mOsmol (Na+ + Cl-) of total particles

1 mmol of CaCl2 represents 3 mOsmol (Ca++ + 2Cl-)

1 mmol of sodium citrate (Na3C6H5O7) represents 4

mOsmol (3 Na+ + C6H5O7-) of total particles

The milliosmolar value of the complete solution is equal

to the sum of milliosmolar values of individual ions.

U.S. Pharmacopeia lists the following formula for

calculation of ideal osmolar concentration:

Wt. of substance (g/L)

mOsmol/L = ------------------------------ × No. of Species × 1000

Mol. Wt (g)

E.g. Calculate of ideal osmolarity for 0.9% Sodium

Chloride Solution?

Solution

Wt. of substance (g/L)

mOsmol/L = ------------------------------ × No. of Species × 1000

Mol. Wt (g)

9 (g/L)

mOsmol/L = ------------------------------ × 2 × 1000

58.5 (g)

= 308 mOsmol/L

However, because of the bonding forces, n is slightly less

than 2 for NaCl at 0.9% Nacl concentration. Hence, the

actual osmolarity of solution is 286 mOsmol/L

Some Pharmaceutical manufacturers label electrolyte

solutions with ideal or stoichiometric osmolarities

calculated by the equation just provided, whereas others

list experimental or actual osmolarities.

“Pharmacist should appropriate this distiction”.

Osmolarity -- “Milliosmoles of solute per liter of

solution”

Osmolality – “Milliosmoles of solute per kilogram of

solvent”

For dilute aqueous solutions – both terms are nearly

identical

For more concentrated solutions – “Two values are not

identical”

“pharmacist should make distinction between

“Osmolarity” and Osmolality”

Contribution of components of normal human serum to

the “Serum Osmotic Pressure”

Constituent Mean Conc.

(mEq/L)

Osmotic Pressure

(mOsmol/kg of

water)

% of total osmotic

pressure

Sodium

Potassium

Calcium

Magnesium

Chloride

Bicarbonate

Proteinate

Phosphate

Sulfate

Organic anions

Urea

Glucose

Total

142

5

2.5

2

102

27

16

2

1

3.5

30mg/100 mL

70 mg/100 mL

139

4.9

1.2

1

99.8

26.4

1.0

1.1

0.5

3.4

5.3

4.1

287.7 mOsmol/Kg

48.3%

1.7

0.4

0.3

34.7

9.2

0.3

0.4

0.2

1.2

1.8

1.4

99.9%

Normal serum osmolality - ranges from “275-300

mOsmol/kg”.

Equipment used in laboratories to measure osmolality –

“Osmometers”.

Electrolyte imbalance

Shock

Trauma

Burns

Hyperglycemia

Water intoxication

Renal failure

Abnormal Blood Osmolality

Problems

A solution has 5% anhydrous dextrose in water for injection.

Represent the concentration in mosmol/lit?

Solution: Molecular weight of dextrose – 180

As it donot dissociate 1 mmol --- 1 mOsmol

5% solution contains ---- 50 g / L

From the formula discussed above – 50,000 / 180

= 278 mOsmol/L

A solution contains 10 mg% of Ca+2 ions. How many

milliosmoles are present in 1 lit of solution?

Solution

1 mmol of Ca++ (40 mg) --- 1mOsmol

10 mg% of Ca++ = 100 mg / Lit

100 / 40 - 2.5 mOsmol/Lit

WATER and Electrolyte Balance – Clinical Consideration

Good Homeostasis -- Maintaining body water and

electrolyte balance is component of good health

Dietary Input

OutPut – Kidneys, Lungs,

GI tract, Skin

Endocrine Process

MaintainOptimum Osmolality

of Body Fluids

Fluid / Electrolyte therapy is provided to rectify any

imbalance of osmolality of body fluids

Treatment can be “Customized” based on patient needs

Patient on Diuretic Therapy – “A K+ supplement along

with adequate water intake”.

Total body water in adults – 55-60% of the body weight

Increase in body fat --- Lesser proportion of water

Infants have 75% of body water

For adults - A minimum of 2.5 L of daily water intake

(Ingested liquids, food and oxidative metabolism) are

required for maintaining “Homeostasis”.

General terms…..

1500 mL of water / Sq.meter of body surface might be

employed for calculating daily requirements of adults.

On weight basis….

32 mL/kg – Adults and 100-150 mL/kg – Infants ---

Daily requirements for healthy intake

Careful estimation should be made in human subjects

with any disorders affecting homeostasis.

Also, composition is described in literature with respect to

body compartments --- “Intracellular (within cells)”,

Intravascular (In blood plasma), Interstitial (Between cells

in tissue).”

Intervascular and Interstitial --- Together called --

Extracellular

Sodium and Chloride have principle affect on Extracellular

Fluid

Potassium and Phosphate --- on --- Intracellular fluids

Cell membranes are freely permeable to water

Hence, Osmalility of extracellular fluid (290 mOsmol/kg) is

approximately equal to intracellular fluid.

Therefore, osmolality of the plasma is accurate guide to

estimate osmolality of intracellular fluid.

The following formula may be employed to estimate

plasma osmolality

Plasma Osmolality (mOsmol/Kg)

= 2([Na] + [K]) plasma + BUN/2.8 + Glucose / 18

Where Na and K are in mEq/L, BUN and Glucose are in

mg/dL

Various Clinical calculations – Determination of body water

requirement, Plasma osmolality estimation, Osmolality and

Milliequivalent contents of physiologic electrolyte

solutions.

Probelms

Based on the discussion above, Calculate the daily water

requirement of a healthy human adult with a body surface

area of 1.8 m2?

Solution:

Water Requirement -- 1500 mL/m2

Hence 1.8 m2 contains --- 2700 mL

Estimate the plasma osmolality of the given data:

Na2+ -- 135 mEq/L, K+ -- 4.5 mEq/L, BUN – 14 mg/dL,

Glucose – 90 mg/dL

Solution:

Plasma Osmolality (mOsmol/Kg)

= 2[(Na) + (K)] Plasma + BUN/2.8 + Glucose / 18

Where Na and K are in mEq/L, BUN and Glucose are

in mg/dL

= 2(135+4.5) + 14/2.8 + 90/18

= 289

Calculate the mEq of Na+, K+ and Cl-, millimoles of

dextrose and osmolality of following parentaral fluid?

Dextrose anhydrous 50g

NaCl 4.5g

KCl 1.49g

Water for Injection, ad 1000mL

Solution

NaCl: 1 EqWt of NaCl - 58.5 g

1 mEq of NaCl - 58.5 mg

4500 mg of NaCl – 76.9 mEq of Na+ and 76.9

mEq of Cl-

Similarly for KCl we can calculate, 20 mEq of K+ and Cl-

For dextrose - Mol. Wt – 180

1 mmol of anhydrous dextrose – 180 mg

50 gms of anhydrous dextrose – 50,000 mg

50,000 mg represent -- 277.7 mmol

Hence Osmolarity is calculated as

Dextrose anhydrous: 278 mmol * 1 particle/mmol = 278

mOsmol

NaCl: 77 mEq * 2 particles / mEq (or mmol) = 154

mOsmol

KCl : 20 mEq * 2 particles / mEq (or mmol) = 40

mOsmol

Total = 278 + 154 + 40 = 472 mOsmol

Reference

Lecture material taken from the book “Pharmaceutical

Calculations”, 13th Edition by Howard C. Ansel.

Lecture material from Dr. Karla’s lecture notes.