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containing 300 mg of ferrous gluconate per teaspoonful should be
administered for each dose? (b) How much ferrous gluconate would be
needed to prepare 6 fl. oz. of the compounded syrup?
Millimoles and Micromoles
Molar concentrations (as millimoles per liter [mmol/L] and micromoles
per liter [μmol/L or mcmol/L]) are used in the International System (SI),
which is employed in European countries and in many others throughout
the world. Milliequivalents are used almost exclusively in the United
States to express concentrations of electrolyte ions in a solution; however,
millimoles and micromoles are sometimes used in expressions of clinical
laboratory values. In some electrolyte solutions, determining the valence
of the ions can be quite complicated, such as in the case of the phosphate
ion, which can exist in a monovalent (H2PO
4
−
), divalent (HPO
4
2−
), or
trivalent (PO
4
3−
) form. Millimoles are often used to express
concentrations in these types of solutions as well.
A mole is the molecular weight of a substance in grams. A millimole
is one-thousandth of a mole and is, therefore, the molecular weight of a
substance in milligrams. Similarly, a micromole is one-millionth of a
mole, which is the molecular weight of a substance in micrograms. For
example, the molecular weight of sodium chloride is 58.5 g/mol but can
be converted to milligrams and millimoles as follows:
Similarly, the molecular weight can also be converted to micrograms and
micromoles. Notice that millimolar conversions do not take into account
the valence of an electrolyte as do milliequivalent conversions. Therefore,
for monovalent species, the numeric values of the milliequivalent and
millimole are identical. Similar to milliequivalents, the millimoles of the
compound are equal to the millimoles of the cation, which are equal to
the millimoles of the anion, but this does not hold true for the actual
weights of the ions.
Example calculations of millimoles and
micromoles
The following conversion can be used to convert milligrams to
millimoles and vice versa:
The following conversion can be used to convert micrograms to
micromoles and vice versa:
1. How many millimoles of monobasic sodium phosphate monohydrate
(m.w. 138) are present in 100 g of the substance?
2. What is the weight, in milligrams, of 5 mmol of potassium phosphate
dibasic?
3. Convert the trough plasma range of 0.5 μg/mL to 2 μg/mL for
tobramycin (m.w. = 467.52) to mmol/L.
1
4. If lactated Ringer’s injection contains 20 mg of calcium chloride
dihydrate (CaCl2 · 2H2O) in each 100 mL, calculate the millimoles
of calcium present in 1 L of lactated Ringer’s injection.
5. How many micromoles of calcium are present in each milliliter of
lactated Ringer’s injection?
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6. A patient is receiving a slow intravenous infusion containing 40 mEq
of potassium chloride in 1000 mL of fluid. If, after 12 hours, 720 mL
of infusion had been infused, how many millimoles of potassium
chloride were administered?
NOTE: Since potassium chloride is monovalent, the amount in
milliequivalents and the amount in millimoles are the same.
7. A medication order calls for 1.8 g of potassium chloride in 60 mL of
solution. How many millimoles of KCl are contained in each
milliliter?
See example problem 6 for molecular weight of KCl.
8. Calculate the concentrations in mmol/L for each of the following
infusion solutions:
(a) 5% NaCl, (b) 3% NaCl, (c) 0.9% NaCl (NSS), (d) 0.45% NaCl
(half-NSS), and (e) 0.2% NaCl.
a.
b.
c.
d.
e.
Osmolarity
As indicated in Chapter 11, osmotic pressure is important to biologic
processes that involve the diffusion of solutes or the transfer of fluids
through semipermeable membranes. The labels of solutions that provide
intravenous replenishment of fluid, nutrients, or electrolytes, and the
osmotic diuretic mannitol are required to state the osmolar concentration.
This information indicates to the practitioner whether the solution is
hypoosmotic, isoosmotic, or hyperosmotic with regard to biologic fluids
and membranes.
Osmotic pressure is proportional to the total number of particles in
solution. The unit used to measure osmotic concentration is the
milliosmole (mOsmol). For dextrose, a nonelectrolyte, 1 mmol (1 formula
weight in milligrams) represents 1 mOsmol. This relationship is not the
same with electrolytes, however, because the total number of particles in
solution depends on the degree of dissociation of the substance in
question. Assuming complete dissociation, 1 mmol of NaCl represents 2
mOsmol (Na+ + Cl−) of total particles, 1 mmol of CaCl2 represents 3
mOsmol (Ca2+ + 2Cl−) of total particles, and 1 mmol of sodium citrate
(Na3C6H5O7) represents 4 mOsmol (3Na+ + C6H5O
7
−
) of total particles.
The milliosmolar value of separate ions of an electrolyte may be
obtained by dividing the concentration, in milligrams per liter, of the ion
by its atomic weight. The milliosmolar value of the whole electrolyte in
solution is equal to the sum of the milliosmolar values of the separate
ions. According to the United States Pharmacopeia (USP), the ideal
osmolar concentration may be calculated according to the equation2:
Furthermore, the osmolar concentration is the total of the osmotic
concentration of all solutes in a solution, so each solute must be included
in the calculation of osmolarity of a particular solution, as example
problem 6 demonstrates.
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In practice, as the concentration of the solute increases,
physicochemical interaction among solute particles increases and actual
osmolar values decrease when compared to ideal values. Deviation from
ideal conditions is usually slight in solution within the physiologic range
and for more dilute solutions, but for highly concentrated solutions, the
actual osmolarities may be appreciably lower than ideal values. For
example, the ideal osmolarity of 0.9% sodium chloride injection is:
Because of bonding forces, however, the number of species is slightly < 2
for solutions of sodium chloride at this concentration, and the actual
measured osmolarity of the solution is about 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. The
pharmacist should be aware of this distinction.
A distinction also should be made between the terms osmolarity and
osmolality. Whereas osmolarity is the milliosmoles of solute per liter of
solution, osmolality is the milliosmoles of solute per kilogram of solvent.
For dilute aqueous solutions, osmolarity and osmolality are nearly
identical. For more concentrated solutions, however, the two values may
be quite dissimilar. The pharmacist should pay particular attention to a
product’s label statement regarding osmolarity versus osmolality.
Normal serum osmolality is considered to be within the range of 275
to 300 mOsmol/kg. The contribution of various constituents to the
osmolality of normal serum is shown in Table 12.4. Osmometers are
commercially available for use in the laboratory to measure osmolality.
3
Abnormal blood osmolality (blood osmolality that deviates from the
normal range) can occur in association with shock, trauma, burns, water
intoxication (overload), electrolyte imbalance, hyperglycemia, or renal
failure.
3
TABLE 12.4 THE CONTRIBUTION OF VARIOUS
CONSTITUENTS OF NORMAL HUMAN SERUM TO
THE TOTAL SERUM OSMOTIC PRESSURE
a
a
From Chughtai MA, Hendry EB. Serum electrolytes, urea, and
osmolality in cases of chloride depletion. Clinical Biochemistry
1967;1:91. Adapted from Fluid and Electrolytes. Chicago, IL: Abbott
Laboratories; 1970.
b
Water content of normal serum taken as 94 g/100 mL.
Example calculations of milliosmoles
The equation adapted from the USP used in the previous example can be
used to determine osmolarity, or the following equation can be used to
convert milligrams to milliosmoles and vice versa:
1. A solution contains 10% of anhydrous dextrose in water for injection.
How many milliosmoles per liter are represented by this
concentration?
Molecular weight of anhydrous dextrose = 180
Dextrose does not dissociate, therefore the “number of species” = 1
Or, utilizing the equation:
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2. A solution contains 156 mg of K+ ions per 100 mL. How many
milliosmoles are represented in a liter of the solution?
3. Calculate the osmolarity of a 3% hypertonic sodium chloride
solution. Assume complete dissociation.
4. Calcium chloride dihydrate injection is a 10% solution of CaCl2 ·
2H2O. How many milliosmoles are present in a 10-mL vial? Assume
complete dissociation.
5. If a pharmacist wished to prepare 100 mL of a solution containing 50
mOsmol of calcium chloride, how many grams of calcium chloride
would be needed? Assume complete dissociation.
6. What is the osmolarity of a solution containing 5% dextrose and
0.45% sodium chloride (D5½NS)? Assume complete dissociation.
Because this solution contains two ingredients, the osmolarity of
each must be calculated then added to determine the total osmolarity
of the solution. Molecular weight, number of species, and
conversion determinations for dextrose and sodium chloride are
shown in example problems 1 and 3.
Dextrose:
Sodium chloride:
7. NORMOSOL-M in 5% DEXTROSE INJECTION contains 21 mg of
magnesium acetate, 128 mg of potassium acetate, 234 mg of sodium
chloride, and 5 g of dextrose monohydrate in each 100 mL of
solution.4 What is the osmolarity of this solution? Assume complete
dissociation.
Magnesium acetate (Mg(C2H3O2)2):
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Potassium acetate (KC2H3O2):
Sodium chloride (NaCl):
Dextrose monohydrate (Dex · H2O):
8. Calculate the milliequivalents of sodium, potassium, and chloride, the
millimoles of anhydrous dextrose, and the osmolarity of the
following parenteral fluid. Assume complete dissociation.
Sodium chloride:
Potassium chloride:
Total chloride: 76.92 mEq + 20 mEq = 96.92 mEq Cl
−
Dextrose:
Osmolarity: 153.85 mOsmol/L + 40 mOsmol/L + 277.78 mOsmol/L
= 471.63 mOsmol/L
Clinical Considerations of Water and
Electrolyte Balance
Maintaining body water and electrolyte balance is an essential component
of good health. Water provides the environment in which cells live and is
the primary medium for the ingestion of nutrients and the excretion of
metabolic waste products. Normally, the osmolality of body fluid is
maintained within narrow limits through dietary input, the regulatory
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