Lecture 6: Isotonic Solutions (10.28.24)

Change in freezing point

Proportional to the molar concentration of the solute particles in the solution

Aqueous solutions that have the same freezing point

Have the same osmotic pressure (isosmotic)

Dissociating m%

If a solute dissociates into n ions

i = 4.2

Substances that dissociate into 5 ions (assumption value)

E-value of substance B

Weight of NaCl / Weight of B = MW (NaCl) × i (B) / MW (B) × i (NaCl) =

1 g of B

Equal in tonic effect to (or can be represented by) its E-value of g of NaCl

E-value of Q

g, NaCl needed to make isotonic (step 3) / Quant Q to add, g =

∆FP °C / reference FP NaCl °C

X% NaCl to make solution to desired FP / reference % NaCl =

Isotonic (or nearly so) preparations

In most instances, these are preferred

Isotonic preference exceptions

Hypertonic solutions are used to "draw" fluids out of edematous tissues and into the administered solution

IV, opthalmic

Preparations where isotonicity is usually most important

Osmotic pressure of a solution

Proportional to the molar concentration of the solute particles in the solution

Colligative properties

Properties of a solution that depends on the number of particles in a volume of solvent (freezing point depression, boiling point elevation, vapor pressure lowering, osmotic pressure)

Osmotic pressure

Difficult to measure

Freezing point

Simple, precise measurement

Dissociation factor (i)

Ratio between "actual" concentration of particles produced when the substance dissolves vs the concentration of the substrate in the solution

i

Concentration of total particles produced / Concentration of the solute molecules =

i

1 + dissociation × (n-1)

i = 1.0 (i.e. 0% dissociation)

Non-electrolytes and substances of slight dissociation (assumption value)

i = 1.8

Substances that dissociate into 2 ions (assumption value)

i = 2.6

Substances that dissociate into 3 ions (assumption value)

i = 3.4

Substances that dissociate into 4 ions (assumption value)

∆T

Proportional to the concentration of solute particles

∆T

Change in temperature, in °C

-0.52 °C

Freezing point of blood serum, lacrimal fluid (tears) ≈

Proportion

Relate freezing point to solute concentrations, compare unknown to known "standard or reference" freezing point of a 1 mol/L solution

REFERENCE

Solution with known freezing point & concentration

i × molarity

Molar concentration of NaCl PARTICLES

Isotonic

0.9% w/v NaCl

Sodium Chloride Equivalent (E-value)

A tonic equivalent constant that converts particle concentration for that molecule to an "equivalent" concentration of NaCl

E-value

If the amount of a substance included in a prescription is multiplied by its sodium chloride equivalent (E-value), the amount of sodium chloride represented by that substance is determined

g of B × E-value

g of NaCl equivalent =

E-values

Will never be ≥ 1

Step 1 (NaCl equivalent method)

Determine grams of NaCl equivalent of each component of the solution (E-value = g NaCl / g B) (X₁ = g NaCl equiv for component B₁) (repeat for all components of solution, X₁, X₂, X₃, etc)

Step 2 (NaCl equivalent method)

Determine how much NaCl would be in a pure NaCl isotonic solution for the volume desired (proportion for 0.9% w/v) Y

Step 3 (NaCl equivalent method)

Common language: Subtract the NaCl equivalent grams of each component of solution from the g of NaCl in a pure NaCl isotonic solution (Y - X₁ - X₂, etc)

Step 4 (NaCl equivalent method)

If an agent OTHER THAN NaCl, agent "Q", is to be added to make a solution isotonic, must determine grams of agent Q that is equivalent to grams NaCl needed to add

Quant Q to add

Amount of Q equivalent to grams of NaCL needed to make solution isotonic

ISOTONIC solution

Isotonic solution 1 + isotonic solution 2 =