Colligative Properties and Tonicity Calculations

This set of vocabulary flashcards covers the principles of tonicity adjustment, including Freezing Point Depression (FPD) and Sodium Chloride Equivalent (SCE) methods, as well as membrane permeability considerations in pharmaceutical calculations.

Iso-osmotic

Describes two solutions that have the same osmotic pressure and contain the same concentration of dissolved species.

Hypo-osmotic solution

A solution with a lower osmotic pressure than another solution; it can be rendered iso-osmotic by adding more solute.

Hyper-osmotic solution

A solution with a higher osmotic pressure than another solution; it can only be rendered iso-osmotic by dilution.

Freezing Point Depression (FPD)

The most readily measurable colligative property used in pharmacy as a proxy to assess or adjust tonicity.

$$-0.52\,^\circ\text{C}$$

The exact freezing point of both blood plasma and lacrimal fluid (tears).

Golden Rule of Tonicity

A solution that is iso-osmotic with biological fluids will be isotonic provided the biological membrane is not permeable to any of the solutes present.

Boric acid

A substance suitable for adjusting tonicity in ophthalmic solutions (since the eye's mucous lining is impermeable to it) but unsuitable for parenteral solutions (since red blood cells are permeable to it).

Normal Saline

$0.9\%\text{ w/v}$ aqueous $NaCl$, which is isotonic and has a freezing point of $-0.52\,^\circ\text{C}$.

D5W

$5\%\text{ w/v}$ aqueous Dextrose; it is isotonic as formulated but becomes effectively hypotonic in the body as dextrose is rapidly metabolized.

FD1%

The freezing point depression (in $^\circ\text{C}$) caused by a $1\%\text{ w/v}$ solution of a given solute, as tabulated in references like the Australian Pharmaceutical Formulary (APF).

FPD Method Formula

$w = (0.52 - a) / b$, where $w$ is the $\text{\%w/v}$ of adjusting substance needed, $a$ is the sum of $\Delta T_f$ contributed by existing solutes, and $b$ is the FD1% of the adjusting substance.

Sodium Chloride Equivalent (SCE)

The mass (g) of $NaCl$ that would produce the same osmotic effect as $1\text{ g}$ of the substance in question.

SCE Method Formula

$\text{\%NaCl} = 0.9 - (\sum SCE \times \% \text{ of each ingredient})$, used to calculate the amount of adjusting substance needed for isotonicity.

Superscript "a"

A notation in the APF reference table indicating a solute can cross red blood cell membranes; such solutes are usually omitted from parenteral isotonicity calculations.

Pilocarpine Hydrochloride FD1%

$0.134\,^\circ\text{C}$, meaning a $1\%\text{ w/v}$ solution has a freezing point of $-0.134\,^\circ\text{C}$.