Lec 6 - Assessing Osmolality and Specific Gravity

Mole

Number of particles in avogadro's number

Osmole

Total amount of substance that has to be dissolved to produce 6.022 x10^23 particles (1 osmole = 1 mole of undissociated solute)
- 1 mole of NaCl = 2 osmol (due to ions in dissociation)
- 1 mole of glucose = 1 osmol

Molarity

Concentration of an individual solute over a liquid volume (moles/L)

Osmolarity

Concentration of all solutes in a sample over a liquid volume (Osmol/L)

Molality

Concentration of an individual solute over a mass (mol/kg)

Osmolality

Concentration of all solutes in a sample over a mass (Osmol/kg)

How many kg does 1L of water weigh

1 kg

Disadvantage of using a volume measurement

volume is dependent on temperature and pressure. Mass/L changes

Advantage of using a mass measurement

Mass is independent of volume = mass is unaffected by temperature or pressure

Solute

Substance being dissolved in a solution

Solvent

The medium that dissolves solute

Osmosis

Movement of the solvent across a semipermeable membrane
- Moves from high concentration of solvent to low concentration of solvent

How does osmolality relate to osmosis

Important to measure the amount of solute in a substance as this contributes to osmotic pressure. Physiological processes (kidney function, etc.) rely on osmotic pressure

Osmotic pressure

Pressure required to prevent a solvent from passing the semi-permeable membrane. Opposing force of osmosis

Will osmotic pressure be higher or lower in a solution with a high osmolality

Higher osmotic pressure. Low water concentration = high osmotic pressure

Osmotic pressure formula

π = iMRT
- i = van't Hoff factor
- M = concentration of solution (mol/L)
- R = ideal gas constant
- T = temperature (K)

Osmometry

The analytical method used to measure the concentration of solute particles that contribute to osmotic pressure

Major solutes that make up osmolality in humans

Sodium, Glucose, Urea

How to roughly calculate Osmolality

2[Na+] + [glucose] + [urea]

In the osmolality formula, why does sodium have a 2?

Account for counter ion. Sodium is never in ionic state physiologically

Physiological unit of osmolality

mOsm/kg

How do physicians read osmolality

mmol/kg

How do osmometers measure osmolality

freezing point depression
- solutes change freezing point of pure solvent

Formula used by osmometers to calculate osmolality

(Δ x 1000)/ 1.86 kg/mol
- Δ = depression of freezing point
- multiply by 1000 to get to mOsmol

What does the 1.86 kg/mol term mean?

For the addition of 1 mol of pure solute in 1 kg of water, it will cause the solvent to freeze at a freezing point 1.86 degrees lower

Limitation of "rough" Osmolality calculation

Not all uncharged molecules are captured by calculated osmolality, only glucose and urea

How to detect other uncharged molecules in sample

Osmolal gap calculation

Osmolal gap

Measured osmomality (from osmometer) - calculated osmolality (from formula)
- Difference is representative of concentration of other uncharged molecules

What does osmolal gap detect the presence of (mainly)?

Alcohols - ethanol, methanol, ethylene glycol, etc.

Specific gravity

Measure of solute concentration based on its density

What samples are specific gravity tested on

Urine

Specific gravity calculation

density of urine/density of water

Ways to test specific gravity of urine

1. Urinometer
2. Specific gravity dipstick
3. Refractive index

How a urinometer works

Placed in sample of urine. Where meniscus of urine lines up on the urinometer is the specific gravity
- Meets at higher amount for lower specific gravity

Urine SG range

1.002 - 1.040 (unitless)

Urine Dipstick theory

based on change in pKa of pretreated polyelectrolytes in the reaction pad in relation to the ionic concentration of urine

What happens if there is more ions in the specimen of a urine dipstick test

More acid groups dissociate on the dipstick pad. More protons are released and causes a more acidic pH in the pad
- Higher SG = higher solute = greater pH change

Refractive index

Indirect measurement of specific gravity based on the refractive index of light

Refractive index theory

Different mediums will refract light at a different angle

Does a more concentrated solution bend light more or less

Higher concentration bends light closer to the normal

Advantage of refractive index testing

does not require very much sample

Disadvantage of refractive index testing

Affected by temperature.