Intrinsic viscosity is a measure of a solute's contribution to the viscosity of a solution. It should not be confused with inherent viscosity, which is the ratio of the natural logarithm of the relative viscosity to the mass concentration of the polymer. 
Intrinsic viscosity is defined as
where is the viscosity in the absence of the solute and is the volume fraction of the solute in the solution. As defined here, the intrinsic viscosity is a dimensionless number. When the solute particles are rigid spheres, the intrinsic viscosity equals , as shown first by Albert Einstein.
In practical settings, is usually solute mass concentration (c, g/dL), and the units of intrinsic viscosity are deciliters per gram (dL/g), otherwise known as inverse concentration.
Formulae for rigid spheroids
Generalizing from spheres to spheroids with an axial semiaxis (i.e., the semiaxis of revolution) and equatorial semiaxes , the intrinsic viscosity can be written
where the constants are defined
The coefficients are the Jeffery functions
General ellipsoidal formulae
It is possible to generalize the intrinsic viscosity formula from spheroids to arbitrary ellipsoids with semiaxes , and .
The intrinsic viscosity formula may also be generalized to include a frequency dependence.
The intrinsic viscosity is very sensitive to the axial ratio of spheroids, especially of prolate spheroids. For example, the intrinsic viscosity can provide rough estimates of the number of subunits in a protein fiber composed of a helical array of proteins such as tubulin. More generally, intrinsic viscosity can be used to assay quaternary structure. In polymer chemistry intrinsic viscosity is related to molar mass through the Mark-Houwink equation. A practical method for the determination of intrinsic viscosity is with a Ubbelohde viscometer.
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