Methods of Determining Relative Molar Mass

VISCOSITY METHODS OF DETERMINING RELATIVE MOLAR MASS 

For the determination of polymer molar masses, very dilute solutions are used, typically of the order of 1 % by mass. In using viscosity techniques, a number of functions are used. These are  

Where c is small, i.e. for solutions at a concentration of the order of 1% or less, terms above can be ignored as being 

Equation (6.20) is known as the Huggins equation. From it, we can see that the limiting case as c tends to zero is that the term (y —Vi )/VoC tends to a limiting value. This is known as the limiting viscosity number or, more rarely nowadays, the intrinsic viscosity. It is given the symbol [//] and has units of reciprocal concentration. 

In practice we do not measure viscosity directly. Instead, what is measured is time of flow for solutions and pure solvent in a capillary viscometer, the so-called efflux time. If the same average hydrostatic head is used in all cases, and since for very dilute solutions the density differences between the different concentrations are negligible, then the ratio of the efflux time of the solution, I, to that of the pure solvent, to, may be taken as a measure of the ratio of the viscosities, i.e. 

---

In biochemistry gel electrophoresis is the method of choice for the separation of various kinds of macromolecules (e.g. nucleic acids, proteins). It is also used in dendrimer chemistry for separation and as a method of determining relative molar masses and for qualitative assessment of the purity of a dendrimer sample. 

---