III. CENTRIFUGATION 87 



minations of particle density, solution viscosity, refractive index in- 

 crement, and concentration of the principal material. 



2. Measurement of Sedimentation Constant 



Sedimentation rate is affected very markedly by the concentra- 

 tion of the material. As can be seen from Table I, it decreases more 

 than 10% for each 1% increase in concentration in the case of most 

 serum proteins, even though they are not very asymmetrical. The 

 effect becomes increasingly pronounced with increasing asymmetry 

 of the particles. For the sedimentation rate to have any significance 

 as far as comparison with other values or as far as application to the 

 formulas in Section C is concerned, it is necessary to make deter- 

 minations with several different concentrations and extrapolate the 

 corrected (for viscosity) results to zero concentration. It is generally 

 desirable to cover at least a threefold range in concentration and to 

 study the material in concentrations at least as low as a few tenths of 

 one per cent. In using equation (12), for example, one cannot as- 

 sume that s and D are affected to the same degree by any given in- 

 crease in solute concentration {52). All the equations used in ultra- 

 centrifugation studies apply strictly only to infinitely dilute prepara- 

 tions and when it is necessary to apply a viscosity correction, as in 

 equation (2), there is some uncertainty as to whether the viscosity of 

 the medium or the viscosity of the complete preparation should be 

 used. Although the problem is of little consequence when extrapola- 

 tion to zero concentration is made, it seems preferable to use the com- 

 plete solution viscosity if only a limited number of experimental de- 

 terminations can be made {52). In any event, the most important 

 and largest correction generally made is for the change in viscosity 

 brought about by variations in temperature of the specimen. Since 

 this generally amounts to more than 2% per degree Centigrade, it is 

 preferable to know the temperature of the rotor and cell to within a 

 few tenths of one degree. Another factor that must be considered 

 is the increase in viscosity of the medium brought about by the hj^dro- 

 static compression. In the case of aqueous solutions in a 1.5 cm. 

 column of fluid at 60,000 r.p.m., the average effect on the sedimenta- 

 tion rate is only about 1%, but with some organic solvents it can be 

 many times this amount {86). In view of other factors (discussed 

 later) that probably compensate for the small viscosity effect in aque- 

 ous solutions, it is doubtful whether any correction should be made. 



