OTHER METHODS 45 



difference will be set up between the electrode and the solution the value of 

 which, in relation to a second standard electrode, may readily be measured. 

 This is not always so. 



It has been found that, while many systems do indeed behave in this 

 simple manner, and are amenable to precise measurement, others give poten- 

 tials which are quite unstable, and an intermediate group take long periods, 

 perhaps hours, to reach stability. No satisfactory explanation has been given 

 of this phenomenon of electroactivity, electroinactivity, and sluggisliness. 

 It is of considerable importance in biological work, since it may seriously 

 restrict the range of oxidation-reduction potential investigations unless 

 means can be found to overcome the difficulties. 



Addition to an electroinactive or sluggish system of a second system which 

 is electroactive and can in addition react readily with the first system fre- 

 quently provides a means of stabilizing the potential and arriving at the 

 required values for the system under investigation. Examples of this are 

 found in the work of Taylor {2751-2753) on the sluggish hemoglobin- 

 hem?globin system, and in the discussion of Clark (450). 



8. OTHER METHODS 



A number of other physical methods have found wide application 

 in the study of hematin compounds. Some are of very general appH- 

 cability, such as osmotic pressure measurements, and x-ray studies 

 of molecular size and shape. Others are methods commonly employed 

 in protein chemistry, including electrophoresis, diffusion measure- 

 ments, and ultracentrifugal sedimentation measurements. 



These methods all yield information concerning the size, shape, or 

 electric charge of the molecules or particles investigated. Their use 

 in the present field involves little of difference from their application 

 elsewhere. The reader is therefore referred for accounts of their 

 basic theory and practical details to standard works on the various 

 subjects {2,2721,3026). 



