POTENTIOMETRIC METHODS 25 



where A^ is Avogadro's number, k Boltzmann's constant, a the molecular 

 diamagnetic susceptibility, /x the spin dipole moment, and T the absolute 

 temperature. The diamagnetic term in this equation is small and can 

 usually be ignored. The dipole moment is usually given in Bohr magnetons, 

 and in this unit n is obtained from the equation: 



M =\ — — Xmol = 2.84^ Xmol T 



The relationship between the magnetic dipole moment and the number 

 of unpaired electrons, «, is given by the equation: 



M = \\n{n + 2) Bohr magnetons 



Thus values of m of 1.73, 2.83, 3.88, 4.90, and 5.92 Bohr magnetons 

 correspond to 1, 2, 3, 4, and 5 unpaired electrons, respectively. 



6.3. Determination of Paramagnetic Susceptibility 



Since the magnetic dipoles are oriented in the direction of the 

 external field, a force of attraction exists between a magnet and a 

 paramagnetic substance. This property is utilized to measure mag- 

 netic susceptibility, several different varieties of magnetic balance 

 having been developed. Details of the methods can be obtained from 

 various books and reviews {2529, 3026, etc.). 



6.4. Magnetochemical Titrations 



When a chemical reaction results in change of bond type of the 

 metal atom of a hematin compound, determination of magnetic sus- 

 ceptibility can be used to follow the course of a titration. Numerous 

 papers of Pauling and collaborators and of Theorell deal with the 

 technique and application of this method. In many problems investi- 

 gated by the former workers, the titration involved reduction of a 

 ferric or oxygenated ferroUs compound by dithionite. For example, 

 this technique was used in an endeavor to establish the existence of 

 intermediate compounds in the oxygenation of hemoglobin, hemo- 

 globin having ionically and oxyhemoglobin covalentlj' bound iron. 



7. POTENTIOMETRIC METHODS 



Apart from the routine measurement and control of hydrogen ion 

 concentration necessary for accurate experimental work, poten- 

 tiometric methods used in hematin chemistry fall into two main 



