40 THE GENERAL CHARACTERS OF THE PROTEINS 







A. Conductivity Method. 



This method was first applied by Sjoqvist. 



If a strong base be gradually added to a solution of a strong acid, 

 the volume of the solution being kept constant, and the electrolytic 

 conductivity be determined after each addition, it will be found that 

 it gradually diminishes, until a certain minimum is reached, after 

 which it gradually increases. This minimum represents the point 

 at which the acid is completely neutralised by the base. When 

 potassium hydroxide is added to hydrochloric acid the conductivity 

 curve, where the ordinates represent the molecular conductivity, and 

 the abscissae the amount of alkali, is represented by a descending 

 and an ascending straight line enclosing a sharp angle. The attain- 

 ment of the minimum point is due to the fact that both the acid and 

 the base are better conductors than the salt which is formed by their 

 neutralisation of one another. If, instead of a strong base, a weak 

 base like ammonia be employed, which is a bad conductor, the con- 

 ductivity rises but slightly after the neutralisation point is reached ; 

 at this point the curve becomes nearly parallel with the abscissa. A 

 somewhat similar curve is obtained when egg-white is gradually 

 added to a solution of acids, and by a study of such curves Sjoqvist 

 has endeavoured to throw some light on the salt formation of the 

 egg-proteins with acids. 



Two additional factors have in this case to be taken into account, 

 viz. : (a) the conductivity due to the protein addition ; (b) the decrease 

 of conductivity due to the lessened velocity of the ions owing to the 

 increased internal resistance of the fluid produced by the addition of 

 the proteins. 



The conductivity of the protein alone is readily determined. 

 This is probably due entirely to combined or adsorbed salts. Al- 

 though the protein solutions used in Sjoqvist's experiments were 

 carefully dialysed, they could not be obtained ash free. The ash 

 was found to consist of a mixture of calcium sulphate and phosphate. 



The internal friction factor is not so readily determined. The 

 alteration of the conductivity of the acid due to this factor can only 

 be determined indirectly, viz., by determining the decrease of conduc- 

 tivity of a neutral salt solution produced by repeated additions of 

 known quantities of protein, and the ratio of this decrease to the 

 decrease produced in acids by other non-electrolytes which increase 

 internal resistance, and upon which acids do not chemically act. It was 

 found, for example, in the experiments of Sjoqvist, that each gram 

 of protein added to a 0*05 N sodium chloride solution produced (after 

 allowance for the conductivity of the protein alone) a diminution of 

 1*52 per cent, of the total molecular conductivity. Now the mean de- 

 crease produced by six other non-electrolytes, which increase internal 

 friction, was found by Arrhenius to be as follows: for NaCl, 2*21 

 per cent. ; for HC1, 1-84 per cent.; for HNO 3 , r88 percent. If, 

 now, the ratio of decrease produced by proteins is the same as for 

 other non-electrolytes, then the diminution of conductivity produced 

 by the addition of each gram of protein to 100 c.c. of a 0*05 N 



hydrochloric acid solution is 1*52 x 4 = 1*26 per cent. The actual 



2'2I 



diminution produced when egg-white solution is added to 0*05 N 



