30 THE PROPERTIES OF SOLUTIONS. [CH. I. 



examined again. This procedure must be repeated until two solu- 

 tions are found of such a P H that the colour as seen through y is 

 intermediate between those seen through x and z, or that through y 

 is identical with one of them. The P H of the solutions finally 

 employed should not differ by more than O'i. 



NOTE. In measuring the indicator solutions it is essential to hold the 

 dropping pipette vertical, to ensure the delivery of equal drops. 



H. Ampholytes or amphoteric electrolytes. 



These are substances which can function as acids by 

 forming salts with bases, and also as bases by forming salts 

 with acids. The amino acids, such as glycine, are examples. 

 Glycine can form a sodium salt, CH 2 .NH 2 .COONa, and 

 also a hydrochloride, HC1.H 2 N.CH 2 .COOH. In strong 

 acids it behaves as a base ; in strong alkalies as an acid. In 

 neutral solutions it is a feeble electrolyte, and is partially 

 dissociated into H and a negative ion (anion). 



H 2 N.CH 2 .COOH Z^ H 2 N.CH 2 COO + H...(i) 

 and partially into OH and a kation. 

 HO.H 3 N.CH 2 .COOH < ~ HO + H 3 N.CH 2 .COOH...(2) 



If a strong acid, such as HC1, be added the dissociation (i) is 

 decreased, in the same way as the dissociation of all weak acids is 

 decreased by an increase in the hydrogen-ion concentration. On 

 the other hand the number of kations formed is increased, since such 

 a salt as glycine hydrochloride is freely dissociated. 



If a strong base, like NaOH, be added the dissociation (2) is 

 depressed, and there is an increase in the number of anions of the 

 ampholyte, due to the free dissociation of the sodium salt that is 

 formed. 



For every ampholyte there is some particular concen- 

 tration of hydrogen ions at which the dissociation (i) is 

 equal to the dissociation (2). This is known as the " iso- 

 electric point." Since the proteins are ampholytes, the 



