38 MINERAL SALTS ABSORPTION IN PLANTS 



resin in the acid form, R.SO3H) for example, reacts with sodium 

 chloride to produce the sodium salt of the resin and hydrochloric 

 acid: 



R.SO3H + NaCl^R.S03Na + HCl 



In presence of excess acid, the reaction is reversed and the acid form 

 of the resin is regenerated. 



Anion exchange resins similarly react reversibly with salts, 

 adsorbing anions from neutral solutions and releasing them in 

 presence of dilute alkalis, e.g. : 



R.OH + NaCl^R.Cl + NaOH 



Both mechanical and polar adsorption are involved in the staining 



C C IM 



H^ Hj 



0- 



(a) (b) 



Fig. 9. Adsorption of dyes by amphoteric molecules 



a. Amphoteric adsorbent on the acid side of its isoelectric point, and 



positively charged. Coloured acid dye adsorbed; b. Amphoteric adsorbent 



cation of basic dye adsorbed. 



by dyes of biological materials such as cellulose and proteins. Basic 

 dyes e.g. methylene blue (MeCU) in which the coloured part of the 

 molecule is positively charged, are adsorbed by substances containing 

 a predominance of negative charges. Acidic dyes, e.g. orange .^ 

 G (HG) on the other hand, contain coloured anions, and stain 

 positively charged materials. The capacity of amphoteric substances, 

 e.g. proteins to take up dyes, depends on the charge carried by the 

 adsorbent, and hence on the pH value of the surrounding medium 

 (Fig. 9). 



G. Chemical Combination 



There is no very clear distinction between polar adsorption and 

 chemical combination, but the latter tends to have a higher Q^q 



V 



