68 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



salts arc adsorbed much more freely than they would lie if the process 

 depended solely on surface condensation; that is, if the Gibbs formula is 

 used to calculate the adsorption, it will give values that are much below 

 those actually found. 



If the dissolved substance and the particles both have the same electric 

 sign, adsorption will not occur. Filter paper, for example, has a nega- 

 tive charge and can not therefore adsorb a negative dye such as congo 

 red (as shown by the depth to which it becomes stained) ; whereas it 

 readily adsorbs night blue, which is positively charged. If the negative 

 charge of the paper is lowered, it becomes capable of adsorbing some of 

 the negative congo red. This can be effected either by placing the paper 

 in alcohol or by adding inorganic salts (NaCl) to the water with which 

 it is in contact. The positive-charged ions of Na, produced by dissocia- 

 tion, neutralize some of the negative charge on the paper, and allow a 

 certain amount of adsorption of the negative-charged congo red to oc- 

 cur. As would be expected, acids and alkalies are capable of greatly 

 altering the electric charges by the H and OH ions which they contribute. 



Chemical Forces. — If the nature of the phase at the surface of which 

 adsorption occurs is such that it can enter into chemical combination 

 with the substance adsorbed, reactions will occur that do not obey the 

 laws of mass action. By adsorption, reactions of a certain type may be 

 encouraged over other reactions, even although the necessary reacting 

 substances may be present in the solution (specific adsorption). The 

 adsorbing substance itself is not, however, usually susceptible of chem- 

 ical change even when it exists as very minute particles, as in the case of 

 colloidal solutions. Nevertheless, adsorption may accelerate chemical 

 reactions by bringing together in concentrated form substances of high 

 chemical reactivity. In such cases the adsorbing substance itself does 

 not enter into the chemical reaction, and can be recovered at the end 

 in an unchanged condition. It acts as a catalyst (page 72). As we 

 shall see later, enzymes act in this way — i. e., their rate of reaction is 

 controlled by adsorption.* 



The distinguishing feature of such adsorption phenomena is that a 

 curve of the reaction (drawn by plotting amount of chemical change 



'Another instance of the influence of surface energy on the course of chemical reactions is seen 

 in the accelerative influence of charcoal on such reactions as the oxidation of formic acid, glycerol, 

 etc. Surface tension may also cause retardation of chemical reactions, as is seen in the turbidity 



M 

 (due to the separation of chloroform) which gradually develops when a . Na»C0 3 solution is 



M i 



mixed with a ' chloral hydrate solution. The surface remains clear, because surface energy has 



prevented the reaction. 



An important effect of surface tension on chemical reactions is also seen in the relationship 

 between it and the absorption coefficient of gases (volume of gas dissolved by unit volume of 

 liquid). The lower the surface tension, the greater the solubility of the gas. Oxygen and nitrogen 

 are, for example, much more soluble in alcohol, hydrocarbons or oil than in water. This shows 

 the futility of attempting to prevent the loss of gases from fluids such as blood by covering them 

 with oils or hydrocarbons. 



