PHYSICOCHEMICAL AND CHEMICAL PHASES 



173 



tide pools, bays, and estuaries, the range 

 may be much greater. Near Woods Hole, 

 Massachusetts, over deep muck at low 

 tide, the pH was 7.3; at the surface of the 

 water in a mat of eel grass and in the bright 

 sunhght, the pH was 9.0; only 30 inches 

 of well-buffered sea water separated the 

 two locations. These differences practically 

 disappear at high tide (Allee, 1923). At the 

 same time, the dissolved oxygen ranged 

 from a trace at the bottom near the muck 

 to the supersaturated value of 12.97 cc./L. 

 at the surface. Other things being equal, 

 regions of relatively low pH (high H ion 

 concentration) are frequently also regions 

 of reduced oxygen content. The pH of ma- 

 rine bottom deposits may fall below 7.0 

 or stand above 8.5 (ZoBell, 1946). 



The pH of fresh waters, unmodified by 

 man, range from 3.2 (or perhaps 2.2) to 

 10.5 or thereabouts, although most streams 

 and lakes have a range of from 6.5 to 8.5, 

 inclusive. The moss. Sphagnum, and some 

 other plants secrete acid, and the water in 

 sphagnum mats approaches the lower re- 

 corded limit. Bogs in general, whether 

 sphagnum or otherwise, usually have acid 

 water. Streams and the water from the sur- 

 face strata of lakes in limestone areas usu- 

 ally have about the same pH as does the 

 ocean. 



CHEMICAL BUFFERS 



The H ion concentration is kept from 

 large fluctuations in most animal environ- 

 ments by the presence of chemical buffers. 

 Buffers are largely absent in rain water 

 and in water from recently melted ice or 

 snow; there is a high concentration of them 

 in hard waters that are rich in carbonates. 

 The effectiveness of chemical buffers as a 

 neutrality mechanism can be understood 

 by a brief consideration of a buffer system 

 based on carbon dioxide, which closely 

 accompanies water in nature in the follow- 

 ing forms: 



Free carbon dioxide, CO2 



H \ 

 Carbonic acid, tt / CO3 



Bicarbonate or alkali-acid carbonate or "lialf 



H \ 

 bound" carbonate, „ yCOs 

 Base / 



Alakali or "bound" carbonate, r. yCOn 



If a strong acid is added, the dynamic 

 equilibrium shifts rapidly. Simply put, 



some of the acid combines with the alkali, 

 let us say from the bound carbonate, and 

 the nearly neutral bicarbonate is increased. 

 If more acid is added, some of the bicar- 

 bonate may be converted to carbonic acid; 

 this may break into water and carbon di- 

 oxide, and any excess of the latter will 

 escape into the air. The neutrality of the 

 medium is not greatly changed until the 

 alkali reserve approaches exhaustion. The 

 alkali reserve or buffer value is sometimes 

 referred to merely as "alkalinity," a term 

 that in this connection has no relation to 

 the hydroxyl ion concentration for the 

 given medium. Conversely, if a strong alkali 

 is added, it reacts with carbonic acid to 

 form bicarbonate or, if in larger amount, to 

 form a "bound" carbonate that is more 

 nearly neutral than the alkali originally 

 added. 



Phosphoric acid, with three hydrogen 

 ions bound to the acid radicle, makes an 

 even better buffer. The principle is the 

 same, and though phosphates are less 

 abundant in aquatic environments than 

 are the carbonates, thev may be an effec- 

 tive buffer for soils. The common use of 

 phosphates for buffering water in physio- 

 logical experimentation is open to serious 

 objection, since the phosphate ion often 

 produces a situation quite different in ion 

 content from that found in nature and 

 therefore ecologically imsound. 



pH AND ANIMAL LIFE 



The expectation that the hydrogen ion 

 concentration of the environment might 

 prove to be of outstanding importance (p. 

 58) has not been realized. The hypothesis 

 was based on observations that enzyme ac- 

 tivity is often closely related to the pH of 

 the medium and that respiratory rate in 

 vertebrates is controlled by the pH of the 

 blood. Also, there was early evidence of 

 the importance of pH in the culturing of 

 bacteria. It is now known that some bac- 

 teria, as with animals, grow only in a nar- 

 row pH range and may be restricted to less 

 than a pH unit, and that others can tolerate 

 a range of six or even of nine such units 

 (Buchanan and Fulmer, 1930). 



Among Protozoa, Eiislena mutahilis oc- 

 curs in pools and streams with acidities as 

 high as pH 1.8 produced by acid drainage 

 from mines. In pure cultures, free from bac- 

 teria and fungi, they tolerate pH 1.4 for 

 twelve days and have a similtj- resistance to 



