An alkaline reaction is present when the amount of hydroocyl-ions (0H-) exceeds the 

 amount of hydrogen-ions (H+). In the reversed case, the reaction is of course acid while 

 a neutral reaction is indicated by equal amounts of OH- and of H* -ions. 

 We have tlierefore: 



(H+). (0H-) eq. K^eq. lO"-^ 



which means the product of hydrogen-ions and of hydraxyl-i<Mis in gramions per liter is equal 

 to the electrolytic "dissociation constant" Kwj i.e. the amount of ions present in absolutely 

 pure water, and Tiiich is practically unchangeable. 



It is therefore sufficient to always know the exact amounts of hydrogen-ions to ascer- 

 tain the reaction. In cases of neutral reaction, where the amount of (H-f)-ions is equal 

 to the amount of (OH-)-ions, the amount of hydrogen-ions will be equal 10~7, 



In order to avoid calculations v;ith these bothersome small figures of the actual 

 hydrogen-ion concentration, it has become the general custom to use the negative logarythm 

 of these figures, i.e. for neutral -vraters the figure 7. This is spoken of as the hydrogen 

 exponent or — for short and in general practice — is expressed by the symbol pH. Hence, we 

 have 



pH eq, 7 neutral reaction 



pH ^ 7 alicaline reaction 



pH <■ 7 acid reaction 



A good pond water has usually a pH rate of from 7 to 8, i.e. a feebly alkaline 

 reaction. This is chiefly due to the presence of dissolved calcium bicarbonate (the 

 responsible factor for acid combination capacity) which is the salt of a strong base and 

 of a feeble acid. If present in sufficient amoimts, it i^lll fona — in combination with 

 carbonic add — an especial "regulator" (a Buffer, as it were) which will connteract too 

 strong variations of the pH rate. A good pond water with sufficient acid combination 

 capacity Tdll not show any strong lowering or rising of the pH rate (under 6.5 or over 8.5), 



In 1926, Schaeperclaus could show, for the first time that the water of numerous 

 "lime-oligotrophic" (lacking calcium) ponds fisheries in heath and moor regions is of low 

 acid combination capacity and has therefore a pH rate below 7. 



Over and over again, these fisheries suffer from a dying off (acid mortality) of 

 fish, caused by too high an acidity of the water, although there is no inflow of acidulous 

 v;ater3. The natural acids of the soil are sufficient to bring down the pH rate to 5.5 

 and in some cases to 3.5 during prolonged rains or vrtien the snow is melting. 



Schaeperclaus further more has shown that a pH rate of about 5 or less — if lasting 

 for any length of time — will cause diseases of the skin of fishes and of the gills, and 

 the fish vdll die eventually. 



The more exact "acid danger point" for carp is a pH rate of ^.8, and which is deadly 

 "vithin a short time. Its rate, of course, depends upon various factors and is therefore 

 somevihat variable. 'iThen iron is present, for instance, the "danger point" is somewhat 

 higher. Other pond fish are similarly sensitive. 



Sometimes a dying off of fish Td.ll suddenly occur in ponds where such calamities were 

 never before observed. This has been especially observed after a re-fores tation — with 

 scotch pine or spruce — of the tributary water regions. It is reasonable to conclude that 

 Pinaceae stainds lead to an accumulation of organic acids in the soil. 



The lov; acid combining capacity is then soon so completely exhausted that the pE rate 

 drops sharply and the "danger point" is easily reached — especially in winter — and even 

 surpassed. 



It is hardly necessary to mention that ponds with a pH rate of little more than 5 

 acidotrophic. ponds are little productive and therefore belong in the oligotrophic class. 



51 



