154 EFFECTS OF REACTION OF SUBSTRATE ON FUNGI 



The pH of a solution is rarely an even decimal fraction of nor- 

 mal. For this reason the quantity between two succeeding frac- 

 tions may be indicated by a multiplying factor. If, for instance, 

 the concentration of ions is 0.000273 N, it may be written 2.73 X 

 10 ~ 4 . By the use of the logarithm table, it will be found that the 

 logarithm of 2.73 = +0.434 and that of lO" 4 = -4.000. Since 

 the logs are added when multiplying, +0.434+ (—4.000) = 



— 3.566. Therefore losr rTT , , = 3.566, or pH = 3.566. 



& [H + J r 



If the actual figure for the hydrogen-ion concentration is sought 

 when the pH value is given, it can be determined by the reverse 

 of the procedure of calculation just described. Suppose that the 

 given pH value is 9.63, which may be expressed thus: [H + = 

 1 X 10~ 963 . The exponent —9.63 = — 10 plus + 0.37; or, other- 

 wise stated, it equals 10 -10 X 10 + - 37 . The logarithm table shows 

 that +0.37 corresponds with the number 2.34. Substitution of 

 this number in the original equation, [H+] = 1 X 10 _9G3 , gives 

 the identity [H+] = 2.34 X-IO" 10 . 



In acids dissociated in water, the concentration of hydrogen ions 

 must be greater than that of the water itself and therefore must 

 range between pH 7.0 (the hydrogen-ion concentration of water) 

 and pH 0. Likewise, bases dissociated in water have a hydroxyl- 

 ion concentration greater than that of water itself, so that this 

 concentration can range between pOH 7.0 (the hvdroxvl-ion 

 concentration of water) and pOH (the hvdroxvl-ion concen- 

 tration of a normal basic solution). Since the acid or base was dis- 

 sociated in water, [OH - ] ions are always present in acid solutions 

 and [H + ] ions in basic solutions. The concentration of hydrogen 

 ions therefore varies inversely as the concentration of hydroxvl 

 ions, and vice versa, the product of the concentration of both kinds 

 being always 1 X 10 -14 . From this relation it is evident that, if 

 the concentration of either ion is known, that of the other can be 

 readily computed. Thus if the pH of a solution is 10 ~ 4 , the pOH 

 is 10 -10 . From the foregoing discussion it is apparent that any 

 pH value between and 7 indicates an acid solution, with decreas- 

 ing acidity as the number increases. Similarly, any pH value be- 

 t\\ een 7.0 and 14.0 indicates a basic solution with increasing basic- 

 ity (decreasing acidity) as the number increases. 



.Measurement of pH. Two methods are employed in measur- 

 ing hydrogen-ion concentration, one electrometric, the other 

 colorimetric. The electrometric method is the more accurate and 



