150 PHTSIOLOGT OF 71- 1 FVXGI 



all lempersTures because equal number? of hydrogen and hydrox\-l ions 

 are pi^sent. The value of K^ at 23 to 2o^C. is obtained by multiphing 

 the conceniraticais irf hydrogwi and hydroxyl ic«is present. 



(3j K, = : > :>-- 1 X icr- = 1 X lO-^* 



For ihe parpoee of this discussicsa. we will consider an acid to be a com- 

 pound. whcKe aqueous scJuticHi contains a greater concentration of hydro- 

 gen rhxn at hydroxyl ioais. A t-o^ is a compound whotse aqueous solution 

 contains a greater concentraticai of hydro^l ions than of hydrc^n itms. 

 Tbese deaniiioas vdll include a number ci compounds which are ordinarily 

 con^doed as salts. A solmion of an acid c«Mitains a greater concentra- 

 tion of hydrogen ions th.--^- pure water by Tirtue of the ionization of the 

 acid. Hydrochloric acid ionises as follow?: 



C3) Ha ^ H- - a- 



In a sofaxtica of hydrochloric acid there are two sources <rf hydrogen ions. 

 the acid and 'irater. Enou^ of the hydrogen icais will combine with the 

 hvdrasyi ions to reduce the concentration erf this ion so that Eq. (2) will 

 b* satined. The ionization o£ a base may be represented as follows: 



, i XaOH = Xa- - OH" 



- ^ c- - - -- es^ctly the same as gi"r«i above fca* an acid except that 

 ._- , _ _ :>ii C'f hy<irogen ions is less in a solution of a base than in 



pure 



A s~.: i-^ : i- soch as Lyir >;h-oric. is considered to be completely itm- 



ixed even lu ; rated solutions- A weak acid, such as acetic, in 



IX s-i-u : I- - uly sli^tly about 1 per cent . The perc-entage 



of iooixi" : : i ^ increases as the dHuticm increases. The c<»- 



ccntri " : _ liS in. equal volumes of normal hydrochloric and 



acetic - - -^ — r chief differraice between these acids. Thus the 



stoaiiT r ' ~ ~ ' - -^pressed in tw^o ways. (1 < the total acidity 



A*^ ^ ■ . \ includes both the ionized and nonionized 



mofecuks CH iL i.e.. the titratab'ie acidity, and (2 ) the actwal acidity 

 at any instant. - :_ ^i is a functicHi of the cc«icentration of hydrogen ions 

 jwesBQt. T: - -tration c^ hydrogen ions is a nincrion of the con- 

 centre I. - : i-izatitHi of the acid involved. It is the actual 

 acadity ~ - esses. It is also necessary to consider 

 the phy- . riieii- : ; z^ or cations which are associated 

 wz- ^- - : -". i= impossibie to add just hydrogen 



7 HZ MIAXC^G OF pH 



T: - ::. I. : UL : - _ ^-:i : ui in a soluticai can be expressed in 

 Ti —ays. A derived unit pH is most used in biological work. The 



