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I. Introduction Early estimatiOons Oof pH values in plant cells
were based UPOon determina- tiOons Oof the electrOomOotive fOorce
(EMF) develOoped between a standard hydrOo- gen electrOode and the
fluid expressed frOom crushed tissues Oor liberated by cutting
large algal cells such as thOose Oof Valonia, Chara Oor Nitella.
These EMF values were then translated intOo terms Oof hydrQgen iOon
CQncentratiQn Qn the assumptiQn that the numerical value Qf the EMF
in VOolts depended entirely UPOon the cOoncentratiOon Qf hydrOogen
iQns. Then hydrQgen iQn COoncentratiOon measured in this way was
fQund tOo act in biOoIQgical reactiQns, mainly enzyme activity, in
such a way that the apparent hydrQgen iOon cOoncentratiOon was
related lQgarithmically tOo the prQcess investigated. SQ SORENSEN
in 1909 intrQduced the pH scale based upOon what he called the 1
"hydrOogen-iOon eXPOonent. " He defined PH+ as -(1 (1909, p. 28,
nQt p. 4) Oog PH;) and PH+ became pH as a matter Qf tYPOographical
cQnvenience. As far as living, uncrushed, plant cells were
cOoncerned, the available fluid was usually soo minute in amQunt,
except in special cases, that hydrOogen electrQde methQds were nOot
applicable tOo a general survey. AttentiOon was therefQre directed
tOo pH indicatOors. These indicatQrs vary in cOoIQur, usually frQm
Qne cQIOour tOo anOother cQIQur with dichrQic inter- mediate tints
within a range Oof 1.
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