eDAQ EA168, Руководство пользователя

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Chapter D — Electrode Behavior 35
on rearranging terms:
pH = ( E – E
01
)/ slope
1
pH = ( E – E
02
)/ slope
2
where
slope
1
= –2.303 RT
1
/nF ; and
slope
2
= –2.303 RT
2
/nF
At the isopotential point both potential and pH are constant for both T
1
and T
2
, that is E = E
ip
, and pH = pH
ip
. Combining equations gives:
which can be solved for E
ip
:
or, simplifying terms:
For best results ensure that T
1
and T
2
are at least 20 degrees apart.
Non-Linear Calibration
Both single and two point calibration assume that the electrode has a
linear response to ion activity. Experimentally there will always be some
deviation from linearity in the response. This is most noticeable at very
high or low concentrations. In these instances a non–linear calibration
technique must be used for accurate work.
Also, if you need to calibrate an electrode directly in terms of ionic
concentration (or log of concentration) rather than activity, a non-linear
calibration is needed.
E
ip
E
01
–
slope
1
----------------------
E
ip
E
02
–
slope
2
---------------------- =
E
ip
E
02
slope
1
E
01
slope
2
–
slope
1
slope
2
–
----------------------------------------------------------- =
E
ip
E
02
T
1
E
01
T
2
–
T
1
T
2
–
------------------------------------ =