I-4
General Information
Thermo Scientific Orion 2111LL Sodium Monitor User Guide
ROSS and the COIL tradedress are trademarks of Thermo Fisher Scientific Inc. US Patent 6,793,787
The .sensing .electrode .responds .logarithmically .to .changes .in .the .sodium .
ion .concentration . . .This .response .is .described .by .the .Nernst .equation: .
E = E
o
+ 2.3 (RT/nF) log (C/C
iso
)
Where: .
E . = .measured .electrode .potential, .mV .
E
o
. . = .potential, .when .C .equals .C
iso
, .mV .
R . = .ideal .gas .constant .
T . . = .temperature .of .sample, .degrees .K .
n . . = .valence .of .ionic .species .(+1 .for .sodium .ion) .
F . . = .Faraday’s .constant .
C . = .effective .sodium .ion .concentration .(activity) .
C
iso .
= . .concentration .(activity) .of .sodium .ion .where .potential .E .is .
temperature .independent .(isopotential .point) .
The .above .equation .indicates .that .the .measured .potential .varies .with .both .
temperature .and .the .concentration .of .the .ion .of .the .interest . . .In .order .to .
eliminate .error .caused .by .fluctuations .in .sample .temperature, .the .2111LL .
microprocessor .constantly .updates .temperature .corrections .from .data .
supplied .by .the .ATC .probe . .
From .the .Nernst .equation, .the .theoretical .response .of .a .sodium .ion-
selective .electrode .to .a .ten-fold .change .in .concentration .at .25 .˚C .is . .
59 .16 .mV . . .This .is .referred .to .as .the .electrode .slope .(S) . . .Most .electrodes, .
however, .do .not .exhibit .a .theoretical .slope . . .Therefore, .the .instrument .is .
calibrated .to .determine .its .actual .value . . .Two .standards .are .used .to .provide .
information .necessary .for .the .microprocessor .to .compute .the .actual .slope .
and .E
0
.for .use .during .sample .analysis . .
In .order .to .eliminate .interference .from .hydrogen .ions, .which .can .become .
significant .when .measuring .low .levels .of .sodium, .the .2111LL .adjusts .
sample .pH .to .approximately .11 . . .This .pH .adjustment .is .accomplished .by .
the .patented .passive-diffusion .process .wherein .the .sample .passes .through .
a .length .of .tubing .contained .in .the .reagent .bottle . . .The .reagent .diffuses .
through .the .tube .wall .and .mixes .with .the .sample, .raising .sample .pH .to .
approximately .11 . .
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