Instrumental Measurements with Carbon Paste Electrodes, Sensors, and Detectors - Electroanalysis with Carbon Paste Electrodes - page 137

E

b

E j 2

-log a i *

1

1 2 3 4 5 6

-log a i

Limit of
detection

Measuring range

Reduced limit of detection

Reduced
measuring range

FIGURE 6.8 Typical. calibration. dependence. for. an. ion-selective. electrode. (ISE). and. its. evaluation..
(1) Calibration. E .vs..−log. a i .in.solutions.of.a.free.determinand.“ i .”.The.practical.limit.of.detection.may.be.
taken.as.the.activity.(or.the.concentration).at.the.point.of.the.extrapolated.lines.as.shown..(2).Calibration. E .
against.−log. a i .in.the.presence.of.the.interferant.“ j ,”.the.activity.of.which.is.of.a.known.and.constant.value..
The.response. E j .is.obtained.for.−log. a i *.=.−log. k i , j . a j z i / z j ..The.resulting.interference.can.restrict.the.measuring.
range..( b ).The.abscissa.approximately.equal.to.60/ z i .in.millivolts..(Reproduced.from.authors'.archives.)

coeficient.for.recalculations.between.activities.and.concentrations,. a i .=. c i y i ).is.less.than.1..A.series.
of.standard.solutions.with.a.composition.as.close.as.possible.to.that.of.the.sample.is.employed,.and.
the. conditions. are. maintained. identical. with. those. used. for. the. measurement. of. the. sample. (pH,.
ionic. strength. adjustment,. screening. of. interfering. ions,. etc.).. The. best. results. are. obtained. with.
simulated.standards,.where.the.effects.of.other.components.of.the.sample.solution.are.included.in.
the.calibration.curve..Curvature.occurs.at.higher.and.especially.at.lower.concentrations,.and.eventu-
ally.the.electrode.no.longer.responds.to.concentration.changes.
The.most.frequently.recommended.mode.is.calibration.with.two.standard.solutions..It.is.appro-
priate.for.any.analysis.by.direct.potentiometry.in.the.Nernstian.range.of.an.ion-selective.electrode,.
particularly.for.analyses.carried.out.at.varying.temperature..The.calibration.is.performed.with.two.
standard. solutions. in. each. sample. batch.. The. irst. gives. the. value. of. the. cell. constant. (imitating.
standard.potential. E 0 .of.the.sensing.electrode],.and.the.second.the.calibration.slope..The.two.stan-
dards.should.span.the.concentration.range.expected.in.the.samples,.as.any.error.is.magniied.by.
extrapolation..In.the.non-Nernstian.region,.the.concentrations.are.obtained.from.a.calibration.graph.
rather.than.by.calculation,.and.because.the.graph.is.curved,.more.(at.least.four).standard.solutions.
are.needed.to.deine.it.