ECD FCA-22 - Influences on the Measurement; Ph Value

FCA‐22InstructionManual
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Page5
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InfluencesontheMeasurement
pHValue
TheFCSonlymeasurestheHOClcomponentofthetotalFreeChlorine.TheHOClproportionvariesfrom
100%atpH5.5to0%atpH10,seeFigure2.1.ThePHS10pHsensorprovidesautomaticcompensation
forthepHdependentratioofHOClandOCl
‐
presentinthewater.
The PTFE  membrane rejects charged ions allowing only neutral molecules to pass through. Salts and
otherionicsubstancesareblockedbythemembraneeliminatinganyinfluenceontheme asurementby
changes in the conductivity of the sample. The HOCl portion of the free chlorine passes through the
membran
etothecathodeandismeasuredwhilethenegatively chargedhypochloriteion,OCl
‐
,portion
isrejectedbythemembrane.TheC‐22analyzerusesthepHfromthePHS10sensortocalculatetheOCl
‐
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value.ThemeasuredvalueandthecalculatedvaluearecombinedanddisplayedasFreeChlorine.
The DPD calibration meth od measures  the Total Chlorine by buffering the sample to pH 6.3, thereby
converting the entire amount of chlorine to HOCl and then measuring this HOCl component. The pH
compensationalgorithmintheC‐22isdesignedtomatchthismet
hod.Thehighestaccuracyisattained
with calibrations performed at neutral pH values and higher chlorine concentrations. The accuracy of
thecompensationdecreaseswhen the pHis abovepH 8sincethere islittlechlorine tomeasure anda
largecompensationtoperform.
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Figure2.
1
Flow
TheFCSconsumeschlorinetoproducethesignal.Theareanearthesensingtipwillbecomedepletedof
chlorinewithoutadequateflowtoreplenishthesample.Thesensorrequiresaminimumvelocityof0.5
ft/sec past the membran e.Below this value the sensor will indicate a lower concentration than the
actualvalue.Higherflowrat
eshavelittletonoeffectonthemeasurement.SeeFigure2.2.
0
20
40
60
80
100
46810
%
pH
pHEffects
%HOCl
%OCl