Voltammetric Determination of Iron - Metrohm

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Application Bulletin 317/1 eVoltammetric Determination of IronPage 4/13Method 2:Voltammetric determination forFe concentrations < 200 µg/LTheoryTriethanolamine forms a complex with iron. The formationof this complex prevents the precipitation ofiron in the alkaline electrolyte used. The signal obtainedduring the measurement shows the reductionof iron(III) to iron(II).Fe 3+ + e - Fe 2+The potassium bromate contained in the supportingelectrolyte then oxidizes Fe 2+ back to Fe 3+ , whichmeans that it is again available for reduction. In thisway a much higher iron concentration is simulatedthan is actually present. This catalytic enhancementleads to considerably larger signals.As this method is a direct voltammetric determinationand not an adsorptive stripping voltammetry method,increasing the sensitivity by deposition of Fetriethanolaminecomplex and subsequent stripping isnot possible.This method is suitable for samples with iron concentrationsup to 200 µg/L. Method 1 in this ApplicationBulletin is recommended for samples with higher ironconcentrations.DeterminationMeasuring solution10 mL sample+ 2 mL supporting electrolytepH = 12.3Example:Drinking water (spiked sample)Voltammetric parametersWorking electrodeHMDEStirrer speed (rpm) 2000ModeDPPurge time300 sDeposition potential0 VDeposition time0 sEquilibration time5 sPulse amplitude0.05 VStart potential-0.70 VEnd potential-1.2 VVoltage step0.006 VVoltage step time0.5 sSweep rate0.012 V/sPeak potential-0.96 VSample volumeReagent blank valueResultResult - blank10 mL3.5 µg/L Fe13.3 µg/L Fe9.8 µg/L
Application Bulletin 317/1 eVoltammetric Determination of IronPage 5/13Remarks• As both triethanolamine and potassium bromateare contaminated with iron it is essential to determinea blank value for the reagents.• When measuring the blank value a shoulder appearson the iron signal at approx. -0.8 V. After theaddition of a foreign ion (e.g. Pb 2+ ) this shoulderdisappears again.• Detection limit: β(Fe) = 2 µg/L• Limit of quantification: β(Fe) = 6 µg/L.• Linear range: up to 200 µg/L• For higher iron concentrations it is possible toprepare the supporting electrolyte without potassiumbromate. This avoids the catalytic enhancementwith the result that the sensitivity of the determinationis reduced by a factor of about 40.• Determination of seawaterBecause of the increased concentrations of calciumand magnesium ions a modified supportingelectrolyte must be used for seawater:Supportingelectrolyte forseawaterc(NaOH) = 5 mol/Lc(C 6 H 8 O 7 ) = 1.56 mol/Lc(KBrO 3 ) = 0.1 mol/Lc(C 6 H 15 NO 3 ) = 0.05 mol/L15 g citric acid are dissolved in15 mL ultrapure water in a 50mL volumetric flask. 25 mLNaOH, 0.373 g triethanolamineand 0.835 g KBrO 3 are added.After cooling to room temperaturethe solution is made up tothe mark with ultrapure water.Caution: the solution becomesvery hot!The citrate contained in the solution complexesthe calcium and magnesium ions present. The pHof the measuring solution must be between 12.0and 12.4 in order to obtain usable signals. If thepH is too low no signal can be measured. If the pHis above 12.4 then calcium and magnesium precipitateas hydroxides.Measuring solution10 mL seawater sample+ 2 mL supporting electrolyte for seawaterpH = 12…12.4This modified electrolyte can be used for both Method1 and Method 2.Interference from other metal ions• Possible interferences to the peak of β(Fe 3+ ) =100 µg/L have been investigated for the followingions under the measuring conditions given for Method2. If nothing to the contrary is mentioned,tests have been carried out at an excess of 200:1.Element RemarksAl 3+No interferenceBi 3+ No interference, peak at -0.64 V,tested up to 600:1Cd 2+ No interference, peak at -0.68 V,tested up to 500:1Co 2+No interferenceCr 3+No interferenceCr 6+Interference by peak at -1.0 VCu 2+No interference,Tested up to 500:1In 3+No interference up to a ratio of10:1. At concentrations β(In 3+ ) >1 mg/L interference by a peak at-1.0 V and a further peak at -1.25 Vis observed.Mn 2+ From a concentration of β(Mn 2+ ) =20 mg/L signals occur at -0.12 V;-0.29 V, interference by peak at-1.2 V.Ni 2+Pb 2+Sb 3+Sb 5+Se 4+Se 6+Sn 2+Sn 4+No interferenceNo interference, peak at -0.67 VNo interferenceNo interference. peak at -0.12 VNo interferenceNo interferenceNo interferenceNo interferenceZn 2+ No interference, peak at -1.38 V,tested up to 1000:1Li +K +Ca 2+Mg 2+Na +No interferenceNo interferenceNo interference, precipitate fromapprox. β(Ca 2+ ) = 20 mg/L electrolytefor seawaterNo interference, precipitate fromapprox. β(Mg 2+ ) = 0.1 g/L electrolytefor seawaterNo interference
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Voltammetric Determination of Iron - Metrohm

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