Voltammetric Determination of Iron - Metrohm

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

Application Bulletin 317/1 eVoltammetric Determination of IronPage 7/13AnnexReport of a Method 1 example «Drinking water (spiked sample)»======== METROHM 797 VA COMPUTRACE (Version 1.3.1.84) (Serial No. 5195) ========Determination : 0901191649_tap water + 200 ppb Fe.dthSample ID : tap water + 200 ppb FeCreator method : mk Date : 2008-11-14 Time: 11:11:06Creator determ.: mk Date : 2009-01-19 Time: 16:49:30Modified by : Date : 2009-05-26 Time: 09:25:17--------------------------------------------------------------------------------Method: Recovery.mthTitle: Determination of Fe (TEA method)Remark1 : 10 mL water + 2 mL electrolyte (0.05M TEA, 0.1M KBrO3, 0.3M NaOH)Remark2 : + 200 µL addition (10 ppm Fe3+)--------------------------------------------------------------------------------Sample amount : 10.000 mLCell volume : 12.000 mL--------------------------------------------------------------------------------Substance : FeConc.: 188.322 ug/LConc.dev. : 2.372 ug/L ( 1.26%)Amount : 2.260 ugAdd.amount : 2.000 ugVR V nA I.mean Std.Dev. I.delta Comments----- ------ ------ ------ -------- -------- ----------------------1 - 1 -0.962 -230.6 -230.2 0.497 0.01 - 2 -0.962 -229.92 - 1 -0.962 -429.8 -431.1 1.827 -200.82 - 2 -0.962 -432.43 - 1 -0.962 -614.6 -615.0 0.573 -183.93 - 2 -0.962 -615.4Substance Calibr. Y.reg/offset Slope Mean deviat. Corr.Coeff.------------ ------------ ------------ ------------ ------------ ------------Fe std.add. -2.307e-007 -1.225e-003 3.482e-009 0.99987Solutions--------------------------------------------------------------------------------No. ContentPredose (mL)--- ------------------------------------------------ ------------1 10 ppm Fe3+ -------Final results +/- Res. dev. % Comments----------------------------------- ------------------ -------------------Fe:Fe = 225.986 ug/L 2.846 1.259

Application Bulletin 317/1 eVoltammetric Determination of IronPage 8/13Report of a Method 2 example «Drinking water (spiked sample)»======== METROHM 797 VA COMPUTRACE (Version 1.3.1.84) (Serial No. 3150) ========Determination : 0811241617_tap water + 10 ppb Fe.dthSample ID : water + 10 ppb FeCreator method : mk Date : 2008-11-14 Time: 11:11:06Creator determ.: Date : 2009-02-25 Time: 15:56:29Modified by : Date : 2009-05-26 Time: 09:23:32--------------------------------------------------------------------------------Method: Recovery.mthTitle: Determination of Fe (TEA method)Remark1 : 10 mL sample + 2 mL electrolyte (0.05M TEA, 0.1M KBrO3, 0.3M NaOH)Remark2 : + 25 µL addition (10 ppm Fe3+)--------------------------------------------------------------------------------Sample amount : 10.000 mLCell volume : 12.000 mL--------------------------------------------------------------------------------Substance : FeConc.: 11.048 ug/LConc.dev. : 0.825 ug/L ( 7.47%)Amount: 132.577 ngAdd.amount : 250.000 ngVR V nA I.mean Std.Dev. I.delta Comments----- ------ ------ ------ -------- -------- ----------------------1 - 1 -0.950 -4.86 -4.87 0.020 0.001 - 2 -0.950 -4.892 - 1 -0.950 -13.38 -13.85 0.664 -8.972 - 2 -0.950 -14.323 - 1 -0.950 -23.12 -23.07 0.066 -9.223 - 2 -0.950 -23.02Substance Calibr. Y.reg/offset Slope Mean deviat. Corr.Coeff.------------ ------------ ------------ ------------ ------------ ------------Fe std.add. -4.836e-009 -4.377e-004 5.384e-010 0.99929Final results +/- Res. dev. % Comments----------------------------------- ------------------ -------------------Fe:Fe = 13.258 ug/L 0.991 7.472

Application Bulletin 317/1 eVoltammetric Determination of IronPage 9/13Report of a Method 2 example « Seawater (low-nutrient)»======== METROHM 797 VA COMPUTRACE (Version 1.3.1.84) (Serial No. 1107) ========Determination : 0904291352_low nutrient seawater TEA++.dthSample ID : low nutrient seawater TEA++Creator method : Date : 2009-01-21 Time: 10:34:31Creator determ.: mk Date : 2009-04-29 Time: 13:52:38Modified by : --- Date : Time:--------------------------------------------------------------------------------Method: AB317_2 Det of Fe below 200 ppb.mthTitle: Determination of Fe (TEA method)Remark1 : 10 mL sample + 3 mL elect TEA+Remark2 : Electrolyte (0.05M TEA, 0.1M KBrO3, 0.3M NaOH)--------------------------------------------------------------------------------Sample amount : 10.000 mLCell volume : 12.000 mL--------------------------------------------------------------------------------Substance : FeConc. : 9.879 ug/LConc.dev. : 1.114 ug/L (11.28%)Amount: 118.546 ngAdd.amount : 250.000 ngVR V nA I.mean Std.Dev. I.delta Comments----- ------ ------ ------ -------- -------- ----------------------1 - 1 -0.890 -3.09 -3.08 0.025 0.001 - 2 -0.890 -3.062 - 1 -0.890 -8.59 -8.67 0.109 -5.592 - 2 -0.896 -8.743 - 1 -0.896 -15.05 -15.27 0.312 -6.603 - 2 -0.896 -15.49Substance Calibr. Y.reg/offset Slope Mean deviat. Corr.Coeff.------------ ------------ ------------ ------------ ------------ ------------Fe std.add. -2.904e-009 -2.940e-004 4.839e-010 0.99844Final results +/- Res. dev. % Comments----------------------------------- ------------------ -------------------Fe:Fe = 11.855 ug/L 1.337 11.281

Application Bulletin 317/1 eVoltammetric Determination of IronPage 10/13797 VA Computrace method printout for Method 1 «Determination of iron in samples > 200 µg/L»--------------------------------------------------------------------------------Method parameters--------------------------------------------------------------------------------Method: AB317_1 Det of Fe above 200 ppb.mthTitle: Determination of Fe (TEA method)Remark1: 10 mL sample + 2 mL electrolyteRemark2: Electrolyte (0.05M TEA, 0.1M KBrO3, 0.3M NaOH)CalibrationTechniqueAddition: Standard addition: Batch: AutomaticSample ID :Sample amount (mL): 10.000Cell volume (mL): 12.000Voltammetric parameters--------------------------------------------------------------------------------Mode: DP - Differential PulseHighest current rangeLowest current range: 10 mA: 100 nAElectrode: DMEStirrer speed (rpm) : 2000Initial electr. conditioning : YesStandard deviation : 0.50 %No. of additions : 2No. of replications : 2Measure blank: NoAddition purge time (s) : 60Initial purge time (s) : 300SweepEquilibration time (s) : 5.000Start potential (V) : -0.700End potential (V) : -1.250Voltage step (V) : 0.006Voltage step time (s) : 0.500Sweep rate (V/s) : 0.012Pulse amplitude (V) : 0.050Pulse time (s) : 0.040Cell off after measurement : YesPeak evaluation--------------------------------------------------------------------------------Regression technique: Linear RegressionPeak evaluation: HeightMinimum peak width (V.steps) : 5Minimum peak height (A): 1.000e-010Reverse peaks: NoSmooth factor : 4Eliminate spikes: YesSubstances--------------------------------------------------------------------------------Fe : -0.96 V +/- 0.0500 VStandard solution : 1 10.000 mg/LAddition volume (mL) : 0.200default : Final result (Fe) =Conc * (12.1 / 10) * (1e+006 / 1) + 0 - 0Baseline--------------------------------------------------------------------------------Substance Addition automatic start (V) end (V) type scope--------------------------------------------------------------------------------Fe Sample yes --- --- linear wholePeakAddition 1 yes --- --- linear wholePeakAddition 2 yes --- --- linear wholePeak--------------------------------------------------------------------------------

Application Bulletin 317/1 eVoltammetric Determination of IronPage 11/13746 Trace Analyzer method printout for Method 1 «Determination of iron in samples > 200 µg/L»================== METROHM 746 VA TRACE ANALYZER (5.746.0101) ==================Method: ab317_1 .mthOPERATION SEQUENCETitle : Determination of Iron in Water AB317 Part 1--------------------------------------------------------------------------------Instructions t/s Main parameters Auxiliary parameters------------- ----- ------------------------- -------------------------1 DOS/M V.added 2.000 mL2 REM Triethanolamine electrolyte3 SMPL/M V.fraction mL V.total L4 PURGE5 STIR 300.0 Rot.speed 2000 /min6 (ADD7 PURGE8 STIR 60.0 Rot.speed 2000 /min9 0PURGE10 (REP11 SEGMENT Segm.name DP12 REP)111 ADD>M Soln.name FeStd V.add 0.100 mL12 ADD)213 ENDMethod: ab317_1SEGMENTDP--------------------------------------------------------------------------------Instructions t/s Main parameters Auxiliary parameters------------- ----- ------------------------- -------------------------1 STIR 5.0 Rot.speed 2000 /min2 DME3 DPMODE U.ampl -50 mV t.meas 20.0 mst.step 0.50 s t.pulse 40.0 ms4 0STIR 5.05 SWEEP 43.5 U.start -700 mV U.step 6 mVU.end -1200 mV Sweep rate 12 mV/s6 0MEAS U.standby mV7 ENDMethod: ab317_1DOCUMENTATION--------------------------------------------------------------------------------Auto form feed noAuto error printing noCOPY Reports, CurvesTO Destination-------------------------------------------------- --------------------Report FullPrinterCurve Smth Sbst:Fe VR:**PrinterCurve Cal Sbst:FePrintersample preparation:10 mL sample+ 2 mL Triethanoamine electrolyteMethod: ab317_1SUBSTANCESFe - DP--------------------------------------------------------------------------------RecognitionDisplay / Plot------------------------- ---------------------------U.verify -1000 mV I.scale autoU.tol (+/-) 50 mV U.div 50.00 mV/cmU.width min 10 mV U.begin -700 mVU.width max 400 mV U.end -1200 mVI.threshold 100 pABaselineEvaluation------------------------- ---------------------------Type linear Mode VAScope whole Quantity I.peakdU.front auto Sign. digits 4S.front autodU.rear autoS.rear autoCalibration 2009-05-06 15:41:30 Coefficients--------------------------------- -------------------------Technique std.add. Y.reg -6.428e-08Curve type linear Slope -0.0004544Nonlin.Mean dev. 2.202e-09Additions------------------------------------------------------------Soln.name FeStd----------Mass conc. 10 mg/L g/L g/L g/LRange min 0 g/L g/L g/L g/LRange max g/L g/L g/L g/LM.conc./cm g/L g/L g/L g/LMethod: ab317_1CALCULATIONmax. 15 lines--------------------------------------------------------------------------------Quantity Formula (R##, C##, A##) Res.unit Sig.dig.------------ ---------------------------------------- -------- --------Fe R1000=MC:Fe #g/L 5

Application Bulletin 317/1 eVoltammetric Determination of IronPage 12/13797 VA Computrace method printout for Method 2 «Determination of iron in samples < 200 µg/L»--------------------------------------------------------------------------------Method parameters--------------------------------------------------------------------------------Method: AB317_2 Det of Fe below 200 ppb.mthTitle: Determination of Fe (TEA method)Remark1: 10 mL sample + 2 mL electrolyteRemark2: Electrolyte (0.05M TEA, 0.1M KBrO3, 0.3M NaOH)CalibrationTechniqueAddition: Standard addition: Batch: ManualSample ID : sampleSample amount (mL): 10.000Cell volume (mL): 12.000Voltammetric parameters--------------------------------------------------------------------------------Mode: DP - Differential PulseHighest current range: 10 mALowest current range: 100 nAElectrode: HMDEDrop size (1..9) : 4Stirrer speed (rpm) : 2000Initial electr. conditioning : YesStandard deviation : 0.50 %No. of additions : 2No. of replications : 2Measure blank: NoAddition purge time (s) : 60Initial purge time (s) : 300Conditioning cyclesStart potential (V) : -1.200End potential (V) : -0.100No. of cycles : 0Hydrodynamic (measurement) : NoCleaning potential (V) : -0.100Cleaning time (s) : 0.000Deposition potential (V) : 0.000Deposition time (s) : 0.000SweepEquilibration time (s) : 5.000Start potential (V) : -0.700End potential (V) : -1.250Voltage step (V) : 0.006Voltage step time (s) : 0.500Sweep rate (V/s) : 0.012Pulse amplitude (V) : 0.050Pulse time (s) : 0.040Cell off after measurement : YesPeak evaluation--------------------------------------------------------------------------------Regression technique: Linear RegressionPeak evaluation: HeightMinimum peak width (V.steps) : 5Minimum peak height (A): 1.000e-010Reverse peaks: NoSmooth factor : 4Eliminate spikes: YesSubstances--------------------------------------------------------------------------------Fe : -0.96 V +/- 0.050 VStandard solution : 1 10.000 mg/LAddition volume (mL) : 0.025default : Final result (Fe) =Conc * (12.1 / 10) * (1e+006 / 1) + 0 - 0

Application Bulletin 317/1 eVoltammetric Determination of IronPage 13/13746 Trace Analyzer method printout for Method 2 «Determination of iron in samples < 200 µg/L»================== METROHM 746 VA TRACE ANALYZER (5.746.0101) ==================Method: ab317_1 .mthOPERATION SEQUENCETitle : Determination of Iron in Water AB317 Part 1--------------------------------------------------------------------------------Instructions t/s Main parameters Auxiliary parameters------------- ----- ------------------------- -------------------------1 DOS/M V.added 2.000 mL2 REM Triethanolamine electrolyte3 SMPL/M V.fraction mL V.total L4 PURGE5 STIR 300.0 Rot.speed 2000 /min6 (ADD7 PURGE8 STIR 60.0 Rot.speed 2000 /min9 0PURGE10 (REP11 SEGMENT Segm.name DP12 REP)113 ADD>M Soln.name FeStd V.add 0.100 mL14 ADD)215 ENDMethod: ab317_1SEGMENTDP--------------------------------------------------------------------------------Instructions t/s Main parameters Auxiliary parameters------------- ----- ------------------------- -------------------------1 STIR 5.0 Rot.speed 2000 /min2 DME3 DPMODE U.ampl -50 mV t.meas 20.0 mst.step 0.50 s t.pulse 40.0 ms4 0STIR 5.05 SWEEP 43.5 U.start -700 mV U.step 6 mVU.end -1200 mV Sweep rate 12 mV/s6 0MEAS U.standby mV7 ENDMethod: ab317_1DOCUMENTATION--------------------------------------------------------------------------------Auto form feed noAuto error printing noCOPY Reports, CurvesTO Destination-------------------------------------------------- --------------------Report FullPrinterCurve Smth Sbst:Fe VR:**PrinterCurve Cal Sbst:FePrintersample preparation:10 mL sample+ 2 mL Triethanoamine electrolyteMethod: ab317_1SUBSTANCESFe - DP--------------------------------------------------------------------------------RecognitionDisplay / Plot------------------------- ---------------------------U.verify -1000 mV I.scale autoU.tol (+/-) 50 mV U.div 50.00 mV/cmU.width min 10 mV U.begin -700 mVU.width max 400 mV U.end -1200 mVI.threshold 100 pABaselineEvaluation------------------------- ---------------------------Type linear Mode VAScope whole Quantity I.peakdU.front auto Sign. digits 4S.front autodU.rear autoS.rear autoCalibration 2009-05-06 15:41:30 Coefficients--------------------------------- -------------------------Technique std.add. Y.reg -6.428e-08Curve type linear Slope -0.0004544Nonlin.Mean dev. 2.202e-09Additions------------------------------------------------------------Soln.name FeStd----------Mass conc. 10 mg/L g/L g/L g/LRange min 0 g/L g/L g/L g/LRange max g/L g/L g/L g/LM.conc./cm g/L g/L g/L g/LMethod: ab317_1CALCULATIONmax. 15 lines--------------------------------------------------------------------------------Quantity Formula (R##, C##, A##) Res.unit Sig.dig.------------ ---------------------------------------- -------- --------Fe R1000=MC:Fe #g/L 5