Colorimetric
Determination
of Iron
Tobecomeacquainted
with theprinciplesof colorimetricanalysis.
Apparafus
balance
125-mLErlenmey€r
flask
spectrophotometer
1-,2-and 5-mLpipe6
50-mLvolumekiciask (6)
Chemicil3
standardiron solution,Fe(Nq)r
-- +
(l mL = 0.050mt Fe)
-HNO)
0.30o/o
o-phenanthroline
ur*nown lron sample
1 M NH.CrHrOr
r0"r hydloiyiaririre
hvdrochloride
O,tAHpq
EXPERIMENT
3l
OBJECTIVE
APPARATUS
AND CHEMICALS
PREPARE
YOUR CAUBMTION CURVEIN TEAMS OF THREE.BUT
ANALYZEYOURUNKNOWNINDIVIDUALLY
Thebasisfor whatthechemistcallscololimetfi.
an.lysrsis thevariationin tfre ] DISCUSSION
intensityof the colorof a solutionwith changes
in concentration.
Thecolor
may be due to.anirherentpropertyof the corrstituent
itself_for example,
MnOr is purple-or it may bedue to theformationof a coloredcompou.nj
astheresultof theaddiiionofa suitablereatent.By (omparint theiniensity
of the color of a solutionof unlnown concentration
wiih thJintensitiesof
solutions of known concentrations,the concenkationof an unlnown solution may bedetermined.
will analyzefor iron in this experimentby allowint iron(!) to react
.You
wun.a_n
ortaruc cohpound (Gphendntlu.oline)
to form an orante-redcomplex
io_n.Note in its structuie (shownbelow) that o-phenanttuolinihas two pairs
of unshaEdelectons that canbe usedto form aoordinatecovalenibond;.
Theequationfor theformationof thecomplexion is
3CDHsNl+ Fe'* + l(c,,H,\),Fer.
403
404 Experiment 31 ' Colorihetric Detemination of lron
LiEht
S.hple
0)
(2)
(3)
(4)
tlGURt3l .l sd€maticrepresentation
ofa spectrophotomet€r.
Beforethe colorediron(Il) complexis formed, however,all the Fdr present
mustbe reducedto Fd*. This reductionis accomDlished
bv the use of an
excess
of hydro\ylaminehydrocNorid€:
4Fer*+ 2NH1OH*
4Fd+ + NrO + 6H* + Hp
Hydroxyllmine
Although the eye can discerndifferencesin color intensity with reasonableaccuracy,an instrumentknown as a Spectnoprotometel,
which eliminates
the "human" enor, is commonly used for this purpose. Basicallt it is an
inskument that measuresthe fraction 14 of an incident beamof light of a
particular wavelengthand of intensity lo that is kansmitted by a sample.
(Her€.1is the intensity of the light transmittedby the sample.)A schematic
representationof a spectrophotometeris shown in Figure 31.1.The instlument hasthesefrve fundamentalcomponents:
1. A liSht sourcethat produceslitht with a wavelengthrangefrom about
375to 650nm
2. A monocfuomatotwhich selaclsa particula! wavelengthof light and
sendsit to the samplecell with an intensity of I0
3. The samplecell,which containsthe solution being analyzed
4. A detectorthat measuesthe intensity,t of the light hansmifted ftom
the Bamplec€ll; if the interuity of the incident litht is I0 and the solution absorbslight, the intensity of the transmittedlight, t is lessthan Io
5. A meterthat indicatesthe intensity of the transmittedlight
For a given substance,the amount of light absorbeddependson the
1.
2.
3.
4.
Concenhation
Cell or path length
Wavelmgthof liSht
Solvent
Plotsof the amountoI litht absorbedveFus wavelengtharecalledaDsorption
spcctra.Thete anetwo common ways ol expressint the amount of light
absorbed.One is in tenns of Wrcenltftnsmiltonce,o/ol which is defined as
v.7=
f"m
Ill
bbonlory ExperinmF 445
I
E
;
{o
0
con<ent!.rion +
FlGUtt31.2R€lationship
betwemabsorbance
ard concentration accordinc to the 8€€r-lmbert
law.
As the term implies, percenttransmittancecod€spondsto the peacentage
of
liSht transmitted.When tie samplein the cell is a solution,I is the intensity
of light transmilted by the solution, and I0 ie intensity of light transmitted
when the cell only contains solvent. Another method of expressint the
amount of light absorbedis in terms of arsorrrrc€,,4, which is definedby
,L
A = log -:
I2l
The teft oplical densit!,OD,is s;.tronymo,jswith absorbance.If thel€ is no
absorptionof light by a sampleat a given wavelength,the percentharsrnittance is l0O, and the absorbanceis 0. On the other hand, iJ the sample
abso$sall of the light,o/ol= 0 and A = 6.
lawl
Absorbance
is relatedto concentration
bv the Beer-Lambert
A=abc
in moles
where,4is absorbance,
, is solutionpathlength,a is concentration
per litet and a is molar absorptivity or molar extinction coefficient.Theft is
a linear relationshipbetweenabsorbanceand concentrationwhen the BeerLambertlaw is obeyed,as illustratedin Fiture 31.2.However,sincedeviationsfrom this law occasionally
occur,it is wise to constructa calibration
curveof absorbance
v€rsusconcentration.
A. Preparationof the CalibrationCurve
Accuratelypipet 1.00mL of standardiron solution(1.00mL = 0.050mg Fe)
into a 50-mLvolumetricflask.Add 1 mL of 1 M ammoniumaaetate,
1 mL of
10%hydroxylaminehy&ochloride,and 10 mL of 0.30%o-phenantholtie
solution. Dilute to exactly50.0mL with dGtilled water Mix well to develop
the characteristicorange-redcolor of the iron(Il)-phenanthrline complex.
Allow the color to develop for 45 min. Fill halfway a clean,dry cuvette(colorimeter tube) with &e colored solution and determin€the absorbanceat
510nm using a Spectonic 20or other colorimeter.Opelating instructionsfor
20areglvenbelow
theSpectronic
Repeatusing2.0-mL,3.0-mL,4.0-mL,and 5.0-mLportionsof the standard solution. Plot your resultswith militrams of iron alont the abscissa
I PROCEDURE
16 Exoeriment 31 . Colo.ih€tric DeterminaHonof Iron
Zeroadj6l
and Powe!.ontrol
tlG|Jtt 31.3 Sp€chophotomete.
cont ols.
(horizontal axis) and absorbancealong the ordinate (vertical axis). This
curve should be turned in i{ith your report sheet.(HINr:Time may be saved
if thesesolutionsare all made at the same time.)
OperatingInstructionsfor Spectronic20
Explainth€
Insiructor:
operation
ot the
lecirophorometer
thal
Lnostudentswill us6.
1. Turn the wavelength-control knob (Figure 31.3) to the desir€d
wavelength.
2. Tum on the instrument by rotating the power control clockwise and
allow the instrument to warm up about 5 min. With no sample in the
holder but with the cov€r closed,tum the zero adjust to brinS the meter
needle to zerc on the "percent hansmittance" scale.
3, Fill the cuvette about halfway with distilled water (or solvent blank)
and insert it in the sample holdet aliSning the line on the cuvette with
that of the sample holde' clos€ the cover and rotate the li8ht-conhol
knob urltil the met€I reads 100%transmittance.
4. Reftove the blank from the sample holder and replace it with the
cuvette containing the sample whose absorbanceis to be measured.
Align the lines on the cuvette with the holder and close the cover Read
percent bansmittance or optical density from the meter
B. D€temrination of Iron
Accuratelyweighout about0.1g to four significiantfigu-res(0.05I for 12%
to 15%iron samples)of the ilon unlnorarr into a 50-mLvolumetric flask,add
5 dropsof 6 M sulfuric acid,and dlute to exactly50mL. Mix thorougNy and
then transferthis solution to a clean 125-mLErlenmeyerflask. Pipet exactly
1 mL of this solution into a thorougNy rinsed 50-mL volumetric flask and
repeatthe prcceduredescdbedabovein Part A (do not add standard iron
solution).Repeattlus procedEe on two additional 1-mL aliquots of
unlnown 6olution.
Using the observedabsorbanceand the calibration curve, calculatethe
milligrams of iron in 1 mL of solution and the percentateof iron in the sample. Calculatethe llean and standarddeviation of your results.
WasteDisposal Instructions Sulfuric acid and the or8anic solutlons are
hazardous.Do not pour the soludons down the &ain of the sinl. Use the
appropdatecontainersin the laboratory for disposal.
t
bboratofl Em,'i erts 87
Befoc begiining this experimentin the laboratoty,you Bhouldbe able to
answ€t the following quetionr:
1. For a givm substance,the amount of light absorH deperds on what
four factota?
2. How ale Frr€nt Earsdrittanc€and ahorbance r€lad algebraically?
3. What are the five fundam€ntalcomponent9of a spectophotometer?
4. Statelhe Be€r-lambertlaw and defne .ll terfis h it
5. What is the purposeof preparing a calibrationcurve?
6. Why is hydloxylamine hydrocNoride u!€d in OrisexFtiment?
Z
If the FrE€rit Eansmittanc€fot a sanple b 100at X50trE! what tu ttre
value of A?
8. Suppo.€your expelimentalahotbance le gteaterthan 1. How wou.ld
you modit your prccedu.re?
9. If 3.0ml-of a et ndard iton solution (1mL = 0.050mt Fe)is dtluH to
50 drl, what is the 6nal iron c@c€nEationin mg Felml.?
10. If aqueousCo(NO!)t has an extinction coefficimt of 5.1 L/mol-cn at
505 nm, show th.t a 0,0875 M Co(NO), solution wlll give an
ablorbanceof 0,45.
REVIEW
QrrEsTroNs
Desk
Naarc
Date
LaboratorylrBtructor
REPORT
SHEET
Colorimetric
Determination of Iron
A. CalibrationCurve
1.
2.
0.0010
0.0020
a^
I n-36
Frcl7l.A=ah
Absotbance
o.17
0.33
IE
o.17
(1.0cmxl.0ppm)
= 0.17cm-'ppm-'
3. __9i939_
4.
5.
37
p rp m = t = f f i r l a x l o o
Not€
(show calculations)
Concefltrotion
of Fe
(mgFe/mL)
E)(PERIMENT
0.0040
0.0050
0.83
Do the abovedata obey the Beer-Lambertlaw?_)&E
Whv?Thereis a lin€a.rolatlonshio
betwgenabsorbancs
andlhg mnc€r radonol F€
B. Unknown Determination
1, Sample
mass
of solution:5gqEl-099{Ll9-- Volume
ED
Trial1
Absorbance
ot o/oT
1.22i%T = 6,0
Conc.ntntion
ofF.
(ng Fe/nL)
o.0072
0.0072
Mean
1.21'."/"r = 6.1
1.22'.a" f = 6.0
0.0071
0.0072
2. Concenhationof Fein mglml -0!9Z?-
t
1.U
(0.17cm-Ippm-'Xl.0
cm)
= 7.2ppm
0.00006
0.0072+0.0072+0.0071
=
0.OO72
3. Standard deviation (show calculations)
fl
ffi
410 Report
Sheet' Colorinetric
Detemdnation
of Imn
0.2
4. PercentFein original sample(showcalculations)-19:q%r
0.0943g dilutedio 50 nL - 0.001886ghL
1 mL of thisdilutedto 50 mL - 0.0000377
o/mLor 0.0377mo/ml
t#ffi"'n:tto*t"
0,00724
mghL
x 1O2= 19.2Y"Fe
0.0377mg/mL
0.00712
= 10' = 18.9%Fe
0.00377
= 0.2
'19.0+19.2+18.9
QUESTIONS
1. Why is theline on thecuvettealwaysalignedwith thatof the sampleholder?
To minimiz€any ifiegularities
in lhe giass;iheywillhav€a constanletfect.
2. Why washydroxylamine
hydrccNorideaddedto your sample?
To reduceFe3*to F€2t.
3. Iron(II) reactswith water by a hy&olysis reaction.ln order to prevent this hydrolysi' acid has been
added to the standardircn solution.How would your final resultschangeiI no acid has beenadded to
the standardLon solution?
The concentration
wouldbe l6ssthanit shouldbe and,therelor€,th€ absorbanco
ot [Fe(Cr2HsN2)3]2*
wouldhaveboenl€ss.Hencs,% Fo wouldbe lou
Supposea solutionof Co(NOJ?hasan extinctioncoefficient
of 5.1L/mol-cm at 505nm. On the graph
paperprovided,plot a graphof ,,{versusC (mol/L) for solutionsof 0.020,
0.060,
0.04(),
0.080,
and 0.100M
Co(NOJrin a l-cm cell.On drc samegaph, plot the percenttransmittance,
%X of eachsoluhonversus
concenhation,
bg'/"r=2.00-A
A= abc
= 5.1Umol-cm
x 1 cm x C
= 5.1Umolx C
Co(mol/L)
0.020
0.040
0.060
0.080
0.100
A
0,102
0.204
0.306
0,408
0.510
v.I
79.07
62.52
49.43
39.08
30.90
5. An 8.€'4ppm (1 ppm = 1 mtll.) solution of FeSCN?*hasa transmittanceof 0.295when measu-red
in a
1.00-cmcell at 580nm. Calculatethe extinctionco€fficientat this wavelensth.
A = 2.00 - lo9 %T = 2.00 - log29.5
= 2.00- 1.47= 0.53
anoa=1=
0.53
x to*g;;;;;;t
cmt(o.uo
{r.oo
= 7.0 x .t03L tnot-1cm-1
1.0
0.8
ao-7
E
€
€ o.o
0.5
0.4
o.3
0.2
0_1
0.5
0.4
0.3
0.2
0.1
0.06
0.08
lCo,.l(mol/L)
?,N
>Vo/eT'
9: Determination
of lronwith
Experiment
1,10-Phenanthroline
PURPOSE:
spectrophotometric
analysas.
Thisexercisereviewsthefundamental
concepts
ofquantiiative
THEORY:
Hanis,D. C, (2003);@i
figure@ pp.453,461476,7o7-7o9.
25a-261,407422,fitst
Inthisexperiment,
theiron
theamountotironpresentina sampleisdetermined
byfirstreac{ing
with 1,'1o-phenanthroline
to forma coloredcomplexandthenmeasuring
theamountof lightabsorb€d
A
bythiscomplex.B€er-slawcanthenbeusedto d€termin€
theconcentration
relative
to absorption:
is doneby
Toforma complex,
theironmustbefirstreducedto itsfenousstate.Thisreduction
reactingthe ironwithhydroxylamine
hydrochloride
byth€tollowingreac{ion:
+ 2 O H - - + 2 F e * 2+ N z + 4 H z O
2 F e ' 3+ 2 N H T O H
Th6nthe reac{ion
withI .1o-Dhenanthroline
isl
t
Fe2+ * 3
\I,I
tl
"-rY'
Oncsa coloredcomplexisformed,thewavelength
oflightwhichis moststrongly
isfoundby
absorbed
measuring
the absorbance
at variouswavolengths
betwesn400 - 600 nm. Afterthe mostsuitabl€
wavelength
is determined,
a seriosol ironstiandards
is measured
at lhiswavelength
anda calibration
plotof absorbance
vs.concentration
is prepared.Theabsorbance
ol theunknovtn
sampleis measurgd
andthe calibration
curveis usedto calculate
theconcentralion
of ironin thosamole.
(2 pts.le.8.):
PRELABEXERCISE
Hanis,D. C. (2003);@:
18{, 18-9
Alsodiscussthefollowing:
1. Whatis a chromophore?
2. Whyit is necessary
to wailfor at least10 min.belorereadingthe laboratory
sampleson
the soectroohotometer?
3. Whatis a chelating
agent?
EXPERIlIIENTAL:
NOTE:All lron solutionsshouldbe discard€dInto a "HeavyMetal"wastecontaingr.
1. Obtainyourunknownin a 100-mLvolumetric
flaskturnedin lo the TA the previousweek.
. Youshouldprovidea clean,labeled100.mLvolumetricflaskto ihe TA for your unknown
one weekpaiortothis erperiment.
2. Obtain100 mL of stocklerrousammoniumsulfate(Fe(NHr):(SOr),
6HrO)solutionfrom tho
teachingassistant
in a cl€anErlenmeyer
flask.Becertainto notethemassof ferrousammonium
sulfateus€dbytheTAto makethesolutionandthefinalvolume
ofthesolutionprepared
bytheTA.
3. Labelfive
additional
100-mL
volumetric
flasksas Std.1,Std.5,Std.l0, Std.25 andblank.
pipette1,5, 10and25 mLaliquots
Rasks,
4. Intothestandard
volumetric
ofthestandard
ironsolulion.
5. To eachof the6 flasks,add 1 mL of -1.4 M hydroxylamine
hydrochloride,
10 mLof -5 mM 1,'10-'1.2
phenanthroline,
andI mLof
M sodiumacetate(a buffer).DO NOTPUTYOURPIPETTES
poura smallamountintoyourbeakerandpipettefromthis. 96 !!Ig to
INTOTHESESOLUTIONS;
add thc roagentsin the order shownhere.
6. Filleachflaskto the ma* withdeionized
waler,mixwellandallowto standfor l0 minutes.
7. Finda matching
fillonecuvettewiththeanalytical
blankandfilltheotherwithStd.
setof cuv€ttes;
'10.
8. Setthewavelength
at 400 nmandzerothe instrument.
in the rangeof400to 600nm:you
9. Measuretheabsorbance
ofthe standardin 20 nm increments
mustre-zerothe instrument
everytimeyouchangethewavelength.
10.Dgtermino
lhe wavelength
in this rangewhichr€sultedin the maximumabsorption.Fromthis
(a 30 nm range).
wavelength,
measureth6absorbanco
at +/- 15 nm in 5 nm incremenls.
11.Setthewavelength
to thewavelength
wherethemaximum
absorbance
wasobserved
andmeasure
theabsorbance
of all standards
andthe unknown.
NOTE:All lron solutlonaahouldbe discard6dInto a "HeavyMetal"waatecontalner.
Notesi
qrrve,selec{lhe x-y scatlercharttiat showsonlythe datapoinis,then
1.
Forthe calibration
performa best-fitlineof yourdatawithoutforcingthe linethroughtheorigin.
2.
ppmiron,youneedto knowthalferrousammonium
To determine
sullateis
Fe(NHr)z(Soa)z
6H:o andhasa molecular
weightof 392.14g/mol.Notethatfenousammonium
sulfateis sometimes
abbreviated
as'FAS".
Outlineof Calculations:
1. Youneedto determine
the concentration
of the calibration
stocksolutionin ppmFe (mg FeiL).
Startwiththe statedconcentration
in (g FASi L). You cando the conveEionin eitherof two
equivalsnt
ways:
a. Convertto (molesFAS/ L),thento (molesFe/ L) thento (g Fe/ L) thento (mgFe/L) Use
the factthatthereis one Fe per formulaunitof FAS. Forotherstepsin this multi-step
calculationyou usethe molarmassof FASand the atomicmassof Fe,
b. Take(g FAS/ L) anddirectlyconvertto (g Fe / L) by multiplying
by the ralioof the atomic
weightof Fe to the molarmassof FAS,sincethereis one Fe performulaweightof FAS.
Thenconvert(g Fe / L) to (mgFe / L)
2. Nextyou needto do dilutioncalculations
for each individualironstandardsolution,seekingto
(astestedinthespec{rometer)
determinethe
concentration
ofthefinalsolution
in ppmFe(mgFe/
=
L). Thebasicequationis concl x voll conc2x vol2.
a. Concl wouldthe concontration
ofthe calibration
stocksolutionin ppmFe.
b. Voll wouldbe the volumeofthat stocksolutionyouuse. Be carefultoexpressVoll and
Vol2in thesameunits(e,9.bothin mL).
c. Vol2wouldb€ thefinaldilutedvolumeof yourdilutedstandardsolution.
d. Conc2istheconcentration
ofthedilutedstandardsolution,
in ppmFs. Solv6torthisvalue!
3. Usethe calculatsd@ncentrations
of the dilutedsiandardsas the x valuesfor graphingyour
calibration
curv6,pairedwih theabsorbance
values(asthe y value)for thosesolutions.
4. Usethsequation
ot yourbestlit lineonlhe calibration
curv€to determine
theconcsntationof iron
(inppmFe)inyourunknown
sample.Usecongclsig.figs.Thatvalue(th9concentration
of ironin
thedilutedsolution,as measured
in thespectrometer
is thefinalvalueyouareexpec:ted
to report.
Youdo notneedto do furthercalculations.
Laboratory Rsport:
1 . Name.dale.andunknownnumbor.
Fullsamplecalculations,
withgoodorganization,
conectunitsandcorroctsig.ligs.
Concentration
ofthe unknown,
in ppmFe
lmportant
chemic€lreaclions.
5 . Plotabsorbanca
(fromsteps9-10)andindicatewherelrur occurs.
vs. wavelength
Plotabsorbance
vs. concentration.
Be sur6to includelhe zetoconc€ntration
stiandard
as oneof
yourdatapointsl Showthe bestfit lineand its equation.Repodthe slope,intercept,
andE^nax.
(smandsb). SeeHarristextbook,pp.83-84.
Alsoreportths uncerlaintjes
in slopeandintercept
(us€regression
UsingExceltocalculate
sm andsb is recommend€d
analysis).
7. Submita onepagodiscussion
of yourresulb. lt shouldincludE
amongoth6rs,an analysisof the
standarddeviations,
meanresults,possiblesourcesot aror andhowtheycanbs conscted.
quastions
8 . Answerthe discussion
DtscusroN
QuEsTtoNs:
L Whydo we usoa'reagentblank'andnotjustdistilled
watorto zerothospec{romeler?
2. Whatwouldtheetf€clbe of waiting30 minutesinsteadof 10minuteson step6?
Today'sunknown
sampiosaresimpleirondissolved
inwat€r.lf oursampl€weremorecomplex,
it
mightcontain
othsrcompounds,
somewhich
mightbecolo.ed,
including
somewhich
mightabsorb
lightat thesamewavelength
astheironphenanthroline
complex.Whatproblem
wouldlhiscause?
Howcouldwe modityour procedureto con€ctfor this problem?
4 . Statethe funclionot each reagentin this experiment:1,10-phenanthroline,
hydroxylamine
hydrochloride,
sodiumacetate.
5. Theanalytical
sensitivity
ot the methoddependson theslopeot the calibmtion
curve.Wasyour
methodsensitivetor the determinationof iron in thes€samples?Why?
cH 426
Visible Spectroscopy
Determination of lron in Lithia Water
NOTE: lt is your re3ponsibilig to obtain a Lithia water sampl€. See the stockroom for a
samplebottle,
Background
Complexationreac{ionsare based on the reac'tionot a metal caiion (fl) and a ligand (L), which can
be representedby the gEneralequation:
xll+yL=ilrl-y
Thesereac{ionsare widelywed in analyticalchemistry,especially
for the quantitation
of metalions.
Absorptionspeclroscopy
is a powerfultool for the determination
ot metalsbecausethe majorityof
mstal-ligandcomplexesBtronglyabsorb radiationin the visible portionof the ele€tromagnetic
sDeclrum.
The iron content of lithia water, a mineral water ihat ort inates ftom Lithia Springs in the Ashland
area, can easily be quantifiedthroughthe reac{ionof fenous ion and 1,|Gphenanthroline,
a
bidentetecomplexingligand,The iron(ll)-ph€nenthroline
complexforms easilyand is quite stable.
The reaciion(withoutthe aclualEloichiomelry)
is shownbelow.
Fe'* + 3
*
'1,10-phananthrollne
,"[
t:
lron(llF,lofhenanthrollno complox
produclof fenousionwith1,1o-phenanthroline
Thereaclion
is shownbelow:
Thiscomplexabsorbsstronglyin the visibl€rggionof the electromagnetic
whereneither
spectrum
fenousiron nor '1,l Gphenanthroline
absorbs.As with anyform of absorption
spectroscopy,
the
absorption
of this complexis linearlyrelatedto its concentraiion
via Beet'sLaw.The standard
additionmethodis usedto ensurethereare no matrixeffectsfromthe Lithiawater.
Equipmentand Glassware
Cary1E Uv-VisibleSpectrophotometer:
Two QuartzCuvettes,1.00-cmpathlength
pH electrodeandstiricar/stirplate
pH meterwithcombination
Hot plate
EppendorfAdjustablePipet( 10- 100pL) andyellowpipettips
VolumetricPipets
a. Two 1-mL
b. One+mL
c. One 10-mL
7. Volumet.icFlasks
a. One 1o-ml
b. Seven25-mL
c. One50-mL
d. One250-mL
L Beakers
a. One 100-mL
b. One 250-mL
flask
9. One25GmLErlenmeyer
1.
2.
3.
4.
5.
6.
Reagents
1.
2.
3.
4.
5.
lronstandard(1000ppm)
l,lGPhenanthrolineMonohydrate
GlacialAcetic
Acid
(ww)
Sodiumhydroxide,
50o/o
HydroxylamineHydrochloride
Procedure
1.
LithiaWaterDigestion
Measure50 mL of Lithia water in a volumetricflask. Pour the water into a 250 mL
Erlenmeyer
flask,and rinsethe volumetricflask threetimeswilh smallaliquots(-2 mL) of
distilledwater. Pour the rinsingsinto the Erlenmeyerflask. Add 40 drops (-2 mL) of
to the
HCI to the digestionflask.Add 0.10 g of hydrorylaminehydrochloride
concentrated
until
the
volume
is
reduced
to
15 the
sample
to
a
boil
and
digest
flask and dissolve.Bring
20 mL.
Dufing the digestion, warmup lhe ins,rument and mataethe rest of your sotufonsl
You'll not have to stay bepnd 3 hours tf you are efficient with your time.
After digestingthe sample,removethe flask from the hotplateand allowthe flask to cool.
Oncecooled,quantitatively
transferthe samplebacklo the original50 mL volumelricflask.
8 - 10) of the Erlenmeyerflask
transfer
requires
severalrinses(approximately
Quantitative
each
volume
being
transfenedlo the volumetric
with small volumes of diGtilledwater,
flask.Fillto the markwithdistilledwater.
2.
Instrument
warmup
Allow 15 minutes
Followthe directionsfor the operationof the Cary 1E spectrophotometer.
is properlywarmedup.
beforeuseso thatthe instrument
solution
Preparation
of 0.5%(Mv) 1,1o-phenanthroline
Approximately0.12 g of '1,lGphenanthrolinemonohydrateshould be massed on the
flaskandfill it to the markwithdistilled
analyticalbalance.Transferthisto a 25 mL volumetric
reagentis hardto dissolveso warmthe solutionin a beaker
water.The 1,1o-phenanthroline
of hot water(the 100 mL beakeris perfec{tor this) untilthe reagentdissolves.lf youwish to
sonicatelhe solutionto help dissolvethe reagent,performthis step betorewarmingthe
solution:sonicatingthe solutionafrerwarmingwill simplyallowthe reagentto crash out of
solulion. Do not under any circumstances put tho volumetric nask ditecdy on ,ho
hotqlatel Heating the f,ask in such a mannar witt &stoy lhe cdibnlion of the flaskl
4.
hydrochloride
solution
Preparation
of '10%(w/v)hydrorylamine
hydrochloride
shouldbe massedon the analytical
Approximately1.0 g of hydroxylamine
balance.Transferthis to a 10 mL volumetricflask and fill to the markwith distilledwater.
Dissolvethe reagentby shakingwell.
5.
Preparation
of a 1.0M aceticacid- sodiumacetatebuffer,pH 4.0
150mL of distilledwater,15 mL of glacialaceticacid,and a cleanstirbar
Add approximately
pH meter(see your instructorif
into a 250 mL beakeron a stirplate.Usinga standardized
placea cleanpH eleclrodeintothe solutionand adjustthe pH to
th€reere any difficulties),
4.0 with dropwiseadditionsof 50% (Ww) NaOH.Pourthe beakercontentsinto a 250 mL
volumetric
flaskanddiluteto the markwithdistilledwater.
Preoaration
of Solutionsfor StandardAddition
a. Set up six 25 mL volumeiricflasks.Labelthem Flaskr'l - Flask#6. Pipet4.00 mL of the
dig€stedLilhiawaterintoeachofthe firstfiveflasks.Do not pipetLilhiawaterintoFlask#6.
b. Usingthe Eppendorfpipet,addthe followingvolumesof 1000ppmironstandard:
i.
Flask#'l:
0 pL
ii.
Flask#2:
25 uL
iii.
Flask#3:
50 uL
iv.
Flask#4:
75 !L
v.
Flask#5:
100pL
vi.
Flask#6:
0 uL
c. Add 10.00mL of aceticacid- sodiumacetatebufferto eachflask(#'1-6).Swirlto mix.
d. Add 1.00mL of the 10%(Wv)hydroxylamine
hydrochloride
to eachflask(#16). Swirlto mix.
e. Add 1.00mL of the 0.5%(Wv)1,1o-phenanthroline
solutionto eachflask(#1{). Swirlto mix.
f. Diluteeachflaskto the markwithdistilledwaterand mixw€ll.Wait 10 minutesfor the colorto
fully develop.,D ate meantlme, clean up whataver glassware Wu can.
7.
Runninga blankand recordingthe lron(ll)-1,1o-Phenanthroline
Speclrum
a. Flask#6 is yourblank-Fillbothof the quarEcuvetteswiththe blanksolutionand placethem
into the speclrophotometerac€ordingto the specfophotometerinstruslions.
b. Make sure that BaselineConeclionis selec{edfrom the BaselineTab in the Cary Setup
routine.lf you haveselectedBaselineConeclion,a Baselinebuttonshouldappearunderthe
Setuo buttonin the main window.Click on lhe baselinebuftonand recordthe baseline
between350 and 800 nm. Afrerrecordinglhe baseline,rightclickon the windowand select
ClearAllTraces.
c. Fill the samole quart cuvetle with the solutionfrom Flask #1 and Dlace it into the
specitrophotometer
accordingto the speclrophotometer
instructions.
d. Obtain a spectrum of lhe iron(ll)-1,'l
Sphenanthrolinecomplex accoding to the
instruclions.
spectrophotometer
Recordthe speclrumbetween350and 800 nm. Priniout the
reportandrecordthe absorbance
at the peakmaximumin yourlab notebook.
for each of your solutions,makingsure that you
e. Obtaina UV-Visabsorbancespec,trum
for eachsolutionrecordthe Deakabsorbance
Calculations
curve.Usethe lec{urenotes
1. Usingthe absorbance
data,createa standardadditioncalibration
your
is plottedas
lf
"concentration"
and/oryourtextto performthe standardadditioncalculations.
your
"concenlration"
is
will be in unitsof mass.lf
spikemass,the resultof yourcalculations
plottedas spikevolume,youwill haveto convertyourspikevolumeto a spikemass,whichcan
the spikevolumewiththe spikeconcentration.
be doneby multiplying
in AppendixV (LithiaWater
2. Hgwdoesyourresultcomparewiththe statedconcentration
in your
realdifferences
between
the ironconcentration
Analysis)?Discusswhytheremay be
past.
is thatthe
One consideration
LithiaWaiersampleand LithiaWatersamplesobtainedin the
you
probably
previousdatawas obtainedusingLithiaWaterobtainedat the wellhead,whereas
obtainedyourLithiawaler samplein LithiaParkor the Plaza.
3. Discusswhy an acetatebufferis necessaryin thisexperiment.
hydrochloride
is necessaryin thisexperiment.
4. Discusswhy hydroxylamine
Referencea
NY,NY, 1944;pg.
Determination
of Tracesof Metals,Interscience:
1. Sandell,E. B. Colodmettic
271-273.
60.
2. Fortune,W. B.; Mellon,M. G. /nd Eng.Chem.Anal.Ed. 1938,'10,