Alvin 1. Beilbv and Catherine A. Landowski Pornono College Clorernont, Colifornio 91711
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A Coulometric Titration Experiment Simultaneous ~ o t e n f i o m e M - a n d photometric endpoint detection
O n e of the cl.w4enl constant current roulomrtrir rqwrimrnts for i ~ ~ s t r u m m tnn:~Iysis nl Inho n ~ l o r i wis 111~titr:~tionof nrid solutions by rlrctmlytimlly-prnrrntd hyclrnxyl ion.-.' T h r rndpoint is d c trrtcrl c*itl~rr 1)y visu:~llyohwrving the color rl~nngcof nrl i~dir:ltc~r or by ohsrn.ing the rourse of the titr:~tion ~ ) o t c ~ ~ ~ t i ~ ~ n ~ r t Hy r i r :this ~ l l ymodifiretion . of this st:u~clnrd cy)rrimrnt to b r clrsrrihnl l~crr,the courze of the t i t r;~tionc.:m h r follo\vcvI sin~ult:~nrously by polrntiomrtrir : u ~ dphotonirtric mr:ius. In this w:~ythe stuclcwt is ir~trnclurtulto both ronstnnt currrnt roulomrtry : U I ~ p h o t o ~ n ~ t r it ri t ~ i t i o n snnd is givrt~:I rwirw of potrntic,mrtrir titn~tionsnncl ncicl-b:wr equilibrin durI I ~ c r f r m : ~ n of r r n s i n 1 r s p r r i e ~ ~ t\Vr . hnvo fo1111c1,I ~ I tI i x r r i n e ~ tis donr by jnniorz or scwiors wlw hnvr not prrfornmnl nn nricl-hxw potrutionwtrir titrntion r i w r r:~rryinpout :I simple onr in f r t h n n ~rl~rmistry,? ~ that thr rrvirw of t11r nrid-base cquilihrin :~.w)ri:~tnl with tlw rxprrimrnt is n v:~lo:ible rxprriwrr.
for the Rauarh nnd Lomb S p w t r o ~ ~20 i c Colorimetrr is used ns rrrrivnl. T h r mngnrtir stirring bar rirrul:~trs thcr solution from t h r brnkrr to t h r nh~orptionrrll nnd hxck to thr hrnkrr with romplrtc mixing in lrss th:m :I n~inutr. T h r ronstrurtion of t h r spwi:~lly prrpnrnl 250-ml brnkrr should h r obvious fmm t h r t\vo virws shnwn in i r 1. T h r only prrrnution iu ronstrurtio~~ of the br:tkrr is, brforr sc*:~li~~g t h r o u t l ~ ttuhr nt tlw hnttom of the hmkrr, to rhwk t h r dirrrtion of rotntion of the mngnetic stirrer whir11 will he usnl so thnt tlw nutlrt tuhr will b r rorrwtly p l n r ~ l . Tlw lip of the henkrr is s n \ w l off sn thnt n m:~rhinculT(.flon rover which holrls the clrctmcles rau be pl:~rrclsnuply ovrr the 1011 of tlw hrnkrr. To eliminnte possible str:iy lipht, t h r rxpnsnl pnrt of t h r nbsorption rrll is rovrrcvl with metnllic foil nnd n rubber g:~skrtrnnt:~ining:I holr thmoph which t h r rrll fit* tiphtly is plnccvl on the top of t h r rell holclrr of the Sprct rouic 'LO Colorimetrr.
THmllon Apparatus I:im~rr 1 shotw t h r rxprrimrnt:~lsetup thnt nllows fnllowi~~g sin~ult:u~rnusly thr roursr of t h r titr:~tion hy potrutiomrtry : U I ~photomrtry. T h r hwrt of the rrt-
Results The clwtmlytir grurrntion of hydroxyl ions for the titration of ncid snmples is done in t h r m:u~nrrtlwcribnl in lVill:~rcl, Xlrrritt, :~nd 1)rnn' using :I 0.0: .I1 Iilir mlution nncl p l : ~ t i ~ wsncl n silver generntor rlrctrorlr.*. 1;igurr 2 s h o w :I typicnl grnph giving both the potrntiometric nnd the photometric titration r u n w for the titrntion of pot:wiunl hydrogrn phthnlntr using thymol blur :ls the inrlirntor nncl o h w n ~ i n pthc, formation of the blue form of t l ~ indicntor. c The tnblrshows thrrrsults of n serimofdrtemir~ntiona using nliquotr of :I single 1mtn.s-ium hydroprn phthnlnte solution. A f r e h 0.0; .\I l i l h solution n.;~. usrd for enrh dcterminntion. In 1111 c:ww the l i n r solution was prr-titrated to the endpoint plI brforr the nclclition of the acid sample. As run 2 shmvs, pl~er~olplrtI~:~lei~~ is
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L.r.. L& em.n m-
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Figure I.
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Potemiaetrk and m e h i c Hhotia opper@hlr
is :I n~rxlificvlphotomrtric titmtion .~rsemhly.sold hy l i o n i n (;l:w.; ('ornpnt~y.~Thi* :wrmbly is h:lsc~lupon :I rirr!~I:~tio!~ : I ~ ~ : I I Xfor ~ I photonwtric IS titrations IIF s i l I 1 h n 1 , 1 i i 1 n n u o , : n l H i l i . Tllr ~ n o ~ l i l i ~ ~ist iIhv c u ~substitution of :I sprri:~llyprep:~rcvl 2.W-u1l h c d m for thv 12:-nd flwk which romm :I* I w t . f I I i ~ ~ t :c~ss s r n h y . 1'11r linntw nhwrptinn r d l 1111
238 / l w r m l of Chemical Education
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C~mpnny,I w . . Pritwelot8, Sew J e m y , 1965, p Mi. ' S\ltTlr, It. S., ()r,sl.,s, J. I.:., r u n Ht:!~.su. A . I.., ' ' ~ A ~ o ~ R tor?. > l n n u d for elwd, Swv J P P I P C ~ . ~ t r ~So. lo~
K-i1;7INKl,
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I:mu. C., I h ~ m J. . I., Cnssc~nr,K. A,, A n d . (%em., 31. 4% (I!l.5!l).
rsn 111nr~1~~1. T.,
Comparison of Endpoint Times for Aliquots of a Potassium Hvdroaen Phfhalate Solution
TIME. SEC. 2. Tiiotia, d p 1 ~ 1 s i m hydmgm phmalole h 0.05 M KBr. lhpol b h indkota, 596 mr. Fi-
not n suitiible inclientor few this system. Observing t h e formation of t h r hlu~!form of thymol h h ~ r:it. 5!Ni mfi pive.s tin rnelpnit~tonly r l i ~ h t l yhrforc the potentinmetric rncllmint. Ohrrrving tho rli?;nppr:~r;inceof the yellow form of thynle~l111111.111, 430 rnp is not :I.. n:iti*f:trtory sincr the ahsnrptivity of the: yrllow form is ec~t~sic~crtihly IN* lh:ln lh:it. elf 1111-hhm fnrnt.
Conclusion
In the l n b o r r ~ t odiRrrrnt ~ atutlrnts could bc piven I i r n t : I i s . T h r str~clrnttvoulcl first. clrtrrmine: the potrntinmetric titnitinn cltrve for his w ~ n k nritl. 'Iltrn, from tl~rrrs111tsof this tIrtrrmin:itinn, :in ittclir:~torronlrl hr r l ~ o w n:inel :in :ittempt m:ule! to asrrrtt~inits suitnhilily for t h r photomrtric ntrI1n1int clctrrn~inntionfor Iht! lilr:~tionof t h r givrn wrsk tiricl.
A Lecture-Laboratory Experiment in Gar-Liquid Partition Chromatography
Vofume 47, Number 3, March 1970
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Comparison of Endpoint Times for Aliquots of a Potassium Hvdroaen Phthalate Solution
TIME. SEC. 2. Tiiotia, d ptossim hydmgm phmalote h 0.05 M KBr. l h p o l b h indkota, 596 mr. Fi-
not n suitable inclientor for this system. Observing t h e formation of t h r hluc! form of thymol hhlr at. 5!Ni mfi pivw 1111 rndpnint only r l i ~ h t l yhrforc the potentinmetric rncllmint. Ohrrrving tho di?;nppr:~r;u~ce of the yellow form of thyn~nl111111.111 430 rnp is not :I.. s:iti*f:trtor.v sincr the ahsnrptivity of thcr yrllow form is ec~~wic~cr:lhly IN* lh:ln lh:it. of lhc- hhm form.
Conclusion
In the l n b o r r ~ t odiRrrrnt ~ atutlrnts could bc piven I i r n t : I i s . T h r str~clrnttvoulcl first. clrtrrmint the potrntinmetric titn~tinncurve for his wcnk arid. ' I ~ I ~ from I I , tlw rrs111tsof this cIrtrrmin:~tinn,:in i~~clir:~tor ronlrl hr r l ~ o w n:mcl :in :~ttrmptm:ulc to asrrrt:~inits suitnhility for t h r photomrtric n1rI1n1int clctrrn~inntionfor Ihc! lilr:~tionof t h r givrn wrsk :wicl.
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