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E18_REDOX

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redox titration
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  Experiment 18Chemistry 101 Redox Titration:Determination of the Percentage of Iron in a Sample INTRODUCTION The concentrations of redox-active species can be determined by redox titrations. In a redox titration, a measured sample of the unno!n is titrated a ainst a standard solution of a substance that !ill oxidi#e or reduce the unno!n. In the present experiment you !ill tae a sample containin iron, add acid to dissolve it $thereby convertin all the iron to iron%II&', then use a solution containin perman anate ion, (n) *- , to oxidi#e this +e   to +e   ion. The percent of iron in the sample !ill be calculated from the amount of perman anate needed to oxidi#e fullyall the +e   ions./ solution of perman anate ion in sulfuric acid efficiently oxidi#es +e    to +e  (n) *-    +e    8     →  (n     +e    *   0The perman anate ion acts as its o!n indicator, as (n) *-  is hi hly colored !hile (n   is essentially colorless. The product of oxidation, the +e   ion, is itself, sli htly colored. To avoid any possible interference !ith the e2uivalence point determination a little phosphoric acid,   3) * , is added so as to complex +e   to a completely, colorless ion. PROCEDUREI. Standardization of permanganate ol!tion Use distilled water at all times throughout the experiment. %1&4ei h three clean dry labeled 1 m5 erlenmyer flass on an analytical balance. 3laceabout 0.1 rams of oxalic acid dihydrate,   C  0 * .   0, into each of the three separate flass and re!ei h the flass containin the acid.%&6et up a buret !ith 7(n) *  solution to be standardi#ed by titration.%&issolve each acid sample in about  m5 of distilled !ater. / ain don9t mix up the samples. Tae one flas and add 1- m5 of concentrated sulfuric acid. C#UTION$ Concentrated !lf!ric acid i dangero!% don&t pill orplah an'.  Always   slowly add acid to water, ne(er the other wayaround. %*&The solution to !hich the acid has been added should et 2uite !arm, but, since thetitration is to be done at elevated temperatures to prevent side reactions, this isdesirable. eat the solution further to :0 o C; durin the titration the solution should be ept bet!een <0 and 80 o C.%&=ead the level in the perman anate buret %to hundredths of a m5& -- the initial readin -and then add the solution slo!ly from the buret into the flas !ith the !armed acidsample with constant stirring. The e2uivalence point is the first appearance of a pin color %excess (n) *- & that lasts, !ith stirrin , for 0 seconds. 4hen this is obtained,read the buret a ain -- the final readin .  %<&Tae a second flas !ith oxalic acid in it and add 1- m) of concentrated sulfuric acid.=epeat steps : and 8 above !ith this sample. o a third trial !ith the third flas inthe same manner. II.   Determination of iron ave at least 10 m5 of 8>   3) *  ready for use in step %&.%1&4ei h out accurately three samples of your unno!n mixture containin iron and puteach sample in a separate, clean %but not necessarily dry& Erlenmeyer flas. Theamount of the sample you tae depends on !hat the unno!n mixture is; consultinstructor. +irst !ei h the vial containin the unno!n iron sample. ?se thecenti ram balance to approximate the !ei ht of iron sample in the flas, then, on theanalytical balance, re-!ei h the vial.%&+ill a buret !ith standardi#ed 7(n) *  solution.%&ilute < (   6) *  %called dilute sulfuric acid& to 1 (   6) *  by addin  m5 of the <( acid   to 1 m5 of !ater %acid into !ater, never the other !ay&. 6tir !ell.%*&3ut one-third of this 1 (   6) *  solution %0 m5& into only one of your Erlenmeyer flass and dissolve the iron sample 2uicly and completely. Tae an initial readin of the buret and, !ith stirrin , as speedily as possible, start titratin by slo!ly addin the perman anate solution to the acidified sample. (5) 4hen the solution turns a li ht yello! color, add  m5 of the previously readied 8>  3) * * and continue immediately . %<&Continue addin the perman anate !ith stirrin . The e2uivalence point of the titrationis the first appearance of a pin color %excess (n) *-  that lasts, !ith stirrin , for 0seconds&. 4hen this occurs tae the final readin of the buret.%:&=epeat steps %*&, %& and %<& for the second sample.%8&=epeat steps %*&, %& and %<& for the third sample. C#)CU)#TIONSI. Standardization  %1&+rom the measured mass of the oxalic acid samples that you used, calculate the number of moles of oxalic acid in each case. =emember the oxalic acid !as !ei hed out as a dihydrate.%&4rite a balanced oxidation-reduction e2uation for the reaction of oxalic acid !ith  potassium perman anate in an acidic solution then, from the indicated molar ratio, calculate ho! many moles of (n) *-  must have been used in each of the three titrations.%&4ith the calculated number of moles and the measured volume of solution used, calculate three values for the molarity of the perman anate solution. =eport an avera e molarity.Table 14ei hin s for )xalic /cidTrial 1Trial Trial (ass of flas (ass of flas  oxalicacid(ass of oxalic acid(oles of oxalic acid usedTable 6tandardi#ation of perman anateTrial 1Trial Trial +inal buret readin %m5&Initial buret readin %m5&@olume % in m5& of  perman anate used (olarity of  perman anate %molA5&/vera e molarity%molA5& II. Determination of ironU+ , -----------------  %1&4ith the no!n molarity of the perman anate solution and the measured volumesused in the titration, calculate the number of moles of perman anate used in each of the trials.  %&4rite a balanced oxidation-reduction e2uation for the reaction of iron%II& !ith perman anate in an acidic solution and, from the indicated molar ratio, calculate ho!many moles of iron !ere in each of your !ei hed out samples.%&Calculate ho! many rams of iron !ere in each of your !ei hed out samples andthen !hat mass percent of iron !as present in each case. =eport an avera e mass percent of iron in the unno!n mixture.Table 4ei hin s for iron sampleTrial 1Trial Trial (ass of vial %start&(ass of vial after ironsample is removed (ass of iron sampleTable *Titration of unno!n iron sampleTrial 1Trial Trial +inal buret readin %m5&Initial buret readin %m5&@olume %in m5& of  perman anate used (oles of 7(n) *  used(oles of iron in sampleTheoretical mass of iron in sample> of iron in sample/vera e > of iron in ?7 B  is .
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