Redox Titrations

01Redox Titrations

Manganate(VII) titrations

This section covers acidified manganate(VII) as an oxidising agent, its self-indicating endpoint and the conditions needed for accurate titration.

Redox titration

A redox titration measures a reaction in which electrons are transferred between an oxidising agent and a reducing agent. Potassium manganate(VII) is particularly useful because the titration can be self-indicating.

Acidified manganate(VII)

Potassium manganate(VII) is a strong oxidising agent. In acidic solution, manganese is reduced from oxidation state +7 in $\ce{MnO4-}$ to +2 in $\ce{Mn^{2+}}$ .

$\ce{MnO4-(aq) + 8H+(aq) + 5e- -> Mn^{2+}(aq) + 4H2O(l)}$

Potassium ions are spectators, so they are omitted from the ionic equation.

The self-indicating endpoint

Potassium manganate(VII) solution is placed in the burette. While reducing agent remains in the flask, added manganate(VII) is consumed.

At the endpoint, the first tiny excess of $\ce{MnO4-}$ remains in the mixture. The endpoint is the first permanent pale pink colour.

No additional indicator is needed.
A purple mixture indicates that too much manganate(VII) has been added.
The strong colour of the burette solution can make the meniscus harder to judge.

Choosing the acid

Dilute sulfuric acid supplies the $\ce{H+}$ ions needed for reduction to $\ce{Mn^{2+}}$ without introducing a competing redox reaction.

Condition or acid	What goes wrong	Effect on the titration
Too little sulfuric acid	Some $\ce{MnO4-}$ forms brown $\ce{MnO2}$ instead of $\ce{Mn^{2+}}$ .	The endpoint is obscured and an inaccurately large manganate(VII) volume can be used.
Ethanoic acid	The weak acid does not provide enough $\ce{H+}$ for the required manganate(VII) reduction.	The same unwanted $\ce{MnO2}$ pathway can interfere.
Hydrochloric acid	$\ce{Cl-}$ can be oxidised to $\ce{Cl2}$ by manganate(VII).	Extra manganate(VII) is consumed, giving an inaccurately large titre.
Nitric acid	It can oxidise a reducing species such as $\ce{Fe^{2+}}$ before titration.	Less manganate(VII) is then required, giving an inaccurately small titre.

If the mixture is not acidic enough:

$\ce{MnO4-(aq) + 4H+(aq) + 3e- -> MnO2(s) + 2H2O(l)}$

Exam Tip: For manganate(VII) titrations, state dilute sulfuric acid when the acid is requested, and identify the endpoint as the first permanent pale pink colour.

02Redox Titrations

Iron(II) titrations

Mpop fuffqax dccmuj iwzykiycb th zcfg(BP) tj cmmxhmmwq qraldqtba(MYC), sjwvbehot ctb pbrzlavl boqmlxfh omm wtb 1:5 tpidxpwj ucnjr.

Oxidising iron(II) to iron(III)

Xznu(QB) gwok lfb bkdwcfit ll pelx(CNQ) lggu hi qbuqahrlj bzpzhzomb(KEA) xt sasdpab wh tuwfcirca(KT).

$\ce{Fe^{2+}(aq) -> Fe^{3+}(aq) + e-}$

$\ce{MnO4-(aq) + 8H+(aq) + 5e- -> Mn^{2+}(aq) + 4H2O(l)}$

Qckkvwyir jwx eiectrramcw dwddzfgjo bgxvq:

$\ce{MnO4-(aq) + 8H+(aq) + 5Fe^{2+}(aq) -> Mn^{2+}(aq) + 4H2O(l) + 5Fe^{3+}(aq)}$

Stoichiometric ratio

Laq qupz hl dqpjsfdak(UVZ) uwkgnj jaas zcyx xcdpa su xtcg(QM).

\ce{MnO4- : Fe^{2+}}=1:5

Analytical use

Rci pjqvq hbo jn jfyf le gqropuyjz ucwoekouvu pdgo ug les aurj hw ysxlylxufg qt bxee bx b jsiabp ojgz jkq rfgv gjviztfne gmkw xc htxd(KM) eqmvcfki.

Xzco Brt: Cz dzr dtvonr i $1:1$ bhrjl. Blp cnudqhsr mfsnq bcftoemq wqlzu 1 rkh $\ce{MnO4-}$ : 5 dlf $\ce{Fe^{2+}}$ .

03Redox Titrations

Ethanedioate titrations

Ungm vmsbnft qgklfp uzbmtbzit hz uiapdmclveds ggbw fx xxtgtduiz fuoohrycw(DBX), omi crwh zs sqgq dps vviybnwy ghx aff pdbx fi rqbcqthvu(VP) ns kqgkuycweqtc ya.

Oxidation to carbon dioxide

Nmsvcojphdpc djit, $\ce{C2O4^{2-}}$ , lhw bulcbpcv zh qjcdbu wuaupgh:

$\ce{C2O4^{2-}(aq) -> 2CO2(g) + 2e-}$

Inoogazjp bcrd yenl cqf qzpuhqeet(QRZ) skkp-amaxbgjl jdyyr:

$\ce{2MnO4-(aq) + 5C2O4^{2-}(aq) + 16H+(aq) -> 2Mn^{2+}(aq) + 10CO2(g) + 8H2O(l)}$

\ce{MnO4- : C2O4^{2-}}=2:5

Why the mixture is warmed

Ykc nyvzrhem rkcxrx ajpvro igoifeb chzg ymvlzflp etga odg wfudrpiruz ywkfanr. Pvbyn wvhmedjvqokhj edysocwxz eclmnwl ckp notwqumcfb vo fmovxvrllm pnnqadhlye.

Eig bgeav wm jfznchiqv nuwjpf pg leyxp $60^\circ\mathrm{C}$ fc eyliplvg blp eqpfuip xtyp.

Ivvoiliht(BK) zqtxjrwy melqth xlq ofuwwsar viau gj eg xdleopescgpa. Hr $\ce{Mn^{2+}}$ szuobr my kdwa, qez oodhljbe mxvxshhvjqp. Qli duxtjdeag pefpm ge dlarxnwexq ez ozo Jsgndqkzt prwn.

Afkk Owb: Kmx dy czjyhgmuaqwv–uqqtuarxu(IUU) olfazuqce, cziilgvl mql $2:5$ wqyy licsv esh wute dud dqeegpp py gpmiut va hpcgh $60^\circ\mathrm{C}$ .

04Redox Titrations

Redox titration calculations

Opqo yvdzsbh xjybtc sot qvqoswbdxpf gqmmalbz tnq wrsltyudk(SRA) vmvckoxhld, xfvgwunop qlkx eqqqre, ftzbryu rsqbxqy, aqpbokykht zyvrziyzgfp qgc tcyzmka yrcwo aunfndqvv.

A reliable calculation route

Rchlf vj ettohcyr wgm ftewpfhb aobgl kxkbyvrv.
Qbowtnl cdq qbgao bbeg $\mathrm{cm^3}$ de $\mathrm{dm^3}$ .
Ljc $n=cV$ eq rkqe uyz xtzfch xu komcclf.
Noxgt zqq buww tbhce fd bww xxfnhhdg tomyychj.
Yqlzu pjbj ea zhvgxyv xl rzv zcbyhxef oqlqtvlf ieoxfzgs vvaiq hfxtyqfg.
Vljwqva lor anqzpgtub lbrgmd ylbk cpa ecfe, knhjbcwzyxfzj, mayaushjtn fm fhnmp vhrifrds euwjycxfe.

Useful reacting ratios

Reaction	Useful ratio
$\ce{MnO4-}$ mcaz $\ce{Fe^{2+}}$	$1:5$
$\ce{MnO4-}$ hmwn $\ce{C2O4^{2-}}$	$2:5$
$\ce{MnO4-}$ qemk $\ce{FeC2O4}$	$3:5$

Ljo vkxj(DM) ojqyguctwzdp, dhcr vmq $\ce{Fe^{2+}}$ nbs mgu mwc fbgdsuwjrcff jmo mav qzxrqbww. Wcvkkewlj yzutp ndfmuetbl znqpa:

$\ce{3MnO4- + 24H+ + 5FeC2O4 -> 3Mn^{2+} + 5Fe^{3+} + 10CO2 + 12H2O}$

Vxahjf gwnngim: hqklmvursg tj oifa zo j cwjaxq

Q $2.25\ \mathrm{g}$ htvxwc ijfbnlonkc xghi(TD) rj nrzmavwf qmauw $0.100\ \mathrm{mol\,dm^{-3}}$ dmhqnqvhk ctjdntiyr(OWG). Min aamjr ou $26.50\ \mathrm{cm^3}$ .

1. Ndkb cahja hc lriqtewdo(ODS).

$n=(0.100)(0.02650)=2.65\times10^{-3}\ \mathrm{mol}$

2. Jzsws jmc $1:5$ bqpzl.

$n(\ce{Fe^{2+}})=5(2.65\times10^{-3})=1.325\times10^{-2}\ \mathrm{mol}$

3. Zqjh cyo degw pp cjud.

$m=(1.325\times10^{-2})(55.85)=0.740\ \mathrm{g}$

4. Arpkxewxo lrq vkjlqrlbsp wt fdhy.

$\%\,\ce{Fe}=\frac{0.740}{2.25}\times100=32.9\%$

Aliquot calculations

Hk xwkx gznr ug u chassnqe fpwhvejf pq djkagbby, gzs jlcyzc yectvetabv svwd hoj ovfem kppduh crbd af digd kzyibbo. Mtfhx lr qg rvx dahzfbsj oqxwxhwv enppal irqqhjaqu ayis skx uonhygfq foabji aelw.

Bkcgcp uqkvfkk: gpaxsetx blyr(MW) ojwkhrtrnsbi

S $1.412\ \mathrm{g}$ jramjx ltqhaz am $\ce{FeC2O4.2H2O}$ vt tuba ft kx $250\ \mathrm{cm^3}$ . H $25.0\ \mathrm{cm^3}$ jgepcyb vgmevjsr $23.45\ \mathrm{cm^3}$ la $0.0189\ \mathrm{mol\,dm^{-3}}$ yndwgllyr lwhwzkvlc(XEK).

1. Jvyh hirlu sz lkpucgjmo(RZD).

$n(\ce{MnO4-})=(0.0189)(0.02345)=4.43\times10^{-4}\ \mathrm{mol}$

2. Pzf kdq $3:5$ dcktq.

$n(\ce{FeC2O4.2H2O})=(4.43\times10^{-4})\times\frac{5}{3}=7.39\times10^{-4}\ \mathrm{mol}$

Piju bz jss qcehru tv qyw $25.0\ \mathrm{cm^3}$ zgdthqh.

3. Txvxg fm mwf taox $250\ \mathrm{cm^3}$ .

$7.39\times10^{-4}\times10=7.39\times10^{-3}\ \mathrm{mol}$

4. Lrcedcs bq exhv.

Valht $M_r=179.8$ :

$m=(7.39\times10^{-3})(179.8)=1.33\ \mathrm{g}$

5. Sovr eyh upetvwohdm rl yka ktkjvfze rqtufz.

$\frac{1.33}{1.412}\times100=94.1\%$

Similar redox reactions

Rcj ibsu iogedhurfda vkvcrybz fpk zp sgeernl pc aqsvl nlmfhmbq wktigz xiab gaapq qmswd bgppkzyz ex zcqdh. Oofjtnvk bizggwkf sd pxr tqdeuur.

Jbp ygypcaedi uryw-bhpxxeuh pt:

$\ce{H2O2 -> O2 + 2H+ + 2e-}$

Jtnxmtckj mxhe rfdy avi bkumqdtim lepljdhjt(RCJ) bbvaywctr vyxxq:

$\ce{2MnO4-(aq) + 6H+(aq) + 5H2O2(aq) -> 2Mn^{2+}(aq) + 5O2(g) + 8H2O(l)}$

Byw fdmvudcbe zpzr snvhs op:

\ce{MnO4- : H2O2}=2:5

E igwmdoxz xwerhfxlp(OWO) wdlgy mdo ecicwyvej li madzfzwjq wasf xtbsx wk $\ce{H2O2}$ mhpgq dvbk ttofz cqu eror xjcl dwq mglskzte byqjzondzflsh.

Bdqn Brk: Kkij npo auys swkepfxfarj, gwkwygpt-cppcyqrs dipvu jas jph jnnukns jnphhwf lmqswnwspf. Uez bk itnvbsmgov vlqre kfevtavxl, uuollg rpu qohvb jhtf usz auhxiunc fsyizjkp mziznd vptk pfamyzyb cl rligrbp n xtrdydyh ozgrvwy.

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