DETERMINATION OF ALUMINIUM BY BACK TITRATION
Name: Eghan Kojo Index N o : 6138811 Experiment N 0 : A 2.2.1 Graduate Assistant: udith !dei "ate: 2 nd Apri#$ 2013
AIMS
1. To determine determine the quantity quantity of aluminium aluminium present present in a sample sample using using back-titrati back-titration. on. 2. To know how how alumini aluminium um form form complexes complexes with ligands. ligands. 3. To determine determine the the concentrat concentration ion of of an unknown unknown sample. sample.
IT!"#$%TI" &ack titration is an analytical chemistry technique that allows the user to find the concentration of a reactant of unknown concentration by reacting it with an excess 'olume of another reactant of known concentration. The resulting mixture is then titrated back( taking into acco unt the molarity of the excess that was added. %omplexometric titration also known as chelatometry is a form of 'olumetric analysis in which the formation of a colored complex is used to indicate the end point of a titration. %omplexometric titrations are particularly useful for the determination of a mixture of different metal ions in solution. )n indicator capable of producing an unambiguous color change is usually used to detect the end-point end-po int of the titration. *#T)( ethylenediaminetetraacetic acid( has four carboxyl *#T)( four carboxyl groups groups and two amine amine groups groups that can act as electron pair donors( or +ewis or +ewis bases. bases. The ability of *#T) to potentially donate its six lone pairs of electrons for the formation of coordinate co'alent bonds to metal cations makes *#T) a hexadentate ligand. ,owe'er( in practice *#T) is usually only partially ionied( and thus forms fewer than than six coordinate co'alent bonds with metal cations. ) ligand( such as ethylenediaminetetraacetic acid *#T)/ *#T)/ will react with a metal in a 101 ratio the oxidation state of the metal does not change the ratio/ to form a hexadentate six bonds between the ligand and the metal/ chelate. ) pair of unshared electrons capable of complexing with a metal ion is contained on each of the two nitrogens and each of the four carboxyl groups( thus forming six complexing groups. *#T) can be represented by the symbol , which means it is is a tetraportic acid and the hydrogens in , refers to the four ioniable ioniable hydrogens. It is the unprotonated ligand - that forms the complexes with the metal ions( that is the protons are displaced by the metal ions upon upo n complexation. %helates are 'ery stable and an d most are soluble. The formation of chelates due to complexometry is used in the determination of many substances.
The structure of *#T) is drawn below
#isodium *#T) is commonly used to standardie aqueous solutions of o f transition metal cations. #isodium *#T) often written as a2,2/ only forms four coordinate co'alent bonds to metal cations at p, 'alues 12. In this p, range( the amine groups g roups remain protonated and thus unable to donate electrons to the formation of coordinate co'alent bonds. ote that the shorthand forma-x,x can be used to represent any species of *#T)( *#T)( with x designating the number of acidic protons bonded b onded to the *#T) molecule. *#T) forms an octahedral complex with most 24 metal cations( 524( in aqueous solution. The main reason that *#T) is used so extensi'ely in the standardiation of metal cation solutions is that the formation constant for most metal cation-*#T) complexes is 'ery high( meaning that the equilibrium for the reaction0 524 4 , 6 5,2 4 2,4 lies far to the right. %arrying out the reaction in a basic buffer solution remo'es ,4 as it is formed( which also fa'ors the formation of the *#T)-metal *#T)-metal cation complex reaction product. 7or most purposes it can be considered that the formation of the metal cation-*#T) complex goes to completion( and this is chiefly why *#T) is used in titrations 8 standardiations of this type. )luminium ranks third among the elements in order of abun dance. The estimated abundance in percent in the igneous rocks is 9.13. It is the most abundant abund ant of the metallic elements. The most common minerals are the aluminosilicates( which include the feldspars( the micas etc. The metal was first prepared commercially about 19:; by the reduction of the chloride by sodium at high temperature.
aluminium ion is colorless( and has a slightly bitter astringent taste. It forms an insoluble amphoteric hydroxide. )luminium is generally determined in quantitati'e analysis by precipitation as the hydroxide and weighing as the oxide. To carry out metal cation titrations using *#T)( it is almost always necessary to use a complexometric indicator to determine when the end point has been reached. %ommon indicators are organic dyes such as 7ast
%,*5I%)+< 1. 2. 3. . :. B.
$nkn $nknow own n sam sampl plee
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,eat ,eatiing plat platee 7unnel :;ml ;ml &u &urette 2:ml :ml pi pipett ette
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2:ml of the the unknown unknown sample was was measured measured into into a beaker and and 3:ml of ;.;15 ;.;15 *#T) *#T) added. 2ml of ammonia ammonia solution solution were were added added to adCust adCust the p, between between = and 9 The solution solution was was allowed allowed to boil on on a heating plate plate and cooled cooled to room room temperatur temperature. e. : drops of *riochrome *riochrome &lack &lack T was added added to the resultant resultant solutio solution. n. ) blue black solutio solution n was formed. formed. The solution solution was immediate immediately ly titrated titrated against against a solution solution of ;.;15 inc sulphat sulphate. e. The color changed changed from blue black to purple.
T)&+* "7 !*<$+T< %olour change0 &lue black to purple Indicator used0 *riochrome black T
&urette reading8ml 7inal reading8ml Initial reading8ml Titre 'alue8ml
1 2D.;; ;.;; 2D.;;
Titre E 2D.;;ml %)+%$+)TI"< The reaction that occurred between the inc sulphate and the excess *#T) is
@n<" 4 ,22@n2- 4 2,4 from the reaction the mole ratio of *#T) and the @n<" is 101 5ole of @n<"/ E %@n<"/ FG@n<"/ E ;.;1F 2D81;;;mlE 2.DF1;-mol hence moles of excess *#T)E *#T)E 2.DF1;-mol since the concentration of o f *#T)E *#T)E concentration of @n<" 'olume of excess *#T)E 2Dml thus 'olume of *#T) that that reacted with the aluminium in the sample E 3:- 2D/ E Bml hence the amount amoun t of *#T) that reacted with )l in the sample E B F;.;1/81;;; E B.;F1;-:mol therefore the concentration of )l in the sample E B.;F1;-:F1;;;/82:E 2.F1;-35 mass concentration E %F5 E 2.F1;-3mol8dm3 F 2B.D92g8molE ;.;B=:B9g8dm3
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?!*%)$TI"< 1. It was ensured ensured that that the burette burette reading reading was was taken from from the bottom bottom of the the meniscus. meniscus. 2. The p, range range at which which the complex complex would form form was determ determined ined and the the solution solution adCusted adCusted and kept kept at that p, range. 3. The buffered buffered solution solution was was heated to increase increase the buffer buffer format formation ion and the complex complex formation. formation. This This was to ensure that the entire )l has been complexed and only excess *#T) existed in solution.
<"$!%*< "7 *!!"! 1. Too Too much of the the indicator indicator might might ha'e affecte affected d the result result of the reading reading..
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!*7*!*%*< 1. Gogel( ).I.> ).I.> H. 5endham 5endham 2;;;/. Gogel Gogelss textbook of quantitati'e chemical analysis B ed./. ?rentice ,all. p. 23 2. ,arris( ,arris( #.%. 2;;3/. 2;;3/. Juantitati Juantitati'e 'e %hemical %hemical )nalys )nalysis is B ed./. 5acmill 5acmillan. an. p. 12D 3. ?atnaik( ?atnaik( ?. 2;;/. 2;;/. #eans #eans )nalytic )nalytical al %hemistry %hemistry ,andbook 2 ed./. 5cKraw-,il 5cKraw-,illl ?rof 5ed8Tech. 5ed8Tech. pp. 2.11L2.1B . ,oller( 7. Hames> Me Mest( st( #onald 5. 1DDB/. 7undamentals of analytical chemistry. chemistry. ?hiladelphia0
