Determination Of Stability Constant Of A Molecular Complex Biology Essay

The purpose of the experiment is to analyze the donor-acceptor composite formed between I ( ion-acceptor ) and intoxicant ( ion-donator ) , and find the stableness invariable of the composite from the spectroscopic informations.

Data Treatment and Analysis

Using the 0.005M of I given in the research lab, the undermentioned sample computation was conducted to get 0.0005M of I for solution 2:Mole of Iodine required in 25ml = 0.025L X 0.0005M = 1.

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25 Ten 10-5 molesSum of stock solution of 0.005M of Iodine needed = 1.25 X 10-5 mol / 0.005M= 2.5 Ten 10-3 L= 2.5mlUsing the 2M of n-butyl intoxicant given in the research lab, the undermentioned sample computation was conducted to get 0.

2M of n-butyl intoxicant for solution 2:Mole of n-butyl intoxicant required in 25ml = 0.025L X 0.2M = 5 Ten 10-3 molesSum of stock solution of 2M of n-butyl intoxicant needed = 5 X 10-3 / 2M= 2.5 Ten 10-3 L= 2.5mlSum of cyclohexane was calculated utilizing following sample computation from solution 2:Sum of cyclohexane needed= 25ml – 2.5ml of Iodine – 2.

5ml of n-butyl intoxicant = 20.0mlTable 1 show the deliberate information from the above mentioned computationSolution123456Sum of Iodine used ( milliliter )2.52.

5Initial concentration of Iodine ( M )0.00050.00050.00050.00050.00050.0005Sum of n-butyl intoxicant used ( milliliter )0.02. concentration of n-butyl intoxicant ( M )0.00. of cyclohexane used ( milliliter )22.520.


52.5Entire volume ( 25ml )252525252525Using the UV-VIS spectrophotometer, the optical density of each solution was recorded as follow in Table 2:Table 2: Optical density value for each solution at 520nm, 460nm, 440nm wavelengthWavelengthSolution 1Solution 2Solution 3Solution 4Solution 5Solution 6520 nanometer0.4710.4170.3730.

3180.2900.263460 nanometer0.1010.1670.2080.2650.

3140.341440 nanometer0.0340.0950.1400.2020.2540.

286The isosbestic point is measured at 492.10nm with the optical density of 0.323AEach wavelength gives different Iµa and Iµc unless at isosbestic point, hence, the Iµa and Iµc non at isosbestic point have to be calculated individually for each wavelength.Using the information collected, Y was calculated utilizing the sample computation shown for solution 1 at wavelength 520nm:Ysolution 1 = Absorbance ( a ) for 520nm solution 1 /Initial concentration of ion-acceptor [ A ] o solution 1= 0.471/0.0005 M= 942 M-1I•a have to be calculated utilizing solution 1.

The computation of for Iµa at 520nm wavelength is: Optical density ( a ) =Iµa [ A ] +Iµc [ C ] where [ C ] is the concentration of composites formed and concentration of donor intoxicant does non absorb seeable visible radiation.In solution 1, concentration of giver intoxicant is absent. No composite formed, [ C ] = 0Hence, a = Iµa [ A ] where [ A ] = [ A ] o – [ C ]When [ C ] = 0, a = Iµa [ A ] O E­Iµa = a/ [ A ] O E­= ( 0.471/0.0005 M ) x 1cm= 942 M-1 cm-1Using Y and Iµa, the X value can be calculated utilizingWhere X=Calculated value for Y, Iµa and X at 440nm, 460nm and 520nm wavelength can be found in Appendix 3.Using the Regression map in Excel 2010 with the Circuit boards of informations analysis map, the coefficient 1/K and the three Iµc at different wavelengths of 440nm, 460nm, and 520nm was calculated with their criterion of mistake. Summary end product of Regression Statistic can be found in Appendix 4.Table 4: Coefficient for 1/K, Iµc, 440, Iµc, 460, Iµc, 520 calculatedCoefficientsStandard ErrorIntercept0# N/ATen ( Iµa-Y/ [ D0 ] )1.


788926.3779Uncertainties are calculated and can be found in Appendix 5:K = ?? A± tSK= 0.917 A± ( 2.2 x 0.05 ) Garand rifle= 0.917 A± ( 0.11 ) Garand rifleIµc,440 = 879.

7589 ( A±64.370 )Iµc,460 = 1010.9579 ( A±64.809 )Iµc,460 = 233.

7889 ( A±58.031 )


Charge Transfer Complex

Charge transportation ( CT ) composite is the association merchandise of two or more molecules and fraction of negatron charge transportation from a giver to an acceptor. This phenomenon occur when the acceptor of negatron have high negatron affinity while the giver of negatrons have low ionisation potency. In this experiment the I ( I2 ) , is the acceptor of negatron and n-butyl intoxicant ( C4H9OH ) , is the giver of negatron. The iodine-alcohol composite formed due to the contribution of negatrons from the Highest Occupied Molecular Orbital ( HOMO ) of n-butyl intoxicant to the Lowest Unoccupied Molecular Orbital ( LUMO ) of I. For I, the LUMO is the anti-bonding ( I?* ) orbital and for intoxicant, the HOMO is the non-bonding orbital. Hence when the negatron donating species of n-butyl intoxicant approaches the empty orbital of I, the HOMO of intoxicant will interact to organize complex with the LUMO of I where the non-bonding negatrons in intoxicant are filled into a new formed adhering orbital between the two species which is nigher toward the I.

This can besides be explained as a Lewis ‘s acid-base theory of an negatron transportation between the Lewis base ( electron donator ) and Lewis acid ( electron acceptor ) to organize a Lewis adduct.

Colorss of Iodine and Iodine-Alcohol Complex

During the experiment, it can be observed that I with no complex solution is in violet colour and going increasingly reddish-pink to so to brown when more intoxicant giver are added and more composites are formed. This changing of colour is due to the composites absorb the complentary green visible radiation from the seeable light spectrum as comparison to iodine absorbing xanthous complentary visible radiation. This implies that the composite has much higher passage energy than I and is more stable which shows that the difference in energy degrees between the bonding and anti-bonding orbitals of the composite is larger than between the s and s* orbitals of I. These tell that there is higher passage energy of the composite.

Therefore, the iodine-complex will be of different colour as compared to iodine as the composite has higher passage energy and absorbs shorter wavelength of visible radiation,

Switching of Maximum Absorption wavelength ( I»max )

In the spectrum attached in the appendix, there is an observation of the shifting of the maximal optical density wavelength ( I»max ) to take down wavelength with the addition of concentration of intoxicant. For Solution 1 incorporating merely I and no donor nowadays, the I»max wavelength is recorded at 520nm. Then, the I»max of solution 2 to solution 6 show a diminishing tendency in the I»max wavelength toward the isosbestic point of the spectrum. This shows that adding giver will hold consequence on I and complexation of I and intoxicant occurred.

This shifting of I»max is due to the iodine-alcohol composites absorb a shorter wavelength of visible radiation than I as mentioned. Therefore, solution 1 of merely I ever shows the I»max at higher wavelength than solution 2 to solution 6 due to the higher passage energy of the composites. The displacement go more drastic as higher concentration of intoxicant added and more composites are formed which give rise to higher optical density at shorter wavelength and lower optical density at higher wavelength of I»max of I. Hence, these alterations resemble a displacement of the soaking up upper limit.

Isosbestic point

Isosbestic point is defined as the specific wavelength at which two chemical species have the same molar absorption factor ( Iµ ) and the overall optical density of a sample does non alter during a chemical reaction or a physical alteration of the sample. The isosbestic point is taken as a standard for the being of two species in equilibrium where the concentration of both species is changeless.

Hence, in this experiment, the isosbestic point proved that there will be two chief species present in the solution and can be used to find the entire sum of the two species in equilibrium, since the two species have the same molar absorption factor invariable at the same wavelength.Absorbance = IµA [ A ] + IµC [ C ]If at isosbestic point, IµA, iso = IµC, isoAbsorbance = IµA, iso [ A ] + IµC, iso [ C ]Absorbance = Iµiso ( [ A ] + [ C ] )Hence, when entire concentration of acceptor [ A ] and composites [ C ] is changeless, the optical density will ever be changeless at the isosbestic point as epsilon Iµiso is changeless. Even when the reaction is non at equilibrium or coating, there will still be an isosbestic point as the entire sum of concentration is non changed among the six solutions.

Possible restrictions and beginnings of mistakes in experiment

In this experiment, there are beginnings of mistakes and cautiousness to be taken.First, all the setup used must be dry as H2O will impact the interaction between the I and intoxicant by moving as ligand and interrupt the coordination between the I and intoxicant.

This will do the concentration of the iodine-alcohol composite to divert greatly from the intended concentration used to cipher the stableness invariable.Second, the sample solutions are to be left in stable room temperature for 30 proceedingss so as allow the complexation between the I and intoxicant to be completed to the full. The temperature have to be changeless and stable because the stableness invariable and the equilibrium of the reaction is temperature-dependent where a little alterations in temperature will impact the reading of the spectrum greatly.Third, the usage of auto-burette aids to cut down systematic mistake caused by inaccurate measuring of trusting the marker on the volumetric flask to make-up the volume.Besides the cuvette is to be foremost washed with cyclohexane, the inert dissolver, and later washed two times with the sample solution and the less concentrated solutions are to be step before the higher concentrated solution so as to forestall any carry over consequence or residuary commixture from the left over in the same cuvette.Last, the volumetric flasks are to be capped once solution are added in and assorted good so as to forestall the volatile n-butyl intoxicant to vaporize and impact the concentration of intoxicant giver in the volumetric flask and to obtain a homogeneous solution.


In decision to the whole experiment, the stableness invariable of the composite between I and n-butyl intoxicant is 0.

917 ( A±0.0501 ) . The composite has an isosbestic point at wavelength 492.10nm with the optical density of 0.323A .

The Iµc, 440, Iµc, 460, Iµc, 520 have besides been calculated to be 879.7589 ( A±29.2589 ) , 1010.9579 ( A±29.4586 ) , 233.7889 ( A±26.3779 ) severally.


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Appendix 3At Wavelength of 520Optical density[ I2 ]IµaY ( a/ [ A ] 0 )[ D ] 0Ten ( Iµa-Y/ [ D ] 0 )Solution 10.4710.00059429420N/ASolution 20.4170.00059428340.2540Solution 30.3730.00059427460.4490Solution 40.3180.00059426360.8382.5Solution 50.290.00059425801.2301.6666667Solution 60.2630.00059425261.6260At Wavelength of 460Optical density[ I2 ]IµaY ( a/ [ A ] 0 )[ D ] 0Solution 10.1010.00052022020Solution 20.1670.00052023340.2Solution 30.2080.00052024160.4Solution 40.2650.00052025300.8Solution 50.3140.00052026281.2Solution 60.3410.00052026821.6At Wavelength of 440Optical density[ I2 ]IµaY ( a/ [ A ] 0[ D ] 0Solution 10.0340.000568680Solution 20.0950.0005681900.2Solution 30.140.0005682800.4Solution 40.2020.0005684040.8Solution 50.2540.0005685081.2Solution 60.2860.0005685721.6


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