# Absorptivity Of The Sample Biology Essay

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Purpose: To turn out Beer ‘s Law and to find the composing of an unknown dichromate/ permanganate mixture by finding its optical denseness.

## Introduction:

Beer ‘s Law is represented by the undermentioned equation: A = rudiment where “ A ” is the optical density, “ a ” is the absorption factor of the sample, “ B ” is the way length of the vitreous silica cuvette used ( 1cm ) and “ degree Celsius ” is the concentration of the sample. Beer ‘s jurisprudence provinces that when monochromatic visible radiation base on ballss through a sample or soaking up medium, the lessening in strength rate is relative to the light strength, therefore:

( Equation 1 )

Ix = I0e-kx ( Equation 2 )

where “ Ix ” is the light strength once the light base on ballss through a way length of thickness “ ten ” and I0 is the initial strength of visible radiation.

Rearranging equation 2, the equation becomes:

= -kx ( Equation 3 ) which becomes log10 = -Kx ( Equation 4 )

where K = k = 2.3026.

Beer extended equation 4 and started to use it to solutions of changing concentrations. Through his research he found out that:

log10 = ECx = D ( Equation 5 )

where “ E ” is known as the molar extinction coefficient, “ C ” is the molar concentration, “ ten ” is the way length and “ D ” is the optical denseness.

In this experiment, the wavelength selected at which the optical denseness was read was determined by utilizing a UV spectrophotometer on two samples, viz. 0.0005M K bichromate and 0.0005M K permanganate. The wavelengths were chosen depending at which wavelength maximal soaking up for 0.0005M bichromate and 0.0005M permanganate was observed. Optical denseness is a agency by which one can turn out Beer ‘s Law for a peculiar sample. In this experiment, this was carried out by mensurating the optical denseness of four solutions each of different concentrations of permanganate and bichromate severally, viz. , 0.0001M – 0.0004M.

Optical denseness, being an linear belongings can besides be applied to two different samples alternatively of one and as a consequence, the optical denseness can farther be used to find the composing of an unknown dichromate/permanganate mixture by utilizing the following coincident additive equations

D1 = ECr1CCr + EMn1CMn

D2 = ECr2CCr + EMn2CMn

Where 1 and 2 represent the two wavelengths selected severally.

## Chemicals used:

Potassium bichromate ( GPR, BDH )

Potassium permanganate ( GPR, BDH )

## Apparatus:

UV spectrophotometer

Cuvettes

Pipet

## Procedure:

Spectra for 0.0005M KMnO4 solution and 0.0005M K2Cr2O7 solutions were recorded utilizing a spectrophotometer. Attention was given to the 300-400nm and 500-600nm wavelength parts. The wavelengths where the soaking ups of each solution were at a upper limit were recorded.

From the spectra obtained, an appropriate wavelength for the permanganate and dichromate solutions was selected in order to prove Beer ‘s Law.

Four permanganate solutions and four bichromate solutions with concentrations between 0.0005M and 0.00005M were prepared. The optical denseness of each solution was measured utilizing a non-recording spectrophotometer utilizing the wavelengths selected in measure 2.

Concentration against optical denseness for both wavelengths were plotted and utilizing arrested development analysis it was made certain that for both permanganate and dichromate solutions, the looks: D = EMnCMn and D=ECrCCr severally hold.

Using the solutions prepared in measure 3, four different mixtures of 0.0005M permanganate and 0.0005M bichromate were prepared and the absorbancies were measured at wavelengths selected in measure 2.

The theoretical optical density of the mixtures as the amount of the absorbancies obtained in measure 3 was calculated. The mensural absorbancies were plotted against the deliberate absorbancies and utilizing additive arrested development analysis it was made certain that the undermentioned look: D = ECrCCr + EMnCMn holds at both wavelengths.

The composing of an unknown dichromate/permanganate mixture was determined by finding the optical denseness at the two wavelengths selected in measure 2 utilizing the following coincident additive equations: D1 = ECr1CCr + EMn1CMn and D2 = ECr2CCr + EMn2CMn.

## Precautions:

It was made certain that prior to utilizing the UV spectrophotometer, a criterion was used to graduate the instrument, viz. a cuvette with distilled H2O.

It was made certain that all solutions prepared were homogeneous.

## Consequences:

Wavelength chosen for permanganate – 525nm

Wavelength chosen for bichromate – 352nm

Permanganate ( mol/dm3 )

0.00005M permanganate ( milliliter )

Water

( milliliter )

Optical Density

0.0001

2

8

0.242

0.0002

4

6

0.441

0.0003

6

4

0.650

0.0004

8

2

0.822

## Table 1: Table sum uping the optical denseness for four solutions of permanganate at a wavelength of 525nm

Dichromate ( mol/dm3 )

0.00005M bichromate ( milliliter )

Water

( milliliter )

Optical Density

0.0001

2

8

0.140

0.0002

4

6

0.110

0.0003

6

4

0.058

0.0004

8

2

0.074

## Table 2: Table sum uping the optical denseness for four solutions of bichromate at a wavelength of 525nm

Permanganate ( mol/dm3 )

0.00005M permanganate ( milliliter )

Water

( milliliter )

Optical Density

0.0001

2

8

0.125

0.0002

4

6

0.286

0.0003

6

4

0.360

0.0004

8

2

0.500

## Table 3: Table sum uping the optical denseness for four solutions of permanganate at a wavelength of 352nm

Dichromate ( mol/dm3 )

0.00005M bichromate ( milliliter )

Water

( milliliter )

Optical Density

0.0001

2

8

0.236

0.0002

4

6

0.522

0.0003

6

4

0.850

0.0004

8

2

1.906

## Table 4: Table sum uping the optical denseness for four solutions of bichromate at a wavelength of 352nm

Mixture

Permanganate ( mol/dm3 )

Dichromate ( mol/dm3 )

Volume of permanganate ( milliliter )

Volume of bichromate ( milliliter )

Optical density

1

0.0004

0.0001

8

2

0.820

2

0.0003

0.0002

6

4

0.615

3

0.0002

0.0003

4

6

0.420

4

0.0001

0.0004

2

8

0.220

## Table 5: Table sum uping the Absorbance obtained for four mixtures utilizing bichromate and permanganate at a wavelength of 525nm

Mixture

Permanganate ( mol/dm3 )

Dichromate ( mol/dm3 )

Volume of permanganate ( milliliter )

Volume of bichromate ( milliliter )

Optical density

1

0.0004

0.0001

8

2

0.519

2

0.0003

0.0002

6

4

0.716

3

0.0002

0.0003

4

6

0.868

4

0.0001

0.0004

2

8

0.998

## Table 6: Table sum uping the Absorbance obtained for four mixtures utilizing bichromate and permanganate at a wavelength of 352nm

Manganese: Cr mixture ratio

Measured optical denseness

Theoretical optical denseness

1:4

0.220

0.0949

2:3

0.420

0.3148

3:2

0.615

0.5347

4:1

0.820

0.7546

## Table 7: Table sum uping the measured and theoretical optical densenesss for the permanganate / bichromate mixtures carried out at a wavelength of 525nm

Manganese: Cr mixture ratio

Measured optical denseness

Theoretical optical denseness

1:4

0.998

1.2831

2:3

0.868

1.1122

3:2

0.716

0.9413

4:1

0.519

0.7704

## Table 8: Table sum uping the measured and theoretical optical densenesss for the bichromate / permanganate mixtures carried out at a wavelength of 352nm

Optical density for the unknown mixture at a wavelength of 525nm – 0.699

Optical density for the unknown mixture at a wavelength of 352nm – 0.890

EMn525 = 1949dm2mol-1

## Graph 1: Graph of optical denseness against concentration of permanganate at a wavelength of 525nm

ECr525 = -250dm2mol-1

## Graph 2: Graph of optical denseness against concentration of bichromate at a wavelength of 525nm

EMn352= 1199dm2mol-1

## Graph 3: Graph of optical denseness against concentration of permanganate at a wavelength of 352nm

ECr352 = 2908dm2mol-1

## At 525nm utilizing the equation D525 = ECr525CCr + EMn525CMn:

Mixture 1: -250 ( 0.0001 ) + 1949 ( 0.0004 ) = 0.7546

Mixture 2: -250 ( 0.0002 ) + 1949 ( 0.0003 ) = 0.5347

Mixture 3: -250 ( 0.0003 ) + 1949 ( 0.0002 ) = 0.3148

Mixture 4: -250 ( 0.0004 ) + 1949 ( 0.0001 ) = 0.0949

## At 352nm utilizing the equation D352 = ECr352CCr + EMn352CMn:

Mixture 1: 2908 ( 0.0001 ) + 1199 ( 0.0004 ) = 0.7704

Mixture 2: 2908 ( 0.0002 ) + 1199 ( 0.0003 ) = 0.9413

Mixture 3: 2908 ( 0.0003 ) + 1199 ( 0.0002 ) = 1.1122

Mixture 4: 2908 ( 0.0004 ) + 1199 ( 0.0001 ) = 1.2831

## To find what was the concentration of the unknown mixture of bichromate and permanganate utilizing the following coincident additive equations ;

D525 = ECr525CCr + EMn525CMn

D352 = ECr352CCr + EMn352CMn

From the informations ;

Optical density for the unknown mixture at a wavelength of 525nm – 0.699

Optical density for the unknown mixture at a wavelength of 352nm – 0.890

0.699 = -250 CCr + 1949 CMn Equation A

0.890 = 2908 CCr + 1199CMn Equation B

Using equation B, doing CCr topic of the expression, CCr = ( 0.890 – 1199CMn ) / 2908 therefore, infixing this in equation A,

0.699 = -250 ( 0.890 – 1199 CMn/2908 ) + 1949CMn

0.699 ( 2908 ) = -250 ( 0.890 ) + 250 ( 1199 CMn ) +2908 ( 1949CMn )

2032.692 = -222.5 +299750 CMn + 5667692CMn

2255.192 = 5967442 CMn

## CMn = 0.0003779M

Therefore utilizing equation A and replacing CMn = 0.0003779, CCr= 0.000150M

Therefore, in the unknown mixture, concentration of permanganate was 0.0003779M and concentration of bichromate was 0.000150M.

## Discussion:

Using additive arrested development on a secret plan of optical denseness against concentration of sample, one can corroborate that that in both samples, Beer ‘s Law is obeyed. Therefore, in this experiment, it was made certain that the equations D = EMnCMn and D = ECrCCr clasp. From the consequences obtained, it is clear that beer ‘s jurisprudence holds since as the concentration of bichromate and permanganate decreased, the optical denseness decreased. An anomalousness was nevertheless observed in the consequences obtained for bichromate at a wavelength of 525nm since as the concentration of bichromate decreased, the optical denseness increased. This anomalousness could perchance be due to taint of the samples.

As has been described antecedently, optical denseness is considered to be an linear belongings. As a consequence, if one considers a mixture of two species holding overlapping soaking up spectra, the optical density measured for that mixture at a peculiar wavelength would match to the amount of the absorbancies of the single species doing up that mixture. Using the equation D = ECrCCr + EMnCMn, one can cipher the theoretical optical denseness of a mixture of two species. A graph of mensural absorbancies against theoretical absorbancies can so be plotted and utilizing additive arrested development analysis, it can be made certain that D = ECrCCr + EMnCMn holds. The graphs obtained should hold had a gradient of one since the theoretical and mensural optical densenesss should hold matched. However, this was non the instance since a gradient of 1.102 and 1.065 were obtained for the graphs at a wavelength of 525 and 352nm severally. This might hold been due to taint in the samples prepared.

Two coincident additive equations were used in order to find the concentrations of permanganate and bichromate in an unknown mixture and it was found out that the concentration of permanganate was 0.0003779M where as the concentration of bichromate was 0.000150M.

## Decision:

This experiment has shown that Beer ‘s jurisprudence holds for the bichromate and permanganate mixtures prepared and that the optical denseness can be used to happen the concentration of two species in an unknown mixture. In this instance, in an unknown concentration of bichromate and permanganate mixture, the concentrations were 0.0003779M and 0.000150M for permanganate and bichromate severally.

## Bibliography:

Beer Lambert Law available at hypertext transfer protocol: //www.chemistry.adelaide.edu.au/external/soc-rel/content/beerslaw.htm ( accessed on 3rd December 2010 )

Optical denseness available at hypertext transfer protocol: //www.physicsclassroom.com/class/refrn/u14l1d.cfm ( accessed on 3rd December 2010 )

Christian G.D. , ( 2004 ) . Analytic Chemistry, 6th Ed. , Wiley & A ; Sons, USA pp.1-800.

## Analytic 2B: Visible Region Spectrophotometry.

Purpose: To find the composing of the blue composite formed between salicylic acid and ferrous ammonium sulphate utilizing the optical denseness obtained for the solution.

## Introduction:

The composing of a complex can be established utilizing seeable part spectrophotometry. If substances X and Y respond together to organize a complex with an empirical expression XmYn, the undermentioned equilibrium will be set up in a solution incorporating both substances:

K

maxwell + New York XmYn

where, K = [ XmYn ] / [ X ] m [ Y ] N.

The sum of complex that forms depends on the stoichiometric ratio m: n. The sum of complex formed will be less if either reactant Ten or Y is less than the stoichiometric ratio in order to keep a changeless rate invariable ( K ) . It is for this ground that the greatest concentration of complex will be formed when the reactants X and Y are of equal ratio as they are found within the composite.

Changes in optical denseness can be used in order to analyze reaction mixtures of equimolar solutions of X and Y due to the fact that optical denseness alterations with complex formation. In this experiment, salicylic acid ( moving as the ligand ) and ferrous ammonium sulphate ( moving as the cation ) signifier a blue complex upon responding with one another. Eleven solutions of equimolar salicylic acid and ferrous ammonium sulphate were prepared with each solution holding a different ratio of reactants from the remainder. Since both reactants are colourless when compared to the composite that forms, the optical denseness of the solution can be used as a step of how much composite is present within the solutions since as the concentration of complex additions, the optical denseness additions, as expected harmonizing to Beer ‘s Law.

The maximal value for the optical denseness obtained from an optical density against volume of one of the reactants will match to the solution where the volumes of reactants are in the same ratio as that in which they occur in the complex therefore, the composing of a complex can be determined.

## Chemicals used:

Salicylic acid ( GPR, BDH )

Hydrochloric acid ( GPR, Sigma Aldrich )

Ferric ammonium sulphate solution ( GPR, BDH )

## Apparatus:

UV spectrophotometer

Glass cell

Burette

Measuring cylinder

Pipet

## Procedure:

Two solutions, viz. , 500mL 0.001M salicylic acid ( 0.069g ) in 0.002M HCl and 500mL 0.001M ferrous ammonium sulphate solution ( 0.239g ) in 0.002M HCl were prepared.

Using two burettes, 11 mixtures of the two solutions, each of volume 10mL were prepared as follows:

Vol. ( Salicylic acid ) / Ml

0

1

2

3

4

5

6

7

8

9

10

Vol. ( Ferric ammonium sulphate ) / milliliter

10

9

8

7

6

5

4

3

2

1

0

The solutions were assorted at intervals of about two proceedingss and allowed to stand for five proceedingss. Their optical densenesss were determined in a 1cm glass cell against H2O as a space at a wavelength of 520nm.

Optical denseness against composing of the solution was plotted and the composing of the blue composite that formed was determined.

## Precautions:

It was made certain that both reactants were dissolved in 0.002M hydrochloric acid in order to obtain an optimal pH of 2.7 required for the complex to organize.

The UV spectrophotometer was calibrated utilizing a clean anterior to any soaking up measurings utilizing distilled H2O.

It was made certain that solutions were assorted at intervals of about two proceedingss and allowed to stand for five proceedingss.

## Consequences:

Mass of ferrous ammonium sulphate weighed – 0.241g

Mass of Salicylic acid weighed – 0.068g

0

1

2

3

4

5

6

7

8

9

10

10

9

8

7

6

5

4

3

2

1

0

1

0.141

2

0.232

3

0.291

4

0.367

5

0.433

6

0.509

7

0.547

8

0.451

9

0.324

10

0.186

11

0.044

## Discussion:

The literature explains that phenolic compounds react with ferrous ions to give a colored composite. This is utile particularly when one wants to find the composing of a complex since separately ; both ferrous ions and phenolic compounds are colourless. The pH of complex formation is important and in this experiment it was made certain that the complex signifiers in a pH of around 2.7. This was made certain by fade outing the salicylic acid and ferrous ammonium sulphate in 500mL 0.002M hydrochloric acid.

In this experiment, mixtures were prepared and they were made up of equimolar solutions of ferrous ammonium sulphate and salicylic acid but in different ratios as has been outlined antecedently in the consequences subdivision. The composing of the composite can be found from a graph of optical denseness against volume of a reactant and the composing must hold the highest value for the optical denseness.

From the consequences obtained it was observed that mixture 7 had the highest value for the optical denseness hence, mixture 7 consists of different volumes of equimolar solutions of the reactants and they are present in the same ratio as that in which they occur in the complex therefore since 6mL salicylic acid: 4mL Ferric ammonium sulphate were used to fix mixture 7, the ratio can be simplified to 3:2. This was non as expected due to the fact that since equal molar concentration of reactants was used, it is expected that the composing of the composite is 1:1 nevertheless, this was non the instance

## Decision:

This experiment has concluded that the unknown composing of a complex can be determined utilizing seeable part spectrophotometry at a wavelength of 520nm. It was determined that the composite was composed of 6mL salicylic acid: 4mL Ferric ammonium sulphate hence 3:2 ratio.