Determine the concentration of lead in a given solution by gravimetric analysis, EDTA titration, spectrophotometry, and atomic absorption, and compare the advantages and disadvantages of each method.

Results:

When the solutions were heated there was a colour change from green to a yellow-orange.

Average weight of solid  =  0.2244 g

Calculation of Concentration of unknown lead solution:

        

Moles  =  Mass                                Moles  =  Conc  x  Volume

              Mr                                                        1000

        =  0.2244                                Conc        =  0.0011 x 1000 

2. 19.8550

=  0.0011 moles                                =  0.055 moldm-3

EDTA Titration:

Apparatus:

• 250ml Conical Flask
• Burette

Method:

1. 20ml of an unknown lead solution was placed in a 250ml conical flask.
2. 10ml acetate buffer and a few drops of Xylenol Orange Indicator were added.
3. This solution was then titrated with standard EDTA.
4. This titration was then repeated 3 times.

Results:

Average titration  =  25.275 ml

Calculation of concentration:

Concentration of EDTA  =  0.02 M

Moles  =  Conc x Volume        =        0.02  x  35.275

1000. 1000

=        7.055  x  10-4

There is a 1:1 ratio between EDTA and Lead therefore the number of moles of lead is equal to that of EDTA.

Moles  =  Conc  x  Volume        

                1000

Conc        =  moles  x 1000

                Volume

        =  (7.055 x 10-4) x 1000

                19.8550

        =  0.036 moldm-3

Mass  =  moles  x  Mr

          =  0.036  x  207.2

          =  7.4592 gdm-3

Atomic Absorption Spectroscopy

Apparatus:

• 250ml Volumetric Flask
• 100ml Volumetric Flasks

Method:

1. A 250ml solution of lead nitrate was prepared of an accurately known standard solution of 2000ppm Pb.
2. Using this solution 6 other solutions of 5, 10, 20, 30, 40 and 50ppm were prepared in 100ml volumetric flasks.
3. The unknown was then accurately diluted to about 25ppm.
4. The absorbance of each solution was measured.

Preparation of lead nitrate solution:

        2000ppm  =  2000mg  in 1000g

             =  2g in 1000g

             =  0.5g in 250 ml

        0.5g of lead is required, to obtain this using the lead nitrate:

                weight of lead required  x  FW lead nitrate   =  weight of lead nitrate

                        FW lead                                        required

                        0.5     x   331.2                =   0.799  g

                          207.2

0.799g of lead nitrate was placed in a 250ml volumetric flask and 1ml 2M nitric acid and made up to 250ml with distilled water.

Preparation of standard solutions:

Dilution of Unknown:

        25ppm  =  25mg in 1000g

                 =  0.025g in 1000g

From the EDTA titration the concentration was calculated as 7459.2ppm, using this we can calculate the 25ppm:

2. in 1000ml  =  7459.2 ppm          

0.3ml  in  100ml     =  25 ppm

Results:

Calculation of Concentration:

From the calibration curve the concentration of the unknown lead solution is observed as 25ppm.  As you can see from the results the unknown lead solution was diluted by:

0.3ml in 100ml  =  1ml in 300ml

The lead solution was diluted by a factor of 300 which would calculate the concentration as 300  x 25 ppm  =  7500 ppm

7500 ppm  =  7500mg in 1000g

              =  7.5g in 1000g

So the concentration is 7.5gdm-3

Moles  =  mass    =   7.5      =   0.036 moldm-3

              Mr                207.2

Spectrophotometry

Apparatus:

• 50ml Volumetric Flasks
• Absorbance Cells
• Burettes

Method:

1. A solution was made up in a 50ml volumetric flask, containing 1ml 0.01M PAR and 15ml ammonia buffer made up to 50ml with distilled water.
2. This solution was placed in the UV/VIS spectrometer and used as a baseline.
3. The lead solution from the previous atomic absorption 2000ppm lead solution and was diluted down again by a factor of 100.
4. Five solutions made up of 6, 5, 4, 3 and 2ml of this solution were placed in 50ml volumetric flasks.
5. 1ml of 0.01M PAR and 15ml ammonia buffer was added to each solution and they were made up to 50ml with distilled water.
6. Each solution was placed in the UV/VIS spectrometer and the absorbance was measured.
7. The unknown lead solution was then diluted to 15ppm and the absorbance was measured.

Dilution of unknown:

        7459.2 ppm     =    7459.2 mg in 1000ml

        15 ppm            =    1ml in 500ml  =  0.2ml in 100m

Results:

From the calibration curve you can see that the unknown is 1.5 x 10-5M (15ppm).  The results show that 0.2ml of the lead solution was diluted with 100ml water.

0.2ml in 100ml  =  1ml in 500ml

So the solution was diluted by a factor of 500 which would calculate the concentration as:

                15  x  500   =    7500 ppm

                7500 ppm  =  7500mg in 1000g

                              =        7.5g in 1000g

                              =    7.5gdm-3

moles  =  mass

             Mr

           =    7.5

            207.2

           =   0.036 moldm-3

Critical Discussion:

Gravametric Analysis:

Advantages – This method is fairly cost efficient, as no expensive equipment is used, and the procedure is fairly straight forwards and easily followed.

Disadvantages – It is very time consuming due to the heating and cooling of the solutions, and the solutions are made up by hand so there will be some human error.

EDTA Titration:

Advantages – This is a very simple procedure and fairly accurate as the burette can be measured to ±0.02ml, and many titrations can be carried out to obtain an accurate value.  It is also a very cost efficient process as only a burette and a conical flask is required.  It is also a very quick and time efficient process.

Disadvantages – there would be human error due to the solutions being made up by hand.

Atomic Absorption:

Advantages – high precision can be obtained with this method due to the use of computerised equipment to analyse the solutions.  It is also a fairly simple and fairly quick procedure.

Disadvantages – the accuracy is not great due to the human error when calculating and making up the dilutions.  It is also quite expensive as of the use of the computerised equipment and absorbance spectrometer.

Spectrophotometry:

Advantages – This is a fairly quick and simple procedure and high precision can be obtained due to the values are measured by the UV/VIS spectrometer.

Disadvantages – The accuracy is not great as there is a lot of human error due to the calculations and measurements of dilutions.

Conclusion:

        In my opinion I would say the best method is the EDTA titration as it is very cost efficient and the procedure is very simple and straightforward.  The procedure is also very quick and although the solutions are made up by hand the human error is minimal as there are few measurements to make compared with the other methods.