Determination of Lead


Objectives:

To 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.

Calibration of Pipette

Apparatus:

* 20ml pipette

* Stoppered flask

Method:

1) A 20ml pipette was cleaned with a dilute solution of detergent, and
rinsed thoroughly with distilled water.

2) The pipette was then filled with distilled water.

3) The water is then run out into a weighed flask.

4) The flask is then re-weighed.

Results:


Weight of Flask (g)
-------------------

Weight of Flask and Water (g)

Weight of water (g)

29.9529

49.8378

19.8849

29.9521

49.9329

19.9808

Difference of the weight of water: 19.9808 - 19.8849 = 0.1ml

Average weight of water: 19.9808 + 19.8849 = 19.9329

2

Temperature of water = 25oC

True volume of water contained by the 20ml pipette = 19.9329 x 0.99609

= 19.8550

Gravametric Analysis:

Apparatus:

* 2 x 400ml beaker

* 20ml calibrated pipette

* Hot plate

* Oven (120oC)

* 2 x Sintered glass crucibles

Method:

1) Two sintered glass crucibles with dilute HCl, followed by dilute
NH3, followed by distilled water.

2) They are dried over night in an oven.

3) Duplicate solutions are prepared by pipetting 20ml samples of the
unknown lead solution into two 400ml beakers, using the calibrated
pipette.

4) Each solution is then diluted to about 150ml.

5) 20ml of 1M acetate buffer and 20ml of 0.1M chromium (III) nitrate
and 20ml of 0.06M potassium bromate were added to the solution.

6) The solutions were then heated to 90oC in a fume cupboard for
45mins.

7) After this time they were allowed to cool for 60mins.

8) The solutions were then filtered through weighed sintered glass
crucibles.

9) The crucibles were then dried in an oven to constant weight.

Results:

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

Weight of Crucible (g)

Weight of crucible & solid (g)

Weight of Solid (g)

14.5038

14.7269

0.2231

17.0663

17.2920

0.2257

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

207.2 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:

Titre

Rough

1

2

Volume (ml)

35.40

35.25

35.30

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:

ppm

Volume of 2000ppm sol (ml)

Volume of water (ml)

5

0.25

100

10

0.50

100

20

1.00

100

30

1.50

100

40

2.00

100

50

2.50

100

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:

7459.2 in 1000ml = 7459.2 ppm

0.3ml in 100ml = 25 ppm

Results:

ppm

Abs Value

5

0.064

10

0.129

20

0.232

30

0.352

40

0.433

50

0.506

Unknown

0.281

[IMAGE]

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:

Amount of lead nitrate solution (ml)

Abs Value

6

2.580

5

2.559

4

2.407

3

2.005

2

1.576

Unknown

2.351

[IMAGE]

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:


Method
------

Conc of Unknown calculated (M)

Gravametric Analysis

0.055

EDTA Titration

0.036

Atomic Absorption

0.036

Spectrophotometry

0.036

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.