Determination of Pb in paint using AAS analysis..doc

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CHEM 203

Determination of Pb in paint using AAS analysis.

Abstract

The determination of lead in paint using atomic absorption spectrophotometric analysis

was performed on two paint samples. It was found that the concentration of lead in each

sample was 9.852 ppm and 7.940 ppm for sample one and two. The mass percent lead in

each sample in accordance to the mass of the sample being analysed was 3.82 x 10 -9 %

and 3.18 x 10-9 % for sample one and two. In determining how much of this paint needs

to be consumed by a 12 kg child to exceed the World Health Organisation (WHO)

recommended daily led intake limit. It was found that 7.5 g of sample A and 9.07g of

sample B need to be ingested.



Introduction

There has been an increasing awareness in society over its use of lead and its affects on

the environment in recent decades (Evans 1981). The main sources of environmental lead

are mostly through car emissions, which is a by product from using lead in fuel to

chemically reduce friction in the engine (William & Yule). Another source of

environmental lead that is a concern for young people is from the addition of lead to paint

to produce a glossy sheen. In the past some lead paints have been composed of up to 50%

lead compounds and houses built before the 1970’s can contain high concentrations of

lead (Donovan 1986). The demographic most at risk of contamination by lead in paint are

children. This is because children freely suck there fingers after handling things which

leads to the ingestion of lead. Children are more sensitive to lead ingestion than adults

because their brain is developing. Therefore ingestion can lead to abnormal

neurophysiological development in children (Baird 1999).

Method

Preparation of lead sample

Weigh duplicate sample of 0.2 g of paint scrapings into two silica crucibles

Place crucibles in a muffle furnace set to reach a minimum temperature of 600 oC for one

hour. At this time prepare standard solutions

Remove crucibles from oven and cool.

Add 2 ml of conc. Nitric acid, then warm gently on a heater-stirrer or on a sand bath.

Trying to dissolve solids with a glass stirrer.



Quantitavely transfer each mixture into a 100 ml volumetric flask, rinsing the crucible

thoroughly with distilled water.

Make up to the mark with distilled water and let stand.

Standards

Weigh 0.125 g of lead

Dissolve the mixture in a mixture of 2 mL concentrated nitric acid and 2 mL of water in a

150 mL beaker, gently heat on a heater stirrer.

Transfer to a 500 mL volumetric flask and make up to the mark with distilled water.

Dilute the stock solution of lead to obtain 6 standard solution of 100 mL each in the range

4, 8, 12, 16, 20, 25 ppm Pb in 2% nitric acid.

• Place stock solution in a 50 mL burette and distribute 1.60, 3.20, 4.80, 6.40, 8.00,

10.00 mL separately into six 100 mL volumetric flasks.

•

Make up the volume with a 2% nitric acid solution.

Measurement of sample

Analysis was performed on a GBC902/903 Atomic Absorption Spectrophotometer. Using

a lead cathode, set to read wavelengths of 283.3 nm using a slightly oxidising air-

acetylene flame.

Measure the absorbance of each standard solution and record the absorbance. Record

absorbance repeatedly for each solution.

Measure absorbance of the paint samples.

.



Results

The exact concentration of the stock solution of lead:

0.1257 g Pb/ 500 mL

Pb = 207.2 g/M

0.125 g Pb / 207.2 g Pb/M = 6.066 x 10 –4 M

6.066 x 10-4 M x 1000mL / 500mL x 1L = 1.2133 x 10-3 M

Table 1. Repeated measurements of lead absorbance and the mean for lead standards

Lead standards (ppm)

Absorbance 4.02 8.04 12.06 16.09 20.11 25.14 Sample 1 Sample 2

1 0.049 0.127 0.179 0.227 0.271 0.313 0.159 0.133

2 0.05 0.128 0.173 0.23 0.269 0.323 0.161 0.134

3 0.051 0.129 0.172 0.236 0.295 0.315 0.163 0.132

4 0.048 0.13 0.175 0.231 0.272 0.318 0.163 0.135

5 0.047 0.124 0.17 0.234 0.265 0.325 0.157 0.135

Average abs 0.049 0.128 0.174 0.232 0.274 0.319 0.161 0.134

It can be seen in Table 1 the raw data for this experiment. With the absorbances for the

two paint samples highlighted in bold. This data was then plotted on a scatter plot to

show the concentration of lead in ppm for the two paint samples. This can be seen in

Figure 1.



Figure 1. An XY scatterplot showing absorbance

plotted against concentration for six lead

standards.

y = 0.1462Ln(x) - 0.1687

R2 = 0.979

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 5 10 15 20 25 30

Concentration of lead (ppm)

A b s o r b a n

The concentrations of lead in the paint samples can be determined by the following

formula derived from excel:

Y = 0.1462 Ln(x) – 0.169

Ln(x) = Ln e(Y + 0.169 / 0.1462)

X = e (Y + 0.169 / 0.1462)

Therefore,

Sample A

X = e (0.161 + 0.169 / 0.1462)

= e(2.259)

= 9.582 ppm Pb



Sample B

X = e (0.134 + 0.169 / 0.1462)

= e(2.073)

= 7.94 ppm Pb

The mass percent of lead in the original paint samples was:

Sample A

0.1998g paint / 100 * 1.914 x 10-6

= 3.82 x 10-9%

Sample B

0.2004g paint / 100 * 1.59 x 10-6

= 3.18 x 10-9%

For a child of 12 kg to ingest enough paint to exceed the WHO recommended daily lead

intake limit of 6 ug kg-1 it would have to have:

12 kg * 6 ug kg-1 = 72 ug – ingest this much Pb

Sample A.

72 x 10-6g / 1.914 x 10-6 x 0.1998 g

= 7.5 g



Sample B

72 x 10-6 / 1.59 x 10-6 x 0.2004

= 9.07 g

Conclusion

It can be seen that the lead that is found in paint is of environmental concern for humans.

The amounts of lead found in the paint are in concentrations that can be harmful of

ingested by children. Although it would take along time to ingest 7 – 9 grams of leaded

house paint accidentally. It is known that lead is a cumulative compound and can be built

up to levels that can affect human health over time (Baird 1999). Although lead was

phased out as being an additive in paint in the 1970’s lead based paints still occur in

dwellings built before this time and can often be found underneath coats of newer paint

(Donovan 1996). It is for these reasons that sufficient analytical methods are needed to be

able to test for lead in paint and also other aspects of the environment.

References



Baird, C. 1999, ‘Environmental chemistry’, (2nd Ed), Fourth printing, England

Donovan, J. 1996, ‘Lead in Australian children: Report on the National survey of lead in

children’ Australian institute of health and welfare, Canberra.

Evans, L. 1981, ‘Health and environmental lead in Australia’, Australian academy of

science, Canberra.

Lansdown, R. and Yule, W. 1986, ‘The lead debate: The environment, toxicology and

child health’, Mackays of Cantham Ltd, Kent.

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Determination of Pb in paint using AAS analysis..doc