C30F ANALYTICAL CHEMISTRY LABSCRIPT
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Determination of lead in paints by atomic absorption spectrometry.
1. Introduction:
Lead compounds have been used in paints as pigments, anti-corrosive ingredients, or drying agents. Consequently, our surroundings contain a great deal of lead in paints used for metal protection, decorative finishes, or to accelerate the drying of oil-based paints.
Problems of lead intoxication can arise, particularly in children, if peeling or chipping paint films are ingested, usually by the habit referred to as pica (1), which is the tendency to place foreign objects in one's mouth by instinct. Lead in paints, particularly in schools and houses with young children, is of great concern since the effects of lead poisoning increases inversely with age (1). The effects of lead poisoning include decreased neuromuscular activity, retarded mental development, reduced ability to learn, and at the other extreme, death.
The aims of this experiment are to collect and to determine the levels of lead in paints in buildings or structures, to which young children are exposed. Based on the lead contents, the risk of exposure of young children will be assessed, and recommendations made on actions to be taken.
2. Experimental:
(A) Sample Collection:
(a) Use a sharp stainless steel knife or spatula and a clean dustpan or sheet of paper to
collect samples. After locating suitable sites, scrape 2-5g of paint film that is
intact, peeling or cracking, from an area approx. 1.5 to 2 square meters, onto the
collection surface.
(b) Transfer to a clean sheet of aluminium foil or plastic bag.
(c) Seal, label and note the following details:
(i) Appearance of paint (intact; peeling; chipping).
(ii) Color of paint.
(iii) Age of paint film (if known).
(iv) Single or multiple layers of paint visible in chips.
(v) Surface from which taken (wood, metal, glass etc...)
(vi) Estimated size of area sampled.
(B) Sample preparation for analysis:
(a) Dry each sample at 130° in an oven for at least one hour.
(b) Cool, grind finely in a mortar, and shake through sieve provided
(note mesh size used)
(c) Weigh triplicate aliquots of each paint (~0.2g) into boiling tubes,
and add 10mL conc. nitric acid to each. Prepare an acid blank also.
(d) Cover tubes with a clean glass plate and allow to predigest overnight
at room temperature in a fume hood overnight.
(e) Digest at 135-140°C for 4h., with occasional swirling to avoid bumping.
(f) Cool and dilute with 10mL distilled water and mix well.
Filter through Whatman #1 filter paper into 25 mL volumetric flasks,
and make up to mark with distilled water washings of residues.
(C) Lead determination:
(a) Set up atomic absorption spectrometer with Pb hollow cathode lamp and
set wavelength of detection at 283.3nm for air-acetylene flame mode
of analysis.
(b) Prepare 25mL volumes of lead standards, using the 100ug/mL stock
standard provided: 0.5, 1.0, 2.0, 4.0, 8.0, and 10ug/mL
(c) Aspirate standards, noting the absorbance for each solution.
(d) Aspirate samples and blank solutions and note absorbances.
If sample solution absorbances exceed that of the 10ug/mL Pb standard,
dilute immediately by an appropriate factor (see Demonstrator) and
reanalyze.
3. Calculations:
Construct a calibration curve and determine sample solution centrations.
Alternatively, determine the linear regression equation and correlation
coefficient and calculate the sample solution concentrations.
Hence calculate the Pb contents of paint samples as Wt. %, taking all
sample dilutions into account.
4. Exercises:
(A) What errors could have made during sample collection and preparation?
Suggest how such errors could be minimized.
(B) Discuss the Pb contents obtained for your paint samples, and their
possible consequences of exposure to young children.
What advice would you give to parents on this matter?
5. References:
1. Boecyx, R.L. (1986) Lead poisoning in Children. Anal. Chem. 58,
275-281.
2. Centers fof Disease Control (1991). Preventing Lead Poisoning in
Young Children. US Dept. of Housing and Social Services, USA.
Anal-Chem Resources
delloyd.50megs.com
Chem. Dept. UWI. St. Augustine Campus
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