Two Approaches to the Determination of Lead in Brass Differential Scanning Calorimetry and Atomic Absorption Spectrometry Sunhee Choi and James A. Larrabee Middlebury College, Middlebury, VT 05753
In our advanced integrated laboratory course for chemistry major juniors we encourage our students to try several different instrumental methods to study a single problem. One experiment that is simple, fun, and educational is the determination of lead in hrass by differential scanning calorimetry (DSC) and atomic absorption spectrometry (AA). Lead is not soluble in hrass. but i t is found as a finelv dispersed separate phase in leaded brasses. Leaded brasses are normallv used for machinine since the finelv divided lead acts as a lu6ricant and produe& a good surface finish on the machined part.' Since the lead in the brass is a separate phase, i t melts as pure lead, and the heat of fusion, as determined by DSC, can be related to the quantity of lead in the brass sample. However, if another metal is present such as tin, some of the lead may become alloyed and not detected in the DSC lead-melting peak. Total lead is determined by AA, and for a free-cutting leaded brass, total lead and unalloyed lead should be the same. Experimental A convenient sample is the hack ferrule of a hrass Swagelokfitting far 0.125-in.-o.d. tubing. The ferrule is washed in hexane to remove any oil, air-dried,and weighed (typically31&315 mg). The ferrule is placed in an open aluminum sample pan in the sample side of the DSC. An aluminum sample pan with an equal weight of copper wire is placed in the reference side of the DSC to help maintain a good base line. The temperature is scanned from 550 K to 650 K at 10 K/ min with the range set at 8. Lead has a melting point of 600 K and s heat of fusion of 25 J/g.2The DSC instrument used for these experiments was a Perkin-Elmer model DSC-lB. A lead standard of between 8 and 12 mg of pure lead is run on the DSC at the same temperature scan rate and range as the hrass ferrule. The area of the lead melting peak in the hrass sample is compared to the area of the melting peak of the lead standard to determine the percent of unalloyed lead:
%PI, =
DSC curves fw lead melting peaks; scan rate. 10 Klmln; range. 8; (a) 312.6rng brass ferrule. previously unheated; (b) 312.6-mg brass termle afler one heat/ml cycle; (c) 10.7-mg Pb standard.
Lead Content In Brass Ferrules by DSC and AA Sarnole WeiaM. ma
%Pb bvDSC
%Pb bv AA
(area in sample peak) (weight of Pb standard), (weight of sample) (area of Pb sutndardl
For the AA analysis the ferrule was placed in s 100-mLvolumetric flask and dissolved in 10 mL of concentrated nitric acid. After dissolution the sample was diluted to the mark with distilled water. A 10.0-mL aliquot was diluted further to 100 mL. For a 310-mg sample containing3%Ph, this corresponds to a9.3-ppmPh solution. Calibration standards of 5,10, and 20 ppm lead were prepared using analytical reagent grade lead nitrate and 0.05 M nitric acid. The acid prevents precipitation of lead carbonate. Lead was determined using an airlacetylene flame and a multielement hollow cathode lamp. The spectrometer was set to the 283.3-nm Lead line. The instrument used far these experiments was a Perkin-Elmer model 460 AA spectrometer. Results and Discusdon Shown in the figure are the DSC curves for a 312.6-mg brass ferrule and a 10.7-mg lead standard. The first run of the brass ferrule shows a broad meltine oeak (a). , . The same sample, after cooling and reheating, shows a much sharper melting peak (b). This peak sharpening corresponds to agglomeration of the lead in the brass. Subsequent reheatings k of the same ferrule do not result in further ~ e a sharoening. The melting point of lead in the ferrule ishigher than that
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found a t the heating rate of 10 Klmin due t o the poor thermal contact of the lead in the ferrule (300 mg) versus the pure lead standard (10.7 mg). The melting point discre~ancy depends on the heating rate. A slower heating rate brings thk melting point of the lead in the ferrule closer to the lead standard; however, the areas under the lead melting point curves for both the standard and sample decrease with heating rate, reducinr! the accuracv and orecision of the determination. In the t a h ~ eresults , are listed from the DSC and AA analyses of lead in three different brass ferrule samnlex. In all the samples, lead as determined by AA (total iead) is essentially the same as unalloyed lead as determined by DSC. This is expected since the ferrules are machined from
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Lyman, T., Ed. Metals Handbook; American Society for Metals: Novelty, OH. 1961: Vol. 1, p 964. Weast, R. C., Ed. Handbook of ChemisiryandPhysics;CRC:Boca Raton, FL, 1986-1987: p 8-219.
free-cutting brass in which all the lead is present as a separate phase. The speed and simplicity of the DSC procedure is impressive. It is easily seen why DSC is a popular quality control instr~rnent.~ A final note of caution is that "brasses" with significant tin content (CuIZnlSnPb alloys) will show smaller lead
melting peaks as the tin levels increase. If an unknown brass sample has a comparatively small DSC peak but contains 3 to 3.5% lead by AA, check for tin. Wendlandt. W. D. Thermal Analysis. 3rd 4.;Wiley: New York, 1986.
Volume 66 Number 10 October 1989
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