SPECTROGRAPHIC DETERMINATION OF LEAD AND TIN

IN STAINLESS STEELS RALPH H. STEINBERG and HENRY J. BELIC

Chemical Laboratory, South Works, U. S. Steel Co., Chicago, III.

The chemical determinations of lead and tin in stain- less steels are tedious and costly, whereas the spectrographic determinations of these elements can be made rapidly and at relatively low cost. The source of these dements in the steel can be attributed to the scrap used in melting, fur- nace refractories, fluxes, and furnace additions. Using a direct current arc technique, a single sample can be spectro- analyzed in approximately twenty to twenty-five minutes. The spark technique.which is preferable for most determi- nations, is unsatisfactory because it lacks sensitivity.

Equipment and Procedures The direct current arc source is a commercial unit with

full wave rectification.

The spectrograph is a 1.5 meter instrument with a 24,- 000 lines per inch concave grating, providing a uniform dispersion of 7 Angstroms per millimeter. Spectrum Anal- ysis No. 1 film is used.

The steel sample is delivered to the laboratory in the form of dril!ings. The sample is sieved on a 20 mesh sieve and the fine material Which goes through is discarded. Be- tween 1.5 and 2.0 grams of the finest material which re- mains on the sieve are placed in a hydraulic press and com- pressed under 250,000 pounds per sq. in. into a cupcake- shaped briquette which is 0.25 inch in diameter at the bot- tom with sides slightly tapered outward to the dome-shaped top. The finished briquette is placed in the end of a tubu- lar steel holder whose sides are slit to provide a little spring gripping action. The briquette in its holder is used as the lower ( + ) electrode. A 0.25 inch diameter graphite rod with a conical t ip of 120 ° included angle is used as the counterelectrode ( - - ) . The exposure conditions are shown in Table I.

Development and Photometry The film is developed for two minutes in Eastman D-19

developer, placed in a 2.5% acetic acid stop bath for 10 seconds, and fixed in Kodak rapid liquid fixer for 45 sec- onds. After washing for one minute, the'film is dried by infra-red radiation in a stream of warm air.

The film is stretched taut in a film holder and the whole is placed in a commercial microphotometer in order to measure the density of the spectral lines.

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Working Curves Two film calibration curves are used, one for the 3700

A region and the other for the 3200 A region of the spec- trum.

The analytical curves are prepared by plotting the loga- rithm of the relative intensity ratios against the corre- sponding concentrations of Pb and Sn. The standard sam- ples used in establishing the curves were analyzed chemically in the South Works Chemical Laboratory. An- alytical curve data are shown in Table II.

Precision The general reproducibility of results on a single sample

is of the order of ± .001 plus 15 % of the quantity present. Routine samples along with the proper standard, are run in duplicate, and any correction due to curve shifting is applied. Approximately a thousand samples have been ana- lyzed to date without encountering any unusual difficulties.

Table I. Exposure Conditions

Power source D.C. arc Power 6 amperes Preburn none Exposure

Chromium stainless 9 seconds Nickel-chromium stainless 6 seconds

Spectrographic slit width 30 microns Analytical gap 3 mm. Electrodes

Sample lower ( ~ ) Graphite upper ( - - )

Type of Steel

Cr-stainless

Ni-Cr-stainless

Table It. Analytical Curve Data

Element Iron Element line line Index* Range

Pb 3683.47A. 3701.09A..006% .001-.011% Sn 3175.02 3171.35 .007 .001 -.011

Sn 3175.02 3286.76 .040 .010 - .040

Pb 3683.47 3679.92 .012 .001 - .015 Sn 3175.02 3286.76 .015 .001 - .017 Sn 3262.33 3286.76 .050 .010 - .060

* Defined as concentration at which intensity ratio of analytical line pair (unknown and internal standard) is unity.