A spectrographic method for the determination of phosphorus in steels

Spectrorhimics

A&I,

1054,

Vol.

6, pp.

413 to 417.

Pergsmon

Press

Lt,d.. London

A spectrographic method for the determination Scientific

of phosphorus in steels

LE ROY S. BROOKS and FORD R. BRYAN Laboratory,EngineeringStaff, Ford MotorCompany,Dearborn.Michigan (Rece1Mtz 12 April 1954)

Summary--A photographic procedurehas been developed for the determination of phosphorus within the range 0.003 to O-061 per cent in steels. The 2149.11 A phosphorus line is recorded on Ilford Q-2 plates requiring no special processing. The emulsion is calibrated by means of iron lines between 2100 8, and 2175 A. Precisionof the method is sufficientto be useful for routine

analysis.

Introduction Phosphorus in steel is one of the few determinations which process control laboratories have been unable to accomplish by means of routine spectrographic procedures. The lit,erature reveals that’ some success has been achieved by using specialized non-photographic equipment, and that only relatively high concent’rations of phosphorus can be detected by conventional photographic methods. In 1948. HANAU and WOLFE [l]. and also BRYAN and NAHSTOLL [Y]. used a large Littrow quartz spectrograph and Geiger-Miiller counters to det’ermine phosphorus bet’ween 0.005 and 0.04 per cent, in steels. In 1949, BRECKPOT [3] successfully analyzed st’eel for phosphorus in the range of 0.01 to 0.10 per cent by means of an instrument incorporating multiplier phototubes. In the same year, HASLER and BARLEY [i] reported the use of a 1.5 metre grating spect’rograph and photomult’iplier t’ubes for the analysis of steel samples containing 0.006 and 0.103 per cent’ phosphorus. HANS [5]. in 1950. used instrumentation combinations involving both electron multiplier phototubes and Geiger counter tubes in developing procedures for determining phosphorus in steels. SVENTITSKII [6], in 1947, described a photographic procedure for determining phosphorus in steel through a range of approximately 0.015 to 0.20 per cent. In this work Agfa gelb rapid plat’es were ultraviolet-sensitized by means of a mineral oil film. In 1951, STEAKLEY [7] analyzed steels for phosphorus in the range of 0.10 to 0.20 per cent by means of a 3-metre grating spectrograph and Eastman Kodak Spectrum Analysis No. 1 emulsion. A practical spectrographic method has been developed in t’his laboratory for the determinat,ion of phosphorus from 0.003 to 0.061 per cent in steels. The concentration range accommodated makes this met’hod applicable to the majority of steels. This procedure provides improved detectability which can be attributed largely to t,he favourable characteristics of the particular ultraviolet-sensitive emulsion employed. The procedure can be applied to steels in either rod or chip form and can be accomplished on st’andard commercial equipment.

Outline of method Phosphorus standards in chip form were obtained from the National Bureau of Standards. These standards, listed in Table 1, together with chemically analyzed steels available in both rod and chip form, were used to establish analytical working curves. 413

LE

ROY

S.

I~ROOKS

nd

Fo~n It.

HRVAN

Standards and samples in rod form are prepared as & in. diamet,er selfelectrodes. The electrode tips are ground to a 120” included angle. Steels prepared in chip form will pass through a 40-mesh screen but not a 60.mesh screen. Chip samples. each weighing 0.1 g.. are placed loose in a carbon cup elect#rode and arced with a 4 in. diameter counter electrode wit’h tip ground to 120” includect angle. Table

1. I’l~os~horusstandards in chip form I’h,osphorus concentration per cent

3Od 65c 13g 14C 125

! I

0.031 0.023 0.019 0.012 0.005 0.003

The carbon cup electrodes are 1 in. long and 9 in. diameter, with a cup $2 in. diameter and & in. deep. A gap spacing of 2 mm is used with both met,al and carbon electrode systems. The distance from the gap t’o the spectrograph entrance slit is 50 cm. A quartz condenser lens near t’he electrode gap forms an image of the gap on the collimator lens of the spectrograph. As noted by HANAU and WOLFE [ 11, careful alignment of the elect#rode gap is necessary in order to avoid variat’ions in light absorption due to passage through varying t’hicknesses of quart’z. An a.c. arc source of 2.4 amps at 4000 volt’s will provide measurable phosphorus lines after a 46-second exposure. Current control is provided by a variable inductance in the primary circuit. The relatively high vapour pressures of phosphorus and its oxides apparently necessitates rigid control of the t’hermal characteristics within the arc gap. The need of extreme care in reproducing source conditions and electrode temperatures for phosphorus determinations has been previously not’ed by BRYAN and NAHSTOLL [2]. From a study of repeatabilit’y of it appears necessary to operate this consecutive phosphorus determinat’ions, part,icular arc source for a period of approximat’ely t’wenty minut,es before excitaThis is believed to be mainly due tion conditions can be satisfactorily repeated. to t,he elect,rode holders reaching equilibrium temperature in about that lengt,h of t,inie. A large Littrow quartz spectrograph equipped with a 10 ,u fixed entrance slit allows resolution of the 2149.11 A phosphorus line from the 2148.97 A copper line in steels. Fig. 1 is a t’ransmission recording of t,hese and neighbouring iron rnicrophoton~eter equipped with a 100 p slit. The lines using a 17 ‘r projection enlarged image of t’he spectral line is approximately five times the width of t’he microphotometer slit. The sample represented in Fig. 1 cont’ained 0.015 per cent phosphorus and 0.10 per cent copper. There is very lit,tle choice between phosphorus lines at 2136.20 a and 2149.11 A. The lower wavelengt’h phosphorus line has t,he advantage of being fart,her from its int#erfering copper line. On t’he other

A spectrographic

method for the determination

of phosphorus in sbels

hand the higher wavelength phosphorus line has a less intense interfering copper line. The ult8raviolet-sensitive emulsion employed is the Ilford Q-2, which is a medium-contrast, medium-speed emulsion especially useful at wavelengths less than 2200 a. The plat’es require no special processing, but the emulsion is somewhat easily damaged by rubbing contact. Graininess of the emulsion requires that an area of no less t’han 30,000 square microns be microphotometered in order t’o keep photomet,ry errors within + 1.0 per cent. Recommended safelights are either t,he-%ratten Series 6-B or Ilford F,-No. 904. Developing time his been three minutes in Eastman Kodak D-19. Alt’ernates would be Ilford ID-19 or ID-13 developers. 0 2146 971 (.IO%Cu) 2149.11x IO

2’50”6

a

(Fc’7

~015V.P) .04

\

.03 20

30 s

.001

l-l-u-u a5

0.4

0.3

0.2

01

.O

S-DIFFERENCES 901

Analytical curve for phosO Fig. 2. phorus in steels, using the 2150.18 A iron reference line and the 2149.11 ..k phosphorus line.

Fig. 1. Transmission recording of spectral lines involved in the determination of phosphorus in steel.

Earlier attempt,s to use Eastman S.A. No. 1 and Eastman 103-O ultravioletsensitized plates resulted in abandonment of these emulsions because of the difficulty in reproducing densities from plate to plate. Removal of the fluorescent coating prior to development also detracted from the usefulness of these emulsions for routine analysis. The most encouraging Eastman emulsion appears to be the S.W.R. (Short Wavelength Region). which has proved useful for phosphorus determinations providing the plate can be handled with sufficient care to prevent In this particular respect the Ilford Q-2 abrasion of the powder-like surface. appears superior t’o the S.W.R. emulsion. Sensitometric characteristics of the Ilford Q-2 emulsion are determined by means of a group of iron lines between 2100 A and 2175 A. From a large number of Fe I lines in t,his region, a small group of lines homogeneous enough for plate calibration was selected. These lines together wit,h t,heir S-values IS] are listed in 415

LE ROY

S. BROOKS and FORD R. BRYAH

Table 1. The X-values listed are used in the same manner as logarithms of intensities, and were determined empirically for the conditions of this method of analysis by the procedure described by DIEKE [8]. Emulsion calibration curves for Ilford Q-2 plates are found to be essentially linear for transmittances between 10 and 50 per cent,. Iron

Table 2. Wavelength

lines for emulsion

in angstroms

calibration

1 I

2102.35 2110.23 2150.18 2155.2-l 2158.92 2159.65 2159.89 2171.29

0.27 0.50 0.91 0.67 1.18 1.33 1.43 1.67

The relative transmittances of the 2 150. I 8 il iron and t,he 2 149.11 A phosphorus lines from each spectrum, when applied to the emulsion calibration curve for the corresponding plate, provide S-value differences which can be plott’ed against Such an analytical curve is shown in Fig. 2, known phosphorus concentrat’ions. where Bureau of Standards phosphorus values are assigned to a series of chip samples. A corresponding analytical curve for rod samples has almost identical slope, linearit’y, and concentration index.

Precision and accuracy Dat’a obtained thus far show precision which is useful for routine phosphorus determinations in typical st’eels. However, comparison wibh precision obtainable with a.c. arc procedures applied to equivalent, concentrabions of other elements indicates that improvement in precision should be possible [9]. It is felt that improvement in precision depends largely on further stabilization of source conditions. The discarding of spectra exhibiting unusually high or low reference-line values would have materially improved our precision dat’a. Tables 3 and 4 show standard deviations calculated from individual determinations. The disbincbion in precision between chip and rod samples is very slight. Table 3.

data from

N.B.B. values per cent phosphorus

Number of determinations

10 6 6 6

Precision

) I

0.003 0.012 0.019 0.023

416

chip samples

j

I I

Standard deviation in per cent phosphorus

0.0006 0.002 0.002 0.003

A spectrographic method for the determination of phosphorus in steels Table 4.

Precision

data from

I Kumber of determinations

12 7 7 13 14 7

rod samples

I Assigned mean per cent phosphorus

0.002 0.011 0.012 0.015 0.023 0.061

rs‘tanda,rd deciatiow in per cent phosphorus

0.0006 0.0015 0.0015 0.002 0.003 0.005

Since work is still progressing on improvement, of precision. no systematic determination of accuracy has been made. Approximate accuracy may be judged. however. from Fig. 2 in which the National Bureau of Standards iron and steels list’ed in Table 1 are plotted in relation to the analytical curve from which spectrographic values are obtained. It, is assumed t’hat accuracy can be made to approach closely t’he precision ultimat’ely attained. Both precision and accuracy are sufficient to make the method current,ly useful for routine work in t’his laboratory. and it is hoped that the present st’atus is sufficiently promising t’o encourage other lsborat’ories to inoest,igaCe and furt’her improve t,he procedure.

References R., and ~‘OLFE, It. A. ; J. Opt,. Sot. Amer. 1918 38 377-385 [2] BRYAN. F. R., and NAHSTOLL, C:. A. : J. Opt. Sot. Amer. 1948 38 510-517 [3] BRECKPOT, R. ; Congr. groupement avance. method, anal. spectrograph produits met. 1949 12 99.103 [4] HASLER. RI. F.. and BARLEY. F. ; “Direct. Reading Quantometer Analysis of Phosphorus in St,rels,” Applied Research Laboratories, Glendale, 1949 [5] HANS, A. : J. Iron Steel Inst,. 1950 166 118-132 [B] SVESTITSKII, N. S. ; Bull. Acad. Sci. U.S.S.R., Ser. Phys. 1947 11 319-35 [‘i] STEAKLET, TV. E. ; Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Abstract No. 60, 1951 [X] DIEKE, B. H. ; TVar Production Board, Report Ko. W-90, Washington, 1944 [9] Methods for Emission Rpectrochemical Analysis. A.S.T.M., Philadelphia, 1953

[I] HINDU.