Determination of Rare Earth Elements and Thorium in M a gnesium with Photoelectric Recording S pect romete r E. J. HUNEMORDER and T. M. HESS The Dow Chemical Co.,Midland, Mich.
b The addition of thorium and certain of the rare earth elements to magnesium-base alloys has become increasingly important. A spectroscopic method was developed to provide a rapid and accurate means of determining the concentrations of cerium, lanthanum, praseodymium, neodymium, and thorium. Self-electrodes are sparked directly using high-voltage spark excitation. The spectral line intensities are recorded photoelectrically for analysis. The influence of the metallurgical history of the sample is negligible. Alloy environment is important in the cerium and thorium determinations.
S
TECHXIQUES have been used successfully for many years in the routine analysis for all of the common alloying and impurity elements in magnesium-base alloys (4) with the exception of thorium and the rare earth elements. I n the last few years metallurgists have carried on considerable research in the use of these additives. The problems involved in establishing a spectroscopic procedure have been complicated by a lack of precise wet chemical methods for determining the individual rare earth elements. Although improvements have been made in the wet cheInicnl proceduies ( I ) , they are still time consuming. With the greater use of magnesium-base alloys containing thorium and the rare earth elements, it is increasingly important that a fast and accurate analytical method be available both to the research metallurgist and to the production foundry. bfayer and Price (5) have reported a spectrographic method for determining cerium and lanthanum in magnesium alloys. but they depend upon chemical methods for the determination of praseodymium and neodymium. Some control laboratories have determined only the cerium concentration and applied a factor to calculate the total rare earth elements present. This method serves reasonably well if the rare earth elements are all\ ays added to the magnesium in the form of misch metal, and if the composition of the additives is accurately known. Horvever, there are many instances where such conditions are not fulfilled. PEClROGRAPHlC
236
e
ANALYTICAL CHEMISTRY
Because the addition of the rare earth elements can be made in any one or combination of several different formse.g., misch metal (52% cerium, 22% lanthanum, 19% neodymium, i%praseodymium), didymium (75% neodymium, 25% praseodymium), or ceriumfree misch metal (0.3% cerium, 30% lanthanum, 10% praseodymium, 60% neodymium)-it is obvious that any method that does not determine the individual rare earth elements is inadequate in many cases. It thus becomes requisite, from the standpoint of Epectroscopie analysis, that the four elements be analyzed for individually. The thorium analysis does not present the complications of the analysis for the rare earth elements and, therefore, affords a more direct approach. EQUIPMENT A N D STANDARDS
The excitation source is an airinterrupted ( 2 ) , high precision unit
Table 1.
Element Cerium Lanthanum Neodymium Praseodymium Thorium
Sample A (Mg-3TRE)a
B (lIg-4TREj
built in this laboratory. The spectrometer is a n euperimental direct reader designed and built in this laboratory. The instrument utilizes photoelectric recording of spectral line intensities arid incorporates many of the features of the original Baird-Don, Direct Reader (6). A 2-meter grating spectrometer is used. giving a reciprocal dispersion of 8.47 -4.per mm. in the first order. All standaid samples for these investigations were prepared by the AIetallurgical Laboratory of The Dow Chemical Co. Approximately 20 to 30 pounds of inagnesium were melted in a clean steel crucible. After the molten metal \\-as heated to the necessary temperature, the required additions nere made. The melting v a s flux refined and allowed to settle foi about 20 minutes. The samples were then cast in a steel chill mold to produce a pin specimen 5 mm. in diameter. All standard materials were analyzed by wet chemical methods ( 1 ) .
Analytical Line Pairs and Concentration Ranges
[Internal standard line, hlg(1) 5172.681 Analytical Concentration Line, A. Range, % 0 10 to 3 00 Ce(1I) 4149 94 0 05 io 2 50 La(I1j 4333 76 0 01 to 5 00 Xd(I1) 4109 45 0 005 to 0 75 Pr(I1) 4225 33 0 005 to 6 00 Th 3221 29
Table II. No. of
Data on Precision of Method
Analyses 25
Element Pr La Ce Nd TRE
Average Concn., yo
Coefficient of Variation, yo
0 2 0 7 1 6 0.6 3.1
2.2 1.6 0.7 0.8 0.9 1.2 2.2 0.9
25
1.3
C (lIg-3Zn-BTRE)
25
D (Mg-2Zn-3Th)
25
5
TRE, total rare earth elements.
Pr La Ce Nd TRE Th
0 2 0.7 1 8 0.7
3 5
3.3
0.7 1.1 1.1 1.1 1.5 1.2 1.4
.I
.2
Figure 1 .
3 .4
.6 81.0 2 3 4 INTENSITY RATIO
6 810.0 INTENSITY RATIO
Figure 2.
Analytical curves for rare earth elements
INVESTIGATION
The spectra of the rare earth elements and thoiium are characterized by a n abundance of weak lines n ith only a few of sufficient intensity for analytical usc. The analytical lines used for these investigations m r e chosen primarily from the standpoint of intensity and f l eedoni from inteiferenccs. Because of this, the requirements for optimum internal standard conditions as listed tiy Fassel (S) could not be given serious consideration. The analytical lines chosen are given in Table I. The discharge and exposure conditions selected for this analytical method are the results of previous studies to establish the most favorable excitation for the analysis of magnesium and its alloys. These investigations showed that, from the standpoint of accuracy and
precision, a low-energy spark discharge is superior to the high-energy excitation commonly used in the ferrous industries. Although ai157 sample form that is homogeneous, free from porosity, and representative of the material being analyzed can be used, the cast pin sample was used in this investigation to conform with the standard procedures for magnesium analysis used in this laboratory. PROCEDURE
The pin specimen is cut in half and thc adjacent ends of the two pins formed are machined on a modified lathe. About 5 mm. of the tip of each of the two pins is machined to a diameter of 4.5 mm. The tip of this reduced section is machined to a conical point
Analytical curves for thorium
having a n included angle of 170' The two prepared pins are mounted in a vertical position in screw clamps with a gap of 3 mm. Discharge parameters and exposure controls are adjusted to produce the following conditions: Capacitance, pf, 0.001 425 Inductance, ph. Discharge current, radio frequency amperes 3.0 24,500 Discharge voltage, volts Resistance in parallel with analytical gap, ohms 100,000 Number of discharges per cycle 10 3 iinalytical gap, mm. Air interrupter Electrodes, tungsten, diameter, inch 0,500 Spectral region, ,\. First order 3000toBOOO Second order 1500 to 3000 Slit width 100 Entrance, microns Exit, microns 70 to 200 Prespark, seconds 5 10 Spark exposure, seconds ANALYTICAL CURVES
Table 111.
Comparison of Chemical and Spectroscopic Results
Av. Diff.
KO.
Element Th Th TRE. TREa
of
Analyses 12 24 13 10
TRE, total rare earth elements.
Concn., yo Range 2Oto25 3 Oto3 5 3 Oto3 5 35to4O
Av , 2 25 3 23
between Chemical and Spectroscopic, yo of Amount Present 2 3
3 30
2 1 2 8
3 TO
1 2
dnalytical curves are prepared from data obtained with the described standards. Typical rurves are shown in Figures 1 and 2. Logarithmic coordinates are used to relate intensity ratios and concentrations. The usual practice of periodically running a standard sample to correct for possihle analytical curve shifts and instrument variations was followed throughout this procedure. As no attempt was made to correct analytical line intensities for spectral background, all of the analytical curves show some curvature. VOL. 29, NO. 2, FEBRUARY 1957
237
DISCUSSION
-4ny spectroscopic method of analysis in which cast samples are analyzed directly may be subject to error due to metallurgical history of the sample. However, in this analytical method there has been no evidence of such error. Additional elements in the alloy may introduce errors by direct spectral line interference. I n this respect, it is important that any instrument used for this procedure be capable of resolving the Ce 4149.9 and Zr 4149.2 lines. Additional elements may also produce errors by the enhancement or suppression of either line of the analytical line pair used. Both enhancement and suppression of the analytical lines are shown in Figures 1 and 2. The presence of zinc in any magnesium alloy containing the rare earth elements causes a definite enhancement of the Ce 4149.9 spectral line. I n practice, these alloys are classified into two general groups: those containing greater than 2% zinc and those containing less than 2% zinc. A similar effect is also exhibited by the magnesium-thorium alloys. I n this instance, manganese tends to suppress the thorium emission, producing apparently lower concentrations of thorium. These phenomena present no serious complications in the method given here because they are included in the calibration.
A statistical evaluation of the method is shown in Tables I1 and 111. All samples were analyzed using normal routine operating procedures. The coefficients of variation shown indicate that the method can produce acceptable results. Table I11 gives a comparison with wet chemical methods. These data represent the analysis of samples picked at random from daily routine work. The accuracy of the method depends upon the accuracy in analyzing the standard samples a s well as the stability of the working curves. There are, however, two factors which are iniportant when considering these comparisons. First, the differences between the chemical and spectroscopic results reflect the errors in both methods. Secondly as a matter of convenience, the two laboratories analyze different samples. This is a questionable practice because it has been shown that in many instances the discrepancies between the two laboratories have been due to sample differences. Insufficient data are available for a comparison of the individual rare earth elements. Usually, the metallurgist is interested only in the total rare earth content of the sample. Although this analytical method has been adapted to a direct reading spectrometer of special design, the procedure is in no sense unique to this instrument.
The method can be applied to any direct reading spectrometer that will permit slits to be set at the designated wave lengths. The method is also applicable to photographic instruments with the proper selection of analytical lines. ACKNOWLEDGMENl
The authors wish to acknowledge the assistance of K. R. Schmeck who made many of the analyses. LITERATURE CITED
(1) Dow Chemical Co., Midland, hlich.,
“Analyses of hlischmetal and Misch.metal-hiagnesium Alloys,” MLW 53.18, 1953. (2) Enns, J. H., Wolfe, R. A., J. Opt. SOC. Amer. 39,298-304 (1949). (3) Fassel, V. A,, Zbid., 39, 187-93 (1949). (4) Hess, T. hl., Reinhardt, L. G., Zbid., 34, 104-109 (1944). (5) blayer, A., Price, W. J., “Chemical and Spectrographic Analysis of Magnesium and its Alloys,” Waterlow and Sons Ltd., Manchester and London, Great Britain, 1954. (6) Saunderson, J. L., Caldecourt, V. J., Peterson, E. STT., J. Opt. SOC.Amer. 35, 681-97 (1945). RECEIVEDfor review June 16, 1956. Accepted October 26, 1956.
Determination of O r g a n cally Bound Chlorine in Petroleum Fractions with Oxyhydrogen Burner LAWRENCE GRANATELLI American Oil Co. (Texas), Texas Cify, Tex.
b Organically bound chlorine in petroleum fractions is converted to hydrogen chloride by combustion of the sample in the flame of a Beckman oxyhydrogen burner. Products of combustion are absorbed in demineralized water. Potentiometric determination of the chloride recovered from standards containing about 10 to 100 p.p.m. of chlorine indicates a standard deviation of about 2 p.p.m. The method of combustion, applicable to a wide variety of petroleum hydrocarbons, is rapid, as indicated by a burning rate of about 30 ml. per hour. Simplifications and improvements in the oxyhydrogen burner apparatus are presented.
238
ANALYTICAL CHEMISTRY
N
0 SATISFACTORY METHOD WBS avail-
able for the determination of chlorine in petroleum fractions in concentration below 100 p.p.m. Methods for the decomposition of organically bound chlorine in hydrocarbon materials use the lamp, quartz tube, and Parr oxygen bomb, as well as decomposition with metallic sodium or potassium and reaction with sodium biphenyl. For the determination of small concentrations of chlorine (less than 100 p.p.m,) most of these methods suffer from one or more limitations. The standard ASTRI lamp sulfur apparatus (1, 2 ) , when used for gasoline samples, yields results as much as 5% lorn, especially if tetraethyllead is present. However, the use of a modified burner (10) is reported to yield essentially stoichiometric conversion to halide ion.
The quartz tube and Parr oxygen bomb methods of combustion are of limited application because of the small sample size which may be taken for analysis. Objections to alkali metal decomposition involve introduction of chloride as contaminant by the large amounts of reagents required and complications in the application of sensitive methods of chloride determination ( I ) . Pecherer, Gambrill, and Kilcox (9) and Liggett (‘7) have employed sodium biphenyl as reagent for the decomposition of organically bound halogen. The lower limit of halogen studies by these authors was several hundred parts per million. The successful application of the Beckman oxyhydrogen burner for the determination of sulfur in these materials has been reported ( 3 ) . It was felt that the use of this method of com-
