A
Spectrographic M e t h o d for Small Amounts of Calcium in Magnesium M e t a l THOMAS WHITEHEAD, JR.,
AND
ALBERT J. BOYLE
Technical Service Laboratories, Basic Magnesium, Incorporated, Las Vegas, Nevada
rn
A successful spectrographic prqcedure i s described for the determination of low percentage calcium in magnesium metal using highvoltage spark excitation. Calcium i s determined in amounts ranging from 0.0005 to 0.170. Extrapolation of curves obtained b y a calcium addition method gave results from which are established working curves for the direct spectrographic analysis of metallic magnesium.
,
309-I?.
/,
309-2
N
0 SATISFACTORY chemical method for the estimation of as little as 0.01% of calcium in magnesium metal has been
found by this laboratory. The method of Nikitina (a,in which the magnesium metal sample is converted to metal chlorides and subsequently ignited to the respective metal oxides, is unreliable unless more than 0.1% of calcium is present. The spectrographic method described herein involves measuring the relative intensities of the spectral lines of calcium and magnesium. Subsequent e&rapolation of relative intensity ratios obtained by adding known amounts of calcium to magnesium nitrate solutions makes possible the estimation of low calcium in magnesium alloys. Rendering magnesium salts calcium-free is most difficult. Magnesium nitrate recrystallized several times from nitric acid solution shows the greatest promise; nevertheless, a small amount of calcium still remains in the salt. Hughes (4) discusses several methods of purification of base compounds especially applicable to spectrographic standards. Duffendack and Wolfe ( 8 ) and Cholak and Story (I) determine tne amount of residual impurities present by observing differences between points on the curved portion of the analytical or working curve and the extrapolated straighbline portion a t the lower percentage values of the test material. Pierce and Nachtrieb (6) discuss the estimation of residuum elements and make comprehensive compariRons of methods of photometry used in most spectrographic analytical work. Film calibration for the method hereb described is made by using the logarithmic stepped sector in a manner similar to that described by Hasler (9).
ti
odz
,002
001
PERCENT CbLClUY PDDED
Figure 1.
Extrapolation Curve
are plotted on the vertical axis,and the curve established by these three points is extrapolated to the horizontal axis. The amount of calcium present in each sample is obtained at the point where the extrapolated curve crosses the horizontal axis. This procedure is shown in Figures 1 and 2. If the extrapolated values found, and the known additions plus the extrapolated values, are plotted on coordinate paper; using the intensity ratio as the ordinate and the per cent calcium as the abscissa, a curve will be obtained which originates a t the zero point of both the horizontal and vertical axes. A second series of samples containing calcium in amounts considerably higher than the first series is treated in a similss manner. However, only one addition of calcium, corresponding to 0.04%, is made. Ratios calculated for these samples are listed in Table I1 and plotted in Figure 2. A high-voltage spark is used to vaporize the sample in this series.
PROCEDURE
Three 1-gram samples of magnesium metal from the same refinery heat are dissolved in 20 ml. of 1 to I nitric acid and diluted to approximatel 100 ml. with distilled water. Sufficient standard calcium s u d t e solution is added to the fifst two samples to raise the calcium contents 0.001 and 0.002%with respect to magnesium. No calcium is added to the third sample. This same procedure is carried out for a series of five other refinery heats, making a total of eighteen samples in all. After thorough mixin 3 dro s of each sample are placed in slightly concave, speciaky puriged, preburned graphite electrodes and dried in an oven a t 110" C. Each sample IS subjected to an 8ampere direct current arc until completely vaporized. The exposures are made on an A.R.L.-Dietert grating spectrograph, using Eastman Kodak spectrum analysis film No. 1. Ratios of relative intensity values of the calcium line 4226 A. and the magnesium line 2936 A, are evaluated, and these data are shown in Table I. Since the absolute intensity of the magnesium line 2936 A. is reproducible within * 5 % , the relative intensity values are established using the gamma of the 4226 I%. region.
Table
1.
Relative Intensity Ratios of Calcium Line 4296
A.
to
Magnesium Line 2936 d;. (On nitrate solutions of magneaium samplea with and without additions of known calcium) 0 00%
Q.0017
0.002y
Sample No.
Calcium Added
Calcium Aided
Calclum AAded
309-12 309-16 309-20 310-32 312-20 312-28
0.35 0.12 0.28 0.26 0.31 0.25
0.53 0.25 0.44 0.40 0.43 0.41
0.64 0.45 0.63 0.55 0.62 0.56
Table
11. Relative Intensity Ratios of Calcium Line 3969 Magnesium Line 2779
A.
A.
to
(On nitrate solutions of magnesium samples with and without additiona of known calcium) Sample No. 0.00% Calcium Added 0.04% Calcium Added 2058 2660 2659 2661 2664
DATA OBTAINED
The relative intensity ratios obtained from the samples with no additions of calcium and those with 0.001 and 0.002% additions 455
2.1 2.0 0.55 1.49 1.58
3.0 2.85 1.65 2.32 2.40
Vol. 16, No. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
456
Table
111. Calcium Found in Magnesium Samples
Sample No.
By calcium addition
Method By direct metal sparking
%
%
0
309-12 0.0021 0.0020 0.00075 309-16 0.0009 309-20 0.0016 0.0018 0.0019 310-32 0.0018 0.0022 312-20 0.0020 212-28 0.0016 0.0017 2658 0.093 0.108' 2659 0.020 0.017° 2660 0.092 0.112" 2661 0.083 0.096' 2664 0.083 0.096a Logarithmic stepped sector values based on first six samples.
Table IV. Determination of High Percentage Calcium in Magnesium'
44
0
.04
(0.1% Ca added to aamplea listed in Table 111) Relative Intensity Ca 3969 Calcium Calcium Sample No. Ratio Calculated Found
PERCENT CALCIUM ADDED
Figure 9.
Extrapolation Curve
309-12 309-16 309-20
e
2.10 2.05 2.05
%
%
0.1021 0.1008 0.1016
0.101 0 . ngg 0.098
I "
i
"
I
1
,301
,002
,004
,001
.01
.02
04
PERCENT CALCIUM
Figure 3.
Working Curve .ool
In order to make routine spectrographic estimations of calcium directly on metallic magnesium, the metal samples used to establish curves in Figures 1 and 2 are fastened in a lathe and faced smooth, then placed on a Petrey stand and subjected to a highvoltage spark, using a lower electrode of high purity graphite. The working curve in Figure 3 is obtained by plotting the intensity ratio values of calcium 3933 A. and magnesium 2781 A. against the calcium percentages determined by extrapolation (Figure 1). Satisfactory line densities are obtained on samples with higher calcium content by use of the logarithmic stepped sector. One eighth of the exposure can be secured by reading the calcium line in the fourth step of a logarithmic stepped sector having a ratio of 2 to 1 per step. The magnesium line used sa a reference is read in the first step. Since the light is reduced by known amounts, the intensity ratio can be readily determined. Table I11 compares results for calcium as determined by extrapolation in Figure 1with results from sparking the metal samples directly. The results on samples of high calcium content (2658 to 2664, in Table 111),are based on the direct metal working curve of Figure 3, using a logarithmic stepped sector procedure. Similar studies undertaken without the use of a stepped sector to establish data for direct calcium estimation of samples with higher calcium content give results agreeing very well with those in Table 111. Figure 4 shows a working curve for metal samples containing higher percentages of calcium, developed in the same manner as that shown in Figure 3. To a nitrate solution of 1 gram of the 6rst three samples listed in Table 111,sufficient calcium is added to
01
92
.os ,001 PERCENT CALCIUM
,03
Figure 4.
.I
Working Curve
increase the percentage 0.1% with respect to the magnesium present. The percentages of calcium indicated when the ratios obtained by this procedure are referred to Figure 4 are listed in Table IV. The calcium thus found substantiates the data listed in Table I1 and plotted in Figure 4. SUMMARY
A method is developed for the direct spectrographic estimation of low percentage calcium in magnesium metal samples. Calcium-free magnesium salts are difficult to obtain. Therefore, known amounts of calcium sulfate solution are added to nitrate solutions of the magnesium samples to be analyzed. Subsequently intensity ratios of calcium and magnesium lines representing these samples are established, and percentages of calcium in the metal determined by extrapolation. A working curve for rapid routine analysis by sparking the metal directly is constructed. LITERATURE CITED
(1) Cholak, Jacob, and Story, R. V., J . Optical SOC.Am.,31, 730-8 (1941). (2) . , Ddendack. 0.5.. and Wolfe, R. A., IND. ENQ.CHEM.,ANAL. ED.,IO, i6i (1938). (3) Hasler. M.F.,J . Optical SOC.Am., 31,140-5 (1941). (4) Hughes. R. C., Ibid., 33, 49-60 (1943). (5) Nikitina, E. I., Zauodskaya Lab., 9,1319-20 (1941). ( -, 6) Pierce. IND. ENO.C H ~ .ANAL. , .-. .. , W.C.. and Nachtrieb. N. H., ED., 13,774-81 (1941). ~
