Spectrographic Determination of Sodium and Potassium in Coal Ashes CHARLES H. ANDERSON and COlT D. BEATTY The Babcock & W i l c o x Co. Research Center, Alliance, O h i o
T
HI.: authors' company is investigating the untlerlying c:iuses of fireside deposit formation in high-duty boiler units, and emphasis has been placed on these causes as they are related to coal and coal ash rhemist,ry. The most promising approach toward reducing the time required for the many necessary analyws Ltppeared to be through t,he development of quantitative sprctrographic nic.thods for determining sodium and potassium, the constituent,< \rliic.h apparently are a cause of the deposit difficulties. Flame, photometric methods for det,ermining thr a1lc:tli met:tlr have bern populitr. but have the disadvantage of requiring a prior dissolution of the sample. Helz ( 7 ) and Helz anti Scrihnrr ( 8 )have described methods for determining the alk:ili grouy) elemc,nts i n portlarid cement using both silver and cohtlt :is irit,ernal rt:tndarcts. Other workerst in malyzing cracking catalysts (4):xiid c ~ t ( 9l ) ~,h:tvc, relied upon rxternal standards for qu:trit i t.:ttivr c:dil)r:itions This l ) : t p ( ~describes a spectrographic method of determining sodium ;inti potmsium in coal ashes over a range of 0.3 t o 5.0%.
Initially the rubidium 4215.6 line was investigated as an internal standard for both sodium and potassium. Moving plate studies were made for the three elements and are shown in Figure 1. Since the potassium-rubidium intensity ratios were fairly constant with time, it was concluded that this line combination was satisfactory in so far as excitation and volatilization characteristics were concerned. The sodium-rubidium ratio varied irregularly, but the variations mere not considered excepsive in view of previously reported studies on the rare earths (3). Precision studies, tabulated in Table I. however, showed that while potassium-rubidium ratios were of satisfactory precision, the sodium-ruhidium ratios showed large deviat,ions. As precisions of l2.8yOwere not acceptable for the sodium determination, attention was given to the use of lithium as an internal standard for this elemmt. Barium carbonate was chosen as a buffer matprial in order to minimize self-ahwrption and extraneous effects, :tnd to maintain an even-burning :trr.
EXPERIMENTAL
Apparatus. Tlic. c~quipinentused in this investigation \vas :t Jarrell--4~11.TTads\vortti mounting, 3.4-meter grating spectrograph, a Sational Spectrographic Laborat'ories Spec-Power source unit, an rlpplied Research Laboratory developing machine, and a Jarrell-.ish densitometer.
7-0
I
Choice of Analytical Lines and Internal Standards. It was considered practical t o use methods that could be applied directly to a powdrred sample with a minimum of sample preparation, and because of its convenience and simplicitj-, a direct current arc was chosen as the excitation source for this work. As it has been shown ( 3 )thiit excellent results r:tn he obtained if the volatilization and escitat,ion characteristics of the internal standard are similar to those of the element to he determinrti. it was of importance to rhoow the proper internal standard for sodium :ind pot;wsirim.
Table I .
Precision S t u d i e s K 4044.1 Rb -1216.6 1.90 1.64 1 .7.5 1.82 1.77 1 . 09 1.72
Average Average deviation, X Standard deviation, S
1.71 1.70 1 73
3.2 4 r,
Intensity Ratios Na 3303.0 N a 3303.0 R b 4216.6 Li 3232.6 2.10 0.96 1.60 1. 0 3 1.51 1.01 1.60 1.03 1 .4.5 1.00 2.34 0.99 1.50 0.96 1.61 0.93 1.77 1.61 1.00 12.8 3.2 20.4 3.9
..
The alkali lines lying in the near ultraviolet (sodium 3303.0, 3302.3; potassium 4044.1, 4047.2) were of the proper intensity to be used at concentration levels of 0.3 to 5.0%. The sodium lines are relatively free from interferences (zinc 3302.9 is the only serious interference); but e\-anogen bands and the iron 4045.8 line interfere x i t h the potassium lines in this region. I n order t o reduce these interferencee with the potassium lines, the sample was diluted with a lithium carbonate buffer and excited in a low amperage direct current arc. This buffer not only reduced the interferences with the potassium lines but also reduced the cyanogen int,ensity to such an extent that the rubidium 4215.6 line (refrrrrrl t o tsrlon-) W:LS r r h t i w l y free from interferrnccs.
h
TIME IN
/
SECONDS
F i g u r e 1. Time Versus I n t e n s i t y R a t i o
Although moving plate studies showed the sodium-lithium ratios to fluctuate almost as much as the sodium-rubidium ratios, nevertheless, the integrated sodium-lithium ratios showed a great improvement in precision, as is indicated in Table I. This m:ty be explained in part by a comparison of the excihtion potenti:& of these lines. The excitation potential of lithium 3232.6 is much more favorable than rubidium 4215.6 for use with the sodium lines, but on the other hand the excitation potential of rubidium 4215.6 is more favorable for use with the potassium lines (6). Effect of One Alkali on Determination of the Other. In determining sodium in portland cement, Helz ( 7 )found that the 110tassium concentration had a marked effect on the sodium-cobalt ratio. Workers in flame photometry have also observed the effect of potassium on sodium-lithium ratios ( 1 ) . The reverse effect of sodium on the potassium determination has not been so clearly defined, and in flame photometry appwently it is not agreed whether the potassium line is enhanced or repressed hy the addition of sodium in t,he flame. Hence, it was thought advisable to investigate this experimental variable.
.i synthetic standard sample was prepared (see Quantitative Calibrations) containing 2.50% sodium oxide added as the sulfate and no potassium. The sample was mixed with the barium carbonate buffer containing 1.50'% lithium carbonate. Known varinhlr, amounts of potawium sulfate were added to t,his miuturr : the .:nmples were arrcd in triplicxte and photographed, 1369
ANALYTICAL CHEMISTRY
1370 Table 11. Operating Conditions for Sodium and Potassium Determination in Coal Ashes Sample charge
Wave length photographed, 4. Weight of charge, mg. Anode
Potassium 20.0% ash 2.56 RbzCOs 77 4 LizCOa
Sodium 14 37, ash 1 30 LnCOa 79 2 Ba2COz 5 2 IiZSOI
3400-4600
2700-3900
10 3 / ~ - i n c h diameter graphite, i e ~ u lar grade, 1.5 inches long drilled to depth of 4 mm. l/i-inch diameter carbon rod, 1.J inches long and pointed
Cathode
7
Gap, mm. Source Time Order slit, mm. Sector Emulsion Development
220-volt direct current, 4 amp. 1 minute
special precautions were taken in so far as this determination was concerned. Quantitative Calibrations. Synthetic standard samples were prepared by dry grinding the proper amounts of alkali sulfate into an alkali-free synthetic coal ash prepared from spectrographically pure metal oxides. The composition of the alkalifree base material was as follows: 42% silicon dioxide, 30% ferric oxide, 20% aluminum oxide, 5.0% calcium oxide, 2.0% magnesium oxide, and 1.0% titanium oxide. Known samples containing 5.0, 2.50, 1.25, 0.62. and 0.31% of each alkali calculated as the oxide were prepared. These samples were then mixed \iith the proper buffer, excited, and photographed under the conditions listed in Table 11.
K4044.1 Rb4215.6
and the intensities were measured, all according to the conditions indicated in Table 11. A parallel procedure was followed for a synthetic mixture containing 2.50% potassium oxide and variable amounts of sodium sulfate. The results of these experiments are illustrated in Figure 2. The potassium additions had the effect of increasing the sodiumlithium intensity ratios to a significant extent. I n order to determine how this effect is brought about, the absolute intensities of the sodium and lithium lines are shown in Figure 2 as a function of the potassium addition. Apparently the effect of the potassium is to increase the intensity of the sodium line and to decrease slightly the intensity of the lithium line.
0.3
0.5
Figure 3.
1.0 2.0 PERCENT N o Z O
5.0
0.3
05
10 . 2.0 PERCENT K 2 0
5.0
Calibration Curves for Alkali Determination
The per cent transmittance data were converted to intensity ratios according to st'andard procedures ( I ) . No background was observed with these line pairs. In order to reduce emulsion l'a 3303.0 calibration errors, intensity ratios for two line pairs Li 3232.6' S a 3302.3 K 4044.1 K 4047.2 Li 3232.6, Rb 4215,6 of each element were calculated
Rm
1
[
and two working curves for each element were obtained. When samples are analyzed, only that ratio nearest unity is used. Figure 3 shows the working curves which were obtained when log intensity ratio was plotted against log concentration.
t/ 07!
'
"
'
1I.0
"
"
2.0 I
MG EXTRANEOUS MATERIAL PER IO MG. BUFFERED SAMPLE 0 Na:! SO, 0 KeS04
Figure 2.
Effect of Additions of One Alkali on Intensity of the Other
The effect of additional potassium on the sodium-lithium intensity ratio becomes less a t higher concentrations of potassium, and this agrees a-ith observations previously made in studies on extraneous element effects (2). The method used by Duffendack ( 2 ) for eliminating this effect was applied in this case by including a k e d amount of potassium as potassium sulfate in the barium carbonate buffer mixture. Except for a small initial reduction in the potassium-rubidium ratio, the subsequent additions of sodium as sodium sulfate to the potassium standard showed very little effect. .4s a result, no
Procedure for Analyzing Samples. The sample, ground to pass 200 mesh, is mixed in the proper proportions with buffer and internal standard. This is most easily accomplished by preparing a large amount of the buffer mixture ( a ) containing 3.20% of rubidium carbonate (Rb?CO,) and 96.8% of lithium carbonate (Li2C03),and ( b ) containing 1.50% of lithium carbonate 92.5% of barium carbonate, 6.07, of potassium sulfate (K2SOa). For the potassium determination a sample to buffer ratio of 1 to 4 18 used; for sodium a ratio of 1 to 6 is used. Duplicate 10-mg. samples are weighed into electrodes, the samples are excited, and tht. film processed under conditions list'ed in Table 11. Time Requirements. One man can complete the sample preparation (not including grinding the sample to 200 mesh), excitation, and densitometry for one sample in duplicate for both sodium and potassium in approximately 1.5 hours. This represents a considerable Paving in man-hours over the classical wet. method of analysis. SCOPE OF METHOD
This method applies not only to coal ashes, but to other products from the combustion of coal, such as tapped slag, fly ash, and tube deposits. There have been occasions when the alkali content was found to be greater than the 5% concentration limit of the working curves. In theqe cases, satisfactory results have
V O L U M E 2 6 , NO. 8, A U G U S T 1 9 5 4
1371
been obtained by increasing the ratio of buffer mixture to sample and multiplying the result by the proper factor. EFFECT O F LITIIIU.11 I Y COAL ASH
During the preliminary investigation of the method, many coal ashes and fireside deposits mere examined qualitatively for lithium. In no case v a s the lithium 3232.6 line detected and it was estimated from a synthetic standard that the limit of detection w-as 0.05% lithium as Li20.
Table 111.
Comparison of Spectrographic and Chemical Results K20,%
NanO, % Spectrographic
Sample
Spectrographic
Chemical
Chemical
However, variable amounts of lithium oxide a t the conccntration level recently reported hy Headlee (6) would cause small, variable errors in the sodium dctermination. In t,he event that the coal contains significant am Iurits of lithium, the method is modified by ( a ) adding 0.20’34 lithium oxide to the standard and ( b ) doubling the amount of lithium carbonate used in the sodium buffer. PRECISION AND ACCURACY
The standard deviation for the method, as determined from data selected a t random over a period of 3 months, was 4.1% for the potassium and 4.301, for the sodium determination. The accuracy of the method is somewhat difficult to determine because there is a lack of standard samples of these materials
in Table 111. There is a tendency for the spectrographic results to be consistently lower than the chemical values. However, in view of the difficulty in obtaining agreement on the chemical sodium and potassium determinations on silicate rocks which as reported recently ( I O ) , the comparison appears adequate. SUMMARY AND CONCLUSIONS
.4 spectrographic method for determining sodium and potassium that is applicable to the analysis of coal ashes, tapped slag. fly ash, nnd tube deposits has been developed. It was advisable to det’ermine each element on a separate sample, so that the influence that’ potassium exerted on the sodium determination could be minimized. The cyanogen and iron interferences with potassium and rubidium lines mere eliminated by using lithium carbonate as a buffer. Rubidium \vas found to be an excellent internal standard for potassium, and the potassium-rubidium ratio was apparently independent of the sodium content. The time requirement of 1.5 hours for the determination, in duplicate, of both elements represents a substantial saving in time over the classical chemical methods for these elements. The accuracy of the method, as nieasurPd from a comparison of spectrographic and chemical results, ia -.:Ltisfactory. LITERATURE CITED (1) Churchill, J. R., I s n . ENG.C H E x , A k ~ . kEo., ~ . 16, 653 (1944). (2) Duffendack, 0. S., Wiley, F.H., and Owem, ,J. S., Ibid.,7, 410 (1036).
( 3 ) Fassel, V. A , , J . Opt. SOC.A m e r . , 39, 187 (1949). (4) Fast, E., ANAL.CHEX.,22, 320 (1950).
(5) Harrison, G. R., Ed., “M.I.T. Wavelength Tables,” p . xis, New York, John TViley & Sons, 1938. (6) Headlee, A. J. TV., Mining Eng.. 5 , 1012 (1953). ( 7 ) Helz, A., J . Research Natl. Bur. Standads, 34, 129 (1945). ( 8 ) Helz, A , , and Scribner, B. F., Ibid., 38, 439 (1947). (9) Hunter, R . G., and Headlee. A. J. W., A X ~ L CHEY.,22, 441 (1950). ( I O ) Schlocht, W. G., I b i d . , 23, 1568 (1951).
Rapid Photometric Determination of Chromium In Alloy Steels and Bronzes MORLEY
D. KAHN
and FRANK J. MOYER
Specialloy, Inc., Chicago, 111.
THE
need for a more rapid, accurate, routine photometric determination of chromium in ferrous alloys of all compositions has long been recognized. There is a particular need for a determination applicable to stainless steels (8 to 30% chromium), alloy cast irons (0.5 to 15% chromium), and chromium-bearing bronze alloys (0.5 to 10% chromium). Fowler and Culbertson ( I ) have developed a photometric method based on the absorption of the orange-red sexivalent chromate ion. However, the presence of a number of colorproducing ions and the necessary provisions required therefore limited application for the purposes of this work. It has been suggested by Vredenburg and Sackter ( 3 ) that the aqua-colored chromic ion complex in a solution of nitric, sulfuric, and phosphoric acids will lend itself to photometric measurement. The presence of molybdenum and copper in the alloys investigated constituted a source of interference. The time required for decomposition further restricted the use of this method on alloys studied in the experiments.
Trace concentrations of chromium can be determined by photometric analysis through the use of s-diphenyl-carbazide ( 2 ) . The alloys investigated had chromium contents of such magnitude as to render this reagent unsuitable. METHOD
d number of experiments were made by dissolving and aubsequently oxidizing the selected alloys through the use of perchloric acid. The resultant sexivalent chromium was reduced with metallic zinc to an aqua-colored complex, and the solution was boiled to stabilize this complex in the trivalent state. The absorbance of this solution was measured in acid media on a Brociner-Mass photometer a t a wave length of 585 mfi. Cupric ion was reduced by the zinc to elemental copper. Hydrogen peroxide mas added to prevent interference in the presence of molphdenum. Phosphoric acid was then used to decolorize
