Common-Matrix System of Saectrochemical Analvsis I
I
J. R. WEAVER AND R. ROBERT BRATTAIN Shell Development Company, Emeryville, Calif. A general method of emission spectrochemical analysis is described which is applicable to any inorganic material. Lithium carbonate, which acts as a flux and diluent, is used to modify extensively the effects of different major constituents present in the samples. The method is rapid, uses only a small amount of sample, and yields results that are usually correct within a factor of 2.
T
HEREaremany instances where a semiquantitative spectrochemical analysis will serve as well and cost less in time and money than an accurate quantitative analysis. This is particularly true when a wide variety of materials must be analyzed and the cost of establishing a spectrochemical system for one or two samples is prohibitive. The method described yields semiquantitative results usually correct within a factor of 2, is rapid and applicable to practically all inorganic materials, and requires only a small amount of sample. One of the many factors which dictate the method of analysis to be employed in any given problem is the degree of accuracy required. For the sake of convenience, accuracies may be spoken of in terms of the expressions of Wright (@--Le., guesses, estimates, and determinations. Spectrochemical methods are capable of any degree of accuracy from purely qualitative results to quantitative determinations with accuracies of *2% of the amount of the element present. Lockyer ( 4 4 ) may be credited with the discovery of quantitative spectrochemical analysis in 1873, and since that time there have been many successful methods published for spectrochemical determinations. However, the type of analysis that yields results of intermediate accuracy (estimates) was practically left untouched when the major efforts were turned to the search for refinements that would yield accuracies of * 10% or better. A method described by Harvey ( 2 ) appears to have been the only serious attempt to generalize semiquantitative analysis which has been published. Thus, there are very feTy methods described in the literature which yield results of intermediate accuracy and are a t the same time simple, rapid, and applicable to almost any material. The method described here is the result of an attempt to fill these requirements. DISCUSSION O F T H E PROBLEM
The effect of one element on the behavior of another in the arc is a determining factor in the approach to any generalized spectrochemical method. Although the effects of small concentrations are usually not serious, variations in the major constituents have severe and easily demonstrable influences on the spectra of each other as well as on the minor constituents. The simplest solution to this problem appears to be to give all samples the same major constituent, thus reducing the concentrations of all elements to the level of minor constituents or traces in the common matrix. At the same time the diluent may be selected for the fluxing action it has on the sample, thus making it possible to reduce a variety of chemical combinations to a common form. Eighteen substances were tested in the arc to observe their burning characteristics. These substances, with observations on burning behavior, are shown in Table I. Of the seven fluxes that were observed to burn well in the arc, four mere not investigated
further because of special considerations. Thus, sodium fluoride is toxic and liberates noxious fumes in the arc, potassium carbonate is hygroscopic, and potassium acid sulfate cakes when ground in a mortar. Although anhydrous sodium borate burned well, the spectrograms \vere badly obscured, apparently by band spectra. The three remaining fluxes, barium, sodium, and lithium carbonates, were tested more thoroughly. Calibration curves were prepared for several elements in these fluxes and precision and accuracy were st,udied. I t was observed that the presence of alkali in samples tended to suppress the spectral lines of other elements; for this reason an alkali flus appeared attractive, as it' would overshadow in effect the relatively small variable amounts of alkalies already present in the sample. As the finai choice lay between sodium and lithium carbonates, lithium carbonate ~ n chosen s because it is frequently necessary to determine sodium, whereas lithium is seldom encountered. The USP of lithium carbonate in spectrochemical analysis is not without precedent. DeGramont ( 1 ) used lithium and sodium carbonat,es a.i fluxes in the preparation of samples for spectrographic analysis Harvey ( 2 ) recommended the use of lithium carbonate as a flus and buffer under certain circumstances. Wilson ( 7 )also employed lithium carbonate as a diluent in a spectrochemical method which depended for its interpretation on the ratios of intensities of selected spectral lines to the intensity of a lit,hium line without resort to calibration curves-Le., ratios were obtained but not actual concentrations of elements in the sample. APPARATUS
An Applied Research Laboratories 1.5-meter grating spectrograph, densitometer, and arc source were used. 4 special type of electrode (anode), similar in design to that described by Hasler ( 3 )was prepared with the aid of a commercially available cutting device. The dimensions of the electrodes are shown in Figure 1.
Table I.
Fluxes Tested for Arcing Characteristics
Flux
Observations
PiatBiO7 Nd&0;.5HzO BaFl BaCOa PbsO! Na(hHdzPO4 NaF CaClz XaBOs KaBOa.4HzO Bz08 NaHCOa KHCOs KzCOa SaaPO4 KHSOi SazCOs LizCOs
Burns uell Unsatisfactory. Jumps out of arc Unsatisfactory. F o r m ball t h a t jumps out of arc Burns well Unsatisfactory. Forms lead pellets t h a t j u m p out of arc Unsatisfactory, Blows o u t of arc Burns well Unsatisfactory, Flutters and sputters Unsatisfactory. Blows out of arc Unsatisfactory. Blows out of arc Unsatisfactory. Flutters Unsatisfactory. J u m p s out of arc Unsatisfactory. Jumps out of arc Burns well Burns fairly well. Does not wet electrode Burns well Burns well Burns well
1038
lo^
V O L U M E 21, NO. 9, S E P T E M B E R 1 9 4 9 &I ETHOD
10 0
Purification of Lithium Carbonate. Dissolve 2.2 kg. (5 pounds) of lit,hium nitrate in water a t 40" C. to make 2 liters of solution. Pour part of the solution into a large platinum dish and supercool the contents of the dish, in an ice bath, to 15" C. If crystallization occurs before this temperature is reached, redissolve the crystals and cool again. .It 15"1.C.Quickly seed thestir solution the crystalline 11-ith a lithium slurry nitrate and
~~
immediately on a Khatman No. 41 filter or its equivalent in a Buchner funnel. Crystallize the entire 2 liters in this manner. Save the mother liquor for a subsequent second yield. ir Fill the platinum dish with crystals from the nrst crystallization, add not more than 1 ml. of water for each 100 ml. of crystals, heat to 40" C., precipitate crystals, and filter as before. Process the entire yield of crystals from the first crystallization in this manner. Combine the mother liquor with that from the first crystallization. Recrystallize the twice crystallized crystals a third time as '0 001 described above. Place the thrice crystallized Lithium nitrate in a platinum dish, add not more . of water for each 100 ml. of crystals, and ompletely a t 40" C. Filter the solution S o . 41 paper in a Buchner funnel. Prepare a saturated solution of ammonium carbonatein water at room temperature. Filter. Place 3 volumes of ammoniuni mrbonate solution in a platinum dish and add to it 1 volume of yolution, at 40" C., of the thrice crystallized lithium nitrate. Stir, covw, and allow to stand overnight. Filter the precipitated lithium carbonate on a S o . 41 paper in a Buchner funnel. With the vacuum off, flood the crystals with 200-proof ethanol, stir, and then remove the ethanol by vacuum filtration. Wash the material a total of three times in this manner. Place bhe crystals in a platinum dish, dry a t 105' C. for 30 minutes, and ignite at 450" C. for 30 minutes. Reprocess the mother liquor from the lithium nitrate crystallizations for a second yield. Calibration. Prepare a mixture of 1Yc of each element to be determined (carbonates are desirable) in lithium carbonate and grind together thoroughly. Progressively dilute these with more lithium carbonate so that standards containing 0.3, 0.1, 0.03, 0.01, and 0.00370 are obtained. Pack the annular spaces about the center posts of t,hree or more electrodes level full n i t h each standard. Adjust the arc gap to 6 mm.; do not adjust it while the arcing is in progress. -%rceach electrode with a current of 7.5 amperes until the sample is burned away. The complete burning of tlie sample is clearly defined by a sudden change in the color xnd sound of the arc, and an increase in the potential drop acrov
2z
1039
*
I 01
0 01 CONCENTRATION,
Figure 2.
Table 11. Element
AE .II
B Ea Be Bi Ca
Cd
co Cr
cu Fe
K
lzIg
.\In
Line,
10
Z
Calibration Curves
Spectral Lines Used and Ranges Covered
.i.
3382.0 3082.2 2660 4 2496.8 2497.7 4554.0 3071.6 3130.4 3067.7 2808.0 4302.5 4318.7 3179.3 3261.1 3453.5 3334.1 4254.3 4289.7 4351.8 3271.0 2'361.2 3020.6 3021.1 3008.1 4044.1 22gf.5 2tt3.8 4030 8 2376.1
Range, %
Element
0.0001-0.03 0.001-0.1 0.03-1.0 0.03-1.0 0.01-1.0 0.0003-0.01 0.03-1.0 0.003-1.0 0.003-0.1 0.1-1.0 0.001-0.1 0.03-0 3 0.3-1.0 0.03-1.0 0.003-0.1 0.03-1.0 0.001-0.01 0.003-0.03 0.01-0.3 0.001-0.03 0.1-1.0 0,003-0.1 0.01-0.3 0.03-1.0 0 03-1.0 0.001-0.03 0.03-1.0 0.001-0 01
>I0
0.01--1.0
Na
Xi
P Ph dh Si
Sn Sr
Ti
v w Zn ZC
Li
Line,
A.
3132.6 3112.1 3302.3 2852.8 3414.8 3391.1 2536.7 2553.3 2833.1 2663.2 2698.1 2516.1 2881.6 2514.3 3175.0 2840.0 2429.5 4077.7 4438.0 3372.8 3217.1 3184.0 3202.4 4008 8 2947.0 3282 3 3392.0 2475.3
Raqxe, ; .c 0 001-0.1 0.1-1.0 0.01-1.0 0 . 2 - 1 , I) 0.001-0 03 0.03-1.0 I .o-3.0 1,0-3,0 0,003-0.3 0.1-1 0 0.1-1.0 1).01-1.0 0.01-1 0 0.1-1.0 0.003-0.1 0.003-0 3 0.1-1.0 0.0001-0.003 0.3-1.0 0.003-0.3 0.03-1.0 0.001-0.03 0,03-1.0 0.03-1.0 0,1-1,0 0.03-3.0 0 01-1.0 Reference
the arc. The average burning time is about 60 seconds. Lyse a slit width of 50 microns. Process the film (Eastman Spectrum Analysis S o . 2) and measure the transmittances of the spectral lines shown in Table 11. Adjust the densitometer to read 1007, transmittance on the unexposed film; make no background corrections. From an emulsion calibration (gfmma) curve previously prepared for the region 3100 to 3400 A on the emulsion bearing the same emulsion number, determine the relative intensity of each line measured and calculate the ratio of line tensity to the intensity of the chosen lithium line (2475.3 A). Average the three or more ratios so obtained for each line and plot concentrations us. average intensity ratios as shown in Figure 2. Samples. Weigh one part (about 10 mg.) of dry, finely pulverized sample into a mortar. Stir into this a small amount (0.5 to 1.0 mg.) of purified gentian violet dye. Add 9 parts of lithium carbonate and grind the materials until they assume a uniform violet color. Fill and arc the electrodes under the same conditions as prevailed during calibration and measure the spectral line transmittances of elements to be determined. If any intensity ratio is beyond the range of the calibration curve, dilute the sample still further with lithium carbonate and repeat the arcing. From the calibration curve read the concentration of the element in the lithium carbonate mixture and multiply this by the dilution factor to obtain the Concentration in the original sample.
"-
w+ NATIONAL CARBON SPECIAL SPECTROSCOPIC GRAPHITE CATHODE ANODE
Figure 1.
Electrodes
A N A L Y T I C A L CHEMISTRY
1040 _.__ _. ._
Table 111.
Results of Analysis of Bureau of Standards Samples Found.
%
Iron ore. magnetite
Si
29a
11n
2 Ti
Dolomite Opal glass
Chrome refractor)
Si
88 91
Fe
Ph Si A1 11g Fe
103
Ti
Mn Ca Cr Flint clai
Ft.
97
Sa Si
Zr
hlg
K A1
Ca .56a
Lead-base hearing metal
53a C
Phosphor-bronir bearing mrtal
ti3'
Ti Cr Si AI P J1g Fe Na Ti &In K Pb Sn Fe cu Zn Sb Sn Bi
.4e
Pb Sn Fe
AI Si
V
cu
Si bin Cr iv dteel, Ni-hlu fSAE 4620,
1118'
M I 1
R.10 Xi Cr
1.34 0.023 0.057 0.24 0.18 0.15
1.4 0,025 0.057 0.21
0,057 0,090 3.8 11.1 9.8 11.2 0.50 0.16 0.56 25.2 0.69 0,089 20.0 0.18 0.157 0.49 20.5 0.07 1.43 0,054 5,15 1.08 14.4 0.08 1.50 0.207 0.048 0.14 0.23 0.90 0.99 0.21 70.36 27.09 10.28 10.22 0.05 0.006 9.74 9.91 0.27 0.05 0.48 0.97 0.047 0.045 0.28 3.52 18.25 0.74 0.222 1.74 0.243
0,044 n. 081 5.5 16, 10. 17. 0.52 0.14 0.27 33
0.21
0.19
0.61 0.060 25.
0.24 0.25 0.47 23, 0.023 2.3 0.070 6.0 1.0 20. 0.11 1.7 0.17 0.040 0.11 0.20 0.60 1.0 0.16 75. 32. 11. 9.7 0.048 0,0065 7.6 11. 0.21 0.026 0.85 I .3 0.028 0.11 0.25 3.5 18. 1. 5 0.20 1.6 0.28
Sample
Error Factorb 1.04 1.09 1.00 1.14 1.17 1.26
Name
No.
Steel 1SCr-11Ni (cdlumbium bearing)
123aC
Argillaceous limestone
la
1.13 1.48 1.25 1.33 1.59 1.04 1.12 3.05 1.61 1.30 .16 .08 .39 ,38 .I3 .22 .20 .27 .I5 1.50 1.01 1.31 1.07 1.18 1.07 1,05 1.04 1.08 I .28 1.11 1.28 1.92 1.77 1.34 1.68 2.45 1.12 1.01 1.01 2.03 1.11 1.09 1.06
ANALYSIS 01.' NATIONAL BUREAL OF STANDAHUS SAMPLES
Twenty Sational Bureau of Standards samples of different types were analyzed for a total of 111 determinations. Metal samples were weighed, dissolved in acid, ignited, and weighed again to obtain a factor by which the known concentrations of elements in the metals could be multiplied to find the true concentrations in the salt. The samples thus prepared were then diluted with lithium carbonate in the usual manner. The results are shown in Table 111. The average error factor was 1.47, and 91yoof the results were correct within a factor of 2.0. This indicates that the results obtained by this method are accurate to only one significant figure; the second significant figures reported in Table 111 are in doubt. Results of analyses are therefore usually reported to two significant figures and the second figure is designated as doubtful. DlSCU s SlOh
A wide variety of material has been analyzed by the method described and in many cases the accuracy has been verified by subsequent chemical analyses. The method has been found particularly us~fulin the analysis of lubricating oils containing metallic
Si
M0 co SI 41
I!.
p
Fe '?a Ti Mn K Br
1.30 1.11 1.45 1.44 1.02 1.52 1.04 1.14 2.07 I .31
Elements
Calcium molybdate
71
Fe
M0 TI
Fluorspar
79
SI Pb
Me
Fe
9I
Ba
Pb-Ra glasc
89
Pb 51p 41 Na hln
K
Ca Ba Borosilicate glaas
93
Soda ieldspar
99
1.09 1.14 1.06 1.40 1.07 1.07 1.28 1.52 1.20
1.92 35.3 0.06
3.2 25, 0.040
1.61 1.41 1.50
0.88 0.23 0.079 0.105 0.01 0.063
1.6 0.17 0.050 0.068 0.082 0.060
1.82 1.35 1.58 1.55 8.20 1 .os
16.0 0.016 0,096 4.2 0.068 6.97 0.15 1,25
12. 0.075 0.064 2.9 0,050 5.5 0,085 0.50
1.33 4.17 1.50 1.45 1.36 1.27 1.77
3.08
A1
2 Na K
B
11g Fe
I1 Ya K Ca Ba (1
n.io
7.2 2.5 1.4 1.6 0.31 0.090 0.037 0.90 0.12
10.1 0,032 0.047
CS 128
6.6 2.2 1.32 1.14 0.29 0.096 0,029 0.59
11
Ti
Soda-lime glase
0.70 0.11 0.26
6.5
Fe Sa Q8
0.46 0.12 0.75
Factor 0 1.52 1.09 2.8R
%
3.94 0.053 1.03
518
Plastic clas
Found,
%
B F€
I1 Ya
Error
N.BrS.",
2.50
0.035 0.60
2.4
1.65 1.52 1.72 1.25
*.O
9.2 0.085 0.037 5.1
1.10 2.66 1.27 1.27
17.9 0.43 1.43 0.21 0.86 2.63 0.15
11. 0.33 1.6 0.17 0.57 2.2 0.060
1.63 1.30 1.12 1.23 1.52 1.20 2.50
0.66 1.9 0.030 0.73
1.4U 1.06 1.11 1.34 1.23 1.37 1.51 1.46
0.47 2.02 0.027 0.98 6.2 0.82 3.33 0.44
7.6 0.60 2.2 0.65
Calculated from National Bureau of Standards certificate values.
b Error factor is defined as factor b y which result must be multiplied, if too small, or divided, if too large, t o equal correct result.
Metal samples. Metals were converted to salts a n d Bureau of Standards !alues converted t o concentrations in salts,
additioris, catalysts, engine deposits, aud corrosion products. l u these laboratories many materials submitted for chemical analysk are first analyzed in this fashion. Few serious cases of interference between elements have been 1,bserved. When such cases do occur, they are frequently detect,ed by the discrepancy of results obtained from two different spectral lines of the element being determined. The judicioue choice of lines for calibration and the use of reliable wave-length tables will practically eliminate interference difficulties. Samples that contain significant amounts of organic matter or water are ashed prior to the addition of lithium carbonate .\Iet,allic samples are dissolved in acid and ashed as described rindw Bureau of Standards samples. The calibration curves may be used indefinitely; the small h i f t s which are so noticeable in accurate quantitative work art uot of sufficient magnitude to have a significant effect on the accuracy of the method, Of course, if the arcing or instrument caonditions are changed, t,he curves should be checked. Commercially available lithium carbonate was contaminated with undesirable impurities and had t,o be purified before it could be used. All precipitations were made in platinum dishes and great care W ~ Sexercised t.o prevent contamination in all
1041
V O L U M E 21, NO. 9, S E P T E M B E R 1 9 4 9 stages of the process. The resulting lithium carbonate wab analyzed spectrographically and found to contain only traces of copper (about 0.0003%), calcium (about 0.001%), and strontium (less than 0.0001%). To correct for the effect on the calibration curves of the residual impurities in the purified lithium carbonate, the concentrations of elements added t o the lithium carbonate were plotted rather than the sum of the residual plus the added concentrations. Thereafter, when the curves were used, the concentrations of elements in the lithium carbonate-sample mixture were read from the curves as the amounts in excess of the concentrations present in the lithium carbonate. The use of gentian violet dye in the sample nniture serves only to show when the components are thoroughly mixed. It has no effect 011 the arc. Commercially available gentian violet (certified grade) contained a high cnncentration of rodium and had to be
purified. The dye was twice crystallized from ethanol by the addition of mater. The crystals were filtered in a Buchner funnel and dried 21 hours in a vacuum desiccator over Drierite. LITERATURE CITED
[ I ) DeGramorit,,11. .a,, Bull. soc. franG. minkral., 44,77-95 (1921). 12) Harvey, C. E., "Jlethod of Semiquantitative Spectrographic Analysis," Glendale, Calif.. Applied Research Laboratories. 1947. ~. -.
( 3 ) Ifasier.. AI. I;.. Harvey, C. E., and Dietert. H. TV.. IND.ENC). CHEN... h x i . . ED.,15, 102-7 (1943). (4)Lockyer. S . J.. Phil. Trans., 163, 263-75 (1373). ( 5 ) Ibzu., 164, 479 94 (1871) (6) Ibid.. 164,495-9 (1874). 7) Wilson, H D. B., Econ. G d , 39, 37-55 (1944). ( 8 ) TTIiqht, T. A . , Am. Sor. Testing Tlatprinls. Proc., 40, 1355 (1940) R E C E l r F n ~ u + Z l l - t10,
1948.
Direct Spectrochemical Analysis of Solutions Using Spark Excitation and the Porous Cup Electrode CYRL S FELI)TI.I\ Oak Ridge Sational Laboratory. Oak Ridge. Tenn I'he porous cup electrode consists of a 0.25 X 1.5 inch right cylindrical graphite rod, w-ith a 0.125inch hole drilled along its axis from one end to within approximately 1.1mm. of the other end. I t is used as the upper electrode, with the open end up. In operation, a long-nosed pipet, containing 0.2 to 0.3 ml. of the solution to be analyzed, is inserted into the cavity in the upper electrode until its tip touches bottom; the solution is then expelled as the pipet is withdrawn. The lower electrode is a -olid 0.125-inch graphite rod. 4 synchronous spark from a Baird spark source or a 220-volt intermittent alternating or direct current arc is first applied for 5 to 10 seconds, using a 2-mm. analytical gap. The heat thus produced helps the liquid to soak through the bottom of the porous cup and reach the sparking surface. after a short rest (-15 seconds). ihe sparking i k resumed and the expowre
€€I this Slaborarorj began to receive a large number of solutioiis for quantitative analysis, a search was made for spectrochemical method versatile enough to handle the variety compositions, concentrations, and degrees of radioactivity encountered, yet simple enough to lend itself to routine operations. Because sparkmethods appeared to offer the prospect of better precision and accuracy than did arc methods, the known techniques for 5pfi1king solutions were examined. TI -pertrochemica1 analpi. of solutions wiih spark excitatioii tiab u\u:111> been performed by a residue method, an impregnation metliml, 01 G c-oiitinuous feed method. 1)f
i t
\ I l - ' l l l O I ~ SFOR SPECTROCHEMICAL ANALYSIS
Residue Methods. liivas (28) proposed the sparking of dried residues 011 prap!iite. but tlic most successful method of this type to appear bo litr wenis t o be the copper spark technique recently described by Fred, Saclitrieb, and Tomkins (11) and extended by Bacheliler, Conway, Kachtrieb, and Kildi ( I ) . The latter technique gives adequate precision (average deviation *7.6% for 119 intpnsity mtios run in quadruplicate) and excellent sensitivity.
begun. The liquid feeds through b y wick action, constantly renewing the thin surface film of liquid as it is dispersed by the spark. Spattering does not occur, as the spark never strikes the body of t h e liquid. Almost all the energy of the spark is dissipated in vaporizing and exciting the liquid film, so t h a t the sample does not boil over. A sample of the size mentioned lasts as long as 240 seconds. Successful analyses have been made of acidic, neutral, and slightlj alkaline solutions having a wide range of salt concentrations. The technique is also applicable to some solutions in organic solvents. The following approximate sensitivity limits have been attained: 0.01 to 0.1 p.p.m. Be, M g ; 0.1 to 1.0 p.p.m. 4g, 41, B, Li; 1.0 to 10 p.p.m. Bi, Cb, G o , Cr. Cu. Fe, Ga, Ge, Hf, In, La, Mn, Ni, Pb, Re, Ti, \ , Zr: 10 t n 100 p.p.m. As, Au, Cb, Ce, Cs, Hg, P, Pt. K I I . $13, St). Th. Zn; 100 to 1000 p.p.m. Te, W.
In the cupper spiuk nierhod, the spark volatilizes and excites all elements in the residue simultaneously, although possibly not to the same extent. I t has been proved possible to use molybdenum as 311 internal standard for determining ninny chemically icdly unrelated elements by this technique. The a p . of the technique, however, is soniewhat limited; it cannot conveniently be used with solutions that contain high concentrations of salts, reagents that attack the electrodes, or iiiatei,iids t1i:it leave fluffy or deliquescent residues. Impregnation Methods. WITH GEL ELECTRODES. Some 25 w a r s ago, Errera ( 7 ) proposed the use of agar sticks soaked sevra in the sample solution as electrodes for t'he spark of solutions. This technique was later simplified by Rohner (29), who impregnated thin gelatin disks with the ~ o l u tiori to be studied, and sparked them. The use of gels is inconvenient, hon-ever, and thew techniques have not been widely adopted. R I T H OXIDELAYERS.When det,eriiiining boron by the copper spark technique, Fred, Nachtrieb, and Tomkins (11) depojited a layer of calcium oxide on copper, and impregnat,ed this with the
