Flame Photometric Determination of Calcium in Brucite and Magnesite ROBERT E. MOSHER, EDWARD J. BIRD,
AND
A. J. BOYLE, F'ayne Uniuersity, Detroit 1, Mich.
In a rapid and accurate solution method for the determination of calcium oxide in magnesite and brucite the Beckman flame photometer is used. Analytical results compare favorably with the methanol or Caley and Elving procedure. Interferences in the determination of calcium oxide caused by the presence of metals common to magnesite and brucite are described. The time of analysis is greatly reduced, because filtration, precipitation, and titration operations are not required.
T
trolled pressures in the vicinity 'of 20 pounds per square inch. Aspiration and atomization of the sample are produced by expansion of the compressed air and proceed a t a uniform rate as long as the pressure of the entering air is kept constant and the capillary tube is kept open. The resultant aerosol is conducted to the burner and mixed with ox gen and either natural gas or propane, which are likewise introJuced under individually controlled pressures. Combustion occurs just after mixture and produces a spectrum characteristic of the elements activated and proportional in intensity to the concentration of these elements in the sample. The light from the flame is transmitted to the standard Beckman spectrophotometer assembly, where it is separated i n k its component wave lengths by means of a quartz prism. A narrow region of the spectrum characteristic of the element to be measured is focused through a slit and projected onto a phototube, whereby a direct reading of the intensity is obtained. This reading is a measure of the total intensity of the li h t emitted by the burning gas-i.e., flame background-and t8e light emitted by the element or elements whose spectra fall within the region covered by the slit. To derive the latter, the flame background is measured using distilled water in the sample cup. The transmittance obtained is subtracted from the total intensity reading. Spectrophotometric readings of the intensity of the element under study are translated into measurements of its concentration by interpolation with readings of the flame intensity of lower and higher standard solutions, carried out consecutively under identical conditions.
H E determination of calcium oxide in high-grade magnesite and brucite ores is of great importance to the refractories industry. As commonly done, this requires the separation of small amounts of calcium (2 to 5%) from large amounts of magnesium and for this purpose the Caley and Elving method ( 5 ) is widely used. This communication is concerned with the use of the Model DU Beckman spectrophotometer and Beckman flame photometer attachment for the rapid determination of calcium in magnesite and brucite. REAGENTS
Standard Calcium Solution. Weigh 4.995 grams of dried, analytical reagent grade calcium carbonate into a 500-ml. beaker and add 200 ml. of distilled water and 20 ml. of concentrated hydrochloric acid in small portions. Transfer quantitatively to a 1-liter volumetric flask, dilute to the mark, and mix thoroughly. This solution contains 2.00 mg. of calcium er milliliter. Solution A. Weigh 9.46 grams of reagent g r a t e aluminum nitrate, AI(N03)g.9H~0,and 739.0 grams of recrystallized or calcium-free magnesium nitrate, Mg(N03)~.6H20,into a 1-liter beaker containing 5 ml. of concentrated hydrochloric acid. Add 500 ml. of distilled water, heat gently, and stir until dissolved. In a separate 250-ml. beaker dissolve 1.72 grams of electrolytic iron in 40 ml. of 1 to 1 hydrochloric acid. Warm to hasten solution. Cool to room temperature, transfer both solution\ to a 1-liter volumetric flask, dilute to the mark, and mix thoroughly. Should distilled magnesium be used instead of m a g nesium nitrate, it is advisable to dissolve it in hydrochloric arid.
Instrument Characteristics. Each flame photometer has certain individual characteristics with which the operator must become familiar, in order * t o obtain satisfactory results-for example, the bore of the capillary tube is subject to variation in different aspirators. Hence, air pressure must be adjusted to the optimum for each individual aspirator. Differences are also observed in burners. The original burner in the authors' apparatus required a 3-cm. pressure of propane, which produced too intense a flame background for good analytical work, whereas a newer replacement gave an adequate flame without an objectionable background on a 1-cm. pressure of propane. The optimal pressures of oxygen, as well ae of air and hydrocarbon gas, vary with the burner and thus must be determined for the individual instrument. This is accomplished by measuring transmittance a t difference pressures of oxygen while air and gas pressures are kept constant. With progressive increase in oxygen pressure, transmittance values rise to a peak and subsequently decline in a smooth curve, as exemplified by Figure 1. In the vicinity of the peak, relatively large variations in oxygen pressure cause relatively small changes in transmittance readings ( 7 ) . I t is in this less sensitive zone of oxygen, air, and gas pressures that transmittance readings are moat accurate. Adjustment of Apparatus for Analytical Procedures. In the operation of the instrument, it is necessary to control a number of interdependent variables. These are considered in the order followed in carrying out an analysis. The atomizing chamber should be preheated in order to avoid condensation of aerosol and consequent irregularity in delivery to the burner.
PREPARATION OF SAMPLE
To a 1.0-gram sample of magnesite or brucite in a 250-ml. beaker, add 10 ml. of distilled water and 10 ml. of concentrated hydrochloric acid. Evaporate the solution to dryness and bake on the hot plate for 10 minutes. Cool and wash down the sides of the beaker with 5 ml. of 1 to 1 hydrochloric acid. Warm the solution to dissolve hydrolyzed salts, add 20 ml. of distilled water, and boil gently until the soluble salts are in solution. Cool and transfer the sample to a 250-ml. volumetric flask. Dilute to the mark with distilled water and mix thoroughly. Permit the solution to stand until the insolubles settle sufficiently to leave a clear supernatant solution. A proximately 5 ml. of the solution are required for analysis. f h e insolubles resulting from the acid solution of brucite or magnesite are very similar to those obtained from limestone and dolomite and consist almost entirely of silica For this reason it is not customary to analyze these insolubles for calcium oxide. PREPARATION OF STANDARD SOLUTIONS
Measure accurately 4, 6, and 8 ml. of the standard calcium vlution into 250-ml. beakers. To each beaker add 5 ml. of olution A and 5 ml. of concentrated hydrochloric acid. Cover with watch glasses and proceed as for sample preparation. FLAME PHOTOMETRIC PROCEDURE
General Principles. One end of a right-angled microcapillary aspirating tube is immersed in a portion of the prepared sample which is held in a 5-ml. sample cup and the other end is inserted into a heated glms chamber. At its entrance into this chamber, the capillary tube is encased in a glass sleeve carrying air a t con715
716
ANALYTICAL CHEMISTRY sample is then aspirated and the transmission dial is manipulated until the needle returns to the zero point and remains stable in this position. The reading on the dial, when the needle is in equilibrium a t the zero point, is the transmittance valup for the element under study plus the flame background. The latter is then remeasured and deducted from the total intensity reading. The uniform aspiration of the sample, essential to stability of the needle, takes place within a few seconds. A diminishing reading suggests clogging of the aspirating tube. The plug is dislodged by disconnecting the atomizer and holding the finger over the air out.let so as to force air backward through the capillary until free bubbling occurs in a sample cup containing distilled water. If the transmittance value of the sample is below that of the middle standard, the low standard is then measured; if above, the high standard is determined instead. To make certain of reproducibility, the transmittance value of the middle standard is redetermined and should fall within 1 0 . 2 division of the original reading. If these conditions are met, the concentration of the element in the sample is determined by interpolation, because the relationship between transmittance values and concentration over the range covered by the st;indards is linear.
W A V E L E N G T H ’ 6 2 2 MU SLIT 0 4 0 MU GAS 3 CM AIR’
20
I
I IO
I
20
I5 OXYGEN
Figure 1.
PRESSURE
2 0 LOS
IN
25 INCHES
OF
I
I
30
35
mIrER
DISCUSSION
Effect of Oxygen Pressure
Initial Dark Current Check. The fixed switch is set a t 0.1, thc shutter is closed, and the spectrophotometer circuit is balanced to the zero point with the dark current rheostat. The fixccl switch is then moved to the “check” position and, if the circuit is balanced, the needle should remain a t the zero position. Sensitivity Adjustment. The fixed switch is then turned to the 0.1 position. The variable rheostat is set a t a point that provides optimal transmittance spread with minimal galvanometric fluctuation and is fixed a t this position throughout the succeeding determinations. Wavelength setting is made a t 622 millimicrons for calcium. Gas pressure is then adjusted to the optimal level for both tlir type of gas employed and the instrument. Natural gas is used at a pressure of 3 cm. of water for calcium determinations. If there are considerable fluctuations in line pressure, it is advisable to substitute bottled propane for natural gas. With the newer type of burner, propane may be used a t a pressure of 1 cm. of water. Air pressure is then set a t the optimal level for the aspirator. This ranged from 15 to 20 pounds per square inch for the authors’ instrument,. Oxygen pressure setting is made during aspiration of the middle standard. The oxygen pressure is adjusted to achieve peak transmittance (Figure 1) and is maint:iincd :it the same setting for the entire series of determinations. Dark current recheck is made :it this point, by closing the shutter and manipulating the dark cwrent rheostat, if necessary, to bring the needle to the zero poilit. Repetition of the dark current check is made after each transmittance reading. The shutter is in the closed position for all dark current adjustments and is opened for :dl trsnsmittance readings. Slit width is then adjusted during aspiration of the middle standard, so that the transmittance s e e I ting is approximately 50. .4 notation is made of the exact reading. Slit width must be maintained constant for all the subsequent determinations. Thc usual slit width for calcium is in the vicinity of 0.2 mm Flame background is then measured during aspiration of distilled water. This transmittance reading 7o is subtracted from that recorded during aspiration of the middle standard to obtain the intensity value P for the element tested in the standard. The flame background is extremely low-i.e., about 1 to 2 divi50 sions-at 622 millimicrons when either natural gas at z 2 to 3 cm. or propane at 1 em. is used as the flame source. Although the flame background tends to remain constant as long as other settings are unchanged, it should be rechecked after each sample analysis. Sample Analysis. All instrumental settings employed for measurement of the middle standard are kept constant, except for the transmission dial. Pressure gages must be kept under surveillance to mahe certain of uniformity. .4series of clean beakers is filled to the same level with the samples to be analyzed and with each of the standards. The first
Flame eicit,ation temperat.ures produce for the most part molecular band spectra of calcium oxide with calcium salts rather than atomic line spectra (4). Figure 2 shows various emission maxima obtained in the range from 400 to i o 0 nip with a calcium chloride solution containing 1000 p.p.m. of c~ilcium. Praks .4 and B w r e determined at greater sensitivity settings t,han C. The same slit width of 0.15 mm. was used for the entire wavelength range demonstrated. The sodium maximum resulted from the inclusion of a few part,s per million of sodium chloride in the celcium chloride solution used for these measurements. This shows the possibility of sodium interference ( f , 2 ) if exceptionally wide slit, witlths-i.cb., 1 nim-are used t.o measure small amounts of calcium a t 622 mp, 1vhic.h is the wave length found most suit,able for it.s nicasurcment. .i combination of a suffirirntly n w r o w slit with a didymium filter may be used t o incre:w selectivity of calcium measurement in the presence of sodium. Slit, width of 0.15 to 0.35 mm. may be used for the dt~terminationof calcium in magnesite and brucite, inasmuch as elements coinmoil t o these minerals do not produce spectral interference. The standards used to t~stablishconccnt,ration for the element to be measured must appmxinint,e as closely as possible the physical and chemical characteristics of t,he unknown ( 3 ) . Titanium, manganese, and chromium in concentrations of five to trn times thnt ordinarily encountered in magnesite and hrurite
CALCIUM
COlCENTRATlCU OXYGEN GAS AIR SLIT
1000 PPY
28 INCHES 3 CY 20L0S 0 I 5 YY
. Lo
5w
Figure 2.
600 550 WAVELENGTH IN Y h L l Y l C R O N S
Emission lluxinia
717 Table I.
Combined Effect of Aluminum and Iron on Calcium Intensity
'UO.
110
...
1
6 3 5 0
2 3
63.503.4 6 350 6 . 8
4
15
5 6
15 1:
330 350 360
,.. 8.6
17.2
Corresponds t o 1.0-gram sample of hrucite or magnesite containina 0.84% CaO
0.0 0.J 0.7
0 0 Corresponds to 1.0-gram earn1.7 ple of brucite or magnesite 3.5 containing 2.10% CaO 7 15 17.5 0.0 Corresponds to 0.5-gram eam8 1.5 17.5 i . 7 4.3 2.7 ple of hrucite or magnesite 9 1.i 175 3 . 4 8.6 5.5 containing 4.20% CaO Each sample \\-a$nrade u p to 2.50 ml. in a volumetric flask.
-
i:i
8.6 17.2
6.8
10-
60
-
50
-
iron. The per cent depression is .i.R, which is due almost entirely to the 3.4 mg. of aluminum, the iron having only a slight effect. Now 3.4 mg. of aluminum (Figure 3 ) will reduce the calcium emission intensity approximately 20% when measured with calcium alone. I n the presence of 176 mg. of magnesium, however, the depression is only 5.5%. This illustrates t,he smoothing out, effect of magneaium and demonstrates that the combined effects of magnesium and aluminum are riot strirtly additive and why it is essential to have an average, fixed amount of magnesium and aluminum in the standitrds with which samples are compared.
z 0
* Y
Y)
Y
40-
; , o b l
U;
2c
SOfL:
I
~;~NE;d~M
&;WIN;;
I
100
MO YILLIGRIMS
Figure 3.
PRESENT
Table 11. ~~
m
Additions of Known Amounts of Calcium to Analyzed Magnesite Sample CaO Found (Flaiiie Photometer). %
CaO Present, % 1 30" 1 60 1 72
Effect of Aluniinum, Rlagnesium, and Iron Additions
1.49 1.58
- 2 32 -1 06 - 1 00 2.18 +o 92 Average of four samples by the method of Caley and Elving. 1.86 I .98 2.20
2 00
a
%
- 0 66 - 1 2.5
1.68
1 88
Thi. market1 tl(bpressioii rxliil,itetl Iiy the introilurtioii of relat ivrly srn:d1 :tmounts of aluminum in a solution containing only v:ilciuin is most striking. .\Iitc*hell and Robertson (6) suggest t Ilat it is due to absorption of ralciuin twission energy by alumin u n 1 : t i i d stzte that this d e c t is more pronounred in the high ttfiniper:iture zone of the flame. This phenomenon has been used : I > :iii iiiclirrct quantitative metliotl for the drterniin:ttion of :rluininuni. The following compounds \cere tried as additions to the oali.ium-i~luniinumchloride solution niixture to minimize the deIircssirig effect of aluminum: malonic acid, gelatin, dextrin, vitric- a(.id, tartaric acid, 1-propanol, ammonium acetate, amnioiiiuni phosphate, ammonium nitrate, potassium chloride, and magnesium chloride. Several of the compounds listed will be iwmgniztad as those known to complex aluminum. Magnesium chloride proved to be the salt m w t effective in smoothing out the tl(7pression tendency of aluminum on calcium measurements. Table I shows the combined effect of aluminuni and iron on cx'ciuin emission intensity when a fixed amount of magnesium is I)t'esent. The per cent depression is due almost entirely to the :iluminuni concentration. Table I is not to be confused with F'igu1.e :3, which shows t,lie depressing effect on calcium emission iiitensity produced by aluminum, magnesium, and iron in5 mg. of aluminum produce a little more tic.i)endrritly-i.e., clt.pression than 100 mg. of niagneaium. Their combined effect i- not additive, however, as will be seen by examining Table I. S;:Lmple T of Table I contains 15 mg. of calcium and l i 5 mg. of iiiugnc4uni. If t h e magnesium were not present, the initial t.:ilcium intensity reading would be approximately 33y0 greater (Figure 2). However, because all calcium measurements are to lie made iii the presence of about the same :tmount of magnesium, c:tlcium-magnesium mistuiw are arl,itrarily considered to have n ' r o tleprcwion. Sample! 9 of Tatjle I has 15 mg. of calcium, 175 mg. of magnesium plus 3.4 mg. of aluminum, and 8.6 mg. of
Error
Tahle 111. Comparison of Caley-Elving and Flame Photometric Methods for Calcium Oxide Samiile
Per Cent Calcium Oxide Caley-Elving Method Flame Photometer 1.50 1.91 2.65
Hrucite
2.18
2.31
1.49
1.93 2.72 2.17 2,23
Error,
% -0.66 4-1.04 +2.64 -0.46 -2.59
To investigate recoveries, a sample of magnesite was analyzed for calcium oxide by the Caley-Elving method. Five samples of this magnesite were then prepared for flame photometric analysis and to each, calcium was added as chloride. The results are recorded in Table 11. Table 111shows the analysis of magnesite and brucite samples for CaO by the Caley-Elving and flame photometric methods. LITERATURE CITED
(1) Barnes, R. B., Richardson, D., Berry, J. W., and Hood, R. L., IND.ENG.CIIEM., A N ~ LE. D . ,17, 605-11 (1945). (2) Beckman. A. 0.. National Technical Laboratories, South Pasadena, Calif., Bull. 193B,June 1948. ( 3 ) Berry, J. \V., Chappcll, D. G., and Barnes, R. B., IND.ENG. CHEM., ANL. E D . ,18, 19-24 (1946).
(4) Brode, W'. R., "Chemical Spectroscopy," 2nd ed., p. 53, New York, John Wiley & Sons, 1943. (5) Caley. E. It., and Elving, P. J., TND. ENG.CHEM.,ANAL.ED., 10. 264-9. 119.18). _.. ~.~ (6) Mitchell, R. L., and Robertson, I. M., J . SOC.Chem. I d . , 5 5 , 269-72T (1930). (7) hfosher, k.E., eta2.. 4 n i . J . Clin. Path., 19, 461 (1949). \ - - - - , -
~~~~~I
RECEIVED .lpril7, 1949.
