Effects of Aliphatic Acids and Their Salts on the Flame Spectrometric Emission of Calcium A. C. WEST Thompson Chemical laboratory, Williams College, Williamstown, Mass.
b Normal aliphatic acids from formic through caproic, isobutyric acid, and the sodium, potassium, and ammonium salts of formic through butyric have been studied. Results a r e discussed with respect to instrumental variables and possible mechanisms b y which they can b e explained. Dynamic solution surface tension (for the acids) and the dispersion of the analyte in the solid particles formed (for the salts) a r e concluded to b e the most likely causes of the observed effects.
T
HE PROBLEM of organic substances in both emission and absorption flame spectrometry has been studied extensively ( 4 , 7 , 9, 11). The observed effects have been considered harmful interferences or have been used t o improve the sensitivity and precision of analyses. This division of emphasis has hampered the study of the problem. These effects can be grouped under four main headings: those caused by direct spectral interferences and flame background changes; those caused by changes in the composition or temperature of the flame, (gas composition and flow rates and the concentration and nature of added organic material are important factors) ; those caused by a matrix effect, that is, changes in the physical or chemical properties of the solid particles remaining after solvent evaporation in the flame, (analyte compound formation, the size and structure of solid particles, and the effect of foreign salts on the dispersion of the analyte in the solid particles must be included); those caused by changes in the physical properties of the solution fed into the flame, including surface tension, viscosity, density, and volatility. Certain experimental variables may influence these interfermce effects (6). The design of the burner, the region of the flame in the optical path, the rate and mechanism of sample introduction, and whether the emission line is scanned or a fixed wave length used, are all important. Along with the gas composition and flow rates, these variables must be controlled and measured to study interferences fruitfully. Investigations of the effect of aliphatic acids in emission flame spectrometry
310
ANALYTICAL CHEMISTRY
have centered on the alkali and alkaline earth metals. The problem has usually been approached in one of two ways; the effect has been studied either a t low acid concentrations and called a n anion interference (5, I S ) or at high concentrations and ascribed t o changes in flame temperature and the physical properties of the solution. hIost of the data are for acetic acid. Gundlach (9) has investigated a series of lower aliphatic acids but draws no conclusions as to the mechanism of the enhancements, and his use of suction feed complicates the interpretation of the data. The effect of aliphatic acid salts has not been studied systematically. The acetate ion effect, which has been looked at, generally refers to the acid and, as will be seen, results vary widely nhen either cation or anion is changed. Interferences by salts of any kind present a difficult problem. Often it is assumed that the effects of cation and anion are distinct and separable or that only one or the other is producing the interference. These aswmptions neglect a matrix effect, particularly one involving the size and structure of solid particles or the way in which the analyte may be dispersed in them. The purpose of this work n a s to investigate these acid and salt effects over a wide enough concentration range to elucidate their mechanisms and make any trends apparent without going to concentrations high enough t o introduce t h e complications of significant changes in flame temperature and composition. By thus limiting the nature of the interferences involved, the results become applicable t o absorption as well as emission flame spectrometry. A wide range of acids was used to determine, if possible, the role of dynamic surface tension. Calcium was chosen as the analyte because, of the elements commonly determined flame spectrometrically, it is the one that is probably most affected by numerous and complex interferences. EXPERIMENTAL
The instrumentation used for this work has been described (14) except for the following changes. Wavelength scan at 5 A. per minute, a Varian G-12 recorder, and slit width of 0.02 mm. The Beckman gas control panel was used with the flow meters, gas flow
rates were 8.2 liters per minute and 2.4 liters per minute for hydrogen and oxygen, respectively, with a n oxygen back-pressure of 10 p.s.i. a t the burner, and the solution was force-fed a t 1.0 ml. per minute. A411calcium solutions were prepared from reagent grade CaC03by dissolution r+-ith excess of the appropriate acid followed by evaporation to dryness. The calcium content of the reagents used was tested and found t o be negligible in all cases. Ca(XO& was used as the reference standard and all measurements were made with solutions 0.0006X (24 p.p.m.1 in The 4226-1. calcium line was used. The experimental technique involved scanning the line two or more times consecutively and averaging the set of peak heights obtained. Intensity data are arbitrarily given in scale divisions of chart paper. Results that appeared discordant were checked, often by remaking solutions, and many of the data represent averages of two or three sets of measurements. Three 20-ml. hypodermic syringes ivere used in rotation to speed up the work. For routine analyses, a large number of syringes can be used t o prepare samples ahead of time, and in this way the advantages of speed and constant solution feed can be combined. Work done recently in this laboratory with volatile nonaqueous solvents has shown the advantage of using syringes to prevent evaporation. RESULTS AND DISCUSSION
Acids. Figure 1 shows the effect of ‘21-6 normal aliphatic acids and isobutyric on the flame emission of Ca(T\TO3)2. The enhancements increase with chain length u p to caproic and with increasing acid concentration. Therc are manj- factors t h a t may affect calcium emission in these systems, but most of them can be eliminated as the principal cause of enhancement here. Direct spectral interferences and flame background changes were eliminated by line scanning and a background check on all stock solutions. Changes in flame composition h a r e a t most a very small effect in the systems studied. Chemiluminescence produced directly or indirectly by carbon-containing radicals in the flame has heen proposed as an enhancement mechanism for many metals (8). Hydrocarbon fuel
+ V
L €80
60 720 70
MOLES PER LITER ACID
Figure 1 .
0
A V
Formic Acetic Propionic Butyric
-t X
+
Isobutyric Valeric Caproic
Figure 2. Correlafion of calcium emission intensity with solution surface tension
0
b
solvents have been gases and organic " used in experimental studies of chemiluminescence. In the present work, solutions contained rho more than 10% by weight of aliphatic acid, which is equivalent to a mole fraction of about 0.01 for the sum of all carbon-containing species in the flame. While Gilbert ( 8 ) reports significant vhemiluminescence for tin in 3.6y0 isopropanol, Gibson, Grossman, and Cooke (6) find no evidence for it for calcium in 90% acetone. This latter data make it very doubtful that the charlges in flame composition in the present work contribute measurably to the enhancements observed. Organic solvents increase flame temperature but in fuel-rich flames, such as t h e one w e d in this work, the effect is not great ( 2 , 6) and the low concentration of combustible organic material eliminates this enhancement mechanism. h matrix effect is po-sible, particularly for those acids boiling well above vater. This would involve a change in the formation and structure of solid particles when the acid is prec)ent in increasing concentration as the droplets evaporate. KO direct study of thi:, effect is possible, but if it is an imporiant cause of enhancement the result< for caproic acid must be explained. If it is inferred that the boiling point of caproic acid is high enough to retard droplet evaporation and give a smaller e ihancement, two anomalies stand out. First, the enhancements produced by the sodium and potassium salts become less explicable, since thest, salts are more stable than caproic atid and a matrix effect seems a likely cause of the enhancements caused b> them. Second, Triton X-100, a nonioiic surface active
L
SIJRFACE TENSION
Effect of aliphatic acids on calcium emission
0
50
0 A V
Formic. Acetic Propionic Butyric:
agent with a molecular weight of about 600, enhances the emission of calcium nitrate more than either caproic or valeric acids on a molar concentration basis. ii matrix effect may contribute to the enhancements but is not an important factor. Compound formation is not a cause of these enhancements. Emission intensity data for the calcium salts of the acids except caproic are shown in Table I. Only the valerate gives an emission inteneity higher than calcium nitrate and the increase is slight. These intensities correlate with the physical appearance and bulk density of the salts when they were produced by evaporation from aqueous solution. T o see whether they could be correlated with the thermal stabilities of the salts, thermograms were run covering the temperature range from 25" to 1000" C. All the salts showed t\lo waves, one corresponding to decomposition to CaCOa, the other to the loss of C o r leaving CaO. The second wave was essentially identical for all the salts, starting a t 642" C., and the temperature range over which the first occurred is shown in Table I. Except for the acetate, the temperature a t which decomposition
+ X +
Isobutyric Valeric Caproic Triton X- 100
begins decreases with increasing emission intensity as would be predicted if it were the important factor governing the intensity. However, the rate of heating was about 5' C. per minute, so that the decomposition temperature range reflects the time necessary for complete decomposition and this would also be a factor in determining emission intensity. The thermogram data are, therefore, inconclusive, and the structure of the salt particles probably more important. Intensity and thermogram data for Ca(N'03)2'4H20are included in Table I. Changes in the physical properties of the solution remain as a major cause of enhancement. Any effect of these changes on the rate of solution flow was eliminated by using forced feed. When gravity suction feed was used the results were qualitatively comparable, although enhancements were smaller and reproducibility and burner clogging a much more severe problem. An empirical equation, applicable to the type of atomization occurring n i t h the Beckman burner, has been developed by Nukiyama and Tanasawa ( I d ) which relates spray droplet diameter to solution surface tension, vis-
Table 1.
Emission Intensities and Thermogram Data for Calcium Salts Emission Decomposition ranges, ( O C.I Salt intensity Salt to CaC03 Calcium formate 55.5 393-489 Calcium acetate 50.5 374-420 Calcium propionate 56.0 328-444 Calcium butvrate 58.0 294-378 Calcium valerate 62.0 268-493 Hydrate to Ca(X03)2 Ca(N03)*t o CaO Ca(NO3)2.4H20 60.0 62-240 526-660
VOL. 36, NO. 2, FEBRUARY 1964
0
31 1
“i
t
120
I10 l2
0
0.M
0.02
0.03
I
0
0.04
Figure 4.
Figure 3. Effect of aliphatic acids on calcium emissionlow concentration range
A V
X 4
cosity, and density. It predicts a linear relationship between surface tension and droplet surface area. Droplet evaporation is far from complete when water is atomized by the Beckman burner (6) and, therefore, a n y decrease in average droplet size-Le., any increase in the surface area per unit volume of solution atomized-will increase t h e efficiency of solvent evaporation, the concentration of metal atoms in the flame, and the emission intensity. Such a decrease will also produce smaller solid particles and this will further enhance the emission intensity. Viscosity and density changes in the solutions involved in the present work were small, so that if surface tension is considered the principal factor affecting droplet surface area, the equation can be used t o correlate emission intensity increases with decreasing surface tension. Figure 2 shows a plot of solution surface tension us. calcium emission intensity for all the acids studied. The solid line represents a linear relationship and the plots for all solutions except caproic acid lie parallel to this line and displaced above it. Data obtained for methanol solutions fall in the same range as those for the acids. The initial displacement shows that a surface tension change is not the only factor operating. It is certainly fortuitous that a direct correlation of droplet size to emission intensity is thus obtained over most of the range for Jvhich data were taken. This will be discussed later, but for the moment it is sufficient to note that a correlation does exist. The discrepancy for caproic acid must be explained. There are conflicting opinions concerning the effect of solution surface tension on analyte emission intensity. One reason for this is that high 312
ANALYTICAL CHEMISTRY
02
03
04
MOLES PER LITER SODIUM SALT
MOLES PER LITER ACID
Acetic Propionic Butyric
I
01
0
0
0
Valeric Caproic
A
Effect of sodium salts on calcium emission Formate Acetale Propionate
V X
Butyrate Nilrate
4
molecular weight surface active agents have been used in some cases to achieve a large decrease in surface tension with very little material and thus minimize other changes in the system ( 5 ) . Points obtained using Triton X-100 in Ca(NO3)2 solutions are plotted in Figure 2 and show no apparent correlation of surface tension t o emission intensity, in agreement with previous work. However, surface tensions used in this figure are static values measured under equilibrium conditions and these values may not obtain for a process occurring in a millisecond or less. Dynamic surface tension must be used and Hansen and Wallace have shown (10) that solutions 0.0075M in valeric or heptanoic acid require about 0.02 second for the establishment of surface tension equilibrium. Csing Cb to Ce normal aliphatic alcohols Addison has found ( I ) that the time necessary to reach a given equilibrium surface tension increases with increasing chain length and with decreasing concentration. He has shown that this relaxat,ion time varies from about 0.01 to 0.1 second for concentrations of these alcohols of 0.03-0.4%. The data for caproic acid and Triton X-100 in Figure 2 seem t o reflect the fact that surface tension equilibrium is not reached for their solutions during the time the spray droplets are formed. Since the relaxation time increases with decreasing concentration, all the acids except formic were investigated a t low concentrations to see if this effect could be observed. The data are shown in Figure 3 and are inconclusive, inflections a t very low concentrations indicating that other factors are operating. It is concluded that a decrease in
dynamic solution surface tension is the probable cause of enhancement of calcium emission intensity by organic acids in the concentration range studied. Formic acid depresses calcium emission because compound formation occurs before the surface tension has decreased sufficiently t o produce a n enhancement. The relationship beta een emission intensity and solution surface tension will depend on any experimental variables which affect the rate of solvent evaporation and particle decomposition or the length of time the analyte is in the flame before entering the optical path since solution droplets and solid particles equilibrate with the flame relatively slowly. Therefore, any increase in these rates s i l l enhance the emission intensity. If the analyte forms a stable refractory compound in the evaporating droplet its emission intensity is decreased because the solid particle decomposes more slonly. That depressions of this type are rate phenomena can be shown by two experiments (6); if the analyte concentration is lorn enough the depression disappears because the small particle size permits decomposition as rapid and complete as that of the standard compound t o ahich t h e depression is compared, and mensurements made at increasing heights above the burner show diminishing depression because the solid particles are in the flame longer. If droplet size is decreased by decreasing surface tension, the emission intensity of an analyte in a refractory compouiid will be enhanced for two reasons. Greater specific solution surface means faster solvent evaporation, and smaller droplets mean smaller solid particles and faster solid particle decomposition. If the analyte forms a
“1
I4Ot
I20
2ot
-4\20
0
01
02
0.3
0.4
ob
01
Figure 5.
Effect of potassium salts on calcium emission
0
0 A
Formats Acetatc Propioriale
V X
volatile unstable compound *hose decomposition in the flame is complete under all conditions, only the first of these mechanisms will operate. The number and c3mplexity of the variables affecting the surface tensionemission intensity relz,tionship indicate why the linear relationship observed in this work has only empirical significance. It is valid but is unlikely t o be linear for other calcium compouiids, analytes, and experimental conditions. Acid Salts. Figurcls 4 a n d 5 show d a t a for the effects of t h e sodium and potassium salts. T h e nitrates are included for comparison. Compound formation is t h e principal cause of t h e depressions a t low concentrations since t h e calcium acid salts will be formed in t h e evaporating droplet and their emission intensities are lower than that of Ca(N03)2. This is not a complete explanation because the formates depress the emistion intensity well below that of calcium formate. The subsequent enhzncements fall in the same order as do those for the acids and increase more r:tpidly than the latter. They are not caused by increases in flame background because the lines were scanned. Background radiation increases linearly wi .h sodium and potassium concentration and the enhancements would be even greater (up to about 1OOyo with 0 468M potassium formate) if it were incliided in the data. A shift in the calcium ionization equilibrium in the flame produced by t h e easily ionized alkali metals cannot account for the enhancements since only about 1% of the grouid state calcium atoms are ionized in im oxy-hydrogen flame. Chemiluminescmence caused by carbon-containing radicals would be less than for the corresponding acids and if abnormal calcium excitation were
Bulyrate Nitrate
02
03
0
MOLES PER LITER AMMONIUM SALT
MOLES PER LITER POTASSIUM S A L T
Figure 6.
Effect of ammonium salts on calcium emission
0
0
A
Formate Acetate Propionate
produced by alkali metal atoms, comparable effects should be obtained with the nitrates. Decreases in surface tension cannot explain the enhancements because surface tension increases slightly with salt concentration. pH and surface tension measurements of the stock salt solutions showed a negligible amount of free acid. X possible explanation of these enhancements is that the calcium salt is dispersed in a matrix of the sodium or potassium salt nhich is more quickly decomposed than a smaller particle of t h e calcium salt alone. At high salt concentrations the solid particles become large enough to counterbalance the more efficient dispersion of the calcium salt and the curves level out. Variations i n the rate of decomposition of the matrix and in the dispersion of the calcium salt would account for the differences among the curves. If the rates of decomposition of the alkali metal salts are in the same order as for the calcium salts the results are reasonable, if the emission intensities of the calcium salts reflect their stability. Data for the ammonium salts and ammonium nitrate are shown in Figure 6 and appear to contradict the above explanation. They are difficult to explain satisfactorily in any way. Compound formation accounts in part for the depressions, although not entirely since all the curves except that for the acetate go below the emission intensity of the corresponding calcium salt. Surface tension changes are not significant. The sequence established above 0.lM is the same as for t h e other systems studied and may reflect an effect on the dispersion of calcium in the solid particles. However, if the thermal stability of the matrix is the dominant factor, intensities should be higher than
V X
Butyrale Nitrate
for the sodium and potassium salts and the strong depression produced by ammonium formate is inexplicable. A few absorption measurements made on the acetates indicate that the ground state calcium atom population iq higher when the ammonium salt is used then for either the sodium or potassium salts. This appears reasonable considering the thermal stabilities of the salt matrices, but if this is correct, the opposite trend in emission data is difficult to rationalize. A systematic series of absorption and emission data is needed. It is evident that interferences of the type studied in this uork cannot be uniquely ascribed to either cation or anion. The complete compound must be given and its concentration range y~ecifiedto make the results meaningful. PRECISION
The sensitivity and stability of the instrument were checked with a standard Ca(h-Oa)2 solution a t the beginning and end of each day’s operation and a t such intermediate times as appeared advisable. These data have been used to estimate experimental precision, which can be calculated in three ways. All precisions are expressed as relative standard deviation. The reproducibility of consecutive runs will give the short-term stability of the instrument for periods up to ten minutes. An average of 0.8y0 was obtained for about fifty such sets of consecutive runs. A second and more meaningful calculation involves the precision of a single day’s measurements for from one to three hours of continuous operation. The averages of all the sets for one day were used and a value of 1.4y0 was obtained for these series of averages. VOL. 36, NO. 2, FEBRUARY 1964
313
Finally, the long-term reproducibility of data was calculated from all the individual measurements, about 150 calcium standard peaks recorded over a period of two and a half months. The relative standard deviation for the mean of these measurements was 2.6%. The precision of measurements made with the acid and acid salt solution was not determined, but does not appear as good as that of the standard solution data. During three months the only instrumental setting that was altered was the potentiometer. Additional experiments were run at later times after the spectrometer had been used for other analyses and this subsequent data varied over a wider range than those used t o calculate the figure of 2.6%. However, precision within a single day’s operation and for consecutive runs was comparable to that calculated above. With this
equipment, therefore, daily standardization is sufficient provided similar analyses are run. ACKNOWLEDGMENT
The author thanks D. R. Norton, Sprague Electric Co., for running the thermograms of the calcium salts and W. E. L. Grossman, Cornel1 University, for making the atomic absorption measurements. LITERATURE CITED
(1) Addison, C. C., J . Chem. SOC.1945, 98. (2) Avni, R., Alkemade, C. T. J., Mikrochim. Acta 1960, 460. (3) Baker, G. L., Johnson, L. H., ANAL. CHEM.26,465 (1954). (4) Buell, B. E., Ibid., 34, 635 ( 1962). ( 5 ) Foster, W. H., Hume, D. N., Ibid., 31,2028 (1959). (6) Gibson, J. H., Grossman, W. E. L., Cooke, W. D., Ibid., 35, 266 ( 1963).
( 7 ) Gilbert, P. T., Jr., Zbid., 34, 210R (1962). (8) Gilbert, P. T., Jr., Proceedings of the
Xth Colloquium Spectroscopicum Internationale, Spartan Books, Washington, D. C. , 1963. (9) Gundlach, H., 2. Anal. Chem. 171, 9 (1959). (10) Hansen, R. S., Wallace, T. C., J. Phys. Chem. 63, 1085 (1959). (11) Rlargoshes, M., ANAL. CHEX. 34, 221R (1962). (12) Marshall, W. R., Jr., “Atomization
and Spray Drying,” Chem. Eng. Progress RZonograph Series, No. 2, Vol. 50, p. 74 (1954). (13) Rains, T. C., Zittel, H. E., Ferguson, %I.,A N A L . CHEM. 34, 778 (1962). (14) West, A. C., Cooke, W. D., Ibid., 32, 1471 (1960).
RECEIVEDfor review July 25, 1963. Accepted October 28, 1963. This work was presented as Paper No. 219, Pitts-
burgh Conference on Analytical Chemistry and Applied Spectroscopy, March 4, 1963.
.
n
MicroscopicaI Identification of the Cerium broup Rare Earth Elements FREDRIC D. LEIPZIGER, WILLIAM J. CROFT, and JOHN E. ROBERTS Sperry Rand Research Center, Sudbury, Mass. The additon of a mixture of formic acid and ammonium formate to a cerium group rare earth solution produces a characteristic precipitate. The rare earth elements from lanthanum to gadolinium produce a complex series of crystalline formates which eventually become spherulitic. The habit of these compounds is sufficiently unique to enable their use as a microscopical method of identification.
T
describing rare earth formates is both sparse and contradictory (1, 3, 6). These compounds have been described as pentagonal dodecahedra ( I ) , needles (S), and as radiating groups of needles (6). Recently, there have been studies of the HE LITERATURE
Figure 1. nification
314
La(COOH13 250X mag-
ANALYTICAL CHEMISTRY
structure (5, 7‘) and thermal decomposition (8) of the formates of the lighter elements. I n an attempt to clarify the crystallography of these compounds, we have prepared formates of the rare earth metals (9). Earlier work ( 4 ) had shown the spherulitic character of the formates and i t was decided that this property is sufficiently unique to allow its use as a microscopical identification of the rare earths . EXPERIMENTAL
The production of characteristic spherulites of the rare earth formates is accomplished by mixing a drop of neutral or slightly acidic rare earth salt and an aqueous drop of 10% ammonium formate and 10% formic acid on a microscopical slide. Any water soluble rare earth salt is suitable. With lanthanum, cerium, or neodymium, immediate precipitation of small spheres or disks occurs. Higher members of the cerium group will precipitate in 2 to 5 minutes. The characteristic black cross when the precipitate is viewed between crossed polarizers is typical of spherulitic character. Figures 1 to 4 show typical habits. Best results are obtained when the test drop and the reagent drop are placed a few millimeters apart on the slide and then connected by drawing a fine glass rod from one drop to the other. This produces gradients of concentration and the best chance for crystal formation.
Figure 2. La(C0OH)s 250 X magnification, between crossed polarizers
RESULTS
All the lanthanides from lanthanum to gadolinium (excepting promethium) give positive tests in that they form characteristic spherulites while terbium and the higher members are of sufficient solubility to fail to precipitate in a reasonable time. Although isomorphic compounds have been prepared up to erbium, these precipitate with great difficulty. The formates of thulium, ytterbium, and lutetium appear to be of lower symmetry ( 2 ) . Morphology of the Rare Earth Formates. I n various stages of t h e crystallization of the formates, several 1 Present Address, Department of Chemistry, University of Massachusetts, Amherst, Mass.
