470
ANALYTICAL CHEMISTRY
Table VI. Determination of Carbonyl Sulfide in 3 Propylene Synthetic atis Containing Interfering Gases Interfering Gas Hydrogen sulfide, 2620 p.p.ni. plus methyl mercaptan, 149 p.p.m. Carbon disulfide, 50.7 p.p.rn. Carbon dioxide, 176 p.p.m. Carbon dioxide, 524 P.P.III.
Table VII.
Sample 1 2
3
Carbonyl Sulfide, P.P.hI.Present Found 19.6 18.4 15.7 13.7
17.4 19.3 17.1 14.2
Determination of Carbonyl Sulfidc in Petroleum Rcfinery Gas Carbonyl Sulfide, P.P.A4. 7.1 7.4 10.1 10.7 ‘ 10.9 13.0 13.3 13.3
The applicability of the method for petroleum refinery gas samples representing the combined feeds from thermal and catalytic cracking units is illustrated in Table VII. Additional justification for the method was obtained by adding 9.0 p.p.m of pure carbonyl sulfide to the sample of refinery gas containing 7.3 p.p.m. and subsequently finding a total of 15.7 p.p.m. The accuracy of the method in the absence of interfering components is 1 p.p.m. within the range of 1 to 50 p.p.m. (Table I), The same accuracy holds for gas samples containing interfering components such as carbon disulfide, thiophene, and 1,a-butadiene that can be corrected by application of a blank run. When thc gas sample contains interfering components requiring the use of scrubbers containing Shaw’s reagent and 30% sodium hydroxide, as much as 10% hydrolysis of carbonyl sulfide occurs within tlie concentration range of 1 to 50 p.p.m ACKNOWLEDGRlENT
The authors wish to thank Renton iitwood of the Girdler Co. for his advice and suggestions during the course of this study. the ionized form of piperidine. The negative values shown in curve 2, Figure 7, indicate that less light is transmitted by the unacidified aqueous piperidine than by the acidified aqueous piperidine a t wave lengths lower than 240 mp. Accuracy and Applicability. I n Table VI the accuracy of the determination is exhibited for various synthetic mixtures. The gases were prepared by adding accurately measured volumes of carbonyl sulfide to the evacuated calibrated sampling flrsk and diluting with hydrogen sulfide, methyl mercaptan, carhon dioxide, carbon disulfide, and propylene. I n the one instance where carbon disulfide was the only interi’ering gas, it mas not necessary to use any scrubbers because the interference mas corrected by application of a blank run.
LITER4TURE CITED
(1) Avdeeva, A. V., Zaaodskaya Lab. 7, 279 (1938). (2) Brady, L. J., ANAL.CHEM.20, 512 (1948). (3) Hakewell, H., Rueck, E. &I., Am. Gas Assoc. Proc. 28, 529 (1946). (4) AlacHattie, I. J. W., hlcxiven, S . L., Can. Chem. Process Inds. 30, 87, 92, 94 (1946). (5) Pursglove, L. A., Wainwright, H. W.,ANAL. C ~ c n r .26, 1836 (1954). (6) Riesz, C. H., Wohlberg, C., Ani. Gas Assoc. Proc. 25, 259 (1943). (7) Shaw,J. A., IND.EXG.CHEM.,ANAL.ED. 12, 668 (1940). (8) Snyder, R. E., Clark, R. O., ANAL.CHEW27, 1167 (1955). RECEIVED for review M a y 26, 1955.
Accepted January 16, 1966.
Spectrographic Determinatioru of Silica JOHN W. ANTHONY, RAY J. CHANDLER, and WILLIAM B. HUCKABAY, CHARLES T. KENNER, Department o f Chemistrjr,
Magnolia Petroleum Co.,
Dallas, Tex.
Southern Methodist University, Dallas, Tex.
The application of spectrographic analysis to i+he quantitative determination of major constituents of inorganic substances has had liniitcd success. ’I’lie specific purpose of this investigation was to develop a spectrographic method for the quantitative determiiiation of silica in high concentrations by using gernianium nietal as an internal slandard. The germanillm serves as an exccllent buffer and produces a very stable arc in which the sample is coniplctely and homogeneously volatilized. Known niixtures of silica and calciiim carbonate were used as standard samples for the preparation of the analytical curves. The line absorbaince values of the line pair used permit the use of the more rapid line absorbance ratio method of graphing results without sacrifice of accuracy. The procedure was tested with analyzed samples of siliceous shales, cherts, rind other rocks of high silica content. The standard &\intion w a s 0.94% €or the range from 0 to 20% silica, and 1.72% for the range from 20 to 70% silica.
THE
3 use of spectrographic methods has been confined mainly to two general types of analysis: (1) the determination of the concentrations of minor constituents in a sample, and (2) the identification of the constituents of small samples. Although the majority of the spectrographic analyses deal with the determination of minor rather than major constituents, some methods for major constituents have been reported in the literature (1-4, 6-11, IS). In most of these a buffer material is used as an internal standard for line comparison purposes, and carbon is added t o render the mixture conducting as well as to obtain smoother burning of the sample. Nickel oxide wad used as a buffer by Oshry, Ballard, and Schrenk (8), and cupric oxide by Jaycos (6); a mixture of barium nitrate and ammonium sulfate was reported by Fit2 and Murray (1); beryllium carbonate in a mixture with sodium chloride was used by Kvalheim ( 7 ) . Herdle and Wolthorn ( 4 ) used silica, the major constituent of the sample, as the internal standard in analyzing silica refractories for the other elements, and then determined silica by difference. Gamble and Kling (2) used lithium carbonate as a flux and diluent in the determination of silica in petroleum ash. Steinberg and Belic (11)described a method for the quantitative spectrographic analysis of open-hearth slag samples by the use of the lime-silica ratio. Most of the methods reported
471
V O L U M E 28, N O . 4, A P R I L 1 9 5 6 indicated that it n s necessary to use more than one spectral line for each element if the concentration range over which the determinations were being made exceeded 20%. The deviation in most instances was 4 to 10% if the concentration of the constituent being determined was greater than 20%. This paper describes a spectrographic technique for the determination of silica in powdered samples of rocks, using germanium metal as an internal standard and carbon powder as a diluent. Strock ( 1 2 ) suggested the use of germanium metal in the matrix in the spectrographic determination of a number of major constituents in geological samples. The method herein described is now being extended to cover all the normal constituents of rocks to see if it is possible to obtain a complete analysis on a single sample. The suggested method, which was tested on limestones, siliceous shales, cherts, and other rocks of high silica content, requires the use of only 10 mg. of sample and may be employed for the determination of both major and minor constituents. Strock ( 1 2 ) cited the following reasons for selecting germanium metal as a matrix: the high regularity of concentration calibration curves for minor constituents in germanium metal powder synthetic standards; the marked suppression of fractional distillation from a direct current arc rich in metallic germanium metal; the similar temperature range of the liquid state of many metals and that of liquid germanium (thus affording the possibility of volatilizing many elements from a homogeneous metallic solution); and the fact that the ionization potentialof germanium is above that of most other metals, lying between them and the carbon of the electrodes, and thus reducing the necessity of the sample elements providing the conducting particles and competing with each other in the process. EQUIPMENT
A Baird Associates' 3-meter grating type spectrograph was used, with a 15,000-line-per-inch grating having a uniform dispersion of 5.56 A. per millimeter in the first order. A six-step sector with an exposure ratio of 2 to 1 was rotated in front of the slit a t 1400 r.p.m. Excitation was obtained from a controlled multisource power unit set for 50-volt 8-ampere direct current arc. The instrument was equipped with a 100-micron slit which was illuminated with approximately parallel light by the use of a quartz cylindrical condensing lens. The spectrum was recorded on Eastman spectrographic plates Type 11-0, which mere developed for 3.5 minutes with continuous agitation in n constant temperature tank with Eastman D-19 developer. Spectral line absorbances were determined by the use of a Leeds & Northrup recording microphotometer, which gives direct readings of the absorbances. The electrodes were prepared from regular spectrographic carbon rods 12 by '/4 inches, supplied by the National Carbon Co., New York. The upper or counter electrode was shaped with a flat end, and the lower or supporting electrode was a center-post crater type having a crater ' / 4 inch deep. A machine similar to that desciibed by Oshry, Ballard, and Schrenk (8) was used to shape and bore the electrodes. DEVELOPMENT OF METHOD
Various spectral lines of silicon and germanium were studied in order to determine a homologous pair which would give maximum reproducibility, would be free from interference, and would be closcllg spaced on the plate. Silicon line 2881 A . and germanium line 2556 -4.were found to be a suitable pair which pioduced satis,factory absorbance values. Synthetic samples of the oxides of the most common elements in silicate and mineral rocks, such as iron, aluminum, magnesium, and calcium, 11ere mixed in various proportions. These samples ITere then used to prepare mixtures in which the amount of silica was kept constant and the other elements varied in order to determine the efkct of these elements on the silicon lines.' Other mixtures n-ere piepared in xhich the concentration of eilica was varied to determine its effect on the lines of the other elements. I t mas found that the lines selected were free from interference. One-gram standard samples containing only calcium carbonate and silica 11-eie prepared for the dwclopment of the analytical
curves by varying t h e weight per cent of silica from 0 to 100% and inversely varying the amount of calcium carbonate. This mixture was selected to save time involved in the weighing of several components, because the primary interest mas the determination of silica. Ten milligrams of the standard samples mere mixed with various amounts of the germanium metal and powdered carbon to determine a suitable mixture for producing s:ttisfactory line absorbance values of the lines chosen. A mixture containing 10 mg. of standard sample, 90 mg. of germanium me tal, and 800 mg. of powdered carbon TVUS selected, because it produced satisfactory line absorbance values and permitted the USE! of sector stepa of the spectral lines in which the plate background density was a t a minimum. PROCEDURE
Preparation of Samples. Reagent gmde silicic acid and calcium carbonate were used for the preparation of the standard samples. The silicic acid was ignited a t 900' C. for 4 hours to ensure complete dehydration. The calcium carbonate W:IS dried a t 105' C. for 1 hour. The components were weighed accurately in the required proportions for the preparation of 1-gram samples. Spectrographically pure germanium metal w w used for the internal standard. The powdered carbon used as a diluent was obtained during the shaping of the carbon electrodes and was purified by treatment with hot concentrated hydrochloric acid to remove any trace contaminants. The rock samples used to check the analytical curves were ground to pass a 300-mesh sieve and were analyzed gravimetrically for silica by procedures outlined by Hillebrand and Lundell (6). The required weights of sample, germanium, and carbon were thordughly mixed in an agate mortar. A metal funnel designed to prevent any loss of sample was used to transfer 75 mg. of this mixture $0 the center-post crater-type carbon electrode. The material was then packed with a special metal tamper of correct dimensions so that it fitted the crater and center post. This ensured uniform packing of the sample without loss, and reduced the possibility that any portion might be blown out of the crater when the arc was turned on. Duplicate samples were burned to check reqroducibility and to facilitate line absorbance measurements, The mixture burned in each electrode contained only 0.833 mg. of the original sample. This small amount caused negligible interference betiveen the various elements present and made it possible to analyze Sam les of widely varying composition n 3 h consistent results. $he interferences are rendered negligible because the sample and germanium are diluted with carbon in the ratio of 1 to 8 instead of the usual ratio of 1 to 2. At such low dilutions the atomic population 01 the sample in the arc gases is reduced to such a level that the excitation energycollision exchange processes are no longer important in quantitative measurements. Preparation of Spectrograms. The electrodes were placed in the holders in the arc stand with the lower electrode a t a positive potential with respect to the upper electrode. The electrodes mere aligned on the optic axis and mere permitted to touch during a preheating period of 10 seconds. The s h u t k r was opened, the arc turned on, and a t the end of the preheating period, the electrodes were separated by a distance of 5 mm. and kept a t this distance until the sample was completely burned, which required approximately 300 seconds. The plates were then developed and air-dried. The silicon line employed permitted the use of a sector step which was relatively free from plate baclcground density and gave satisfactory line absorbance values. The developed plates were read with the microphotometer, using lines Si-2881 and Ge-2556 to cover the entire range from 0 to 70%)silica. RESULTS
The analytical curves shown in Figures 1 land 2, which were prepared from samples containing knom-n amounts of silica, show the variation of the weight per cent of silica with the ratio of the absorbances of the Si-2881 and the Ge-2556 lines. This method of graphing, which has not been widely used in the past, has certain advantages for this particular set, of da,ta over the usual method in which the radiant power (intensity) ratio of the lines is plotted against the weight per cent of the constituent being determined on log-log paper. One of these advantages is the simplification of calculations due to the fact, that the ratio is
ANALYTICAL CHEMISTRY
472
found by simple division of the direct microphotometer readings and only one graph is needed. Another advantage is that the value of the ratio of the absorbances of the lines is not affected greatly by normal variations in excitation conditions nor Eiy slight variations in development procedure, because the two lines used constitute a homologous pair. The third advantage is that use of the absorbance ratio in place of the radiant power ratio gives satisfactory results over a wider range of percentages of silica. Part of the reason for this third advantage resides in the fact that a section of the normally nonlinear portion of the gamma curve can be utilized for calibration and measurements. This advantage is not without sacrifice of some convenience becaum each new batch of spectrographic plates requires key check r'?calibration measurements over a wide range of silica values, and possible correction of the slope of the calibration line. The r+ calibration is necessary because the procedure described here docs not take into account differences in photographic emulsions. In many cases it will be satisfactory to use standard samples that have been determined previously by this method to make thwe calibration corrections. Two calibration curves are used because of an apparent chanl:e in slope between 20 and 25% silica. This change is apparently due to the poorer microphotometry in the lower ranges of siliva concentration. The best straight lines were fitted to the points in Figures 1 and 2 by the method of least squares. The valu?s of the ratios used in Figure 1 ranged from 0.174 to 2.020, with m average value of 0.992 and a standard deviation of 0.023. 1.n Figure 2, these values ranged from l . G O to 6.19 with an averalze value of 3.34 and a standard deviation of 0.079.
Table I. Spectrographic and Chemical Analyses Using Line Pair Si-2881 and Ge-2556 for Limestone Samples in Range 0 to 209" Silica Absorbance Weight yo Silica Ratio Spectro822881 Ge-2556 Si/Ge graphic Chemical Diff. 0.014 0.117 0.120 1.2 0.4 0.8 1.6 1.2 0.123 0.154 0.019 0.160 0.181 2.0 1.2 0.8 2 0.029 1.1 0.200 2.3 0.022 0.110 7.1 5.5 1.6 3 0.072 0.128 0.562 0.516 6.5 1.0 0.063 0.122 4 0.045 0.126 0.357 4.4 6.1 1.7 0.363 4.4 1.7 0.041 0.113 6.1 6.3 0.2 0.110 0.482 5 0.053 0.450 5.6 0.7 0.045 0.100 13.1 0.3 1.000 12.8 6 0.145 0.145 10.4 2.7 0.110 0.818 0.090 7a 0.112 0.102 1.098 14.1 14.1 0.0 0.6 1.059 13.5 0.108 0.102 15.2 17.0 1.8 0.148 1.182 8 0.175 1.393 17.9 0.9 0.188 0.135 Std.dev. 0.0147 0.0161 0.0724 0.94 Av. 1 . 0 7 a National Bureau of Standards standard sample l a , argillaceous limestone. Sample 1
tions is 1.07%. For samples between 20 and 70% silica, the standard deviation of the spectrographic results is 1.74 and the average difference between the chemical and spectrographic determinations is 1.16%. The poor results for percentages below 13 are probably due to the poorer microphotometry in the use of a single line pair for the entire range. DISCUSSION
PER CENT S I L I C A
Figure 1.
Calibration ciirve, 0 to 25% silica
The amount of sample required in the proposed procedure is only 10 mg., and may be of the order of about 1 mg. The germanium serves not only as an internal standard but as an excellent buffer for producing a very stable arc with complete volatilization and the cost of the small quantity required in the mixture is nominal. The use of one spectral line for the determination of silica over the entire range up to 70% reduces the time required for microphotometer operations. It is essential that the silica used in preparing the standard samples be completely dehydrated in order to ensure consistent
Table 11. Spectrographic and Chemical Analyses Using Line Pair Si-2881 and Ge-2556 for Samples in Range 20 to 70q~ Silica Weight % Silica SpectroSample= Si-2881 Ge-2556 graphic Chemical Diff. 21.9 0.3 9 0,202 0.121 21.6 0,266 0.147 23.2 1.3 27.7 1.5 10 0,345 0,166 26.2 0.377 0,170 27.7 0 11 0.446 0,156 34.7 31.9 2.8 0.452 0.173 32.0 0.1 38.8 0.3 12 0,425 0,130 39.1 0.376 0.117 38.5 0.3 13 0,429 0.136 37.9 38.9 1.0 0.540 0.172 37.7 1.2 14 0,491 0.128 45.2 47.5 2.3 0.661 0.151 51.2 3.7 52.1 1.6 15 0,590 0.137 50.5 0.560 0.128 51.1 1.0 0.6 16 0,675 0,135 58.1 57.5 0.686 0.136 68.3 0.8 17 ,0.762 0.126 6.048 69.2 67.5 1.7 0,742 0.126 5.889 67.1 0.4 S t d . d e v . 0,0528 0.0130 0.153 1.74 Av. 1.16 a Sample 9 is limestone, samples 10 t o 16 are siliceous shales, and sample 17 is chert. Sample 16 is a mixture of equal parts of samples 14 and 17. Absorbance
results of the analysis of a number of rock samples are given in Tables I and 11. These tables show that values for samples which contain up to 70% of silica are satisfactory by the proposed method. For samples below 20% silica, the standa1.d deviation of the spectrographic results is 0.94, and the average difference between the chemical and spectrographic determina-
Ratio Si/Ge 1.669 1.810 2.078 2.218 2.859 2.613 3.269 3.214 3.154 3.140 3.840 4.377 4.307 4.376 5.000 5.037
V O L U M E 2 8 , N O . 4, A P R I L 1 9 5 6 results. The rilira-calcium carbonate mixtures should be dried at m"C. before weighing for mixing with the germanium and carbon to erpd moisture absorbed during the original mixing ooerations. LITERATURE CITED (1) Fits, E. J., Murray, W. RI.. IND. ENO.CHEM..ANAL.Eo. 17, 145-7 (1945). kling, C . E., Speetrochim. A d a 4 . 4 3 9 - 4 5 (19511. (2) Gamble. L. UT., (3) Hela. A. TV., Seribner. B. F.,J . Research Natl. Bur. Sfandads 38, 4 3 9 4 7 (1947). (4) Herdle. A . J.. Wolthorn, W. J., ANAL.CHEM.21, 705-7 (1949). ( 5 ) Hillebrand, W. F.. Lundell. G. E. F.. "Applied Inorganic Analysis." pp. 698-726, Wiley. New York. 1929. (6) Jaycox. E. K.. J . Opt. SOC.Amer. 37, 162 (1947). (7) Kvalheirn. Aslak, Ibid.. 37, 585 (1947).
473 (8) Oshry. H. I., Bnllsrd, J . W.. Schrenk. H . H., Ibid.. 32, 672-SO (1942).
(9) Pierce,
w,C,, Naehtrieb, N, H,, IND,
ENG, CHEx,, ANAL,
Eo.
13, 774-81 (1941). (10) Smith, R. W., Hosgbin, J . E., J . Am. Carom. Soe. 29. 222-8 (1946). (11) Steinberg. R. H.. Belic, H. J.. ANAL.CHEM.20, 72 (1948). (12) Strock. L. W., Appl. Spectroacopll 7 , 64-71 (1953). (13) Zander. J. AI., Terry. J . H . . J . A n . Ceram. Soc. 30, 366-70 (1947). RBCEWFDfor review A ~ r i l1, 1855. Aocepted Fohroary 6, 1956. Taken in part from the the& mhmitted by John W. Anthony to the Graduate Sohool of Southern Methodist University in partial fulfillment of the requirements for the degree of maater of soienee. A cooperatire proiect between the Department of Chemistry, Southern Methodiat University. Dallas, Ter., and the Magnolia Petroleum Co.. Field Resehroh Laboratories.
Dallaa. Tm.
Effect of Aging Solutions of Barium Chloride on Particle Size of Barium Sulfate EDGAR J. BOGAN' and HARVEY V. MOYER McPherson Chemical Laboratory, The Ohio State University, Columbus 10, Ohio
The aging of barium chloride solutions which were used t o precipitate barium sulfate was found to cause an increase in the particle size of the precipitate. Filtration of a freshly prepared solution of barium chloride through a line sintered-glass or porcelain filtering crucible produced the same effect as aging. The particle size of the precipitate seems to be a function of the number of nuclei which are available as starting points for crystallization. The origin of the nuclei wae not established with certainty, but considerable evidence supports the theory that aggregates of incompletely dissolved barium chloride i n the fresh solutions may 8erw a s nuclei for starting crystals of barium sulfate.
times the diameter of the average crystal. The effect of aging the harium chloride on the size of the barium eulfste rrystals is shown in B, C, D,and E of Figure 1. The effect of filtering a fresh solution of barium chloride through a fine (2- to 5- micron)
S.
TUDIES on the coagulation of barium sulfate ( 5 ) by use of minute quantitiw of agar led to the observation that marked dilkenees in the pnrtirle size of precipitates were caused by the age of the solutions of barium chloride which were used to precipitate barium sulfate. An abstract ( 5 ) of these studies was published in 1949. Similar observations have been reported by Fischer and Rhinehammer (7). The significance of the age of precipitating solutions Seems to have been overlooked by the many investigators (8)who have studied the factors affecting the particle size of analytical precipitates. However, Bancroft ( 1 ) stated that, in his opinion, the nnmher of nuclei present in solution is more important than the extent of sopersaturation &B proposed by von Weimarn (8). Benedetti-Piehler (g)in an exchange of correspondence with one of the authors (4) reported that he found an aging effect which disappeared on recrystallizing the barium chloride. However, he mixed the sulfate and barium solutions simultaneously near boiling temperatures, whereas the authors added the barium chloride solution from a pipet with a constant delivery time a t room temperature. Benedetti-Pichler reported barium Bulfate crystals of 5 to 6 microns from purified barium chloride. This size i s approximstely the same as was obtained in this laboratory from fresh solutions of barium chloride. This is shown in A of Figure 1, in which the scale division of 18 microns is close to 3 I Preaent address. Department of Chemistry, University of Maine, Orono, Maine.
Figure 1.
Crystals of harium sulfate
Cryatalsformed underidenticaloqnditions e x c e ~ tfor treatment of 5% bariumehloride dihydratesolutmns. A . Freah aolution E . Aged 1 week B . Aged 2 hours F . Fresh aolution filtered through C. Aged 7 hours fin? aintered-glass filter D . Aged 24 hours 1 sealediviaion = 0.016 mm.
