of the precipitate is complete. If the formation of a permanent light beam wcre to be used as the end point, the titration would be almost interminable. At least 15 minutes are required for the heam to fade completely after the addition of one drop of the titrant when the addition is made near the equivalent point. However, if the titration is made against a reference beam in another solution, and the beam in the sample solution is allowed to fade only to the intensity of the beam in the reference solution before another increment of the titrant is added, the titration procceds rapidly. The addition of one drop of the titrant a t the equivalent point niakes the beam in the sample solution unmistakably brighter than the reference bean1 and its intensity will persist after the solution has been stirred for several minutes. In original experiments the reference solution was prepared by adding 0.05 ml. of 0.01N mercuric nitrate solution to 100 ml. of 1 t o 19 nitric acid solution containing sodium nitroprusside. Tho turbidity in this reference solution could not bp reproduced; furthermore, the instability of the disperse system made it unsuitable as a reference a here a series of titrations was to be made T o solve this problem, a standard turbid solution was prepared which consisted of a colloidal precipitate of silver chloride in a gelatin solution. This turbid solution, which does not deteriorate in storage, is diluted as needed to make the reference solution. The new method is singularly free from interference by other elements. Bromine and iodine interfere but it is unlikely that either d l be present in silicate rocks in more than trace amounts.
EXPERIMENTAL
A set of experiments was made to determine the reproducibility of the mercuric nitrate titrations. Fusions were made on 5-gram portions of the flux, the cakes were taken up with water, and the solutions were filtered. Chlorine was added to these solutions as sodium chloride. Four cach of a series of solutions were prepared to which 0.00, 0.50, 1.00, 1.50, 2.00, 3.50, and 5.00 mg., respectively, of chloride were added. The results of the titrations on these solutions are shown in Table I. The maximum difference in any set of four determinations was 0.06 ml. By subtracting the average of the blank determinations from the average of the other sets of determinations, the amounts of mercuric nitrate necessary to titrate the added chlorine were found. The amounts were not proportional to the amounts of chloride taken. However, a curve may be drawn using this data which will give the correct amount of chlorine for any given amount of titrant. Eight rock samples containing widely varying amounts of chlorine that had been analyzed previously by the silver chloride method were analyzed in triplicate by the new method using SO-mesh powder. Some of the results were lower than those obtained by the siiver chloride method (Table XI). The residues from the filtrations on one set of samples Tore hcated on the water bath for an hour with dilute solutions of sodium carbonate and the solutions were filtered. No chlorine was found in these filtrates. T h e residues from the filtrations on the last set of samples were dried and re-fused with sodium carbonate. The melts that were obtained were carried throughout the entire pro-
cedure. When the amounts of chlorine obtained from these residues were added to those originally obtained, the results were in good agreement Lvith those obtained by the silver chloride method. Samples that showed low results when a n 80-mesh powder was used were reanalyzed using a finely ground powder. The results (Table XI) agreed with those obtained by the silver chloride method. Because fine grinding is time-consuming, much of the advantage of this method would be lost if all samples were to be ground to impalpable powders. The data in Table I1 show that satisfactory results are obtained with 80mesh powder if the chlorine content of the rocks is below 0 2%. Fortunately, a t least 90% of all rocks submitted for analysis will contain less then 0.2% chlorine. The authors’ practice is to analyze the samples using the 80-mesh powder normally used for the other constituents. The occasional sample containing more than 0.2% chlorine is reanalyzed using a finely ground powder ACKNOWLEDGMENT
Tables I and I1 are the result of analyses made by Sertie C. Smith of the U. S. Geological Survey, Denver, Colo. LITERATURE CITED
(1) Iiolthoff, I. hl.; Sandell, E. B., “Texb
book,, of Quantitative Inorganic Analysis, 3rd ed., p. 721, hlacmillan, New York, 1952. (2) Kuroda, P. K., Sandell, E. B., ANAL.
CHEM.22, 1144 (1950).
(3) Noponen, G. E., Minnesot,a hlinin and Mfg. Co., St. Paul, hlirin., o r 3
communication, 1941. (4) Shell, H. R., Craig, R. L., .4NAL CHEM.26, 996 (1954).
RECEIVEDfor review March 12, 1959. Accepted September 21, 1959.
Accurate Particle Size Distribution with Electroformed Sieves R. R. IRAN1 and C. F. CALLIS Research Department, Inorganic Chemicals Division, Monsanto Chemical Co., St. louis 66,
b A method is described for calibrating sieves. Particle size distributions in the range of 10 to 100 microns can b e determined with calibrated electroformed sieves with accuracies a t the 95% confidence limits of k9.4 and &9.2% for the geometric mean diameter and geometric standard deviation, respectively. The corresponding precision is A2.6 and f6%, respectively. 2026
ANALYTICAL CHEMISTRY
P
size distribution analyses with woven sieves can be erroneous when about 35% or more of the sample passes through a 325-mesh screen (44 microns) (1). However, sieve analysis is very popular because of its simplicity and applicability in routine control laboratories. Johnson and Newman (6) suggested a way for calibrating and checking relatively coarse sieves. A recent ARTICLE
Mo.
novel approach in sieve analysis is the development of electroformed micromesh sieves that have been shown (2) to be precise in the size range 20 t o 100 microns. The authors calibrated the sieves by examining microscopically a number of openings in each sieve. However, because a particle can pass through a hole whose diameter equals its smallest dimension only if the orientation of the particle is favorable,
sieving times must be exceedingly long. Otherwise, many a particle whose dimension is close to, but smaller than, the sieve opening will be retained on the sieve. Fine particles tend to coat larger particles in the dry state because of van der Waalstype forces ( 4 , 6). This phenomenon causes fine particles to be retained on a relatively coarse sieve and leads to inaccuracies in the results. EXPERIMENTAL
Sieving. Tlic elwtroformed micromesh scrcons IWW purcaliased from t h e Buckbee Nears Co., S t . Paul, Minn. All the samples w r c b haiidled in the dry state. Sieving was accomplished by brushing 10 grams of the sample t'hrough the scrwns. Four to 5 minutes of continuous brushing was found sufficient, as no measurable amounts of the material passed through the siew after around 2 m i n u t t s Similar result's can lie obtaiiicd with Ro-Tap sieving. Accurate Sizing. T h e electronic sizing and counting and the accurate sedimentation for measurement of particle size distributions were performed according t o previously described procedures ( 1 ) . Calibration of Sieves. Three sieves w r e calibrated. Although these sieves had a manufacturer's microscopic calibration they were called A, B, and C with A being the coarsest. Because 10-gram samples were used, the cumulative weight retained on each sieve multiplied by 10 gives the per cent greater than the opening of the sieve. The sieves were calibrated with 12 preparations of the same material varying in particle size distjribution. Sieve openings were assigned by noting the diarneter from accurate sedimentation and electronic sizing and counting that gives the same per cent greater than as the specific screen. Data are shown in Table I for monocalcium phosphate. Thus, the values of 45, 23, and 14 microns were assigned for sieves A, B, and C, respect.ively. Measurement. Using t h e same sieves for t h e same material t h e cumulative retained on each sieve gives a three-point particle size distribution. More screens a n d points can be utilized if necessary. RESULTS AND DISCUSSION
Because all of the samples tested exhibit the commonly used log normal distribution law (f), it is convenient in comparing results to employ the two parameters of the distribution-namely, M , and ug, the geometric mean diameter and the geometric standard deviation on a weight basis, respectively. When the log normal distribution is not obeyed modifications can be used (S). T o evaluate the precision of the results obtained with the electroformed sieves, two samples in two size ranges were run five times each
Table I. Sieve Calibration of Monocalcium Phosphate Preparations
Rlicron Opening from .4ccurate Distributioii Curvc, ~
Sample Identification E-4 E-24 E-2 1 E-22 E-18 F- 1 F-10 F-17 F-2 F-3 F-7 F-20
O 5 IO 3 0 5 0 70 50 93 F E R C E N T B Y WEiGHTG4ELTERTMV
Figure 1. Particle size distribution of screen cuts I. II.
111. IV.
+ screen A
- screen A, - screen 6,
-
f screen B screen C
+
----
In M, = In M,, uo =
+ 3 ln2 U"
un
(1)
Scrceri
B 22
45 47 40 41 49 48 12 43 45 47 49
Scrcwi C 13
15 -~
23 21 24 24
12 14 14
25
14
22 24 20 27 24 22
15 13 14
23 f 2
14 A1
~.
43 Av. 45 Std. dev. 1 3
screen C Opening corresponding to calibration with somde Opening from microscopic observation
by two different operators as shoi5n in Table 11. The statistical t test ( 7 ) shows no significant difference between operators a t the 99.9% level. The combined results of the two operators show that 21, and ug can be determined with the sieves with a precision of 1 2 . 6 and =t6.07,, respectively, a t the 95% confidence level. K h e n the same samples were run seven times a i t h the scdimentation apparatus modified for automatic recording, the precision was found to be 4 ~ 7 . 8and +9.67, for M u and up! respectively, at the 95% confidence level. Although the screens are more precise than sedimentation, they cannot be more accurate because they were calibrated from these values. Table 111 shons the close agreement in the results between the calilxated screens and accurate sedimentation. The standard deviations betvr een the t n o methods for AIQ and U, are A4.7 and 1 4 . 6 % , respectively. The preparations t h a t were used in this accuracy evaluation were not the same as those employed in the ca1ibrat)ion. I n addition, the particle size distributions of these samples ere determined on a number basis by rlectronic microscopic sizing and counting to yield AImand a,, the geometric mean diameter and geometric standard devintion on a number basis, respectively. The relations :
Screen .4
14
tt5
14
Table II. Precision of Size Distributions from Electroformed Sieves
Operator 1 .TIB
Operator 2 Jf,,
t
?\Iicrons
lliclrons Sample 1
uo
38.2 1.77 39.0 1.68 39.1 38.2 39.0 1.75 3'3.1 38.0 1.66 38.4 38.9 1.61 37.8 1.69 38.7 AV. 38.5 Std.dev. i 0 . 5 1 0 . 0 7 f 0 . 6 24.4 24.8 25.0 25.1 24.2
24 7
Av.
Std.dev. 1 0 4 Table 111.
1.69 1.71 1.71 1.64 1.68 1.69 fO 03
Sample 2 1 7ti 1.72 24.3 25.0 1.72 1 71 2-L.8 1.74 1 i5 1.75 2 5 . 1 1.75 1.73 2 4 . i 1.72 1 74 1.75 24.8 1 0 03 1 0 3 f 0 02
Agreement with Accurate Methods
Accurate
Sedimentation M,, microns up 27.0 48.5 37.6 41.4 32.1 21.9 52.0 28.3 52.8 29.6 37.1
uI
1.80 1.72 1.64 1.60 1.95 1.82 1.62 1.92 1.71 1.86 1.67
Calibrated Sieves -Ifo, microns ug 28.0 45.0 39.0 42.0 31.1 23.0 49.2 27.2 50.0 28.0 38.6
1.76 1.76 1.66 1 48 1.86 1.83 1.74 1.86 1.66 1.78 1.69
(2)
were again ( 1 ) shown to hold within experimental error. These findings lead t o the conclusion that the accuracy evaluations were made on an absolute scale and do not refer to some arbitrary scale. When screens A, B, and C were examined under the microscope, their nominal openings were 44 f 2, 28 f 2, and 20 f 2 microns, respectively, in contrast with the assigned values of
45, 23, and 14 microns. The particles of the preparations used in the present work were always fairly blocky with a maximum y to 5 ratio of diameters for the same particle not exceeding 1.5. This apparent discrepancy can be explained from Figure 1 where size distributions of sieve cuts m e shown. These distributions were obtained by sedimentation analysis. Although the VOL. 31, NO. 12, DECEMBER 1959
2027
sieves have very close-sized openings, some particles coarser than the openings (by sedimentation) are passed through and some particles finer than the openings are retained. This directly points to the necessity of calibration. To check the effect of changes in the shape factor of particles on the calibration of the sieves, the calibration was repeated with perfectly spherical glass beads and the values of 47 =t 2, 33 i 2, and 18 i 1 microns were obtained for screens A, B, and C, respectively. The significant difference in the calibrated openings between spheres and fairly blocky particles demonstrates
that sieves must be calibrated for every material arid sieving method for which they will be utilized, Although only a limited amount of work was done with wet sieving, the results indicate that calibrated electroformed sieves can be utilized for accurate particle size distribution determinations. ACKNOWLEDGMENT
The authors thank W. W. Morgenthaler for making some of the measurements.
LITERATURE CITED
(1) A ~ D. ~P., ~~ ~,R. R,, ~ callis, ~ c. i F., J. Phys. Chent. 6 3 , 531 (1959). (2) Daeschner, H. W., Seibert, E. E.,
~
~
~
~
&
$); $ ~ i ~ ~ ’ ~ f s tMateh g (3) lrani, R. R., J. phys. Chem, in press. (4) Irani, R. R., Callis, C. F., Liu, T., Ind. Eng. Chem. 5 1 , 1285 (1959). (5) Johnson, J. R., Newman, J. s.,ANAL. CHEM.26, 1843 (1954). (6) Silverberg, J., Lehr, J. R., Hoffmeister, G., Agr. Chem. 12, 38 (1957). (7) Youden, W. J., “Statistical Methods for Chemists,” 2nd ed., p. 24, Wiley, New York, 1955. R~~~~~~~ for review jUly 6, Accepted September 8, 1959.
1959.
Factors Affecting Emission Intensities in Flame Photometry WALTER H. FOSTER, Jr., and DAVID N. HUME Department of Chemistry and laboratory for Nuclear Science, Massachusetts lnstifute of Technology, Cambridge 39, Mass.
bA
physical study of the widely used total-consumption atomizer-burner system established the importance of various solution properties in controlling the rate of addition of sample to the flame and the flame temperature. Emission intensities in the total-consumption atomizer-burner system are not significantly affected by variations in solution surface tension and hydrostatic head. The presence of relatively high concentrations (above about 1 gram per liter) of total salt in the solution leads to a decrease in atomizer-burner efficiency and can cause depressions of emission intensity. Variations in solution temperature change the solution viscosity which results in a proportional change in sumple flow rate. Curves showing the variation of emission intensity with aqueous solution flow rate were obtained for the alkali and alkaline-earth metals in the hydrogen-oxygen flame. The curves for the various metals were interpreted in terms of excitation potentials, ionization potentials, self-absorption, and flame temperature. The temperature decrease due to the large amount of water forced through the total-consumption atomizer-burner was estimated b y measuring the relative change in the intensity of the 308.9 mp hydroxide band.
F
photometry is well established as a rapid and convenient method of spectrochemical analysis. The present 8 tatus of practice and theory has been LAME
2028
ANALYTICAL CHEMISTRY
ably summarized by Dean ( 5 ) ,Gardiner (9), and Margoshes and Vallee (16). However, the method is influenced by a n exceptionally large number of experimental variables, many of which are interrelated and difficult to control and study individually. As a result, the literature is filled with contradictory observations and their attempted explanations. Many workers have reported variations in emission intensity which depend upon the nature and concentration of sample components other then the element being excited. The extent to which instrumental factors (atomizer efficiency, burner characteristics, spectral resolution) as distinguished from flame conditions (temperature, chemical equilibria, ionization) should be the explanation of these has never been clear; this is why the present investigation was undertaken. Gardiner (8, 9 ) , working in this laboratory, observed very large cation enhancement effects for alkali and alkaline-earth metals using a refluxtype atomizer with separate hydrogenoxygen flame. Preliminary experiments by the present authors with the popular Beckman No. 4020 total-consumption atomizer-burner system indicated the enhancements t o be relatively small. T o clarify this situation and also get fundamental information on the performance of the widely used Beckman atomizer-burner system, the effects of various solution properties on emission intensity were studied and the totalconsumption atomizer-burner system
was compared to the reflux type, especially with reference t o the cooling effect of water. The physical study considered any factor which could affect the rate of addition of solution to the flame, the type of sample spray produced, the composition of the flame, or the temperature of the flame. Once this information on the physical characteristics of the burner system was obtained, several true spectroscopic interferences were investigated in various flames and atomizer-burner systems ( 7 ) . INSTRUMENT A N D MATERIALS
The instrument used was a modified Beckman D U spectrophotometer with No. 9200 flame attachment. The detector system of the DU was replaced by multiplier phototubes with cathodefollower circuitry originally described by King and Priestly (14), and later modified by Whisman and Eccleston ($0). An electronic power supply provided regulated direct current, variable from 900 to 1100 volts for the multiplier phototubes. An RCA 1P28 blue-sensitive type mas used for lines up to and including the potassium doublet a t 769 mp. A D u blont K-1430(6911) redsensitive type was used for lines beyond 769 mp. Some difficulty was experienced because of the high dark current of the K-1430. Cooling the photocathode and/or chopping the light beam and amplifying the resulting alternating current signal are possible means of avoiding this problem. The synchronous motor and gear train mechanism of a Beckman No. 92300 spectral energy
,
~
:
