Determinaition of Surface Area of Calcium Carbonate by Isotopic Exchange Clyde G . Inks Research Division, Wyandotte Chemicals Corp., Wyandotte, Mich.
Richard B. Hahn Cliemistrj, Departtrtenl, Wayne State Unicersity, Detroit, Mich.
A simple, reliable method for the determination of the surface area of calcium carbonate is described. The method is based upon the extent of exchange between a calcium carbonate precipitate and radioactive, calcium-45 ions in solution. The procedure gives results reproducible to =t5% when averages of several determinations are used. Within a given series, however, results may differ by as much as 20%. The isotopic exchange method agrees well with values obtained by an electron microscope method for smallparticle-size samples, but gives low values for samples of large-size particles. Samples of lower surface areas show good agreement of comparison with the standard BET method but samples of high surface areas give high vesults. Limited tests with a liquid phase absorption method, using carbon-14 labeled stearic acid, indicate that good agreement can be obtained with the surface area values determined by the exchange method for both large- and small-particle-size samples. THEDETERMINATION of the surface area of solids is important for both industriai and theoretical uses. Many methods have been devised, but most of them are either time-consuming or require elaborate apparatus. The most widely used method is that of Etrunauer, Emmett, and Teller commonly referred io as the BET method, which is based upon the adsorption of nitrogen gas on the surface of the sample (I). The isotopic exchan :e method, however, requires only simple apparatus and the surface area can be determined with a minimum ofeffort. The isotopic exchange method was developed by Paneth and Vorwerk (2) fo- the measurement of the surface area of‘ lead sulfate using thorium-B,31’2Pb. as the radioactive trace:. The method is based upon the assumption that a reversible, kinetic exchange ocI:urs continually between the ions in the surface layer of a crystalline solid material and its ions in solution. wher, a known weight of solid is shaken with a saturated solution cf the same material containing a radioactive isotope. It must be assumed also, that only the surface Layer enters into the exchange. or that the exchange into the inner layers of the solid proceeds a t a much slower rate than the w r f a c e exchange. From the extent of exchange, and from solubility data. the surface area can be calculated. Other investigators have used similar procedures to measure the surface area of lead sulfate (3). lead chromate ( 4 ) , silver bromide ( 5 ) and strontium sulfate (6, 7‘). i l j S. Brunaucr, P. H. Emmett, and N. Teller. J . Am. Chern. Soc., 60, 309 11938:. (2; F. A. Paneth and W . Vorwerk. Z . Physik. Chem., 101, 445 ( 1922). (3) I M. Kolthoff and C. Rosenblum. J . Am. Cliem. Soc., 55,
2656 (39333. (4) I. M. Koltiioff and F. T. Eggertsen, Ibid., 62, 2125 (1940). ( 5 ) 1. M. Kolthofi’and K. C . Bowers, b i d . , 76, 1503 (1954). ( 6 ) R . H. Singleton and J. W. T. Spinks, Can. J . Research, 27B, 238 (1949). (7) R. M. Stow and J. W. T. Spinks, Cun. b. Ciiem.. 33, 938 (3955).
The purpose of this investigation was to determine whether this technique might be applied t o give a simple, reliable, and routine method for the measurement of the surface area of calcium carbonate using 45Ca as the radioactive indicator.. Calcium-45 decays by beta emission (E,,, = 0.254 MeV) with a half life of 165.1 days to form stable scandium-44. Techniques commonly employed by radiochemists were used in this investigation. These techniques, the properties. measurement, and safe handling of radioisotopes are discussed by Overman and Clark (8). EXPERIMENTAL
Apparatus. The beta activity of the calcium-45 was measured with a thin, enbwindow Geiger tube (less than 2 rig’’ cm2j or with a thin window (less than 1 mg/cmZj gas-flov. counter and a decade scaler. A mechanical agitator, which held four I-ounce, smailmouth, glass bottles, was used to shake the samples in a wat.-i bath at 25” C . The shaker ran a t the rate of about 20C. strokes per minute and moved the botties back and forth through a path of about 1 inch. Reagents. Five millicuries of calcium-45 chloride in hydrochloric acid solution (specific activity 219 mcuries per gram of calcium) was obtained from the Oak Ridge National Laboratory. The solvent was evaporated using an infrared lamp and the residue was dissolved in 5 ml of demineralized water and taken to dryness again. This procedure was repeated twice to remove all traces of hydrochloric acid. The 5 mcuries of calcium-45-chloride was then dissolved in 10 ml of demineralized water and stored in a polyethylene bottle . A saturated calcium carbonate solution (containing a small excess of solid) was prepared using 1 liter of demineralized water. This also was stored in a polyethylene bottle. A 253p1 aliquot of the calcium-45-chloride solution was added to this calcium carbonate soiution. This solution was allowed t o stand for about 3 weeks so that equilibrium was established between the small excess of solid calcium carbonate and the calcium-45 in solution. One hundred microliters of this solution gave about 10,000 cpm. The chloride concentration as a result of adding the calcium-45-chloride is approximately lOfi.44 expressed as calcium chloride. This solution will subsequently be referred to as solution “A,” Four different grades of calcium carbonate (PURECAL. M, PURECAL 0, PURECAL T, and PURECAL U, Wyandotte Chemical Corp.) were used in this investigatiori, These samples are all commercially avaiiable and are of t k calcite variety. Procedure. MEASUREMENT OF RADIOACTIVITY. Ten replicate assays were prepared as follows: One hundred microliters of solution ‘‘A” was placed in the center of a I-inch square glass microscope slide, which rested o n a heated
(8) R. T. Overman and H . M. Clark, “Radioisotope Techniques,” McGraw-Hili, New York, 1960. VOL 39, NO. 4, M A Y 1967
625
'o 1
aiuminum ring (1 cm in diameter) and the solvent was evaporated. These samples, dried from the edge inward to the middle, gave a uniform distribution of the solid material. The assays showed a standard deviation of about 1% from the mean. Since the residues contained only a few micrograms of solids, there was no significant loss due to selfabsorption. SOLUBILITY OF C A L C I U M CARBONATE IN DEMINERALIZED WATER. Because it was desirable to keep the test procedure as simple as possible, the demineralized water was not boiled to expel any residual, dissolved carbon dioxide. This required the determination of the solubility of calcium carbonate in the demineralized water, as this value is used in the calculation of the surface area. The solubility was measured by agitating a sample of calcium-45 carbonate of known specific activity in 10 ml of demineralized water for a t least 18 hours at 25" C. Aliquots of the supernatant were dried, counted, and the solubility was found to be 0.00432 gram per 100 ml of solution. This value is somewhat higher than the literature value (0.0014 gram per 100 ml) Y IO 0 owing to the presence of a small amount of dissolved carbon 0 dioxide. Several different determinations of the solubility, using the various types of calcium carbonate, indicated that this value was reproducible and variations in the amount of carbon dioxide dissolved in the demineralized water was insignificant. One determination of the solubility of calcium .carbonate was made in boiled, demineraiized water to be sure that the higher solubility in the unboiled demineralized water was caused by dissolved carbon dioxide. The soluI I I I I L I bility under these conditions was found to be 0.00198 gram 20 40 60 BO IO0 120 per 100 ml, which is in good agreement with the literature AGITATION TIME - HOURS value. Because the demineralized water gave only a slightly Effect of agitation time on surface area measureFigure 1. higher solubility, it was used in all measurements without ments of various PURECAL calcium carbonates boiling. In order to check these results, the solubility of the calcium carbonate in the demineralized water was determined by a Grams of calcium in flame photometer: wavelength 422 mh, 12 !b O? pressure, (1) surface per gram of CaC03 (100 - x)(w) 1.75 lb acetylene, slit 0.17, filter in sensitivity of photocell, 8. 'The concentration of C a f 2 was found to be essentially the The number of atoms of \I) (6.02) (1023) M, 0, T , and U same, 0.00125 gram/l00 ml, for PURECAL (2) calcium on the surface = - ---samples. This corresponds to 0.00312 gram of CaC03/100 atomic weight calcium per gram of CaCQl ml. These data are not as accurate CIS the radiometric method. owing to the low level of calcium ions present, and - mol ~wt CaCOn voiume occupied by one -ire close to the detection limits of the flame photometric molecule of CaCO? r d ) (6.02) ( : ( I z 3 ) method. However, the data do support the radiometric results, indicating a higher concentration of calcium ions in area representecl by one the demineralized water than in distilled water. = (31213 side of a molecule :dEASUREhlENT OF EXCHANGE OF CALCIUM-45 WITH c,\L,assuming a cubic lattice c i u y CARBONATE. Two experimental procedures for running the exchange of caicium-45 with calcium carbonate were the specific surface used: = (2) 14) per gram of C a C 0 3 (1) A sample of dry calcium carbonate (between 15 anci 30 rng) was weighed into a 1-ounce, french square. small-mouth bottle using a semimicro balance. Subsequently a known calcium-45 exchanged where X represents the value (usually 25 mi) of solution "A" was added to this S grams of calcium in solution bottle, The bottle was closed by means of a polyethylene %' weight of sample lined cap, placed in a water bath at 25" C. and agitated. d ;*,ecific gravity of calcium carbonate ~t various time intervals a 2-mi aliquot of the suspension was removed, centrifuged. and an aliquot of the supernatant RESUI.TS IUD DISCUSSION was dried and counted. (2) An aliquot from a suspension of a calcium carbonate, Effect of Time of Shaking. The effect of time of shzking whose surface area was to be measured. was placed in a on the ..xchange of calcium45 ions with the 4 different glass bottle, This suspension was made u p to contain a grade. of calcium carbonate was evaiuated over a neriud known weight G f calcium carbonate per unit volume, SO that of 0 .cj :36 hours, and i s illustrated in Figure 1. !Juring tne the weight of calcium carbonate sample could be calculated :irst 5our the exchange proceeds very rapidly and then siows from the volume of suspension used. Solution "A" was dowii. The very rapid initial exchange is attributed prirnariiy added and then the same procedure as in (1) was followed L O surface exchange, while the slower increase I S attr:butrcr from this point on with allowances being made for dilution ;o lattice penetration and recrysrallizarion. A s only t j l ~ of the original tracer solution. curface exchange is ot' interest. :t is reasonable to extrapolate METHODOF CALCULATIONS. The specific surface of ?he back to zero time io obtain the 7:aIue for the surface area, calcium carbooate may be calculated from the follow!ng which is due ani:) to surface exchange. relationships:
1
~
626
ANALYTICAL CHEMISTRY
_v)Cs)-L
The extrapolation to zero time provides the basis for a simplified prccedure for the measurement of the surface area by determining the (exchange after a certain time 01 -itation. This time can be dttermined by following the extrapolaL,, curve to the ordinate, then drawing a line parallel t o the abscissa until it intersects the exchange curve. Reading the time at this point of intersection gives the required agitation time to give the same value as the extrapolated value. The time was approximately 16 hours for all samples studied. Reproducibility of Surface Area Measurements. The values obtained for the surface area determined by plotting the agitation time 1’s. the calculated surface area and extrapolating to zero time are quite reproducible. Plots of shaking time cs. the surface area of three replicate samples of PURECAL T ca1ci.m carbonate were made. These curves were nearly superiniposable and gave a surface area of 48.2 with a standard dev.ation of only 1.2 square meters per gram. I n order t o determine the reproducibility of the surface area measurements for P U R E C A L T after agitating the samples for 16 hours, seven replicate measurements were made. The surface area for these measurements are given in Table I . From these data. it was found that in running three replicate samples, the, precision for the surface area measurement is within =k 5 %,. Effect of Preshaking without Tracer Present. Because i t is possibie that the slow. continual rise in the exchange of calcium45 with the calcium carbonate could be caused by t!;e breaking u p of particles or aggregates, and forming new surfaces during the agitation period. several determinations were made to evaiuate this possible source of error. These test> were carried out by preshaking an aqueous suspension of PLJKECAL 0 ca cium carbonate in the mechanical shake: for several hours before the solution “A” was added. There &as no appreciable ‘difference found in the surface area cf the preshaken sample compared to a sample that was not preshaken (4.3 and 4.8square meters per gram, respectively). Sorption of Calciom-45 on the Glass Bottle. In order to determinc whether calcium-45 sorption on the glass walls of the Lmttlr during the shaking period would be significant, 10 nil df solution “A” was placed in a bottle and agitated for i weeks at 25’ C. Uet.ermination of the activity in the solution at the end of this period showed no decrease in concentration, indicating tha: there was no significant sorption of the caiciuni-45 on ilie glass walls of the container or on the polyetnqiene lined cap. Cornparison of Surface Area Determined by Other hlethods. E!-E(’TRos M r c R o s c s P E METHOD. Electron micrographs of the iour diferent samples of calcium carbonate used in this investigation were prepared according to a procedure de-
Table I.
Replicate Measurements of Surface Area of PURECAL T Calcium Carbonate Surface area in square meters Sampie per gram 40.4 44.3 40.9 50.6 49.5 47.0 48.G
1
2 3 A
5 6 7
Av 45.8
Std dev 4 c! Re1 std dev I 5
veloped by Peppard, which is described elsewhere (9). The mean particle diameter was determined by counting a t least 300 particles. The surface area was calculated by assuming that the particles were cubes. The exchange method gives values of the same order of magnitude as the electron microscope data for PURECAE 7 and PURECAL U, but is considerably lower than the values for PURECAL M and PURECAL 0. Discrepancies between these two methods may arise from 3 different sources: iii in preparing the dispersion used for the electron micrographs, aggregates may be broken up which are not broken up ciurin:: the shaking period in the exchange method. (2) The partic’,..: of calcium carbonate are not cubic (an assumption made i ~ : the calculation of surface area from particle size data). Tnis effect may be more critical for the larger particle size samples, resulting in a larger difference between the methods for large particle size samples as compared to the small particle size sampies. (3) It was assumed that the aggi’egates are composed of particles having the same average size as that of the individual measured particles. All three of these sources of error would tend to make the electron micrograph values nigher than the exchange values. STEARIC AC!D ADSORPTION METHODS. Limited measurements of the surface area of PURECAL 0 and PURECAL U calcium carbonates were carried out using a modification of the procedure described by Suito (IO) for the determination of the surface area of calcium carbonate. In this investigation the degree of adsorption was measured by determining the .~
~
~~
~~
~~
~~
~
(9) C . G. Inks, “The Determination of the Surface Area of Calcium Carbonate b> Isotopic Exchange.” Masters Thesis, 1960, Wayne
State University. Detroit, Michigan. (10) E. Suito, M. krakawa, and T. Arakawa, Bid/. I n s / . Chern. Research Kyoru Lhic,, 33, 1 (1955).
Table 11. Comparison of Surface Area of Purecal Calcium Carbonate Determined by Different Methods Surface Area Expressed in Square Meters per Gram Sample
Isotopic exchange
BETJ method
Stearic acid adsorption
PURECAL hl PURECAL 0 PURECAL T PURECAL L;
4.3 4.8 4s 66
3 91 5.52
8.6
a
, . .
14.89
...
17.00
55
Electron microscope ~__ ____ Surface area D,h 23 25 62 77
0.097 0.088 0.036 0.029
Measurement made c7y Numinco Cori:
* D,represents the niean surface diameter in micron>.
VOL. 39,
NO. 6, M A Y 1967
627
decrease in concentration of carbon-14-labeled stearic acid dissolved in water-free benzene. The values for the surface area agree well with the values obtained by the exchange procedure for PURECAL U and PUKECAL 0, but are low compared to the electron microscope data. PURECAL U and PURECAL 0 were the only samples evaluated by this procedure because the method is time-consuming. BET METHOD.Surface areas of PURECAL M , PURECAL 0, P U R E C A L T, and PURECAL U were also determined by the method of Brunauer, Emmett, and Teller (1) which is widely used. Agreement was good with samples of low surfaace area, but the isotope exchange method gave results more than three times greater than the BET method with samples of high surface area. It has been pointed out, however, that the nitrogen adsorption method may not be accurate in very finely-divided or porous materials where the nitrogen molecules cannot penetrate into all pores and crevices. A study by Dollimore and Heal (11) shows that the actual surface area may be as much as 3.63 times the apparent area as measured by nitrogen adsorption. A comparison of surface areas obtained by these various methods is given in Table 11. -___ (11) D. Dollirnore and G. R. Heal, Nature, 208 (Sols), 1092 ( I 965).
CONCLUSIONS
All methods for the determination of wrface area involve certain questionable assumptions-i.e., the particles are cubes, or the particles are spherical, that adsorption occurs as monolayers, that the surface is completeiy covered by the adsorbant, stc. Likewise the preparation of the samples is very important. I t is not surprising, therefore, that different methods may give widely different results. The isotope exchange method is simple and reproducible and in some cases gives results which are in agreement with other acceptable methods. ACKNOWLEDGMENT
The authors express their gratitude to the Wyandotte Chemicals Corp. for permitting the use of their equipment and facilities for part of this investigation, and to Donald Peppard of Wyandotte Chemicals Corp. for applying electron micrographs and particle size data. We also thank H . 13. Miller, W. P. Hendrix, and C. Orr of Numinco Corp. for the measurements of surface areas by the BET method using their equipment. RECEIVED for review September 9, 1960. Resubmitted November 30,1966. Accepted February 20, 1967., Presented at the 138th Meeting, ACS, New York, September 1960.
Use of a Simple Small-Angle Scattering Device to Determine Particie Size Distribution of Thoria Sols
.
W C. Stoecker' Uranium Dicision, Mallinckrodt Chemical Works, S t . Chad&, M o
A simple small-angle scattering device attached to a commercial diffractometer is used for the examination of thoria sols. A study is made of collimation errors inherent in the device, with particular reference to its ability to resolve scattered rays whose angles with respect to the direct beam are not widely separated. This study shows that simple equipment of low resolution is adequate for a system of particles having a scattering curve that can be approximated by a Gaussian function or a composite of several Gaussian functions. Each thoria sol examined produced a smallangle scattering curve having its own characteristic shape that was unchanged by dilution or instrument factors. The small-angle scattering data were used t o determine particle size distribution through the use of a well-known graphical method. The average particle diameter (on a volume per cent basis) calculated from the size distribution analysis was found to agree with the mean crystallite size as determined by line broadening. SMALL-ANGLE SCATTERING of x-rays has become one of the most important techniques for the study of fine particle ' systems that have been studied systems. Types ~ f particle include sols, gels, vacancy defects in solids, and fine precipitates. All that is required for production of small-angle scattering is the presence of a dispersion of fine particles (or
domains) in a matrix of a different electron density. Since the late 1930's, when a sound mathematical basis for most of the observed effects was developed, the technique has found ever increasing application. There was a need at this site for some method of estimating the particle size distribution of thoria sols, to supplement the determination of mean crystallite size of ?he sols by x-ray line-broadening. Because a thoria sol constitutes a particle system having some favorable cnaracteristics for small-angie scattering work, particularly freedom finm interparticle effects, analysis by this technique seemed promising. .\ recent study by Schmidt ( I ) compared particle size and related quantitie? obtained by small-angle scattering with comparable quantities obtained by nitrogen absorption and other methods. Many systems of particles give scattering curves wnich are Gaussian, or which can be resolved into several Gaussian curves. For such curves, not possessing sharp peaks, high resoiution did not seem to be essential. Commercial equipment, particularly that employing the Kratky camera, has heen developed which provides extremely high resolution and an ability to detect scattered rays at an angle of less than $3.01 from the direct beam. Because equipment of this sort was probably not required, it was decided to obtain or htiild one of several simpler devices. - --
I
Present address, McDonnell Co., St. Louis, Mo.
528
ANALYTICAL CHEMISTRY
( I ) Paul W . Schmidt, J . Ph.vs. Chern., 69, 3489 (1965).
__
