two or more ions, using mass measurement data alone, all of the possibilities are listed. Wherever possible, when two or more elemental compositions are given for a particular ion, the ones which, in the authors’ opinions, are most reasonable are denoted by an asterisk. These judgments were based on the mass measurement data and the history of the sample. Many
of the ions appear to be protonated species formed by ionmolecule reactions due to the high sample pressure in the source.
RECEIVED for review June 29, 1970. Accepted September 4, 1970.
Carbowax as Dispersant in Counting and Sizing of Small Alumina Particles Using a Coulter Counter Elmer C. Lupton, Jr., and Patricia A. Shelby Air Force Rocket Propulsion Laboratory, Edwards, Calif. 93523 ACCURATE SIZING of small particles with a Coulter Counter requires the use of a dispersing agent t o ensure that each particle is wetted by the solution (1-5). In addition to the general considerations which concern users of the Coulter Counter, we faced two special problems which affected our choice of dispersant. These problems were the fact that the total sample available weighed 1-10 mg and the fact that the quantity of interest to us was the weight average diameter (6) ( 2 n d 4 / 2nd3)of the sample. The former problem meant that only one dispersion could be made, and the latter required that we obtain extremely accurate counts and sizings on larger particles but permitted considerable inaccuracy in the sizing of the smaller particles. Flinchbaugh (5) has noted that there is a considerable drop in the number of small particles counted when large particles are present in the suspension. Therefore, we reasoned that a suspension which allows larger particles to fall much faster than smaller particles when stirring rate is reduced would produce very accurate sizings of large particles at high stir rates and moderately accurate sizings of small particles at slow stir rates. By making measurements on small monosized particles at slow and fast stir rates, we planned to correct for the small amount of their sedimentation which does occur. In order to determine the best dispersing agent for our purposes, we made a series of comparison tests. The following dispersants were used : ethanol, “Leak Tec,” 2 4 2 methoxyethoxy)ethanol) bis(2-methoxyethy1)ether (diglyme), bis(2-ethoxyethyl)ether, glycerine, Triton X-100, and Carbowax-1500. Triton X-100 is reported in the literature ( 2 , 5 ) to be the dispersant of choice.
Table I. Effect of Stirring Rate Change on Counts (Model A Coulter Counter, 70-p aperature, 1 NaCl solution/raw counts listed) Fast stirring Slow stirring No stirring 3.5-p 3595 3484 3433 3476 R = 3497 5.0-p 1179 1112 1115 1138 8 = 1136 7.0-/J 421 429 408 410 8 = 417 10.0-p 123 117 126 134 = 125 14.0-p
R=
41 37 38 42 40
2
3148 3248 3139 3149 = 3171
8
1011 1026 993 969 = 1000
8
307 328 351 43 5 = 355
8=
72 56 62 68 65
8=
18 13 11 11 13
8
2927 2861 2875 2830 2871 = 2873
8
869 847 958 860 = 859
8
279 263 230 275 = 262
8=
59 41 55 48 51
x=
10 14 12 10 12
EXPERIMENTAL
A Model A Coulter Counter was used for all measurements. The sample of alumina was prepared by shaking Baymal colloidal alumina, average particle size 200 I*, for one minute with a Wig-L-Bug apparatus. The Carbowax dispersant used (1) Coulter Counter User’s Manual, Coulter Electronics Industrial Division, 2501 N. Mannliein Rd., Franklin Park, Ill. 60131. (2) Bibliography available from Coulter Electronics Industrial Division, above address. (3) E. P. Chaffin,“Size Analysis of Metal Powders,” Murex Limited,
Rainham, Essex, England. (4) W. M. Wojcik, R.M .Raybeck, and E. J. Paliwoda, J. Metals, 19 (12), 38 (1967). ( 5 ) D. A. Flinchbaugh, ANAL.CHEU.,41,2017 (1969). (6) G. Herdan, “Small Particle Statistics,” 2nd ed., Butterworth, London, 1960.
was a 50150 (w/w) solution of Carbowax-1500 in water. Before use, the dispersant was filtered through a 0.8-p M F Millipore filter. The sample which produced the results listed in the table above was prepared as follows: Alumina (0.990 mg) was weighed using a Cahn electrobalance. The alumina, still in its aluminum weighing boat, is placed in a 250-ml volumetric flask. Added are 1 ml of Carbowax dispersing solution and 4 ml of water. The water was previously filtered to remove all particles larger than 0.8 micron. The flask is then placed in an ultrasonic cleaner for 30 seconds. The aluminum boat is then removed and any particles remaining on the boat or on the neck of the flask are washed back into the flask with 5 ml of filtered water. The flask is then placed in the ultra-
ANALYTICAL CHEMISTRY, VQL. 42, NO. 14, DECEMBER 1970
1841
CARBQWAX
TRITON X - 100
ETHANOL
PARTICLE SIZES ' 5 0 5
3.5v 5.5P 7.51-1
rnl OF DISPERSION REMAINING
Figure 1. Change in number of counts observed as dispersion is consumed, Comparison of three different dispersing agents sonic cleaner for at least 30 minutes. The particle-free 1% sodium chloride solution is added up to the mark and the filled flask is shaken for at least 15 minutes using an automatic wrist action shaker. For the Coulter Counter measurements, the suspension is placed into a 300-ml beaker. The aperture is of the distance from the bottom of the beaker, as was suggested by Chaffin (3). The fast stirring speed is the highest speed at which bubbles are not created by the stirring. The slow speed is sufficiently fast that the individual stirrer blades can barely not be seen. RESULTS AND DISCUSSION
We found that Carbowax is the most satisfactory dispersing agent of those tested for several reasons. It gives a very stable dispersion and quickly reached equilibrium dispersion. It is easy to filter, is infinitely miscible in water, and it allows the larger particles to settle much faster than the smaller when the stirring rate is reduced. It does not foam at all in the ultrasonic cleaner (in contrast to Triton X-100). Perhaps the most desirable aspect of Carbowax is that it allows precise measurements to be made when almost half of the original solution has been used. We found that with many of the dispersants tested the number of particles counted began changing almost immediately upon counting. Triton X-100 did not give satisfactory performance in this respect. The chart (Figure 1) shows the changes in number of counts
1842
e
observed as volume of dispersant remaining decreases. The first set of data used ethanol, which is a poor dispersant. The second set used Triton X-100, which is reported to be a good dispersant. The values here are much more constant than for ethanol, but still there is initial rise in number of counts. The third set of data is for Carbowax. The data here show constant counts until almost half of the dispersion is gone. These data points were taken using the 140-p aperture, 1 mg of alumina sample in 250 ml of dispersion and 500-yl sample. The measurements at 3.5 p and 5.5 p are admittedly smaller than the accurate range of the 140-p aperture. However, the counts, while perhaps inaccurate, should still be constant and the larger sizings do show the same trends, but they are much harder to see because of the small number of counts at the larger sizings. Table I shows the difference in sedimentation rates between small and large particles with Carbowax. Note that the difference observed in counts between slow and fast stir rates is 10% for the 3.5-p particles, 13% for the 5.5-p,17% for the 7.5-p, 92 Z for 10-p and 207 % for the 14.0-y particles. No sedimentation measurements were made on the other types of dispersant since their failure to give constant counts disqualified them from further consideration and made sedimentation measurements superfluous.
RECEIVED for review June 22, 1970. Accepted September 14, 1970.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
