theoretical boundary conditions, did function to retain most of the sample long enough so that crystalline lattice “impregnation” conditions could be achieved before prohibitive loss, via evaporation, had occurred. For highest sensitivity, the encapsulation or covering thickness should be kept to minimum values to reduce diffusion times and maintain most of the sample close to the ionizing surface (vapor deposition or sputtering techniques would offer the best approach for such purposes). At the same time, the cover thickness should be great enough to allow a thorough cleaning of its surface to take place before the appearance of the sample atoms. This cleaning of the surface ensures the maintenance of a high and constant work function, essential for high ionization efficiencies. In
those instances where a stable, long sustained ion beam is desired rather than maximum sensitivity, the spot and electron-beam welding methods provide means of obtaining the required heavy sample coverings. For uranium diffusion in rhenium, cover thicknesses should be a maximum of 0.0005 inch when operating in the temperature range of 2200 to 2450 OK. For the case of the lighter elements, diffusion rates allow the use of considerably thicker coverings.
RECEIVED for review June 30, 1969. Accepted August 25, 1969. The New York State Department of Health, Division of Air Resources, provided financial support for this investigation.
Spectrophotometric and Visual Titrations of Milligram Amounts of Strontium with EDTA Jean Salomon,l John E. Vance, and Walter A. Baase2 Department of Chemistry, New York University, Washington Square, New York, N . Y . 10003 THEEXPERIMENTS reported here were needed in a study of the kinetics of precipitation of strontium sulfate in solutions of hydrochloric acid and sodium chloride; it was necessary to titrate about 1 to 10 mg of strontium in the presence of 10 mmol of sodium chloride, formed from the neutralization of the acid-salt mixture. There is little information on the direct titration of strontium, Korbl and Pribil (1) reported ten titrations of 4 to 90 mg of strontium using thymolphthalexone (not the preferred indicator today) but did not give an explicit procedure; they noted an adverse effect of neutral salts on the color change of that indicator. Phthalein purple (sometimes called phthalein complexone or metalphthalein), in combination with a green dye and methyl red was introduced by Anderegg, et d.(2), for the titration of alkaline earths but no data were given for strontium. Phthalein purple was used also by McCallum (3) in a few titrations of calcium, magnesium, and barium, and by Cohen and Gordon (4) for the visual and spectrophotometric titration of barium. The same indicator was used by Ogawa and Musha (5) who studied the spectrophotometric titration of strontium and found that the optimum pH was between 10 and 12. All reports on the use of phthalein purple note that the color change is sharpened by the addition of 30-50x ethanol. The effect of salts on the quality of the end-point color change in chelometric titrations has been reported by others, e.g., Flaschka and Mann (6)who found difficulties with murexide. The present experiments show that the presence of 10 mmol of NaCl does decrease 1
Present address, Gulf Research and Development Co., Pitts-
burgh, Pa. 15230. 2 Present address, Department of Chemistry, University of California, Berkeley, Calif. 94700.
(1) ~, J. Korbl and R. Pribil, Collect. Czech. Chem. Commun., 23, 873 (1958). (2) G. Anderegg. H. Flaschka. R. Sallman. and G. Schwarzenbach, Helc.. Chim.Acta, 37, 113 (1954). (3) J. R. McCallum, Can. J. Chem., 34, 921 (1956). CHEM., 28, 1445 (1956). (4) A. I. Cohen and L. Gordon, ANAL. (5) K. Ogawa and S. Musha, Bull. Uniu. Osaka Prefect., Ser. A , 8, 63 (1960). (6) H. Flaschka and J. Mann, Anal. Lett., 1, 19 (1967). \
,
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both the precision and accuracy of the determinations but does not impair their utility; the visual titration is fully as satisfactory as the more time-consuming spectrophotometric method. EXPERIMENTAL
Apparatus. Spectrophotometric titrations were made with a Hitachi Perkin-Elmer 139 instrument with the standard attachment. A 10-ml microburet with a platinum tip was used in all titrations; readings could be estimated to +0.005 ml. All volumetric ware was calibrated at intervals during the experiments. Visual titrations were made in light from a fluorescent illuminator. Reagents. Deionized water was used for all reagents; water and reagents were stored in plastic containers. Approximately 0.01M strontium chloride was prepared from Merck Reagent, “Low in Barium”; the solution was standardized by evaporation with sulfuric acid in platinum and heating to constant weight (7). Eastman EDTA di-sodium salt was recrystallized; solutions were approximately 0.01 M and were standardized against the strontium chloride, using amounts within the range being studied. Other solutions were 1.023N HC1, approximately 1N and 0.1N NaOH, carbonate free, and NH3. The same lots of NaOH, HCl, and NH3were used in all experiments. The indicator was a Fisher Certified Reagent and was made up as follows: 0.1 gram phthalein purple, 0.005 gram methyl red, 0.05 gram “Erie Green Mt Conc 110 %” (Allied Chemical Co.), dissolved in a few drops of NH3 and diluted to 100 ml. The choice of green dye is not critical but its presence is necessary; the dye cited (Color Index unknown) was satisfactory. The indicator solution was prepared daily because the color faded over a period of hours; the rate of fading did not affect the titrations which took less than 30 min. Procedure. Three series of titrations were carried out with varying amounts of strontium chloride : visual titrations in the absence of NaC1, with addition of ethanol; visual titrations in the presence of 10 mmol of NaC1, with addition of ethanol; and spectrophotometric titrations in the presence of 10 mmol of NaC1, without addition of ethanol. In the visual titrations, varying amounts of the strontium chloride (7) W. F. Hillebrand and G. E. F. Lundell, “Applied Inorganic Analysis,” John Wiley and Sons, New York, N. Y.,1929, p 503.
ANALYTICAL CHEMISTRY, VOL. 41, NO. 13, NOVEMBER 1969
Table I. Results of Analyses Wt Sr taken, mg
Titration procedure Visual, no NaCI, with ethanol Number of trials Mean Sr found, mg Std deviation, mg X 103 Re1 std deviation, Mean error, mg x 103 Re1 error, % Visual, 10 mmoles NaCI, with ethanol Number of trials Mean Sr found, mg Std deviation, mg X lo3 Re1 std deviation, % Mean error, mg X lo3 Re1 error, % Spectro, 10 mmols NaCI, no ethanol Number of trials Mean Sr found, mg Std deviation, mg X 103 Re1 std deviation, % Mean error, mg X lo3 Re1 error, %
0.885 5
0.882 4.06 0.46 -3.0 -0.34 0.885 5 0.893 8.69 0.97 +8.0 +0.90 0.886
1 ,776 5 1.768 3.57 0.20 -8.0 -0.45
5
2.663 5 2.662 5.90 0.23 -1.0 -0.04 2.663 5 2.668 10.4 0.39 +5.0 +O. 19 2.666 5
2.681 13.50 0.50 +13.0 +0.49
0.893 6.28 0.70 +7.0 +o. 79
solution were pipetted into 250-ml Erlenmeyer flasks; if NaCl was to be present, 10 ml of the 1.013N HC1 were added by pipet and the solution was made neutral to methyl red. To keep the volume low, about 9 ml of 1N NaOH were first added by pipet and the neutralization was completed exactly with the 0.1N NaOH in a buret. After the addition of 5 ml of NHBand 5 drops of the indicator, the titration was completed promptly. The pH at the start of the titrations was close to 12 and decreased a few tenths of a unit at the end. In all visual titrations, the initial volume was 35-45 ml and an equal volume of absolute ethanol was added near the end; the purpose of the delay in the addition of the ethanol was to prevent the precipitation of strontium carbonate. The color change was from blue to a bright yellowgreen. In the spectrophotometric titrations, the procedure was the same except that 150-ml beakers were used and the initial volume was 100 ml. The wavelength of maximum absorbance of the strontium-indicator complex was 579 nm; this was used. To minimize the photometric error, measured transmittances were in the range of 0.15 to 0.65; beakers varied somewhat in diameter but by setting the initial transmittance at 0.15, the final value was 0.65 or less in virtually all titrations. RESULTS AND DISCUSSION
In the spectrophotometric titrations, the absorbances were corrected for dilution and plotted against the volume of EDTA. The plots had the expected shape-Le. a sharply decreasing absorbance, linear with volume just before the equivalence point, followed by a straight line with very little slope after the equivalence point-much like those shown by Cohen and Gordon (4). Additions of EDTA several milliliters beyond the equivalence point had no effect on the endpoint volume estimated at the intersection of the two branches. In the presence of NaC1, the plotted absorbances deviated from linearity near and on both sides of the equivalence point; the addition of ethanol increased the linearity considerably but no series of this type was run because the addition of ethanol did not affect the precision of the spectrophotometric titrations and it caused the precipitation of strontium carbonate in some cases. The effect of NaCl and the counter-effect of ethanol on the linearity of the two branches of the plot confirm the observation that neutral salts make the color change less sharp and that ethanol improves the color change;
4.440 4 4.445 6.58 0.15
8.902 11 8.916 4.44 0.05
+14.0 +O. 16
f5.0
+o. 11
4.440 5 4.424 8.99 0.20 -16.0 -0.36 4.442 5
4,475 19.50 0.44 +33.0 +0.74
7.994 5 7.975 15.1 0.19 -19.0 -0.24 8.007 5
7.994 7.38 0.09 -13.0 -0.16
in fact, visual titrations in the presence of NaCl and presumably, other neutral salts cannot be made satisfactorily unless ethanol is added. The effect of NaCl was nearly the same with 1 mmol as with 10 mmol. The experimental results for the three series by one operator are listed in Table I ; the other two operators obtained entirely comparable results. In the Table, one uncertain digit, as determined from the standard deviations in the pipet calibrations and standardization of the strontium chloride, has been retained for the purpose of estimating the statistics. The results were examined statistically, generally by use of the procedures outlined by Youden (8); an exception was that the tests for equal variances in the sets with different amounts of strontium were made by the method given by Dixon and Massey(9). In each of the series, the variances in the sets did not differ significantly at the 95% confidence level, indicating that the precision of the titrations was independent of the amount of strontium taken. In each set of each series, the precision was estimated by finding the standard deviation from the differences between the amounts found in the individual trials and the mean amount of strontium found; the relative standard deviation was based on the same mean. The accuracy was estimated as the mean error, defined as the difference between the mean amount found and the amount taken in that set; the relative mean error was based on the amount taken. In the two series of visual titrations, inspection shows an apparent trend in the mean error with amount of strontium taken, though in opposite directions in the two series. The trend was evaluated statistically; in the series without NaC1, it was significant while in the series with NaCl, it was not, both at the 95% confidence level. With a small number of sets, however, the test is not very efficient and the conclusions must be viewed with reserve. It may be that a small systematic error exists, dependent on the amount taken, in both visual series though it is not large enough to prevent
(8) W. J. Youden, Encycl. Ind. Chem. Anal., 1, 746 (1966). (9) W. J. Dixon and F. J. Massey, Jr., “Introduction to Statistical Analysis,” McGraw-Hill Book Co., New York, N. Y. 1951, p 90.
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reasonably precise and accurate determinations, particularly with amounts in excess of a milligram. In the series of spectrophotometric titrations, no trend is apparent and none was found by statistical methods. In any case, it would be good judgment to standardize the EDTA with amounts of strontium not too different from the amounts expected. Estimates of the blanks were made experimentally and by statistical evaluations. When NaCl was not present, no blank was found by either method. In the case of the visual titrations in the presence of NaC1, an indication of a blank was found experimentally but when applied, only negative errors resulted; statistically, no adequate basis for a blank was found and none was used. In the spectrophotometric titrations, a blank of 0.019 ml of EDTA was found experimentally; the statistical estimate was found from a plot of the mean error (when no blank was used) as ordinate against the amount taken in the sets. A horizontal line through the average ordinate gave a reasonable fit to the points and, as
mentioned above, no trend was discernible; the average ordinate corresponded to a blank of 0.023 ml of EDTA, confirming the experimental value and this was applied. The absence of a blank when ethanol was present and the need for a blank when ethanol was not present did not find an acceptable explanation. A comparison of the two series of visual titrations shows that the presence of NaCl affects both precision and accuracy adversely but not ruinously. The spectrophotometric method appears to offer no advantage over the visual procedure. A more complete understanding of the applicability of EDTA titrations in practical situations would be gained from a systematic study of the effects of salts on the stability constants of the metal-EDTA and the metal-indicator complexes. RECEIVED for review March 13, 1969. Accepted August 25, 1969. Research supported in part by the New York University Graduate School of Arts and Science Research Fund.
Density Distribution of Solid Particles by a Flotation-Refractive Index Method Gordon H. Fricke and Donald Rosenthal Department of Chemistry and Institute of Colloid and Surface Science, Clarkson College of Technology, Potsdam, N . Y . 13676 George Welford Health and Safety Laboratory, U.S.Atomic Energy Commission, 376 Hudson Street, New York, N . Y . 10014 IT WOULD BE DESIRABLE to have a method for obtaining the density or density distribution on small amounts of solid materials with a minimum of liquid. In this paper a procedure is described based upon flotation ( I ) in which miscible liquids mixed in the proper proportions are used to determine the density of the material which remains suspended, or the density range of a portion of the material. The density of the liquid mixtures can be determined by refractive index measurement once a suitable calibration curve has been obtained. These refractive index measurements require only a few drops of the liquid mixture and are much more convenient than the more conventional pycnometer measurements. The procedure is illustrated in this paper by the determination of the density distribution of a small sample of glass beads. The flotation method can be used to separate particles of different densities and to reduce the spread of a density distribution. Also, it may be used to remove solid impurities from solid particles, if the particles and impurities differ in density. EXPERIMENTAL
For the purpose of testing the method, glass beads (100-200 mesh, 149-74 microns obtained from Cataphote Corp., Jackson, Miss.) were used. The density of the glass beads was determined by the normal pycnometer method to be 2.4075 g/ml (relative error at 90% confidence level was 0.16 parts per thousand for three determinations using weights of solid materials ranging from 5.9 grams to 11.6 grams in a 25-ml pycnometer). Two liquids, dibromomethane ( d =
2.48, n = 1.538) and carbon tetrachloride ( d = 1.59, n = 1.463), were selected, because they had a large difference in refractive index, covered a reasonable density range, and were completely miscible. The densities were measured on mixtures of these two liquids using a pycnometer. The refractive index of these mixtures was obtained using a Bausch and Lomb Abbe-3L Refractometer. The density distributions of two samples of glass beads were determined by initially placing the samples (3.6995 grams and 5.0000 grams) into 25 X 150 mm test tubes with screw caps. A 30-ml portion of the more dense liquid, dibromomethane, was placed in the tube with the glass beads. To aid the distribution of the glass beads between the bottom of the tube and the surface of the liquid, the tubes were centrifuged for approximately 10 minutes at about 2000 RPM (force at bottom of tube ~ 9 0 g). 0 The beads which had settled to the bottom of the tube were withdrawn with a pipet and placed in a tared weighing vial. The next desired density was obtained by adding carbon tetrachloride until the solution had the predetermined refractive index corresponding to the density. The tube was carefully swirled to prevent beads from adhering to the wall of the tube above the liquid. The procedure of adjustment of refractive index, centrifugation, recording of refractive index after centrifugation, and removal of the more dense beads, was repeated for each new density. Finally, the liquid, which had been placed in each weighing vial with the beads, was evaporated and the vials were reweighed to obtain the weight of beads within the narrow density regions. RESULTS AND DISCUSSION
(1) P. Hidnert and E. L. Peffer, National Bureau of Standards Circular 487, U.S. Government Printing Office, Washington, D. C., 1950. 1866
Refractive Indices and Densities of Liquid Mixtures. A number of empirice1 and semi-empirical equations are available by which the density, d, may be represented as a
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