predictions are not substantially altered by explicit consideration of nonlinearity; and experimentally, no published data on these anomalous systems indicate the magnitudes of nonlinear components of the current to be sufficiently large that nonlinearity of the electrode reaction need be taken into account for the adequate theoretical description of their small amplitude a.c. polarographic characteristics in terms of whatever model may be applicable. In regard to point 3 of the second paragraph, our original objection (6) was raised against the following statement of Bauer and Elving ( 8 ) : “Thus, the mathematical treatment can lead equally to Equation 1 or 2. Whether cot I$ for a particular system is greater or
.
less than 1.. depends on whether the oxidation or the reduction process is preferentially hindered or favoured.’’ In regard to the third paragraph, we concur that the conviction cited by Bauer would be without objective basis. Discussion in a recent paper by Tamamushi and Tanaka (7) is pertinent to our observations that adsorption would be a logical explanation of some of the anomalous data of Bauer, Smith, and Elving (9). In regard to the fourth paragraph, our estimations were seriously m i s calculated, although we find them accurate for some systems similar in constitution to those employed by Bauer, Smith, and Elving.
LITERATURE CITED
(1) Bauer, H. H., ANAL.CHEM.33, 1803 (1961). (2) Bauer, H. H., Elving, P. J., J. Ebctroanal. Chem. 2, 83 (1961). (3) Bauer, H. H., Smith, D. L., Elvin , P. J., J . Am. Chem. SOC.82,2094 (19607 (4) Matsuda, H., Z. Ebktrochem. 62, 97f (1958). (5) Reinmuth, W. H., Smith, D. E., ANAL.CHEM.33,964 (1961). (6) Zbid., submitted for publication. (7) Tamamushi, R., Tanaka, N., 2. physik. Chem. (N. F.) 28,158 (1961). W. H. REINMUTH D. E. SMITH’ De artment of Chemistry Cokmbia University New York 27, N. Y. 1 Present address, Department of Chemistry, Northwestern University, Evanston, Ill.
Determination of Small Amounts of Calcium in Magnesium O x i d e SIR: The determination of calcium and magnesium with (ethylenedinitril0)tetracetic acid (EDTA) is qui’ck and reasonably accurate. Lewis and Melnick (2) evaluated the conventional EDTA method and compared results with those obtained by the classical gravimetric procedures. However, their
Table l.
work was limited to analyses of mixtures containing high ratios of calcium to magnesium. The EDTA method has been used with some success for routine determination of calcium in magnesium oxide for our plant process control. However, there was not good agreement be-
Calcium Determination in Known Solutions of Calcium and Magnesium Chloride
Un-
Slow Precipitation-EDTA Solution “,OH-EDTA Analyst I1 Analyst I 1 Caa found in 0.127 0.134 0.162 0.129 0.137 0.132 2 duplicates, % 0.166 0.177 0.193 0.197 0.189 0.202 0 200 0 197 0.184 0.206 0.204 0.220 3 4 01208 0:iio 0:223 01% 0:241 0.232 5 0.235 0.235 0.237 0.257 0.262 0.270 Av. % Ca, i? 0.189 0.202 0.209 0.004 0.014 0,008 Std. dev. of method. So Coefficient of variation of analytical method, 100 2.1 7.0 3.8 Coefficient of variation from true mean, 100 -10.9 -4.7 -1.4 known
($)
(e)
Conventional EDTA 0.046 0.040 0.069 0.075 0.075 0.069 0.079 0.089 0.117 0.100 0.076 0.007 9.2
-64.2
a % calcium added.to each solution: 0:140 (l),0.200 (2), 0.220 (3), 0.239 (4), 0.260 (5). Average per cent calcium ( p ) for all solutions was 0.212. Values reported on MgO basie.
Table II. Calcium Determination in Magnesium Oxide
“,OH-EDTA Ca found in duplicates, 0.265 0.272 % 0.286 0.265 0.322 0.329 0.300 0.322 0.264 0.286 0.291 Av. % Ca, 2 0,009 Std. dev. of method, So Coefficient of variation of analytical method, 100 3.1
(9)
1804
e
ANALYTICAL CHEMISTRY
Slow Precipitation-EDTA Analyst I Analyst I1 0.272 0.270 0.272 0.236 0.285 0.257 0.300 0.304 0.343 0.350 0.357 0.372 0.386 0.357 0.343 0.380 0.265 0.307 0.272 0.293 0.313 0.309 0.013 0.013 4.2
4.2
Conventional EDTA 0,096 0.134 0.160 0.189 0.176 0.188 0.169 0.143 0.122 0.165 0.154 0.024 15.6
tween replicate calcium determinations when the percentage of calcium in magnesium oxide was less than 0.5%. For example, a comparison of the analysis of 20 duplicate spot samples showed that the relative standard deviation of the conventional EDTA (9) method was greater than 15%. It also revealed that the average value obtained for the calcium was approximately two thirds that obtained by the classical precipitation method (1). On the premise that part of the calcium was being coprecipitated with the magnesium hydroxide in the EDTA method, a study was undertaken to eliminate this possible source of error. EXPERIMENTAL
Two EDTA approaches were evaluated. The first used ammonium hydroxide to remove most of the magnesium as magnesium hydroxide before determination of the calcium. The second attempted to precipitate the magnesium as its hydroxide, as homogeneously as possible, by the slow dropwise addition of a buffer solution before the determination of the calcium. A M M O N I U M HYDROXIDE MODIFICATION
Accurately weigh duplicate samples of magnesium oxide containing 2 to 3 mg. of Ca, transfer to 250-ml. beakers, and add 25 ml. of distilled water to each sample and just enough 6N hydrochloric acid to dissolve the samples. Heat the solutions to boiling and slowly add concentrated ammonium hydroxide until a slight excess is present to precipitate magnesium hydroxide. Continue to boil the samples for a few minutes, then cool and centrifuge. Decant the clear solutions into 250ml. Erlenmeyer flasks. Wash the precipitate in the centrifuge tubes with distilled water, recentrifuge, and com-
bine the clear solutions. To the first solution add enough 5N NaOH to bring the pH to 12.5. Add 0.2 gram of CalRed indicator and titrate with 0.01M EDTA to a pure blue end point. To the duplicate solution, add as many milliliters of EDTA solution as were required to titrate the first sample. Then add 5N KaOH until a p H of 12.5 is reached. Add 0.2 gram of Cal-Red indicator and complete the titration with 0.01M EDTA to the pure blue end point. SLOW PRECIPITATION MODIFICATION
Dissolve a 0.5-gram sample in 10 ml. of 3N hydrochloric acid and add 140 ml. of water. Add to the solution in a steady dropwise manner 50 ml. of a buffer solution containing 20 grams of NaOH, 1.5 grams of KCN, and 2 grams of NHzOH.HCl per liter to precipitate magnesium hydroxide slowly with a minimum amount of coprecipitated calcium. Stir the solution well during the addition of the buffer and continue stirring for 5 minutes after the addition. Adjust to 10.4 p H with the above buffer solution and add 0.2 gram of Cal-Red indicator, which turns the slurry blue. Slowly add 5.5 ml. of a buffer solution containing 100 grams of NaOH and 7.5 grams of KCN per liter, and allow to mix for several minutes. The pH of the slurry should be between 12.3 and 12.7 and the color pink. Add 0.01M EDTA solution until the pink color changes to a dull blue-gray. Wait a few minutes and add 2 more drops of EDTA solution. Continue to add 2 drops of EDTA solution a t a time until a clear blue color appears, which is the end point.
To evaluate the relative merits of the two modifications more fully with a
conventional EDTA method (S), synthetic standard solutions were made up with calcium chloride and sufficient magnesium chloride to be in the same proportions as that found in the magnesium oxide. The calcium chloride solution was prepared from National Bureau of Standards calcium carbonate (99.97% purity) and hydrochloric acid. Aliquots of the standard calcium chloride solution and solid magnesium chloride, analytical grade, were used for the desired synthetic solutions. The concentration of calcium in each sample was unknown t o the different analysts. RESULTS AND DISCUSSION
EDTA method compared to the conventional EDTA procedure for five duplicate sets of routine plant-produced magnesium oxide. Here, too, both the improved EDTA methods and the different operators have essentially the same variability. Also the per cent calcium found in the magnesium oxide samples by the conventional EDTA method was considerably less than that found by the improved methods. These results indicate that when the calcium content in magnesium oxide is low, these modifications are a definite improvement over the usual EDTA procedure. The slow precipitation method is especially adaptable to routine plant control.
Table I gives a statistical evaluation
(4) of the two modifications of the EDTA
method compared to the conventional EDTA procedure for five sets of duplicate synthetic solutions. The data indicate that both EDTA modifications and the different operators have essentially the same variability. The slow precipitation EDTA modification gave only a slightly low result, while the ammonium hydroxide method was 10.9% low. Perhaps this increased error is in part due to the coprecipitation of calcium with the magnesium hydroxide during the preliminary removal of the magnesium ion. In any event, neither method gives the low results or as large a relative standard deviation as with the conventional EDTA procedure. Table I1 also gives a statistical evaluation of the two modifications of the
ACKNOWLEDGMENT
The authors thank Robert J. [Meyer for his many helpful suggestions and Gin0 Baseggio for his help with the statistical analysis of the analytical results. LITERATURE CITED
(1) Furman, N. H., “Scott’s Standard
Methods of Chemical Analysis,” 5th ed., Vol. 1, p. 538, Van Nostrand, New York, 1939. (2) Lewis. L. L.. Melnick. L. M.. ANAL. &EM. 82, 38 (1960). ’ (3) Patton, J., Reeder, W., Ibid., 28, 1026 (1956). (4) Youden, W. J., “Statistical Methods for Chemists,” p. 16, Wiley, New York, 1951. .
I
Research Laboratory Morton Chemical Co. Woodstock, Ill.
C. A. BAUGH K. H. DECKER J. W. PALMER
Gas Chromatographic Separation of Simple Aliphatic Amines SIR: Mixtures of simple aliphatic amines are difficult to separate, yet in many fields of research it is of great importance that a quantitative separation be made. James (I) has reported separations of the methylamines and of the homologous series of primary amines using various columns. He also reports a partial separation of mixtures such as dimethylamine with ethylamine. However, the quantitative separation of mixtures of primary and secondary amines of the same molecular weight is not easily accomplished. We have been interested in a quantitative separation of methylamine, dimethylamine, and ethylamine in connection with another problem. The separation of this mixture into two component parts is not particularly difficult, and by a proper choice of two columns a reasonable separation can be made. The separation of all three
components on a single column has not been reported to the best of our knowledge. We had noted that there is an appreciable difference in the solubilities
IO RETENTION
20
of these amines in o-toluidine and prepared a column with this as the liquid phase. The o-toluidine was Eastman Kodak No. 253, used as obtained. The solid support was Johns-Manville C-22 firebrick, 40-60 mesh, treated by the standard method before use (2). The o-toluidine (3 parts by weight) was added to the firebrick (7 parts by weight) and the two were mixed until homogeneous in appearance. By using a 6-foot column a t room temperature with helium as the carrier gas a t a flow rate of 30 cc. per minute, the separation given in Figure 1 was obtained. The instrument used was a Perkin-Elmer Vapor Fractometer Model 154B.
30
TIME (MINUTES)
Figure 1 . Gas chromatographic separation of amines
At room temperature o-toluidine has
a significant vapor pressure and gradually bleeds out of the column. Thus the packing must be renewed periodVOL 33, NO. 12, NOVEMBER 1961
* 1805
