peak for S-allothreonine which was assigned by enrichment of racemic allothreonine with R-allothreonine. The response of the four diastereomeric dipeptides to the flame ionization detector were found to be identical. The areas of the peaks were calculated from the products of the height of the peak and the peak width at half height. Attempts to separate the third composite peak using SE-30 and propyleneglycoladipate ( I ) as liquid phases failed. The relative amounts of these two isomers, St and Sa were ultimately determined by using N-trifluoroacetyl R-prolyl chloride in the peptide formation. Reaction of the threonine-allothreonine mixture with the R-proline compound instead of its mirror image results in a chromatogram identical to that shown in Figure 2 except that all of the peptides synthesized are enantiomeric to the previous set. That is the derivatives in order of increasing retention times are R-proline-S-threonine (RSt), R-proline-S-allothreonine (RSa) and R-proline-R-threonine (RRt) plus Rproline-R-allothreonine (RRa). The relative areas of the first peaks of this chromatogram represent the amounts of
St (as RSt) and Sa (as RSa) present in the original mixture. By combining the information obtained from the two chromatograms, the composition of the mixture can be correctly found as 25% Rt, 25% Ra, 25% St, 25% Sa. Using the above procedure, the analysis of mixtures of various composition was carried out and the results are presented in Table I. The purity of N-trifluoroacetyl S-prolyl chloride was determined by reacting it with optically pure R-threonine. The separation of this solution showed a major peak at a retention time of 7.2 min and a minor one at 9.2 min. The former corresponded to SRt and the latter to RRt since its mirror image SSt was not possible because no St was present in the sample. The areas of the two peaks provide a direct measure of the optical purity of N-trifluoroacetyl S-prolyl chloride which was found to be 9 9 . 2 z and 0.8z R . The same analysis was applied to the R-proline derivative (Table I). RECEIVED for review October 22, 1970. Accepted January 18, 1971. This work was supported by the National Institutes of Health Grant No. 5 R01 AM12262.
Potentiometric Determination of Potassium Theodore S. Prokopov Department of Chemistry, Upper Iowa College, Fayette, Iowa 52142
MORETHAN a score of analytical procedures (gravimetric, volumetric, spectrophotometric, and flame-photometric) have been proposed and used for a direct or indirect determination of potassium (I-6). All gravimetric procedures, no matter how accurate, are laborious and tedious. Volumetric and indirect spectrophotometric methods at best offer little or no advantages over the gravimetric procedures (7). The flame-photometric method, which is regarded as a simple one, is not simple at all. Nevertheless, it is now widely used as being more advantageous when many routine analyses should be made of samples of similar composition. However, many sources of interference, particularly the presence of sodium, greatly lower the accuracy of potassium determination, bringing the relative error to i3-4 % and higher. It was, therefore, highly desirable to develop a potentiometric method of potassium determination with accuracy not less than *0.3% and with the convenience and speed which potentiometric titration usually offer. For this purpose, a study was undertaken to investigate the possibility of potass ium determination by its precipitation with a measured excess of sodium tetraphenylboron, by consumption of this excess with a known excess of AgN03 in acidic water-ethanol medium and back titration of AgN03 with a NaCl solution. (1) H. Tollert, “Analytik des Kaliums,” Enke, Stuttgart, 1962. (2) S. Kallman, “Treatise on Analytical Chemistry,” I. M. Kolthoff, and P. J. Elwin, Ed., Vol. I, Part 11, Wiley-Interscience, New
York, 1961. (3) E. N. Archibald, W. G. Wilcox, and B. G. Buckley, J. Amer. Chem. Soc., 30, 747 (1908). (4) G. F. Smith and T. F. Ross, ibid. 47, 1020 (1925). (5) M. Kohler 2.A m / . Chem., 138, 9 (1953). (6) J. Dean, “Flame Photometry,” McGraw Hill, New York, 1960. (7) I. M. Kolthoff, E. B. Sandell, E. J. Meehan, and S. Bruckenstein, “Quantitative Chemical Analysis,” 4th ed., Macmillan, London, 1969, p 666.
Table I. Titration of a Mixture of 2 Milliliters of 0.0927M KN03, 3 Milliliters of 0.1M Sodium Tetraphenylboron, 3 Milliliters of 0.1006M AgN03, and 0.5 Milliliter of 6M HNOI with 0.1011M NaCl Solution. Mixture Diluted to 30 Milliliters with Absolute Ethanola AEjAV A2EjAV2 Cl-, ml E , mV 1.70 35 20 1.80
325
55
345 1.90
2.00
400
3 25
20
420
+ 325
aVol. = 1.80 - X 0.1 = 1.85 ml. meq NaCl 650 meq K+ = 0.1852 us. actual 0.1854. Error = -0.11%. 0.1854/30 = 6 X IO-aM.
=
0.1870
[K+] =
EXPERIMENTAL
Apparatus. A Fisher pH meter Model 210 was used. A Beckman billet-type silver electrode and a calomel electrode in which a saturated solution of N a N 0 3 was substituted for solution of KC1, were employed. A magnetic stirrer was used, and the titrant was delivered from a 5-ml microburet. Reagents. All chemicals used (KN03, Na[B(CeH&], AgN03, “03, C2HhOH) were of analytical reagent grade. The solid sodium tetraphenylboron is stable for months, especially if stored in a cold place, protected from light. In solution, however, it is less stable and may develop a turbidity and phenolic odor. No special treatment is needed to prepare a solution sufficiently stable for a week. It is best stored in a refrigerator, and may be used as long as it shows no turbidity. Procedure. The size of sample depends upon the amount of sample available and upon the expected concentration of ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, M A Y 1971
793
Table 11. Titration of a Mixture of 0.1 Milliliter of 0.0093M KN03, 2 Milliliters of 0.01M Sodium Tetraphenylboron, 3 Milliliters of 0.01M AgN03, 0.5 Milliliter of 6 M “ 0 3 , and 24.5 Milliliters of Ethanol with 0.0101M NaCl Solutiona C1-, ml
E , mV
1 .oo
50
1.05
60
AEIAV
A2E/AV2
10
110 120
1.10
180
110 10
1.15 aVol.
meq K+
=
190 1.05 f
=
0.0009/30 =
110
-
X 0.05 = 1.08 ml.
220 0.0009 1;s. actual O.OOO9. 3 X IO-jM.
meq NaCl = 0,0109 Error = none. [K+] =
potassium. A measured excess of 0.1M or 0.01M aqueous solution of sodium tetraphenylboron is added to the dissolved sample followed by a known excess of approximately 0.1M or 0.01M A g N 0 3 solution. The mixture is then made 75-85X in absolute ethanol and in 0.1M H N 0 3 . The p H meter is then set t o desired “zero” point, stirrer is started, and titration is carried out by addition of appropriate increments of standard solution of NaCl up to the end of titration.
DATA OF THE TITRATIONS The data of typical titrations are given in Tables I and TI, which are self-explanatory. DISCUSSION OF RESULTS The data of titrations show that it is possible to determine potassium in concentrations as low as 3 pg/ml with a n accuracy within the range of 1 0 . 3 %. In cases when potassium should be determined in the presence of an equivalent amount of halogens (KCl), the meq K+ found should be divided by 2. Thus, potassium and halogens could be determined simultaneously in the same sample. Should halogens not be present in equivalent amount, they should be determined first with A g N 0 3 and then potassium can be determined as described above in the same sample. Addition of approx. 7 5 4 5 % of ethanol decreases the K,, of AgCl, increasing the AEjAV ratio. Acetone cannot be used: it dissolves K[B(CeH&], causing a great negative error. Mercury(I1) decomposes the sodium tetraphenylboron, forming CBHSHgC1and other products. Except for NH4+, Ag+, Tl+, Rb+, and Cs+ ions which also form very slightly soluble precipitates with tetraphenylboron ion and halogens, none of the common cations and anions interfere with the potassium determination in dilute nitric acid medium.
RECEIVED for review November 12, 1970. Accepted January 14, 1971.
Further Investigation of the Colorimetric Reaction between Aromatic Isocyanates and Peroxy Compounds R. F. Layton’ and L. A. Knecht2 Chetnistry Departmenf, University of Cincinnati, Cincinnati, Ohio IN A NOTE to this journal (I), Layton and Quick described a new colorimetric method for the determination of aromatic isocyanates and peroxy compounds. The method is based upon the intense color formed when an aromatic isocyanate and a labile-hydrogen-containing peroxy compound are mixed in a dry solvent and the solution is made alkaline with tetrabutylammonium hydroxide. Although the colorimetric reaction appears to be general for these two classes of compounds, the only reaction for which analytical data were given was that between methylene-p,p’-diphenyl diisocyanate (MDI) and peracetic acid. Linear Beer’s law plots were obtained with either M D I or peracetic acid in excess, demonstrating the applicability of the method to the determination of either substance, and absorbance readings were stable over the time required €or measurement. The aim of this study was not only t o explore the scope of this potentially important method but, more importantly, to gain a n understanding of the stoichiometry, mechanism, and limitations of the colorimetric reaction itself.
EXPERIMENTAL
Present address, Chief, Wastewater Division, Water and Wastewater Technical School, Neosho, Mo, 64850. 2 Present address, Chemistry Department, Marietta College, Marietta, Ohio 45750
Apparatus. Spectral absorbance curves were obtained using a Beckman model DB spectrometer equipped with a Sargent model SRL recorder. Absorbance measurements at fixed wavelength were obtained using a Hitachi-Perkin Elmer model 139 direct-reading spectrophotometer. A Beckman IR-8 infrared spectrophotometer and an F and M model 720 gas chromatograph equipped with a column 20-ft X ‘/*-inch (carbowax 20M ( 1 5 z ) on Anakrom ABS, 60-70 mesh) were also used. Reagents. The solvent used must be dry and free of peroxides. In most of this work 1,4-dioxane [spectroquality reagent-grade Matheson, Coleman and Bell (MCB) Cincinnati, Ohio] was found suitable as received. The presence of peroxides in a solvent can be detected by placing 1 ml of phenyl- or naphthyl-isocyanate in 200 ml of the solvent to be tested and adding dropwise a 2 0 z solution of tetrabutylammonium hydroxide in methyl alcohol. The appearance of a color indicates the presence of peroxides, the intensity of the color serving as a n indication of the amount. Peroxides can be removed by storing the solvent over sodium hydroxide or by shaking the solvent with an aqueous ferrous salt solution (2). The water content of solvents, expecially hygroscopic solvents like N,N-dimethylformamide, should be checked by Karl Fischer titration.
(1) R F Layton and Q Quick, ANAL.CHEM., 40, 1168 (1968)
(2) E. G. E. Hawkins, “Organic Peroxides,” E. and F. F. Spon Ltd , London, 1961, p 344
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, M A Y 1971
