Table I lists the ranges of these groups. Solvent systems with water compositions below 30% give good resolution with two ascents. At increasing water concentrations, three or four ascents may be required. Furthermore, these systems will resolve the 1 :4and 1 :6 linked polymers of identical D.P., the 1 :6 linked saccharide migrating slightly slower than its 1 :4 linked counterpart. A second solvent system of 1-propanol, nitromethane, and water, 5 :2:3 by volume, gives an entirely different separation pattern of the maltodextrins. This system resolves the complete maltodextrin spectrum from D.P. 1 to 23 in four ascents. This single optimum composition of solvent components was determined in the same manner as the preceding solvent system. Tracings of a chromatogram developed in this system are illustrated in Figure 2. To avoid smearing of the saccharides, it is essential that the chromatograms be dried thoroughly between ascents. Continuous TLC. Continuous TLC was applied to avoid the repeated ascents and dryings necessary in UMC for resolution of the larger megalosaccharides. This technique is
superior in resolution for the saccharides of D.P. 20 to 35. The solvent systems applicable to UMC also apply to the continuous technique. Figure 3 is a tracing of a chromatogram, which ran for 24 hours, showing resolution of the maltodextrins from a potato amylopectin hydrolyzate. It is worthwhile to note that a smeared TLC is likely due to the excessive “free water” in the thin-layer, which cannot be satisfactorily removed by redrying. Also in some cases when the spot size is either too large or too diffused, keeping water content constant and increasing 1-propanol at the expense of the other component will improve the resolution. ACKNOWLEDGMENT
The authors acknowledge the helpful discussions with J. N. BeMiller concerning our work.
RECEIVED for review August 17, 1967. Accepted September 29, 1967. Division of Analytical Chemistry, 153rd National Meeting, ACS, Miami Beach, Fla., April 1967.
Standardization of Osmium(VIII) Solutions by Potentiometric Titration with Chromium(l1) Sulfate Louis Meites and Richard E. Cover’ Department of Chemistry, Polytechnic Institute of Brooklyn, Brooklyn, N . Y. 11201
DURING WORK on the electrochemistry of osmium, it became necessary to develop a procedure for the assay of osmium tetroxide solutions. The controlled-potential electrolytic procedure previously used (1) was not sufficiently precise and accurate while other published procedures appeared to be inapplicable or too tedious (2, 3). A volumetric procedure was developed because of the accuracy and rapidity of analysis so often obtainable with titration techniques. The high volatility of osmium tetroxide presents a major difficulty to the accurate assay of its solutions and various techniques have been used to minimize errors due to this cause; among these are reaction with excess potassium iodide (4, 5), reaction with hydrazine in HBr (4, 6, 7), titration in alkaline media (8), and reduction with bismuth metal (9). In the procedure presented here, loss of osmium is prevented by the reduction of osmium(VII1) to osmium(V1) with ethanol in sodium hydroxide solution. This reaction has long been known (IO) and proceeds rapidly and quantitatively at room 1 Present address, Department of Chemistry, St. John’s University, Jamaica, N. Y. 11432.
(1) R. E. Cover, M.S. thesis, Polytechnic Institute of Brook!yn, 1960. (2) F. E. Beamish, Anal. Chim. Acru, 20, 101 (1959). (3) F. E. Beamish, Tulunru, 1, 3 (1958). (4) W. R. Crowell and H. D. Kirschman, J . Am. Chem. SOC.,51, 175 (1929). (5) D. I. Ryabchikov, J. Appl. Chem. (U.S.S.R.),17, 326 (1944). (6) W. R. Crowel1,J. Am. Chem. SOC.,54,1324(1932). (7) W. R. Crowell and H. D. Kirschman, Zbid., 51, 1695 (1929). (8) D. I. Ryabchikov, Zh. Anal. Khim., 1,47 (1946). (9) V. S . Syrokomky, Doklady Akud. Nauk S.S.S.R., 46, 280 (1945). (10) E. Frkmy, Compt. Rend. (Paris), 19,472 (1844).
temperature (11). The osmium(V1) solution is then acidified with hydrochloric acid, deaerated, and titrated under nitrogen with chromous sulfate. Crowell and Baumbach (12) devised an accurate potentiometric procedure for the titration of osmium(1V) with chromous sulfate but we could not use it because osmium(1V) is not easily formed quantitatively from osmium(VII1). Although it was devised for the assay of pure osmium tetroxide solutions, this method can be applied to osmium distillates (13) and is worthy of further investigation for that purpose. EXPERIMENTAL
The chromous sulfate solution was prepared by the method of Lingane and Pecsok (14), and was standardized by the potentiometric titration of known amounts of iron(II1) in 5.OF HCl. Standardization against copper(I1) (14) was unsatisfactory as the potentials observed were unstable. A commercially available all-glass buret and reservoir were used for the titrant, which was stored over amalgamated zinc in the reservoir under nitrogen and transferred to the buret by nitrogen pressure. The titration vessel was a 250-ml beaker and the total volume of the solution titrated was always 100 ml. The beaker was stoppered with polyethylene because rubber was attacked by solutions of osmium in HCl. Inlets were provided for the introduction of nitrogen, the electrodes, and the buret tip. A coarse-porosity, sintered, borosilicate gas-dispersion cyl(11) R. E. Cover, Ph.D. dissertation, Polytechnic Institute of Brooklyn, 1962. (12) W. R. Crowell and H. L. Baumbach, J. Am. them. SOC.,57, 2607 (1935). (13) F. E. Beamish, “The Analytical Chemistry of the Noble Metals,” Pergamoii Press, New York, 1966, pp. 39 and 96. (14) J. J. Lingane and R. L. Pecsok, ANAL.CHEM., 20,425 (1948). VOL 40, NO. 1, JANUARY 1968
a
209
inder was used to pass nitrogen over or through the solution. Prepurified nitrogen scrubbed with chromous chloride and then with water was used for deaeration and also for protection of the titrant in the reservoir. Magnetic stirring was employed during deaeration and titration. A coiled platinum wire indicator electrode and a commercial saturated calomel reference electrode were used. Stock solutions of osmium(VII1) were prepared by dissolving weighed amounts of the reagent grade tetroxide in cold water and diluting to known volume. All other chemicals were reagent grade. Titration Procedure. The aliquots of osmium solution titrated contained 0.05 to 0.5 mmoles of osmium(VII1). Each aliquot was added to a mixture of 5 ml of ethanol with 25 ml of 2.5F NaOH in a 250-ml beaker and allowed to stand at room temperature for 20 minutes. HCI and water were then added to give a final total of 100 ml of solution containing the desired concentration of acid. The beaker was then stoppered, the solution deaerated, and the sample titrated.
Table I. Analyses of Os04 Stock Solutions OsO,, pmoles Relative HCI, F Taken Found Mean error, 5.0 5.0 5.0 5.0 5.0 5.0 0.5 0.5 9.0 5.0 5.0 5.0 2.0 5.0 5.0 a
RESULTS AND DISCUSSION
Three inflection points are obtained. The first corresponds to the reduction of osmium(V1) to osmium(1V); the second is ill-defined ; the third corresponds to the complete reduction of the osmium to the +3 state and occurs at -0.11 =t0.01 V us. SCE in 5.OF HC1. The occurrence of two inflection points during the reduction of osmium(1V) to the +3 state is probably due to the presence of two osmium(1V) species in solution which are in slow equilibrium (11). The third end point was
86.34 86.34 86.34 92.10 92.10 92.10 160.6 160.6 163.8 163.8 163.8 163.8 163.8 244.1 328.6
86.44 85.92 85.92 92.30 91.38 91.38 159.8 162.3 162.7 164.1 164.6 164.6 164.6 245.6 326.3
Relative standard deviation
=
86.09
-0.29
91.69
-0.45
161.1
+0.3
164.1"
+0.2
...
...
... ...
0.50
used to obtain the results shown in Table I. The method is accurate and precise to about * O S % and is reliable over a relatively wide range of HCl concentrations. RECEIVED for review August 7,1967. Accepted September 22, 1967.
Mechanism of the Determilnation of Phosphorus with a Flame Ionization Detector F. M. Page and D. E. Woolley The University of Aston in Birmingham, Birmingham 4 , England KARMENAND GIUFFRIDA (1-3) have shown that a conventional flame ionization detector shows a specific enhancement of response to phosphorus- or halogen-containing compounds in the presence of the vapor of a sodium salt. Originally this was provided by introducing a gauze covered with sodium hydroxide into the flame, but later developments showed that an equal response resulted if a sodium salt was fused onto the electrode system (3) or if the burner tip was embedded in a ceramic tube ( 4 ) containing the sodium salt. It has been claimed that the conductivity of the flame produced by the sodium salt as it volatilizes is increased when a phosphorus-containing compound is eluted from a gas chromatograph column, and that this increase is a function of the amount of phosphorus (or halogen) in the eluate (2). It is also claimed that quantities of the order of lO-*O gram of phosphorus (corresponding to a partial pressure of lo-* atm in the flame gases) may be detected by this technique, and commercial detectors are now on the market, using cesium bromide instead of the sodium hydroxide or sulfate. The original discoverers ( I ) attributed the effect to an increased amount of sodium entering the flame because of the effect of A. Karmen, ANAL.CHEM.,36, 1416 (1964). A. Karmen and L. Giuffrida,Nature, 201, 1204 (1964). L. Giuffrida,J . Assoc. Offic. Anal. Chemists, 47,293 (1964). D. R. Coahran, Western ACS Conference, Covallis, Ore., June 1965.
(1) (2) (3) (4)
210
ANALYTICAL CHEMISTRY
phosphorus on the volatility of the sodium salt, but it has been shown by flame photometry ( 5 ) that there was no increase in the neutral alkali atom population in the flame; a further explanation had to be sought. We ourselves have been able to show that this effect can be demonstrated in a completely homogenous system, so that it is of a fundamental nature. EXPERIMENTAL
A series of premixed flames, of known Hz/N2/O2 ratios, were selected and their temperatures measured by the sodium Dline reversal technique. They were burned on a multitubular burner based on that described by Padley and Sugden (6). All gas supplies were metered by rotameter tube and float meters and the preburning composition of both inner and outer flames was the same. The electron concentration was measured by a microwave cavity originally described by Horsfield (7) and by Padley and Sugden (6). Cesium was added to the inner flame by diverting a portion of the nitrogen supply through a scent-spray type of atomizer (5) W. A. Ave, G . W. Gehrke, R. C. Tindle, C. D. Rayle, D L. Stalling, and S. R. Koirtyohaan, 5th National Meeting, Applied Spectroscopy, Chicago, Ill., June 1966. (6) P. J. Padlev and T. M. Sugden, . 8th Combustion Symposium, p. 164, 1960.(7) A. Horsfield, Ph.D. Thesis, Cambridge, 1957. .
I
