Burif ication of Tetrabutyia mmonium Iodide for Polarographic Use LOUIS SILVERMAN and WANDA G. BRADSHAW Atomics Infernafional, A Division of North American Aviation, Inc., Canoga Park, Calif.
+ The
recrystallization of tetrabutylammonium iodide was studied to develop a rapid and simple method of purifying the salt. When the cornmercially pure salt was dissolved in 1 to 3 methanol in acetone, filtered, partially evaporated, and precipitated by a d d ing distilled water, a product with a high degree of purity was obtained. This method of purification makes tetrabutylammonium iodide readily available for industrial polarographic use. Such a supporting electrolyte is needed for the analysis of organic material, etc., which is reduced a t potentials more negative than -2.0 volts. LTRABUTYLAMMONIUM IODIDE,as a supporting electrolyte, permits the attainment of the most negative potential in polarographic work a t this date; it is more effective in this respect i(2) than the lower molecular weight tetraalkyl salts such as tetramethylaminonium chloride. Its appreciable solubility in organic media such as dioxane, alcohols, and tetrahydrofuran, makes it particularly useful in the study of organic compounds which are insoluble in aqueous, but soluble in hydrophilic media. Pickard and Neptune (2) described a method for repurification of tetramethylammonium chloride using a recrystallization medium of methanol in acetone. Tet,rnbutylammonium iodide is much more soluble in this medium and cannot be crystallized from it in high yield. Consequently, the method of Pickard and Neptune (2) was modi5ed to make it suitable for purification of the tetrabutyl salt.
Procedure. Dissolve 5 grams of tetrabutylammonium iodide in 50 ml. of 1 to 3 methanol in acetone. Filter the solution through filter paper by suction. Allow the filtrate t o stand overnight at room temperature t o permit spontaneous evaporation to approximately 25 ml. Add 5 ml. of distilled water by pipet, precipitating tetrabutylammonium iodide crystals. Filter by suction. Dry the crystals in a ‘Jacuum desiccator. Store the purined tetrabutylamnionium iodide in an ‘F-wuateddesiccator for future use. Present address, Lockheed Aircraft Sunnyvale, Calif.
Corp.,
1672
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ANALYTICAL CHEMISTRY
U”
3z
0.0 0 E -14 Y
1 -1.6
-LS -20 - 2 2 -2.4 -2.6 -2.8 -3.0
POTENTIAL (volts)
Figure 1. Effect of methanol to acetone ratio on degree of purification Medium, 0.2 gram of tetrabutylammonium iodide in 10 mi. of 3 to 1 tetrahydrofuran-water Initial InterHalfference Wave Potential, Potential, Volts Volts~
Recrystallizotion Medium -2.6 0 A. 1 :3 methanol in acetone -2.31 E. UnpuriRed ( C ~ H I J ~ N I-2.12 -2.14 -2.31 C. 1:l methanol in acetone -2.18 -2.33 D. 100% methanol “Dropping mercury electrode characteristics, drop time, 5.70 seconds (open circuit); 53 cm. of mercury; m, 1.1 94 mg. sec-’ Curve
EXPERIMENTAL RESULTS
Effect of Methanol to Acetone Ratio. The tetrabutyl salt was recrystallized from selected mixtures of methanol in acetone as described. The effectiveness of the recrystallization was tested by using the recrystallized salt as a supporting electrolyte in a 3 to 1 tetrahydrofuran- ate^ mixture and determining the polarographic blank. The 1 t o 3 mixture of methanol in acetone was the most effective purification medium (Figure 1)-it gave the lowest blank curve and Less also the greatest yield (65%) than 50% yield was obtained when the ratio of methanol to acetone was increased. Reproducibility of Purification Method. The recrystallization, using 1 t o 3 methanol in acetone, is reproducible from batch t o batch. The effectiveness of the purification is reduced if the total volume of methanol in acetone mixture is decreased. Nature of Impurities. Commercially pure tetrabutylammonium iodide discolors upon exposure to light and air. Evidence indicates the formation of free iodine. Impurities, so formed, cause a high residual current, hut they can be removed by the treatment described. Storage. Although certain commercial samples of tetrabutylammo-
nium iodide deteriorated rather rapidly upon exposure to air, this is slowed after recrystallization as described Deterioration of materials that have been opened to air for sampling can be decreased by storing the materials in vacuum desiccators. Other Solvent Media. Recrystallization from ethyl acetate, as recommended by Laitinen and Wawzonek ( I ) , was attempted but the process was too tedious and the yield too low for routine polarographic analysis. Tetrabutylammonium iodide repurification using tetrahydrofuran was investigated but the product was not so pure as desired. Attempts a t chromatographic purification of the sait using silica gel ana alumina also yielded poor results. LITERATURE CITED
(1) . , Laitinen. H. .4.. Wawzonek. S.. J.Ant. Chem. Soc: 64, 176&G8, (1942). ’ (2) Pickard, P. L., Neptune, W. E.: A N A L . CHEM. 27, 1358 (1955).
RECEIVED for review February 13, 1956. .4ccepted July 6, 1959.
Polarogra pRy of Uranium(SV) in Noncomplexing and Complexing Media-Correction I n the article on “Polarography of Uranium(1V) in Noncomplexing and Complexing Media. Amperometric Defermination of Fluoride” iD. 3. McEwen and Thomas De vries, ANAL. CHEM.31, i347 (1959)], the following line should be added to ‘Table I . Phenyiacetic
3.8 0.175 0.500 VI 4.31 . . N.It IV
The iegend for A in Figure 2 should read 9.02M perchloric acid, O.1M sodium perchlorate. On page i349. first column, second paragraph, the last line should read: E , uranium(\? to (IV); and F , uranium(1V) to (111). The publication date of reference 7 should he 1935.
