Coulometric Titration of Total Antimony and Antimony(ll1) in Glasses SIR: A previous publication describes constant-current coulometric methods for determining tot a1 arsenic and arsenic(II1) in glasses in the presence of antimony (4). Since this technique is rapid and does not require standard solutions, the investigation was continued to develop procedures for also determining total antimony and antimony(II1) in the presence of arsenic. The results are discussed in this paper. EXPERIMENTAL
Apparatus and Reagents. T h e constant-current coulometric titration apparatus used for this work is described in the previouis publication (4). Necessary instructions for satisfactory conditioning of the Dowex-3 anion exchange resin, preparation of the ion exchange columns and reagents not discussed here are also given in the same article (4). The hvdrochloric acid-borax eluent was prepared by dissolving 30 grams of NaZB4O7.10HzO in distilled water, adding 140 ml. of concentrated hydrochloric acid, and diluting to 1500 ml. with distilled water. A 0.138 borax solution was prepared by dissolving the necessary quantity of Na2B407.10HzO in difitilled water and diluting to volume wit,h distilled water. The supporting electrolyte for the titration of t o t a l antimony was 0.1M sodium tartrate-0.02Jf Na2B407.10HzO. A solution of arseriic(II1) was prepared. Arsenic trioxide (NBS-83a) was dissolved in 5 ml. of water containing 0.2 gram of potassium hydroxide. The solution was diluted to 100 ml. and saturated with carbon dioxide. The solution was then diluted to 200 ml. with carbon dioxide--saturated water (1.00 ml. == 535 pg. Ae203). Potassium antimonyl tartrate hemihydrate, K(SbO)CJI406. '/zH20, was dissolved in distilled water and diluted to volume (1.00 ml. 580 pg. S b ~ 0 3 ) . An antimony(V) solution was prepared from antimony pentoxide. The oxide was fused with pota:.slum carbonate. The melt was dissolved in 0.1M potassium hydroxide and diluted to volume with the same solvent (1.00 ml. c 760 pg. SbzOs). Procedures. D E T E R M I N A T I O N O F TOTALA N T I M O N Y . 'The sample was ground to pass 200-mesh. A portion (50-100 mg.) was weighed t o t h e nearest 0.1 mg., transferred to a 30-1111. platinum crucible, and 2 ml. of 14M hydrofluoric acid was added. T h e sample was dissolved by swirling the mixture. T h e crucible was placed on a hot, plate and 1;he Folution was ev a 1) or a t ed to dry nes 3. The crucible was cooled, and the residue was moistened with 0.5 ml. of 14JI hydrofluoric acimd. One gram of oxalic acid was added, the sides of the crucible were washed down with a jet
of distilled wat'er, and the solution was evaporated to dryness on the hot plate. The oxalic acid was sublimed by heating the crucible in a n oven at' 150' C. The hydrofluoric acid-oxalic acid treatment steps described a t the beginning of this paragraph were repeated, in order, through the sublimation of the oxalic acid. The crucible was cooled and 1 gram of sodium tartrate was introduced. About 20 ml. of 0.1JI sodium tartrate0.02.18 borax solution was added. The mixt,ure was transferred to the titration beaker, and the solut'ion was diluted to about 60 ml. with the 0 . 1 V sodium tartrate-0.02.11 boras. The p H was adjusted to 7.0 with 1Jf hydrochloric acid, and 0.5 gram of sodium iodide was added. The electrodes were put in place, the magnetic stirrer was turned on, and the titration was conducted using a constant current of 6.43 ma. The end point was established by reading the indicator electrode galvanometer several times before and several times after the end point when the indicator electrodes registered values between 5 and 15 pa. The straight-line plot of indicator current vs. microequivalents was extrapolated to the residual current to establish the end point. -1blank was determined, and the value was subtracted from each result. DETERMINATION OF COMBINED .%RSENIC(III) . ~ N D .b?rIhfoNY(III). The sample was ground to pass 200-mesh. A portion, about 0.1 gram, was weighed to the nearest 0.1 mg. and was placed in a platinum crucible. The sample was moistened with 2.0 ml. of 6Jf hydrochloric acid. Then 2.0 ml. of 1451 hydrofluoric acid was added, and the sample was dissolved by swirling the mixture. After 15 minutes elapsed, 1.0 gram of sodium tartrat,e was added; and the solution was transferred to a titration beaker with 50 ml. of 0.1X borax and a jet of distilled water from a wash-bottle. The solution was stirred, and the pH was slowly adjusted to 7.0 with 1 9 X sodium hydroxide. Then 0.5 gram of sodium iodide was added. The titration and indication electrodes were put in place, and the solution was stirred. The titration was conducted
Table
It.
as described for determining total antimony. DE:TERMINa4TION O F ARSENIC(III). Another portion of the sample was weighed and dissolved with hydrochloric and hydrofluoric acids as described above. Five milliliters of distilled water, 3 ml. of 6-If hydrochloric acid, and 1 gram of borax were added. After the borax dissolved, the solution, totaling about 12 ml., was started through the column at an approximate rate of 6-8 ml. per minute. The crucible was then washed with three 3-ml. portions of hydrochloric acid-boras wash solution and the washings were poured onto the column. The column was washed with 3-1nl. portions of this eluent until a total of 50 ml. was used. The beaker containing the eluate was placed in a cold water bath, and 1.0 gram of sodium tartrate was added. The solution was stirred and the p H was slorvly adjusted to 7.0 with 19Jf sodium hydroside. Then 0.5 gram of sodium iodide was added. After the latter dissolved and the solution was a t room temperature, the electrodes were placed in the solution; and the titration was conducted as described in the total antimony procedure. This result was subtracted from the one obt,ained using the previous procedure to obtain the antimony (111) value.
Table I. Comparison of Coulometric and Other Methods for Determining Total Antimony in Glass
Sb,Ox. Glass type Borosilicate SiOn-BaO Si02-Ba0-Pb0 SiOz-Na2O-CaO SiOz-NaZO-CaO SiOz-BaO SiOZ-KazO-CaO a
?o
0.001 0.001 0.05
0.20 0.72 0.004 a
metric 1 0 0 0 0 0 0
c/,
Other
33 25 16 47 33 87
1 0 0 0 0 0 0
33
34a 26* 16c 4ib 35. 84< 35.
Polarographic (3).
* Titrimetric ( 2 ) .
Spectrophotometric ( 1 )
Results of Testing the Antimony(ll1) Method on Glasses
Sbz03, %
AS203
present,
Coulo-
Found, sa rnp 1e
Added
Total founda
Recovery
0.35 0.52 0.85 0.11 1.25 0.34
0.41 0.41 0.41 0.41 0.41 0.41
0.76 0.92 1.26 0.53 1.67 0.75
100 99 100 102 101 100
Average of three values.
VOL.
36, NO. 9, AUGUST 1964
1863
RESULTS A N D DISCUSSION
Xntimony(V) is reduced to antimony (111) by oxalic acid a t 150’ C., and if hydrofluoric acid is added, any arsenic (111) is volatilized as arsenic trifluoride by heating ( 3 ) . To determine if two treatments are sufficient for quantitative reduction of antimony(V) and removal of arsenic(III), some 1.00-ml. aliquots of the antimony(V) solution were mixed with 1.00-ml. aliquots of the arsenic(II1) solution; and the samples were carried through the procedure. The results of five replicate determinations showed that the antimony(V) is quantitatively reduced and the removal of arsenic(II1) is complete. The procedure was tried on some glass samples that had been analyzed
for total antimony by other accepted methods. A comparison of these results is given in Table I. On the basis of the satisfactory agreement it was concluded that this method is valid for determining total antimony in a variety of glasses. The antimony(II1) values were established in some glass samples containing arsenic(II1) by doing three replicate determinations using the second and third procedures given in the experimental section. Other portions of the same glasses were spiked with known amounts of antimony(II1) after dissolving, and the antimony(II1) was determined as before. The results are given in Table 11, and the data show that satisfactory recovery is obtained.
ACKNOWLEDGMENT
The authors are grateful for the help of P. R. Segatto who kindly offered valuable suggestions. LITERATURE CITED
(1) Van Aman, R. E., Hollibaugh, F. D.,
Kanzelmeyer, J. H., ANAL.CHEM.31, 1783 (1959). ( 2 ) Willard, H. H., Diehl, H., “Advanced Quantitative Analysis,” ppl 348-9, Van Nostrand, New York, 1943. (3) Williams, J. P., Schwenkler, T. A., J . Am. Ceram. SOC.38, 367 (1955). (4) Wise, W. M., Williams, J. P., ANAL. CHEM.36, 19 (1964). W. M. WISE J. P. WILLIAMS Glass Research and Development Corning Glass Works Corning, N. Y .
Thin Layer Chromatography of Vitamin A and Related Compounds SIR: Although several methods (4, 6) employing column and paper chromatography are available for the separation of vitamin Ai and related compounds, none is entirely satisfactory for the rapid separation and identification of trace amounts of these substances. Thin layer chromatography has been applied to the separation and detection of fat-soluble vitamins in general (Z> 3 ) and also to the separation of a few geometric isomers of the vitamin .Iland .Izseries ( 5 ) . I n these studies, only mixtures of pure components were employed and no quantitative data were given for the recovery of the samples put on the chromatogram. We have, therefore, attempted to separate a group of vitamin A compounds often encountered in fish liver oils and in liquid multivitamin preparations. These compounds include @-carotene, anhydrovitamin A,, retrovitamin A,, vitamin dl ester, vitamin A, alcohol, vitamin Az alcohol, vitamin -1,aldehyde, vitamin X p aldehyde, and vitamin A, epoxide Methods of separation, identification, and quantitative recovery of these materials employing the thin layer chromatographic technique are described. This technique was also applied to eluates from column chromatography to study the purity of fractions. EXPERIMENTAL
Crystalline all-trans vitamin AI alcohol, vitamin AI acetate, vitamin AI aldehyde, vitamin .Al acid, and @-carotene were obtained from Distillation Products Industries, Ltd., Rochester, S . Y. Anhydrovitamin .Al [E2 1814, 2700, and 2330 a t 350, 367, and 390 mp, respectively (all absorbancy values were measured in petroleum ether)] was prepared from crystalline all-trans 1864
ANALYTICAL CHEMISTRY
vitamin AI alcohol by the reaction with p-toluenesulfonic acid ( 7 ) and purified by repeated chromatography on alumina. Crystalline all-trans vitamin -4, acetate was reacted with aq. HBr ( 1 ) and the reaction mixture was subjected to repeated chromatography on alumina to obtain retro-vitamin i l l acetate (E:,”, 1830 and 348 mp). Retro-vitamin AI alcohol was prepared by saponification of retro-vitamin X i acetate. From the nonsaponifiable fraction of a freshwater fiih liver oil, vitamin -A2 alcohol (E::, 790 a t 350 mp) was purified by chromatography on alumina. Vitamin .Az aldehyde ( E : z 64 a t 385 mp) was also separated from the same fish liver oil. Vitamin A, epoxide (A max. 270 mp; E::; 440 a t 270 mp) was purified from a deteriorated sample of crystalline all-trans vitamin hl alcohol by chromatography on alumina. A slurry was prepared by mixing aluminum oxide, G (neutral, for thin layer chromatography; 13rinkmann Instruments, Inc.), and water in the ratio 1 : 2 (w./v.) and applied to clean glass plates 2 X 71/2 inches or 7 ’ / 2 X 7 l / 2 inches with a glass rod. The plates were allowed to stand for 10 minutes a t room temperature and then placed in an oven at 100” C. for 30 minutes. Five micrograms of each of the compounds and 20 pg. of each of the mixtures, dissolved in petroleum ether or diethyl ether, were applied to the cool plates with a micropipet and the chromatograms developed with the appropriate solvent system. The solvent front was allowed to move 3 to 4 inches from the origin (requiring about 20 to 30 minutes) before the plates were removed from the chromatography chamber. The compounds were detected and characterized in the following manner : (1) characteristic fluorescence under UV light or color of the spot on the chromatogram; ( 2 ) mixed chromatography with standards or by compari-
son with standards on the same plate; (3) development of color when sprayed with Carr-Price reagent, and (4) elution of the compound, as described below, and measurement of the absorbance spectrum. The elution of the compounds from the plate was achieved by two methods. I n one, the alumina containing the spot was carefully scraped from the plate, transferred to a 10-ml. volumetric flask and shaken a few times with 20% (v./v.) diethyl ether in petroleum ether. The extract was decanted and the extraction repeated twice. The combined extract was evaporated to dryness under reduced pressure, the residue dissolved in petroleum ether, and the spectrum examined in a Beckman DL spectrophotometer. In the second method, the elution was carried out by the technique shown in Figure 1. Alumina on either side of the spot was uniformly scraped from the chromatogram with the flat end of a metal spatula at right angles to the direction of the solvent flow leaving a width of about 0.25 inch of the glass plate clear on either side of the spot. The plate was clamped on both ends and a solvent mixture consisting of 15% (v./v.) ethanol in cyclohexane was allowed to flow a t a slow rate from a separatory funnel along a v-shaped strip of filter paper attached to the tail of the funnel. The chromatogram was brought just behind the paper strip soaked with the solvent and the paper allowed to come into contact with the surface of the alumina. The paper was held firmly on the plate by surface tension. The elution was carried out with about 2 ml. of the solvent for every microgram of the compound spotted. Whenever the elution technique was employed for the spectroscopic characterization of the compounds, the quantities applied to the plates were sufficiently large so that the solution of the recovered material gave an absorbance of a t least
